lithospecific piezometer manual

7/29/2019 Lithospecific Piezometer Manual

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Government of India & Government of The Netherlands

DHV CONSULTANTS &

DELFT HYDRAULICS with

HALCROW, TAHAL, CES,

ORG & JPS

MANUAL

DESIGN AND CONSTRUCTION

OF

LITHOSPECIFIC PIEZOMETER

September 2002


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September - 2002 TOC

Table of Contents

Preface i

Scope of the Manual ii

1 Introduction 1

1.1 General 11.2 Characteristics of geological formations 21.3 Significance of lithology in the construction of piezometers: 31.4 Groundwater monitoring in India- an historical perspective 41.5 Updating the existing network- based on current objectives 51.6 Macro-level planning 61.7 Micro-level planning 71.8 Desk studies 81.9 Field investigations 91.10 Finalisation of piezometer location 12

1.11 Reporting of field investigations 131.12 Approval for piezometer construction 141.13 Discussion and interaction with local community 14

2 Drilling preparation 15

2.1 Planning 15

3 Construction of piezometer 19

3.1 Selecting the appropriate drilling technique 193.2 Deciding the depth of piezometers 193.3 Diameter of piezometer 213.4 Actions to be taken prior to drilling 213.5 Piezometer construction in unconsolidated formations 21

3.6 Sampling procedures during drilling 233.7 Down hole inspection 243.8 Piezometer Completion 24

4 Piezometers construction in consolidated formations 29

4.1 DTH drilling characteristics 294.2 Sampling procedures for consolidated rocks 29

5 Measuring water levels 31

6 Groundwater sampling 33

7 Documentation of piezometer construction 35

8 Piezometer nest 37

Annexure I Hydrogeological Frame Work of Peninsular India 38Annexure II Geo-physical bore hole logging 48

Annexure III Aquifer parameters and well characteristics 53


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September - 2002 Page i

Preface

With the commencement of World Bank Assisted Hydrology Project in nine PeninsularStates of the country, a sizeable programme for establishing groundwater monitoring

network has been taken up. The main objectives were to augment the existing network ofobservation wells by construction of dedicated piezometers for monitoring groundwaterlevels and quality. Many of these piezometers have been provided with Digital Water LevelRecorders (DWLRs) enabling recording of high frequency water level data. Thesepiezometers are intended to serve as primary stations for monitoring purposes. This hasnecessitated formulation of guidelines for location and siting of piezometers, theirconstruction and design so that the primary stations truly reflect the groundwater regimebehaviour of the aquifer under monitoring. Besides, there is a need to lay down the precisepractices for the design and collection of data during drilling. The hydrogeological setting ofthe Peninsular India represents a varied environment with differing lithological settings,especially in consolidated formations, which are predominant in the HP States. The differentlithological environment in conjunction with climatological and land forms call for a separate

procedure to follow. Hence the manual is considered essential for reference to the fieldworkers and practicing Hydrogeologists.

A Manual on Guidelines for Implementation of piezometers has already been released bythe Hydrology Project, during July 1998. The present Manual seeks to present the practices,which should be followed during selecting the location of the piezometer, drilling,construction and design in the different geological formations commonly encountered in atypical hydrogeological environment. The Manual also deals with the methods of sensitizingthe piezometer to respond to the aquifer inputs and out puts; maintenance and rehabilitationof the piezometers.

One of the main aims of the Hydrology project is to install scientifically designed andcorrectly installed piezometers to monitor piezometric head of shallow unconfined anddeeper confined aquifers. Though the design criteria and field operations are well known toall the field practitioners, certain aspects need to be well understood and assimilated into thepractice of implementing a piezometer. The manual gives the guidelines, which are expectedto assist the professionals in realising the necessary reorientation of their drilling experienceand expertise.


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September - 2002 Page ii

Scope of the Manual

An earlier Manual on Guidelines for the Implementation of piezometers was prepared andreleased by the Consultants in July 1998. The Manual dealt with the optimal network design

and the details of piezometers in unconsolidated and consolidated formations. However, asmajority of the participating HP States fall in hard rock areas of the country, a need has beenfelt to deal with various aspects of piezometers in different lithologies along with methods ofdrilling, design and pumping test of piezometers.

The manual is intended to serve as a practical guide to the groundwater field workers and asa tool to visualise the hydrogeological situations and ground realities of the piezometer siteand what results could be expected. And remedial measures to be adopted to revitalise apiezometer.

This Manual seeks to highlight the concept of lithospecific Piezometers, criteria for

prioritisation of areas for location and site selection using Remote Sensing and othermethods. The procedures of Drilling and Design of Piezometers along with an account ofmethods of analysis of pumping test data of the piezometers for different types of aquifershave been described. Also topics on Development of piezometers and their Maintenancehave been discussed using inputs from various sources. Finally selection and installation ofappropriate type of water level recorders in tune with the requirements for litho specificpiezometers has been discussed in the manual. It is hoped the manual will meet theguidelines for hydrogeologists engaged in planning, execution and field operations and dataretrieval from piezometers as per their requirements.


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September - 2002 Page 1

1 Introduction

1.1 General

Groundwater monitoring is an essential tool to obtain the information on groundwaterquantity and quality data through representative sampling. This helps in recording theresponse of groundwater system to a natural or artificial input and out put. Any planning forgroundwater development should be guided by regime monitoring indicators such as waterlevels and quality changes over a period. Groundwater is a dynamic resource requiringcontinuous monitoring of its quantity and quality data for updating and assessment ofavailable resource potential. Such an updating can be made possible by using a soundobservational database from a scientifically well established network monitoring system.

Groundwater observation monitoring network stations or piezometers are constructed torecord the response of groundwater regime to the natural and artificial recharge and

discharge conditions. Keeping in view the regional and local requirements, the planning anddesign of such a network depends upon hydrogeologic, physiographic and climaticsituations, purpose of the study, stage of development, as well as political and socialdemands (UNESCO, 1977). The various types of observation networks can be setupdepending on the objectives e.g., hydrogeological, water management, baseline waterquality and for specific purposes. In the ongoing Hydrology Project, the objectives of theobservational network mainly include high frequency groundwater level and groundwaterquality monitoring.

A few of the salient features of the groundwater monitoring system are as under:

Strengthening of the existing network through construction of purpose built observationwells (piezometers) through identifying gaps in the data.

Ensuring integration of networks of Central and State Groundwater Organisationsavoiding any duplication.

Achieving optimum observation network density in the given area.

Installation of high frequency water level measuring devices like Digital Water LevelRecorders (DWLRs) on piezometers at key /nodal locations.

Establishment of Data Centres at Unit level, Regional level and National level to handle,storage, validation, synthesis, retrieval and dissemination of data generated to useragencies

Development of Hydrological Information System (HIS)

To ensure transparency in the availability of demand driven groundwater data requiredby the User community.

Ascertain the data needs, data type (historical, real time etc.), parameters of datarequirements of user community through Hydrology Data User Group (HDUG) meetings.

For implementation of the observation network monitoring, CGWB and the StateGroundwater Agencies of the participating States have constructed a sizeable number ofpurpose built observation wells (piezometers) with the provision to install DWLRs at keylocations and suitable pumps for water sampling. Some of the piezometers constructed arereplacement to old, defunct existing open wells, due to de-saturation of aquifer, disuse, andaging, among other factors.


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The observation wells/piezometers are usually of small diameter so as to accommodate thewater level measuring device and the water-sampling pump. In unconsolidated formations,piezometers are provided with screens tapping the zone of interest; where as in theconsolidated rocks, piezometers are left open ended (uncased) beneath the loose soil/looseover-burden where the hole has to be provided with a casing. Up-gradation/strengthening ofthe observation well network is a continuous process which would require replacement ofnon performing open wells with dedicated piezometers as well as construction of deeppiezometers to cover aquifers that have not been previously monitored. Improvement in thedensity of the network would also arise with time. All this would involve construction of manymore piezometers. The present manual aims to serve as a reference guide duringpiezometer construction. It is expected that this manual would also help the differentagencies to formulate Protocols and Procedures for construction of piezometers. Thedetailed piezometer construction procedure must contain number of elements plus anyadditional site-specific elements, which may be required. This manual describes thesignificance of different elements in the piezometer construction

1.2 Characteristics of geological formations

The unconsolidated geological materials are generally composed of sand, gravel, and clay invarious proportion as alternate layers and are characterised by occurrence of primary(interstitial) pore spaces which provide the main loci for storage and movement ofgroundwater in the saturated zone. These materials are often assumed to behave ashomogeneous and isotropic media. Yet, the homogeneous aquifers seldom occur in nature,with most aquifers being stratified to some degree. Due to this, the hydraulic conductivity isfound to differ in horizontal and vertical directions.

