using lithologic modeling techniques

GRMENA II, 2007, p. 355-382 355

Using Lithologic Modeling Techniques for Aquifer Characterization and Groundwater

Flow Modeling of Sohag Area, Egypt

Ayman A. Ahmed Geology Department, Faculty of Science, Sohag University, Sohag, Egypt

Abstract

Three dimensional lithologic modeling techniques are used in this paper for detailed characterization and groundwater flow modeling of Quaternary aquifer system of Sohag area, Egypt. The study depends on using the well logs data and techniques of lithologic modeling available in Rockworks software capabilities and tools. Well logs data were used to obtain lithologic models representing the different lithologic materials of the Quaternary aquifer system of the study area. Groundwater flow model, using MODFLOW 2000, is built using results of the lithologic models. Results showed that the obtained lithologic models honored the well logs data and showed the complex sedimentary system which is mainly composed of six lithologic categories, which are clay, clay and sand, fine sand, coarse sand, sand and gravel, and gravel. Inter-fingering and presence of lenses is a main characteristic of the sedimentary basin represented in the study area. A wide range of hydraulic conductivities is present which vary spatially and controls the groundwater flow. Heterogeneity of aquifer system is spatially represented in the study area where different hydraulic conductivity fields are found in the different directions. Sandy layers tend to be connected to form a certain flow continuation. Hydraulic continuity is represented by clusters of sandy materials within the aquifer system. Flow model results showed that the River Nile acts as a discharging line for the Quaternary aquifer where groundwater levels are higher than those of the Nile. Flow regime in the study area includes a downward flow from the top layer due to infiltration from excess irrigation, flow toward the River Nile, flow out of irrigation canals, flow into drains, inflow from the southern boundary, and outflow from the northern boundary.

INTRODUCTION

Aquifer characterization is a matter of importance in groundwater modeling applications where representing spatial variability has a substantial influence on the behavior of the system. The difficulty of characterizing subsurface heterogeneity with commonly sparse data sets severely limits the accuracy and realism of transport models (Weissmann et al 1999). Driller’s lithologic logs data may provide excellent information about the vertical variability of the sediments but only limited lateral variability information. Different methods were used for modeling heterogeneity and spatial connectivity including the methods of Indicator Point Kringing and Conditional Indicator simulations (Ritzi et al 1994), Transition Probability-Based Indicator Geostatistics (Carle and Fogg 1996), One and Multidimensional Continuous-Lag Markov Chains (Carle and Fogg 1997), transition probability geostatistics with a sequence stratigraphy framework (Weissmann and Fogg 1999, Weissmann et al 2002 ), facies models with genetic approaches and

356 Ayman A. Ahmed Geostatistical, Boolean or Markovian methods (Marsily et al 2005), structure-imitating, process-imitating, and descriptive approaches (Koltermann and Gorelick 1996).

The previous works dealing with the configuration of the Quaternary aquifer in the study area used one-dimensional one-layer, two-dimensional one layer and two-dimensional two layer models (Attia 1985; Abdel Moneim 1992). In addition, heterogeneity is not treated in these models where assumptions are applied to use homogenous, uniform thickness, and unique hydraulic properties for layers. Although three-dimensional models are also used in recent literature (Shamrukh 2001; Ahmed 2003), uniform layer thicknesses and uniform hydraulic conductivities for the different aquifer layers are assumed due to scarcity of data about the number of layers, their thickness, and elevations of top and bottom surfaces.

Complex stratigraphy can be difficult to simulate in MODFLOW models. MODFLOW uses a structured grid that requires that each grid layer be continuous throughout the model domain. This makes it difficult to explicitly represent common features such as pinchouts and embedded seams in a MODFLOW model (Jones et al 2002). Consequently, The present study aims at applying lithologic modeling techniques for characterization of the Quaternary aquifer system and building a groundwater flow model for Sohag area, located in the middle of the Nile Valley, Egypt, between latitudes 26° 00/ and 27° 00/ N and longitudes 31° 15/ and 32° 15/ E (Fig. 1). The study depends on using the well logs data and techniques of lithologic modeling available in Rockworks software package (Rockware, 2002). Rockworks is used for accomplishing this study due to its friendly user interface, capabilities of managing the pore holes and well logs data, and ability of using alternative methods of building the lithologic models. The obtained models were used for representing the aquifer system and building a groundwater flow model. The generated lithologic models are used for building a MODFLOW model based on the generated three-dimensional lithologic framework of the study area. The lithologic models are converted to a layered three-dimensional finite difference mesh that honors the horizontal boundaries of the stratigraphic layers and matches the stratigraphy defined by the lithologic models.

MATERIALS AND METHODS Geologic framework

The area of study is a part of the Nile Valley which is geologically investigated by many authors such as Sandford and Arkell 1939; Shukri 1950; Butzer and Hansen 1968; Said 1962, 1975, 1981, 1983, 1990; Wendorf and Schild 1976, 1980; Issawi et al 1978; Issawi 1983; Paulissen and Vermeersch 1987; Issawi and McCauley 1992; Mahran 1993; and Omer 1996. The sedimentological sequence of the study area is mainly represented by the Lower Eocene limestone, Plio-Pleistocene sands, gravels and clays and the recent sediments (Fig. 1). The Lower Eocene rocks are represented by Thebes Formation (thin bedded limestone with chert bands and flent nodules) forming the plateau bounding the Nile Valley on both sides. The Upper Pliocene-Pleistocene sediments form three belts extending nearly parallel to the course of the River Nile (Mahran 1993) with

Using Lithologic Modeling Techniques for Aquifer Characterization 357

Figure 1: Regional geologic map of Sohag area (Modified from Said 1962). different facies distributions (Fig. 2); The inner belt (nearer to the Eocene limestone scarp) and is composed at the lower part of fine grained sandstone, siltstone, and claystone intercalations (Madmoud Formation), followed upward by conglomerates dominated sequence (Armant Formation) intertonguing with breccia and conglomerates (Issawia Formation) at the top. Sediments of the inner belt started with accumulation of flood plain facies of Madmoud Formation, followed with upward by deposition of three interfingering facies; the alluvial fan and the lacustrine facies of Armant Formation and the talus facies of Issawia Formation. The middle belt is represented by sandstones dominated sequence (Qena sandstones) which are overlain by polygenetic conglomerates and sandstones of Abbassia Formation. Sediments of this belt were accumulated under conditions ranging from fluviatile channels at the base (Qena sandstones) to alluvial fans at the top (Abbassia Formation). The outer belt (nearer to the cultivated land) and is composed of shales, laminated sandstone, siltstone, and claystones (Dandara Formation) which are overlain and laterally intertonguing with El Gir conglomerates.The sediments of this belt

358 Ayman A. Ahmed exhibit three interfingering facies; the alluvial fan facies comprises El Gir conglomerates, the flood plain and the lacustrine facies constitute Dandara Formation. Recent sediments are represented by the alluvium (cultivated lands) located between the Plio-Pleistocene sediments and the River Nile.

Figure 2: Facies distribution in the study area (Mahran 1993). Hydrogeologic framework

The study area belongs to the arid region of North Africa which is generally characterized by hot summer and cold winter with low rainfall. Air temperature ranges between 36.5 oC (summer) and 15.5 (Winter), relative humidity ranges between 51 – 61% (Winter), 33-41% (Spring), and 35 – 42% (Summer). Surface water hydrology of Sohag area is represented by the River Nile, irrigation canals, and drains (Fig. 1). The area gets the irrigation water from the River Nile and the main irrigation canals which take water from the River Nile upstream of Nag-Hammadi Barrages. These canals are West Nag-Hammadi and East Nag-hammadi canals with a total length of 130 and 150 km respectively. Other large canals that take water from the main canals include El-Baliana, El-kasra, El-Girgawia, and Tahtawia canals. The drainage system in the study area is mainly represented by Main Girga drain, El-Baliana drain, Akhmim Main drain, Tahta drain, El-Kasra drain, and El Kheikh Marzok drain. These drains are running from south to north parallel to the irrigation canals. There are other lateral minor drains include bani Himail, Bar Khael, Bakri, and Awlad Ali drains. The irrigation canals and drains in the study area cover an area of about 8.5 km2. The Quaternary aquifer system in the study area is formed by the alluvial deposits of the Nile and consists of two layers having distinct hydraulic properties (Fig. 3). The upper layer is the clay-silt member, which has low horizontal and vertical permeability. It functions as a semi-confining layer to the underlying aquifer. The clay-silt member is laterally extensive, having greater thickness near the river channel and vanishing near the valley fringes where it is overlain by desert sands. The lower layer, the graded


sand member, forms the main aquifer having high horizontal and vertical permeability. The lower boundary of the aquifer may be considered impervious due to the presence of extensive and thick deposits of the Pliocene clays of very low permeability. The lateral boundaries along the sides of the valley are not impervious. The horizontal permeabilities of the formations and hydraulic gradients, however, are small enough to justify the assumption that no flow occurs across these boundaries (MPWWR 1988). The aquifer geometry differs from one locality to another (Attia 1974). The thickness of this aquifer as well as its width differs from one locality to another (Farrag 1982). The water in this aquifer is found under semi-confined conditions, and in other localities it is present under unconfined conditions where the Nile silt is absent. The aquifer is essentially restricted to the valley and also to the bottom of the adjacent desert valleys.