Rock

group

Rock types Mode of occurrence Main features

important forgroundwateroccurrence

Crystallinerocks

Non-volcanic igneous andmetamorphic rocks, viz.Granites, gneisses, schists,slates and phyllites, etc.

Large size massifs andplutons; regional metamorphicbelts

Weathered horizon,fractures andlineaments withsecondary porosity

Volcanicrocks

Basalts, andesites andrhyolites

Lava flows at placesinterbedded with sedimentarybeds

Fractures, vesiclesand inter-flowsediments

Carbonaterocks Limestones and dolomites Mostly as chemicalprecipitates with varyingadmixtures of clastics in alayered sedimentary sequence

Fractures andsolution cavities

Clasticrocks

Consolidated sandstonesand shales

Interbedded sedimentarysequence

Inter-granular porespaces andfractures

Table 1.1: Hydrogeological Classification of Consolidated Rocks(after Singhal & Gupta, 1999)


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The consolidated geological formations dominate the Peninsular India. These formations aredevoid of primary porosity and permeability, but tend to acquire some hydraulic conductivitythrough joints, fracturing, weathering and other geological processes. From hydrogeologicalpoint of view, these are classified into Crystalline rocks, Volcanic rocks, Clastic rocks andCarbonate rocks. Groundwater occurrence in these rocks is mainly dependent upon the

degree of weathering and consolidation of fractures and fissures, which form the main flowconduits. Table 1.0 gives chief rock types and brief mode of occurrence of groundwateralong with main features of occurrence of groundwater in each such formations.

The table shows that in contrast to the dominant primary porosity as a main feature forgroundwater storage and movement in the unconsolidated formations, distinctly differenthydrogeological frame work and flow features characterise the consolidated formations. Thelocation and depth of the network observation wells and/or piezometers in such formationssolely depends on the factors like the thickness of weathered zone, occurrence andcharacteristics of fractures and related hydrological features. In the case of uniformly anddensely fractured rocks, the site selection and construction of such piezometers can be more

or less similar to that in unconsolidated aquifers. However, in case of non-uniform fracturing,or in weathered zones of crystalline rocks, in carbonate aquifers with solution cavities and inbasaltic aquifers with lava vesicles and tubes, the decision on the placement and depth ofpiezometers may require detailed studies of the hydrogeological situation. Thehydrogeological framework of peninsular India is described in details in Annexure-I.

1.3 Significance of lithology in the construction of piezometers:

Groundwater occurs in the aquifers either as an individual horizon or as multiple layers. Forproper accounting of resources and judicious planning of exploitation, it essential to monitorthe water levels which are indicators of its potential at different times. The quantity and

quality of water occurring in the aquifers depend upon its mineralogical and geochemicalconditions at the particular level. For this independent observation wells are constructedknown as piezometers.

All water level monitoring programs depend on the design of piezometer. Decisions madeabout the design of the piezometer and its location are crucial to water data collectionprogram. Ideally, the piezometer constructed as part of the monitoring network need toprovide data representative of the different geology, lithology and groundwater developmentenvironments. Decisions about the real-areal distribution and depth of completion ofpiezometers should take into consideration the physical boundaries and geologicalcomplexity of the aquifers under study.

Water level monitoring in complex geological and lithological environment may requiremeasurements of water levels in multiple piezometers (nested) constructed at differentdepths tapping different aquifer units representing varied lithological and geological units inthe area. Large geological/lithological units that extend beyond the state boundaries requirea network of piezometers that have representation beyond the states distributed among oneor more states. One of the purpose of a network is to monitor ambient groundwaterconditions or the effects of natural, climatic-induced hydrologic stresses, the piezometernetwork will require monitoring structures that are representative of regional geological,lithological units that have lateral and vertical continuity and represent the horizontalgroundwater flow regime without any major gaps. The aim should be to ensure that there areno mixing up of information due to improper piezometer design. These and many other

technical considerations pertinent to the design of a piezometer focussing on lithological andgeological units is discussed in detail.


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Commonly overlooked is the need for study of geological map and available reports of thearea giving details on the mineralogical and lithological information before deciding about thelocation and design of the piezometer. Good understanding of the lithology of the area helpin designing the appropriate piezometers that enable collection of accurate, authentic andprecise water level data, which will reflect true conditions in the aquifer being monitored andprovide data that can be relied upon for many intended uses. Therefore field and officepractices that will provide the needed levels of quality assurance for water level data shouldbe carefully thought out and consistently employed. In the construction of piezometers theprincipal objectives should be:

to monitor the water levels and water quality of independent aquifer

to understand the relationship between different aquifers

to understand the hydraulic characteristics of different aquifers.

to evaluate groundwater regime characteristics

to understand the regional flow characteristics

to refine groundwater resources assessment

The procedure and protocol for design and construction of piezometers shall be dictated bya number of factors including the geology, hydrogeology, lithology, aquifer geometry notforgetting the objectives of the monitoring network. Thus prioritisation of the piezometer siteas well as their design and construction should have a clear bearing and perception of thegeology, lithology and aquifer type. A geological map, lithological cross section, structuralmap, geomorphological map and geophysical survey reports are the important tools that willhelp in understanding the regional geological control on the groundwater system which is animportant consideration for the piezometer design. This brings to the fore the need toconsider different lithologies separately for hydrological studies necessary for identification ofrepresentative piezometer sites. This has lead to introduction of the concept of lithology-specific-piezometers, commonly referred to as Lithospecific Piezometers.

1.4 Groundwater monitoring in India- an historical perspective

The Central Groundwater Board in 1968 started groundwater monitoring as part of itsactivities with one observation well for each toposheet over the entire country. In all 68observation stations were established. Gradually with the need more number of stations withlithological representation were also added. Mostly existing open wells owned by farmers orutilized for drinking water were included in such monitoring systems. With the operation ofgroundwater exploration and resource evaluation projects under UNDP and other addedprojects many observation network stations were established tapping shallow as well as

deeper aquifers and amalgamated in the regular groundwater monitoring system.

These were mostly on the basis of availability of wells as a sort of compromise and not onthe basis of requirements at the specific locations. The water levels were measured initiallytwice, pre-monsoon and post monsoon period, which subsequently was converted to fivetimes in a year falling in the months of January, March, May, August and November months.From 1986 onwards 4 times in a year is measured in the months of January, May, Augustand November. The data collected is utilised in specific reports for reporting on fluctuationsand assessing water resources for the administrative divisions.

By 1972 the State Groundwater departments also came into establishment and gradually

groundwater monitoring was taken up. The density of network observation wells in alluviumwas about one well per 100 sq. km on an arbitrary basis, while in hard rock it was more than


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that mostly in localized areas with groundwater development. Also in exploratory areas withpossible scope for groundwater development, monitoring was enhanced with addition ofpiezometers constructed for well field studies both in soft rock as well as hard rock areasunder various national /international added projects in the country. At best these monitoringnetwork stations served as indicators on baseline water level and water quality data. Theinformation emanating from such networks has generally permitted conceptualisation of thegroundwater system and its resource evaluation. These were in essence need-basedpiezometers rather than scientifically required for country monitoring system.

The data generated was mostly utilised for use in the internal report preparation by thedepartments and evaluation of groundwater resources for administrative units for the countryas a whole. The depth to water level maps were prepared and interpreted for response ofaquifers to various natural inputs from rainfall and canal/irrigation returns in terms of mapsboth for pre-monsoon and post-monsoon season. Also maps on water quality coveringelectrical conductivity and iso-chloride and total dissolved salts were prepared andinterpreted.

The data generated were with certain inaccuracies as the monitoring wells were one thoseused for drinking as well as irrigation, as a result exact water levels were not possible.Subsequently, with the advent of tube wells and bore wells in hard rock areas which werefitted with electrically operated pumps, the water levels started declining and many of thedug wells went dry during summer period. As a result, lowest water level data could not berecorded. Some of the old wells went into disuse or were dumped with garbage and as suchdata collection was not possible, leading to data gaps.