Figur 3: General hydrogeologic section of Sohag area (RIGW 1990)

Lithologic modeling of Quaternary aquifer system The investigated subsurface lithologic well logs for the study area

indicate that the area is composed of six categories which are clay, clay and sand, fine sand, coarse sand, sand and gravel, and gravel. The sedimentary sequence encountered in the study area revealed the complexity of the sedimentary basin and heterogeneity of Quaternary aquifer system. 3D representation of lithologic logs indicates that direct correlation between well logs can’t be obtained from the conventional 2D representation (Fig. 4). Consequently, the present paper used the lithologic modeling techniques based on the “solid modeling” concept provided in Rockworks software package (Rockware, 2002) in which a true three-dimensional gridding process, used to create a “box” of regularly-spaced nodes from irregularly-spaced data by interpolating measured values of lithology types. The Geological Utilities Solid / Model tool creates solid models from X, Y, Z, and G data listed in the main

360 Ayman A. Ahmed datasheet. The Borehole Manager Lithology menu tool is used to create solid models from lithology. Once it knows the dimensions of the study area, the program divides it into three dimensional cells or "voxels" which their dimensions are automatically or user-determined. Each voxel is defined by its corner points or nodes. Each node is assigned the appropriate X, Y, and Z location coordinates according to its relative placement within the study area (Fig. 5).

Lithologic modeling methods

There are several methods offered to perform the three-dimensional interpolation of data. Each operates differently, and each has strengths and differences as follows (Rockworks, 2002):

1) Closest Point: The most basic solid modeling method, in which the value of a voxel node is set to be equal to the value of the nearest data point.

2) Distance to Point: This method assigns each solid model node a value equal to the distance to the closest control point. The distance is recorded in your X,Y, Z units.

Figure 4: Boreholes showing 3-D representation of lithology in the study area.


Figure 5: Three dimensional cells "voxels" to be obtained by interpolating measured values of lithology types.

3) Inverse Distance: Isotropic, Anisotropic, or Weighted. This method

assigns a voxel node value based on the weighted average of neighboring data points, either all points or those directionally located, using fixed or variable weighting exponents.

4) Isotropic: The program will use all of the available data points when computing a voxel node’s value, useful when modeling uniformly distributed data in nonstratiform environments.

5) Anisotropic: Instead of using all available control points for the Inverse-Distance modeling, the program will look for the closest point in each 90-degree sector around the node, useful for modeling drill-hole based data in stratiform deposits.

6) Weighting: Uses all data points, but weights them differently based on their horizontal and vertical positioning from node. Useful for controlling the lenticularity of the model.

7) Directional Weighting: This functions like the Inverse Distance method except that you can specify a trend direction and strength, and the program will vary the weighting exponent so that points along the trend influence the node more than closer points perpendicular to the trend.

8) Horizontal Biasing: This method functions like the Inverse Distance method except that the user can define a vertical distance from each voxel node beyond which points will no longer be used in computing the node value

9) Horizontal Lithoblending: This method should be used for creating lithology solid models (for Profiles, Fences, and Models) in the Borehole Manager

Lithologic model dimensions

The “Grid Dimensions” options are used to establish the number of nodes to be created in the grid model and the boundary coordinates of the model. The model dimensions for the present lithologic models are set as 72 X nodes with X spacing of 1000 m, 80 Y nodes with Y spacing of 1000 m, and 37 Z nodes with Z spacing of 5 m.

362 Ayman A. Ahmed Creating a three-dimensional lithology block diagram

The Borehole Manager's “Lithology / Model” tool is used to create a three-dimensional block diagram that illustrates lithology types of the modeled area. The lithologies will be color-coded based on their background colors in the Lithology Table. During the process of building the block diagram, the program will create a solid model of the lithologies using the “lithoblend” algorithm. Optional surface filtering is available to zero-out nodes above a unit or ground surface, and/or below a unit. Generating the lithologic models of study area In this part, lithologic modeling techniques are applied to characterize the Quaternary aquifer system of study area and using the obtained lithologic models for building the groundwater flow model using the different capabilities and tools of Rockworks. Two main steps are used for creating lithologic models as follows: a) Preparing the project database

The “Borehole Manager” tool is used for preparation of the project database including the well locations, elevation, and depth to top and bottom of the different sedimentary materials for each boreholes. Fifty boreholes available in the study area were used for building the lithologic models. The depth of these wells is ranging from 15 to 48 m below ground surface. Elevations of ground surface of the used boreholes ranges from 41.95 to 66.32 m. The sediments encountered from top to bottom include clay, clay and sand, fine sand, coarse sand, sand and gravel, and gravel. b) Creating the lithologic models