Topics include in this document are: network review, site investigations, piezometerconstruction, development, discharge measurement, performing aquifer tests, and waterquality sampling.

1.5 Updating the existing network- based on current objectives

The first task before construction of new piezometers is to review the existing monitoringnetwork at the micro level i.e; drainage basin, geological basin and in limited circumstances,considering only the administrative boundary as a unit. The review has to integrate themonitoring wells of all the agencies involved with water level and water quality monitoring.The review has to be necessarily be aquifer wise. The review should be based on allavailable data. The evaluation should lead to identification of the data gaps (spatial andvertical).

The review has to be based on the data available from the networks related to; aquifer wisedensity, depth of the aquifers and water level plus water quality. The review should alsoevaluate the areas where the data generated from the existing network has been used i.e.;

groundwater resource assessment, understanding the groundwater flow dynamics, delineation of recharge/discharge areas, regional groundwater quality variations over space/time

Based on the review the adequacy of the network has to be evaluated; areas showing gapsin understanding have to be identified; areas showing more than adequate numbers ofobservation wells have also to be identified and duplicate observation wells, if any, have also


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to be considered for elimination or reducing the monitoring frequency. In case an existingnetwork (with respect to a specific aquifer) is found to be inadequate, additional piezometers,tapping that specific aquifer, need to be provided.

The first step towards planning to the enhancement shall comprise of macro-level planning,

i.e., estimating the required number of additional piezometers and their location at a macro-level (say on a map of scale 50,000). The subsequent step shall involve pinpointing the sitesfor the additional piezometers on the ground, i.e., micro-level allocation.

1.6 Macro-level planning

Depending upon the intended use of the data from the network, the macro-level planning ofthe network enhancement can be accomplished using the statistical tools in the dedicatedsoftware

1.6.1 Coefficient of variation method

The method requires the user to specify the maximum permissible error in the estimate ofthe mean water level. Subsequently, based upon an analysis of the data from the existingnetwork, the required number of the piezometers is computed, from which the additionalnumber of the piezometers are derived.

The following procedure is adopted for locating the additional piezometers within thespecified area.

Employing the concurrent data from the existing network, draw contours of water level at

a uniform interval. Divide the entire area into zones, each zone representing an area falling between twosuccessive contours.

Divide the required number of piezometers equally among all the zones. This will ensurea greater densityof the piezometers in the regions of steeply sloping piezometric headand vice versa.

Count the number of existing piezometers in each zone and hence estimate zone-wise,the required number of additional piezometers.

Locate the additional piezometers in each zone in such a way that the piezometers(existing and additional) are uniformly distributed within the zone.

1.6.2 Kriging

Kriging is a powerful tool for evaluating an existing network. It also assists in the macro-levellocation of additional piezometers, in case the existing network is found to be inadequate.The steps involved are as follows:

Specify the level of permissible interpolation error. Conduct kriging on the concurrent piezometric data from the existing network. This shall

yield contours of piezometric head and of the interpolation error.

Study the error contours and hence identify the regions where the error is in excess of

the specified permissible level. Additional piezometers are to be allocated to theseregions.


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Locate additional piezometers in the identified regions tentatively, generally ensuringthat the increase in the network density is consistent with the error excess.

Conduct kriging on the tentatively enhanced network and plot contours of the error. Itmay be noted that kriging permits generation of such contours, even though the datafrom the newly introduced piezometers do not yet exist.

Study the modified error contours and check whether the error everywhere falls belowthe specified limit and the enhancement has not been over-done. An over enhancednetwork shall display interpolation errors far less than the prescribed limit.

Modify the network further, if necessary by repeating the relevant steps

1.7 Micro-level planning

After having decided the location of the piezometer sites on the map, it is essential topinpoint the site exactly on the ground. Certain micro-level deviations may be necessary toaccommodate various hydrogeological and logistical considerations.

1.7.1 Hydrogeological considerations

These considerations originate from the primary expectation out of a piezometer, i.e. itshould record harmonized natural behaviour of groundwater rather than local micro-trends.This can be ensured by keeping in mind the following:

The site should show no impact of any external inputs such as from canal, tank,perennial river and irrigation return flows, except in special cases where interest is thestudy of the influence of these parameters on groundwater system.

The site should not fall within the radius of influence of a well, which is under pumping;

but it should be capable of recording the effects of the pumping as a regionalphenomenon.

The piezometric head/water quality at the site should not be influenced by localrecharge/pollutant sources.

1.7.2 Logistical considerations

There could be many general as well as area-specific logistical considerations such as:

No other agency is considering constructing a piezometer tapping the same aquifer, in

the vicinity. The site is approachable by the rig and support vehicles. Adequate space is available at the site for setting up drilling equipment, digging mud pit

and draining the discharge, while the site should be clear of trees, overhead electriccables, under ground cables/ pipelines/ drainage lines etc.

The ownership of the site is clear and agreements have been made for drilling thepiezometer and for continued monitoring.

The site should be safe from vandalism, as a costly DWLR will be installed. The site should be neither too close nor too far off from the road.


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1.8 Desk studies

Based on the network review and identification of the area where additional informations arerequired piezometer site selection has to be initiated. Desk studies need to be carried out inthe office through review of topographical maps, geological maps, geophysical surveys data,geological cross sections, drilling data, water table/piezometer maps, water quality maps etc.Data emerging from the desk studies should be systematically organised, location wise, forcarrying to the field for field review and investigations.

1.8.1 Remote sensing interpretation map

The Hydrology Project has been involved in the creation of GIS data sets in which thethematic maps are generated using satellite data. The thematic maps, should be used duringthe desk review for the locating the appropriate sites for the piezometers. The Remotesensing maps have to be the basis for delineating the faults, lineaments, study the geology,

hydrogeology, land use etc. Using the GIS capabilities different themes should be overlaid tozero on the most appropriate location. Based on the GIS studies and remote sensinginterpretations inference on the subsurface soil moisture, recharge potentialities need to beestimated. The Remote sensing interpretations should be used to interpret features like karsttopography, dykes, reefs, unconfirmities, buried channels, salt encrustations, tide levels,alluvial fans and abandoned channels etc.

In the hard rock terrain's the remote sensing studies should help in understanding the spatialdistribution of rock out-crops, the catchment characteristics, the presence of structures anddrainage systems influencing the groundwater movement, the nature of the land form andthe slope based on which interpret is the likely thickness of regolith/overburden, the generalgroundwater potentiality and the most preferbale location for constructing the piezometer.

For this purpose, the GIS datasets related to geology, its structures, geomorphology,drainage and soil should be integrated and interpreted.

The satellite imageries provide a good idea of drainage network for computing drainagedensity. Drainage density exhibits a very wide range of values in nature depending upon therelief, climate, and resistance to erosion and permeability of rock material. In general, lowdrainage density (1.9-2.5 km-1) is characteristic of region of highly resistant or highlypermeable surface and low relief. High drainage density (12.5 19.0 km-1) is found inregions of weak or impermeable subsurface materials, sparse vegetation and mountainousrelief. In areas of low relief, drainage density may be more indicative of permeability ofsurface material and therefore, could be used as a criterion for the selection of suitable sites

for piezometers. The drainage analysis is utilized to differentiate the terrains into highlydissected plateau (HDP), moderately dissected plateau (MDP) and poorly dissected plateau(PDP) (see figure 1.1).


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Figure 1.1: Map showing lineaments in the hardrock area in Salem area,of Tamil Nadu

Study of lineament is the most important aspect of satellite image interpretations forgroundwater studies in the hard rock terrains. It has been established that the groundwater

structures constructed close to fractures of tensional origin, or close to their intersections,have proved extremely successful. Siting of piezometers near such favourable structuresshould be considered and such areas clearly marked on the toposheet of 50,000 scale andinspected in the field.

On the satellite imagery, the lineaments can be easily identified by digital image processingas well as visual interpretation, using tone, colour, texture, pattern, and association. Theautomatic techniques of digital edge (or line) detection can be applied for lineamentdetection (Singhal and Gupta, 1999). However, fracture traces having low dips, which havemore potential for groundwater may not be very easily deciphered. Staff with extensive fieldexperience would be able to make such interpretations easily.

1.9 Field investigations

The Field investigations consists of a number of elements including, geological,hydrogeological, geomorphological and hydrological investigations

1.9.1 Geological investigations

Geological map of the area on 1: 50,000 or 1:250,000 scale prepared by national agencieslike Geological Survey of India or State Mines and Geology departments which has beenconverted to digital format as part of the GIS data set preparation should be printed andcarried to the field these maps help to visualize the occurrence of rock formations, theirdisposition, sequences and structures, faults, dykes etc. Surfacial distribution of rocks and


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their regional continuity should also be visualized. The different lithological and structuralfeatures like joints, lineaments, foliation, discontinuities, degree of susceptibility of rocks toweathering, from Dug well sections need to be studied.

Field investigations should also include information about the orientation and density of

fractures, although their subsurface distribution may be different which can be decipheredfrom subsurface investigation. A kinematic analysis of fracture pattern and lineaments isoften useful in delineation of their tectonic origin. Whereas, plotting of the dip and strike ofjoints on Schmidt's Stereo net and as rose diagrams can bring out synoptic, structurallyweak zones.

Data about the thickness and composition of the weathered zone (regolith) is particularlyimportant in crystalline rocks. The minerological composition of weathered products,particularly presence of interstial clay matrix or its absence is important. The texture ofquartz grains with respect to their roundness, sphericity, angularity and abundance of willindicate in-situ deposition or transported deposition. The abundance of orthoclase,

anorthosite minerals give clue to the extent of weathering in the rock as these are mostdissoluble minerals. Similarly, mica is also unstable. The recharge, discharge zones withgeomorphic locations and drainage system help greatly in identifying the suitable location.

In volcanic rocks presence of vesicular basalts, its thickness and geomorphic locations areimportant from the view point of groundwater occurrence. The vesicular and amygdloidalbasalt is most susceptible to weathering. The vesicles with tubular structures form goodwater conduits in basalt. Added to this, fractures and lineaments enhance the potential of therock unit. The hard basalt with fractures underlying the vesicular basalt also forms potentialwater bearing zones in basalt. Attention should also be paid to the palaeodrainage,characters of individual flow units including their dips and inter-flow formations. The surfacedrainage plays an important role in basaltic rocks. The recharge-discharge zones should

also be identified. The above mentioned details will help greatly in identifying suitablelocation of a piezometer in basalts.

In carbonate rocks, mapping of various solutions (karst) features are of special importance.In carbonate rock areas, the geological map with occurrence of karstified and dolomitic typeof rock disposition, better groundwater potential can be visualized better. Presence ofsinkholes and valley depressions form main recharge zones. Presence of springs givesclues of solution channels. However, flaggy and bedded disposition of lime stone withmonotonous topography display low potential zones.

In unconsolidated and semi-consolidated formations nature of deposits are important. Valleyfill deposits tend to be of assorted nature, river/fresh water deposits are likely to be withfrequent variations in textures of grains, even though there may be continuity in sequence ofbed, but will be with variations in lateral porosity. Sudden truncation or swelling of aquifersare common. This needs to be properly visualized through lithological cross sections,prepared on the basis of existing drilling data.

Based on exploratory drilling and well log data, following subsurface maps and sections areprepared viz. fence diagrams, isopach maps, structural contour following maps etc. This isdone to able to project the subsurface distribution and configuration of aquifers, aquitardsand aquicludes. Water table/piezometric contour map if available should also be studied foridentifying gaps and location of suitable site for piezometer.


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1.9.2 Hydrological Investigations:

Drainage pattern is the spatial arrangement of streams and is, in general, very characteristicof rock structure and lithology. These drainage patterns reflect the hydrogeologicalcharacteristics of the area and therefore can be useful in the location of piezometer sites.Figure 1.2 gives common drainage patterns in consolidated and unconsolidated formations.

Figure 1.2:Common drainage patterns (A.D. Howard)

The drainage maps have been created as part of the GIS data set perpetration. Thedrainages have been digitised from the toposheet of 50,000 scale and updated using thethematic maps. The drainage maps have also been used in delineating the differentdrainage order, from the major basins, down to watershed units.

During the field investigations the position of the piezometer location has to be ascertainedwith respect to recharge area/run off zone/discharge area.

1.9.3 Geomorphological investigations

Geomorphological map of the area on 1: 50,000 or 1:250,000, scale available as part of theGIS data sets, should be printed and taken to the field for visualizing the various landforms.Genetically, the landforms are divided into two groups: erosional, and depositionallandforms. Erosional landforms are typically associated with the resistant hard rock terrains.They comprise: (a) residual hills, (b) inselbergs, (c) pediments, (d) buried pediments withweathered basements, and (e) valley fills. Depositional landforms, developed by depositionalprocesses of various natural agencies, (e.g. river and wind) are typically made up ofunconsolidated sediments and may occur in the regional setting of hard rock terrains.

Favourable landforms that contribute significantly to groundwater recharge should be


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identified in the field. The possibility of locating the piezometer in such areas should beexamined.

1.9.4 Geophysical Surveys:

Geophysical surveys need to be carried out as a standard procedure for getting a clearunderstanding of the following depth to bed rock, thickness of weathered zone, extent ofsaturated zone, approximate quality of water in the saturated zone, thickness of differentlayers in layered formations and type of layered formations. Influence of structures like fault,unconfirmities and dykes can also will be evaluated. Occurrence of saline and fresh waterlayers with probable depth of occurrence also will be indicated.

Electrical resistivity survey is the most commonly used method to identify the verticallithological layering distribution in an area. New approaches using the VLF method, Electro-Magnetic methods, Gravity Methods have to be used wherever possible. The mainobjectives of geophysical surveys are to provide information on:

Depth, thickness and extent of aquifers in stratified formations.

Depth, thickness and extent of weathered and fractured zones.

Depth to water table.

Selecting the site of a piezometer, out of the several target areas.

Gross Salinity distribution and contamination.

1.9.5 Hydrogeological investigations

Hydrogeological investigations should include detailed well inventory of 2-4 sq.kms around

the proposed area. All the groundwater abstraction structures need to be inventoried and theinformation to be collected should include the depth of the well, aquifer position, rate ofpumping, pumping duration, drawdown, rate of recuperation, area irrigated, lithologyencountered while construction, static water level, water quality details etc. Collect watersample and carry out field analysis for pH and EC. Collect two sets of representative samplefor detailed laboratory analysis.

The existing monitoring wells/piezometer around the proposed site needs to be visited andthe variations if any with the proposed site understood. Examine the water level hydrograph.Examine the water table elevation contour maps and depth to water table maps generatedusing two sets of data (pre-post monsoon).

Prepare lithological cross section/ fence diagram using the data from the inventoried wells,delineate the prominent aquifers in the area with their thickness and areal extent. Carry outpumping tests/ geophysical downhole logging where adequate information cannot begathered during well inventory.

1.10 Finalisation of piezometer location

Based on all the studies and keeping in mind the logistical and safety considerations thepotential site has to be identified. Where more than one site is considered then a joint teamof hydrogeologists should visit the area and identify the most favourable location. The siteselected should be verified for its true representation of the area specific lithology and


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regime system. The interference from pumping wells, surface water sources, pollutingsources, seepage from return flows should be avoided at all costs.

1.11 Reporting of field investigations

Based on the field investigations a feasibility report has to be prepared. The followinginformation must be documented as a file giving details of the procedures followed indeciding the piezometer site. The report should include:-

A sketch showing the identified site and important landmarks in the vicinity. The sketchshould incorporate the north direction and the distance of the site from the landmarks.

Locate the site on the toposheet of 1:50,000 scale. Record its longitude, latitude and thereduced level as read from the toposheet. Use the hand held GPS wherever availablefor getting the geographical co-ordinate values.

A narrative of the geographic setting of the piezometer site with administrative details.Details pertaining to sites adjacent to or in the vicinity of school, sub station, police

station, floodplains, wetlands should given. A narrative describing the regional lithologic, stratigraphic, structural, and hydrologic

settings of the area.

A narrative must be provided which describes field procedures used to characterizegeologic and hydrologic conditions of the site. Standardized field procedures may bereferenced. Details of the site-specific geology and hydrology based on data collectedshould be explained. The narrative must describe the proposed piezometer design.Interpretations of results must be presented in a clear and concise manner. Allpublished information sources used in the compilation of the hydrogeologic investigationmust be listed.

Appendices of the report must include: Compiled logs of all borwells and piezometers. The raw data for any and all tests (e.g., geophysical survey, bore hole logging,

water quality analysis, pumping tests).

Water level hydrographs of monitoring wells in the neighbourhood

Water table elevation contour maps

Hydrometerological data of the area

All additional information that may facilitate the clearance of the proposed site.

The exact location should be marked on the ground with paint. Lithologic cross-sectionsmust be constructed or inferred. At least one cross-section must be constructed parallel to

groundwater flow. The subsurface conditions of the site must be illustrated in these cross-sections. Where more than one interpretation may be reasonably made, conservativeassumptions must be used. A clear picture has to be given of the thickness, depth andlateral extent of the aquifers in the area with a clear definition of the aquifer to be monitoredand the geo-hydrologic conditions. The type of monitoring required and the need if any for aDWLR and sampling pump should be brought out. The report should clearly bring out theneed for the Piezometer at the proposed site with a justification for the expenditure to bemade in establishing and running the network. The utility of the information emerging fromthe piezometer should be highlighted.

An estimate should also be prepared which should include site preparation, drilling, casing/

screen installation, gravel pack, sealing, development, pump test, platform and well headconstruction.


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1.12 Approval for piezometer construction

The site selection report from the field offices should be forwarded to the head quarters forapproval and clearance. It is expected that the justification for the construction of thepiezometer would be reviewed by a committee of senior officers at the head quarters, who

will look at the requirement from a national/state perspective as well as from a localperspective. The location of the piezometer should be superposed on the existing networkand its utility assessed. The aquifer to be monitored has to be verified on the cross section.The added value from the new piezometer should be verified from a technical, managementand financial perspective. On complete satisfaction of the utility of the piezometer thefinancial estimate has to be examined. While standard rates should be the norm, deviationsshould also be considered on case by case basis. The sanctions for depth of drilling, casingdepth, screen position should be based on the field report, which should come up forratification after completion of drilling. In the case where drilling contractors are to be hiredfor drilling the piezometers the procedure for hiring drilling contractors should follow theestablished norms. The tender document for inviting the drilling contractors should clearlymention that a qualified Hydrogeologist should be part of the drilling team and his/her CV

should be part of the enclosures. The utility of hiring more than one contractor when thepiezometer locations are far part should be examined seriously. Drilling Contractors whenused the terms of the contract should clearly specify the obligations of the contractor as wellas the department. Drilling being a seasonal task the procedures for selection of contractorsshould not be cumbersome. Acceptance of State Govt Approved Rates can reduce theprocess of selection. Since rain, water and mud are major hindrances, it is normallyrecommended that the most difficult holes be drilled first if they are accessible, saving themost convenient holes for last or to drill when the others can't be reached.

1.13 Discussion and interaction with local community

On obtaining the clearance for construction of the piezometer from head quarters, a meetingshall be convened in the village where the piezometer site is proposed. The invitees shouldinclude the village elected representatives, village officials, elders, farmers, women,schoolteachers and youth. The services of NGO groups active in the area should be used forconducting the meeting. The meeting should address the local groundwater issues and theneed for groundwater monitoring. The proposed plan for establishment of the piezometerand the most favourable site location identified need to be discussed. Any suggestions fromthe community should be considered and animated in detail. The agency should alsopromise the community to make available the interpretations of the data collected. As afollow up to the discussions, an agreement should be obtained from the community to makeavailable the required co-operation for safeguarding the piezometer as well as upkeep of the

area.


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2 Drilling preparation

2.1 Planning

Successful piezometer construction requires careful advance planning to be conducted inthe most expedient manner. Proper drill site selection and preparation are essential to avoiddrilling at wrong site, minimise wastage of drill time and other associated costs. Landclearance is an essential item that cannot be taken lightly or ignored. Disputed lands canresult in a tremendous litigation and liability to the department. The following are somedetailed items to consider prior to commencing drilling of piezometer.

2.1.1 Site Preparation

Drilling sites need to be prepared prior to arrival of the drilling rig. The site has to be levelledin order to drill a vertical hole. Inclined bores considerably reduce the diameter and depth

calculations become enormous. Prior to extensive site work, the driller must visit the site andclearly place his requirements. Overhead area must be clear of obstructions. Sites with treesand overhead power line should be avoided. If it is necessary to work closer to power lines,the drill crew should inform the electrical authorities either to shut down the power supply orto make the working environment safe. Underground laid infrastructure such as water lines,sewer lines, electrical/telephone cables, if any, should be checked before commencing work.Roots are a major problem, they force their way into the piezometers,. In such areas properpreventive care should be taken by increasing the casing depth or identifying the root pathand treating them.

It has to be ensured that the drilling rig has access to the site upon arrival. Problems have

arisen in the past from hostile villagers and uncooperative landowners, which can beavoided if the village meetings are conducted and local communities are taken intoconfidence. Bridges/culverts to be crossed must be inspected to check whether they havethe required width/ soil strength and have the capacity to take the weight of the rig, alongwith the spares.

2.1.2 Supervision of drilling

It is important to monitor the drilling and ensure that all procedures adopted should help inconstructing a quality piezometer. The piezometer on completion should be providing thetrue picture of the water level and water quality without any bias. The drilling of the

piezometer, geophysical down hole logging, development and pumping test need to becarried out under the supervision of an on site hydrogeologists. Where the work issubcontracted to a drilling contractor, then the drilling contractor should be responsible foremploying the site hydrogeologist who will be available at all times till the piezometerconstruction is complete. The site hydrogeologist shall be responsible to record the drillingdetails, examine and interpret the drill cuttings, describe and record the physical andlithological characteristics of the geological material, supervise the well design, welldevelopment, measure the discharge and collect the water samples.


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2.1.3 Essential tools for field hydrogeologist

Field tools assist the field hydrogeologist in giving an accurate description of the drillcuttings. It is recommended the field hydrogeologist have these basic items (see Figure 2.1)which include:

Pocket knife to cut the samples for testing hardness and exposing fresh surfacesMillimeter scale to determine the size of the particles

Dilute hydrochloric acid to aid in recognizing calcium carbonate materials suchaslimestone, chalk, or dolomite

Magnifying glass (a 10x) to make a better identification of materials by enabling closerinspection

Figure 2.1:Field tools for drill cuttingsexamination

2.1.4 Field notes

Field logs and notes on drilling should be prepared at the drill site itself.

The field description of drill cuttings should be simple and orderly so that the use of the

terminology is uniform. A good field description of the drill cuttings is very important for thedesign and preparation of vertical sections. The site hydrogeologist and the drill crew are theonly people who witness the drilling and the material obtained. Therefore a reasonableamount of accurate information must be logged. At a minimum, the field hydrogeologistmust, in the field, note on a descriptive log the following: The field hydrogeologist must makesure to note the following on descriptive log:

Start and stop times for drilling Names of field personnel Drill cuttings details-Colour, Texture , shape, mineral assemblage, rock type Diameter of drill bits

Depth at which water encountered and discharge variations with depth Drilling rate


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Casing depth Drill completion depth Screen position Gravel pack position Well completion depth Water bearing zones Development time Discharge after development Water quality details pH, EC Depth to water upon completion

A Standard data collection format should be adopted. All field data should be computerisedsystematically as soon as the drilling is complete and the field data brought to theDistrict/Regional Data Centre.

2.1.5 Description of drill cuttings

The descriptions of the drill cuttings should be as simple as possible (see table 2.1). Everysmall variation does not necessarily warrant description on the log. The description shouldinclude:

Principal constituent: First determine the major constituent in the sample. If a significantportion (greater than five percent) of a secondary material is present then describe andidentify it.

Colour: Describe the primary color and restrict description to one colour. If one main colourdoes not exist in a sample, make a simple description of the multicolouration.

Texture: Mention the texture of the primary material under three to four main cateogoriessuch as Coarse-grained, medium grained, Fine-grained, Highly organic etc.

Shape: Cateogorise the most dominant shape of the drill cuttings under rounded, subrounded or angular.

Hardness: should be mentioned with respect to Mohs 'Hardness Scale

5.5 10: Rocks that will scratch the knife: Sandstone, Chert, Schist, Granite, Gneiss, someLimestone

3 - 5.5: Rocks that can be scratched with the knife blade: Siltstone, Shale, most Limestone

1 3: Rocks that can be scratched with fingernail: Gypsum, Calcite, Evaporites, Chalk,some Shale

Cementation: Identify the degree of cementation if any.


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Descriptive adjectives: Use any descriptive adjectives that might further aid in theunderstanding.

Log form: To promote consistency, use the standard log form which is consistent with thedata entry system.

Depth

to (m)

Lithoologicaldescription

Colour Texture Shape Remarks

0.2 Laterite red hard subangular-subrounded

6.5 Laterite verigated/wuggy

red medium subangular to angular

17.1 Lateritic clay red fine rounded

17.5 Basalt weathered black medium subangular-subrounded

29.5 Basalt weathered/fractured

black coarse subangular to angular

51 Basalt hard black fine subangular-subrounded

52 Clay black fine rounded

83 Basalt hard black fine subangular-subrounded

83.9 Basalt weathered/fractured

black coarse subangular to angular Water toucheddischarge0.2cum/hr

86 Clay Ash fine rounded

87 Sand White fine subangular-subrounded

Table 2.1: Sample description of a drill cuttings during the construction ofpiezometer


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3 Construction of piezometer

The purpose of constructing a lithospecific piezometer is to obtain complete lithological dataand not just to drill a monitoring well. In order to obtain data of maximum accuracy, the field

hydrogeologist must work closely with the driller and consult with him whenever changes arenoticed in penetration rate, slow returns, change in colour of samples, reduction in dischargeetc. The hydrogeologist must recognize the reasons for such changes. The difficulties indrilling, such as caving, boulders, caverns, etc. Whenever encountered, must be clearlyrecorded.

Construction of lithospecific piezometers must ensure that the piezometers meet the designcriteria for water level and water quality monitoring. Factors to be considered for piezometerconstruction shall include the following aquifer to be monitored, nature of materials thatmake up and overlie the aquifer (for example, unconsolidated or consolidated materials; ifconsolidated materials whether fractured or have cavities caused by dissolution); the depthto water, the type of drilling equipment required; access to the site; well casing and screen

materials, length, and diameter, and cost. In unconsolidated deposits, the piezometerdesign, including the well screen, casing, annular space, back fill, gravel and surface seals.

Specific aspects of design however, can vary depending on specific requirements to meetlocal variations, site conditions encountered, and the drilling method used.

3.1 Selecting the appropriate drilling technique

Drilling technique for construction of piezometer will depend upon the type and nature offormations likely to be encountered below at the selected site. The technique to be adopted

for soft and unconsolidated sediments shall be rotary, with bentonite mud or any otherbiodegradable mud as the drilling fluid. In the hard rocks, DTH drilling rigs are best suited.The DTH drilling technique uses air to bring the cuttings to the surface, as well as cleansesthe hole. Availability of high-pressure compressors makes drilling very fast. In suchsituations the fines get deposited in the fractures. The drilling in such cases should befollowed up systematic development. In the soft rocks, with poor accessibility and in riveralluvium, hand rotary drilling can be adopted as in parts of Orissa, Tamil Nadu and AndhraPradesh. In hard rocks, with heavy overburden having boulders the drilling has to be doneusing a combination of rotory and DTH rigs.

The drilling should ensure that it is capable of recording faithfully the harmonized areal

behaviour of groundwater of the targeted aquifer in the area, instead of a local micro trend.The piezometer should not be effected by wrong drilling techniques which can bring inexternal contaminants such as, poor quality water used in the mud pit, thick bentonite mud,drilling oil etc.

3.2 Deciding the depth of piezometers

The depth and diameter of piezometers are two important factors, which not only decidetheir best suited design, but may also affect the cost/economics of the piezometerinstallation.

In the unconsolidated formations, the aquifer horizon for construction of piezometer has tobe based on good understanding of the different vertically distributed aquifers, and the


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specific aquifer of interest. In case all the aquifers need to be monitored, piezometer nestshave to be constructed. Systematic collection of drills cuttings and recording of drill time log,followed by electrical logging of the borehole, is very important in delineating the exactthickness of the aquifer. In the event of construction of nests, the deepest aquifer should bedrilled first. The identified zones should be correlated with the regional aquifer system,distributed in the sub-basin or basin, and accordingly the piezometer depth is then decided.

In crystalline rocks, the depth of the piezometer should be decided on the basis ofoccurrence of aquifer(s) to be monitored in a given hydrogeological environment. Threerypical situations are discussed

Case i: Weathered zone is made up of quartz and the fractured rock immediately underlyingit. The weathered zone acts as a good storage zone with its inter-granular connection, whilethe underlying fractured zone forms the main flow/conduit zone. In such a case the overlyingpermeable zone recharges the fractured zone and hence the two zones can be consideredas part of the same aquifer. The piezometer should be then drilled down to the fractured

zone.

Case ii: Fractured zone is overlain by clayey weathered zone. The weathered and fracturedzone exhibit different permeabilities. In such situations both the weathered and fracturedzone are to be considered as independent zones. The monitoring should be doneindependently for the weathered as well as the fractured zone. The shallow weathered zonecan be monitored using an existing open dug well while the fractured zone is monitored byconstructing a piezometer.

Case iii: Weathered zone is clayey and impermeable, the recharge to deeper fracture zonemay be from a distant recharge area. In such case the piezometer has to be installed against

the fractured zone only. The extent and thickness of the fractures do not follow a systematicfashion, hence the need for greater care in identifying the fractured zone by thoroughlymonitoring the drilling.

In case of basaltic rocks, occurrence of multiple aquifers is common. Generally, the upperweathered zone in such rocks is totally clayey and impervious and the first aquifer in suchformations may occur at different depth as vesicular zones. Each vesicular flow should betapped by an independent piezometer. In areas where more than one vesicular flow has tobe monitored, piezometer nests or a group of piezometers within a limited area (village,watershed) need to be installed, tapping different aquifers. Care has to be taken in properlysealing the upper aquifers while tapping the deeper zones. Typically, contractors who drilldrinking water wells are not the best suited for drilling such piezometers. Departmentaldrilling rigs should be mobilised for taking up such drilling.

In the case of hard sedimentary rocks, good understanding of the stratigraphy is critical inunderstanding the different potential aquifers. Sandstone, shale and limestone occur insequences. The sandstone in many cases have copious supplies. The limestone rockspossess both primary and secondary porosity in the form of fractures, solution cavities andcavernous zones. Shale have limited discharge. Good understanding of the startigarphy,combined with judiciously used geophysical surveys and profiling, the depth of the aquifer tobe monitored can be inferred. Confined aquifers when met with produce artesian free flowingwells, should be, anticipated at the design stage itself. Methods to monitor the pressurechanges should be part of the design.


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The depth has to be accurately measured after the piezometer construction is complete byusing a weighted tape. The measurement should also be compared with the total number ofdrill rods used during the piezometer construction.

3.3 Diameter of piezometer

A piezometer is a non-pumping well and ideally needs to be as small in diameter aspossible. The diameter should be such that it shall facilitate measurement of water tableusing a variety of measuring devices and collection of water sample. The diameter will alsobe dictated by the diameter of the measuring device, such as the probe of the Digital WaterLevel Recorder, diameter of the water quality sampling pump. Piezometers having adiameters of 100 mm are the most suitable. Shallow piezometers having diameter of 50 mmare also uncommon. The utility of the piezometers, to carry out pumping tests, geophysicaldown hole logging and hydrofracturing should also influence the diameter of the piezometer.In the case of deep tube wells (>100 mtrs) in the alluvial areas, the cost will be a majorconsideration in deciding the diameter of the piezometer. In such situations telescopic

design of 100-150mm down to 30 mtrs followed by 50mm dia till the bottom should also beseriously considered. Inclined piezometers can reduce the diameter considerably and causemajor problems during lowering of DWLR probe or the sampling pump etc.

The diameter of the hole is often critical and is recorded based on the diameter of the drillingbit. The hole diameter is best measured using a calliper log.

The piezometer is intended to be vertical, however it does not always stay vertical but driftsfrom verticality. To understand the drift use of a mirror should be made to reflect sunlightdown the hole to enable a visual check on the straightness of a hole. Visibility of half hole isan indication of loss of verticality. The exact point of deviation can be checked by measuring

the depth with a tape, while reflecting light down the hole.

3.4 Actions to be taken prior to drilling

Confirm landowner's/concerned government agencies, permission to enter the propertyfor drilling.

If the location is within a school/office/hospital discuss with the authorities to confirm theappropriate time when the drilling can be carried out without disturbing their functioning.

Check the marking at the site and confirm with the geographical co-ordinates. Locate any subsurface power lines, waters lines, telephone cables, sewer etc. Locate water sources for drilling purposes and secure permission for their use. Prepare the drainage channel for draining of water.

3.5 Piezometer construction in unconsolidated formations

Unconsolidated formations in peninsular India are largely localised to coastal tractscomposed of beds of sand and clays, and sedimentary beds of Gondwana and Tertiaryformations made of alternate layers of sandstone and shales. Piezometer construction inthese areas is through rotary drilling. In the unconsolidated formation, rotary drilling has tobe adopted. Rotary drilling makes use of viscous bentonite mixed fluid as medium of drilling.The mud fluid acts as coolant to the rotating drilling bit as well as a medium for bringing out

drill cuttings outside the borehole. Use of bentonite clay has been banned for water well


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drilling in many countries, as they are not bio-degradable. Organic materials like guar gumare replacing bentonite clay as popular bio-degradable drilling fluid.

The main components (see figure 3.1) of a piezometer in an unconsolidated formation are:

Borehole: This is the primary component of a piezometer and acts as a host to the othercomponents.

Well assembly: This is essentially the hardware of the piezometer and is accommodated inthe borehole and also protrudes above the ground. Depending upon location of the aquifer inthe vertical section, it may comprise one or more of the following parts:

Figure 3.1:Piezometer components in unconsolidated rocks

Blank casing pipe:A blank casing pipe is provided to serve one or more of the followingobjectives:

To prevent caving-in/sloughing of the drilled formation. To prevent a hydraulic connection between the piezometer and the drilled formation

other than the aquifer to be monitored.

To collect the fines entering into the screen. As debris sump.

Screen:A screen provides a hydraulic connection between the piezometer and the aquiferto be monitored.

Gravel pack and seal: Gravel is provided in the annular space between the borehole andthe well assembly around the screen and beyond, extending preferably over the entirethickness of the aquifer to be monitored. The gravel pack serves the following purposes:

inhibits the entry of the fines into the screen. enhances the hydraulic connection between the piezometer and the aquifer


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Cement seal:is provided just above and just below the gravel pack to preempt any hydraulicconnection between the piezometer and the overlying/ underlying formations, through thegravel pack and screen perforations.

Sanitary seal: A 50cm thick concrete seal is provided at the ground surface to prevent the

entry of surface water into the piezometer. The seal should be in the form of a cone aroundthe casing to drain the water away from the well. The seal is underlain by a clay fill/packingfor a more effective isolation of the aquifer to be monitored.

3.6 Sampling procedures during drilling

Examination of drill cuttings is very critical part of piezometer design in the un-consolidatedformations. Some formations are better aquifers than others. Grain size have to beinterpreted during the examination of the lithology (see figure 3.2).

Clean gravel have large pores and hold large quantities of water.

Sand and gravel mixture are very good aquifers. When percentage of gravel to sand is veryhigh the aquifer will permit copious discharges.

Coarse sand are potential aquifers

Figure 3.2:Grain size classification

Fine sand are poor aquifers

Clays hold lot of water but cannot flow. In some situations the clays when tapped can yieldpoor quality water.


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Sandy aquifers when overlain by thick impermeable clay and when penetrated by thepiezometer can result in flowing wells.

Standarised sampling procedures have to be adopted by all agencies:

Collect the samples for every meter. Lay the samples in succession, as obtained, and mark the depth Dry the samples for accurate identification and classification. Describe the samples precisely before and after washing and record any additional

information.

Look out for fossils and identify them Compare all samples with previous samples. Place the samples in plastic wrap and label legibly for any future identification/test. Sample boxes with pigeon hole windows are best suited to transport and for preserving

the samples.

3.7 Down hole inspection

In order to take a decision on the design the piezometer assembly, downhole geophysicallogging needs to be carried out. Logging should be used for providing additiona informationthan gained from examination of drill cuttings. Th details to be collected shall include theformation penetrated, formation characteristics modified as a result of drilling, condition ofthe hole, the exact depth and thickness of the aquifers and water quality of the aquifers. Thestandard probes to be used shall be electric, SP, Gamma, calipper, temp and fluidconductivity (refer Annexure-II). The geo-physical logging, examination of drill cuttings andthe objective of monitoring should form the basis for finalising the piezometer design.

3.8 Piezometer Completion

Piezometer completion should ensure that the hydraulic head measured in the piezometer isthat of the aquifer of interest. Ensures that only the aquifer of interest contributes water tothe piezometer and prevents the annular space from being a vertical conduit for water andcontaminants. Such completion steps are critical to the long-term goals of groundwatermonitoring. It has to be remembered that the investments made in the construction ofpiezometers are part of network monitoring programme that have to last for decades. Wellcompletion in unconsolidated deposit rocks consists of installing the well casing and screen,filling and sealing the annular space between the well casing and piezometer wall.

3.8.1 Piezometer Design

Good design and careful well construction can only ensure good hydraulic flowcharacteristics in the aquifer. The screens should be lined up exactly with the permeableportion of the aquifer. The screens should provide the same hydraulic conductivity of theaquifer. The design should prevent entry of fines and sand particles into the piezometer.The piezometer should completely seal the aquifer which are not to be monitored. The wellassembly should be able to withstand any corrosion or physical damages during pumpingand logging.


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Unconfined aquifer For monitoring the piezometric head of an unconfined aquifer, thepiezometer essentially comprises a cement seal at its bottom followed by the well assembly,resting on the seal, comprising of (starting from the bottom) a bail plug, screen and finally awatertight casing pipe extending above the ground surface

Confined/leaky-confined aquifer: For monitoring the piezometric head of a confined/leaky-confined aquifer, the piezometer essentially comprises a borehole drilled through theoverlying formation and the entire thickness of the aquifer, into the lower formation toaccommodate a cement seal at its bottom. The well assembly, resting on the seal,comprises (starting from the bottom) a bail plug, screen and watertight casing pipe extendingabove the ground surface

3.8.2 Screen length

The well screen should be long enough to ensure that the piezometer records the verticallyintegrated piezometric head of the investigated aquifer. Thus, there must be a perfect

hydraulic connection between the piezometer and the aquifer over the entire aquiferthickness. Ideally, this requires a fully penetratingpiezometer, that is, the screen providedover the entire thickness of the aquifer.

In case of thin aquifers, a fully penetrating piezometer may be provided. However, in case ofthicker aquifers, a fully penetrating piezometer may not be economically feasible, and assuch, a partially penetrating piezometer may have to be provided. But even a partiallypenetrating piezometer can provide an almost perfect hydraulic contact, if it is surrounded bya fully penetrating (that is, extending over the entire aquifer thickness) gravel pack of largeenough thickness and hydraulic conductivity. The length of the screen, in such a case mustbe large enough to ensure a free inter-flow of water between the piezometer and the aquifer

through the gravel pack. A screen length of two meters surrounded by a fully penetratinggravel pack may provide the necessary hydraulic contact and ensure the free inter-flow.

The gravel pack should have a greater grain size than that of the aquifer material in thevicinity of the screen. The gravel pack grain size and gradation should be so designed tostabilize the aquifer material adjacent to the screen and permit only the finest grains to enterthe screen during development, finally providing sediment-free water into piezometer (seefigure 3.3). The gravel pack must not intersect multiple aquifers and should not cross-confining units, otherwise they would establish vertical, hydraulic connection along theannulus between the two aquifers, thus defeating the whole concept of piezometersmonitoring single aquifers.

Figure 3.3:Idealised arrangement of gravel around thefilter assembly for increasing porosity andhydraulic conductivity


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Specific details of completion require consideration of several hydrogeologic factors,including the depth to water, to the top of the aquifer of interest, and to the zone in theaquifer to be monitored;

the nature of materials that make up the aquifer to be monitored and that overlie theaquifer

expected water-level fluctuations expected direction of the vertical head gradient--down ward, whether the aquifer is confined or unconfined

3.8.3 Design of gravel size and screen slot size

Particle sizes are to be determined in the field after sieve analysis of the aquifer material(see figure 3.4). Before sieve analysis the samples need to be dried and weighed. Thestandard sets of required sieves need to be placed one above the other in the order of

increasing sieve diameter. The sample should be placed in the top sieve and shaken toseparate the various grain sizes. The weight of the material retained in each sieve should bemeasured and expressed in percent of the initial weight.

Figure 3.4:Standard sets of sieves

The percentage weight should be plotted against the sieve size, on a logarithmic scale. Theresultant curve that is obtained gives information about the uniformity of the aquifer material.Use of screen having a median size of the aquifer material is generally preferred. Sincepiezometers are not pumping wells the main concern should be top have a good hydraulicconnection while at the same time preventing any entry of fine material into the piezometer.

The slot size of the screen should be so designed that the aquifer material does not enterinto the piezometer. Assuming that fractions greater than or equal to the d60 of the aquifermaterial are to be retained, a slot size of d60 may be provided. (d60/d10) gives theuniformity coefficient The higher the uniformity coefficient, the higher would the efficiencyand vice versa. Thus, depending upon the uniformity coefficient and the extent of theexpected well development, the usually recommended slot size is d40 to d60 of the aquifermaterial.


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The average size of the gravel should be 4 to 6 times the d50 size of the aquifer. The gravelshould be as uniform as possible to avoid segregation during the placement.

3.8.4 Annular seals

Annular seal(s) are to be installed from above the gravel pack to near land surface, in orderto seal the annular space between the casing and borehole wall. These seals should prohibitvertical flow of water between aquifers and prevent mixing and cross-contamination ofaquifers. They also protect against infiltration of water and contaminants from the surface.

3.8.5 Surface Seal

The surface seal prevents surface runoff down the annulus of the well and, in situations inwhich a protective casing around the well is needed, holds the protective casing in place.The depth of installation of a surface seal can change from area to area. The surface sealshould be a mixture of cement and gravel.

3.8.6 Protective Cover

A protective cover should be installed around the piezometer to prevent unauthorizedaccess, house the measuring device as well as to protect the piezometer from damage. Theprotective cover should be installed at the same time as the surface seal and should extendto below the ground. Many designs of protective casing are already available. Essentially itshould be a large diameter casing or a GI sheet with locking protective cover and weep hole,which permits condensation to drain out.

3.8.7 Development

The development of the piezometer, is primarily aimed at ensuring an efficient hydraulicconnection between the aquifer and the piezometer. The development e is very crucialsince the drilling mud, which inevitably sticks to the walls and invades into the aquifer inhibitsthe hydraulic connection between the aquifer and the piezometer. The invasion of the drillingmud and thickness of the cake depends upon the hydraulic conductivity of the aquifer. Thehigher the hydraulic conductivity, the higher is the mud invasion and mud cake thickness.The development should completely remove the invaded/sticking mud and also the fines(see figure 3.5). Under-developed piezometers will fail to provide the true information of theaquifer being monitored and the water level data emerging from such piezometers can lead

to wrong understanding of the system.

Figure 3.5:Effective development by pumpingwater under pressure through thescreens


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The mud cake around the screen should be dissolved using sodium tripolyphosphate.Sufficient volume of solution of sodium tripolyphosphate should be made and circulated todisplace mud around the screen area as well as a portion of the casing for disaggregatingthe clays. The polyphosphate solution should be allowed to act for at least 24 to 36 hours.The solution should be circulated through the well screen that effectively acts on the mudcake. This should be followed by washing.

The development should be carried out through air compressor by alternatively surging andpumping with air. The air should be injected into the piezometer to lift the water. As the waterlevel reaches the top of the casing, air supply should be shut off allowing the aerated watercolumn to fall. Use of eductor lines is recommended when the static water level is deep.

High velocity jetting is another development technique that consists of a jetting tool fitted tothe bottom of the drill string. The jetting tool should be lowered and washed all along thescreen length using fresh water. This should be followed by airlift. Careful jetting of thescreened area is required. Jetting combined with airlift should be continued till pumped water

is free from fine sand and bentonite, and the discharge from the piezometer stabilizes.

Development can also be done through back washing. In back washing, there is a reversalof flow through screen opening, which agitates the sediments and leads to the removal ofthe finer fraction and rearrangement of the formation particles. As a part of back washing thewater column should be alternatively lifted and allowed to fall back. The pump should initiallybe started at a reduced capacity and gradually increased to full capacity.

Mechanical surging needs to be carried out at times using surge blocks attached to drill rods.The surge block forces water into and out of the screen similar to a piston in a cylinder. Thesurging process at times forces fine material back into the screens and hence the fines

should be removed before taking up surging.

3.8.8 Pumping Test

A pumping test is conducted with constant discharge or variable discharge with constanthead for estimating hydraulic parameters of the aquifer tapped in the piezometer. The testinvolves monitoring of the time variation of drawdown in one or more observation wells inresponse to a pumping at a known discharge, from the piezometer. The observation must bein the vicinity of the piezometer and must be tapping the same zone. If no such observationwell is available, the drawdown can be monitored in the piezometer itself. Details of pumpingtest procedures is enclosed as Annexure-III


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4 Piezometers construction in consolidated formations

The drilling of piezometers in consolidated formations are different from the construction ofunconsolidated and semi-consolidated formations. The groundwater occurrence in the

consolidated rocks is in the weathered zone and fractured zone. The consolidated rockshave negligible primary porosity and it is only the secondary porosity, like fracturing andweathering, that provides the porosity and permeability necessary for the storage and flow ofgroundwater.

Groundwater discharges are largely dependent upon the rock type. In granite, gneiss andkhondalites highly productive groundwater zones are found in the vicinity of largelineaments, fractures and deep weathered areas. The lava flows are mostly horizontal andoccasionally gently dipping and as such, groundwater occurrence is controlled by the waterbearing properties of the vesicular zones. In carbonate rocks like limestone, marble anddolomite, solution cavities serve as large repositories of groundwater. In all these rocks thedrilling is usually carried out by the Down The Hole (DTH) drilling technique or a combination

of DTH and rotary drilling.

For monitoring the piezometric head of an unconfined aquifer, the design should be a casedborehole drilled through the top collapsible/weathered rock zone, overlying the unconfinedformation to be monitored and is hard enough to stand on its own without the casing. Thecasing should stand above the ground by 0.3 to 0.5 m.

For monitoring the piezometric head of semi-confined aquifer, which has differentpermeability from the top weathered zone then the design should be a cased hole, drilledthrough the entire weathered rock zone, overlying the fractured/hard formation to bemonitored. The depth of drilling should be such that it taps the most productive part of thefractured zone. Geophysical resistivity surveys should provide the value for the depth ofdrilling. DTH drilling is very fast and completion of one piezometer of 100-m depth takesonly 12-18 hours.

4.1 DTH drilling characteristics

The drilling being very fast, supervision of DTH drilling becomes very important. The sitehydrogeologist has to ensure that the compressor is in good condition to deliver the requiredair pressure and that the drill bit is of the required diameter. The site hydrogeologist has toensure that the drilled hole is constantly cleaned of the drill cuttings. During the change of

the drill rod as well as when a water bearing zone is met, the well should be adequatelydeveloped and the discharge measured using a V notch. The drill cuttings should becollected and studied continuously. At the end of drilling to the desired depth, the well shouldbe cleaned for at least two hours. The cleaning should lead to de-clogging of all the fracturesdrilled through, and removal of all fines and cuttings.

4.2 Sampling procedures for consolidated rocks

The drill cuttings should be sampled for every one-meter frequency and whenever there is achange in lithology. The samples obtained in the DTH drilling are due to the action of the drillbit, which should be kept in mind while examining the sampled cuttings. Further, the depths

of the formations as revealed by the cuttings may not always be accurate - though they canbe generally relied upon. The drill cuttings have to be classified on the basis of megascopic


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observations using hand lens, both for texture and mineral constituents. The descriptionshould identify the rock, colour, grain size, shape, fossils, trace minerals, etc. The drillcuttings should be dried, packed in polythene bags, marked with well number and depthinterval, date and time. The samples should be stored in a box with numberedcompartments. A correct procedure for collection and storage of drill cuttings ensures goodcorrelation between the drillers log, VES interpretation, downhole logging and samplescollected. The recorded drilling data should include the following:

A drill log (time taken for drilling each meter of the drilled depth) A description of drill action (such as nature of drilling noise and motion of the rig) Depths at which moisture is struck Depth at which water flows Depths at which discharge increases Colour, pH and EC of the water

4.2.1 Removal of fines during drilling

Air drilling causes plugging of fractures and crevices with fines of drill cuttings. The cloggedmaterial until removed the hydraulic conductivity with theaquifer cannot be established. The

lithospecific piezometer manual

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