For creating the lithologic models of Quaternary aquifer in the study area, different models are created to show the three-dimensional representation of lithology in the different zones including terraces and floodplain. The Borehole Manager's “Lithology / Model” tool is used to create a three-dimensional block diagram that illustrates lithology types of the modeled area. The lithologies will be color-coded based on their background colors in the Lithology Table. Five models are created as follows (Fig. 6): Flood plain facies “model 1”: This model is created using all boreholes located in the flood plain “cultivated land”. 27 boreholes are used. Terraces “models 2, 3, and 4”: Three models are created to represent lithologic types for terraces east and west of the river Nile. At these sites there are few boreholes used for building the lithologic models. 7 boreholes are used for “model 2”, 8 boreholes are used for “model 3” and 6 boreholes are used for “model 4” respectively. Quaternary aquifer system “composite model”: This model is created using all boreholes (50 boreholes) in the study area to represent the overall configuration of the aquifer system. The ground surface is used as the top boundary of the


model to cutoff any interpolated lithology above ground surface and to obtain a close representation of the real world in the study area. Fence diagrams and lithology cross sections To study the Quaternary aquifer characterization, heterogeneity, and connectivity, fence diagrams and cross sections are created from the lithologic models using tools and capabilities of Rockworks as follows: Lithology fence diagrams:

Using capabilities of Rockworks, three-dimensional fence diagrams were constructed to show the change in lithology types at different directions in the study area. Distribution of lithologic types, horizontal and vertical relations were illustrated by constructing fence diagrams for the study area (Fig. 7). Fence diagrams showing only the sandy layers were also constructed to show connectivity of sandy materials (Fig. 8).

Figure 6: Lithologic models showing the three dimensional representation of the

sedimentary system in the study area: Model 1; flood plain. Models 2, 3 and 4; old alluvial plains (terraces). Model 5; a composite model for the whole area.

364 Ayman A. Ahmed

Figure 7: Lithologic fence diagrams showing aquifer heterogeneity in the study area. Lithology cross sections:

Using capabilities of Rockworks, seven lithologic cross sections were drawn across the Nile Valley in the study area to show the change in lithology and configuration of the aquifer system at different locations (Fig. 9).

Figure 8: Lithologic fence diagrams showing continuity of sandy layers in the study

area: A) Continuity between layers of clay and sand, fine sand, coarse sand, sand and gravel, and gravel. B) Continuity of coarse sand. C) Continuity between fine and coarse sand. D) Continuity between layers of fine sand, coarse sand, and sand and gravel. E) Continuity between layers of fine sand, coarse sand, and sand and gravel in the different directions.


Figure 9: Lithologic cross sections extracted from lithologic models. Building groundwater flow model

A groundwater flow model was built to show how to use lithologic models in groundwater flow models and to illustrate its benefits in model conceptualization and aquifer characterization. Results of the lithologic models are used to build the groundwater flow model. Other necessary input data are used from different sources such as hydraulic properties, recharge, pumping rates, boundary and initial conditions. Groundwater Vistas was chosen as a pre-and post-processor to build the groundwater flow models. Groundwater Vistas is a graphical user interface and data visualization package developed by James Rumbaugh and Dough Rumbaugh 1996 (Environmental Simulations Inc., 1996), supports many groundwater models including MODFLOW. The U. S. Geological Survey’s (USGS) MODFLOW (McDonald and Harbaugh, 1988) is a modular, three-dimensional, finite-difference groundwater flow model and which is widely applied in groundwater modeling and used in the present work. MODFLOW 2000 (Harbaugh et al. 2000) was used to model the area Exporting lithologic models to MODFLOW

For building the groundwater flow model of study area, the composite lithologic model (model 5) is used as it represents the overall configuration of study area. To obtain a well defined representation of lithologic materials, the composite lithologic model is exported as slices of 5 meters thickness. The total slices exported are found to be 36 slices which will require the MODFLOW grid discritization to be 36 layers in the vertical direction whereas the horizontal grid configuration will remain the same as used in the lithologic models. The lithologies are extracted in terms of their thickness and topography for each surface of the slices (Fig. 10).

366 Ayman A. Ahmed

Figure 10: Representing lithologic models in MODFLOW. Adjusting model grid and importing lithologies

The MODFLOW model grid is adjusted to coincide with the lithologic model dimensions. The model grid was constructed as 80 rows by 70 columns with grid spacing of 1000 m, and vertically, the model was divided into 37 layers (5 m each) to coincide with the exported 37 lithologic slices representing the real representation of lithologic types in the modeled area. The lithologies were imported into MODFLOW in a sequence of steps to represent the lithologic model. Topography of each slice was used to represent the different elevations of the hydrostratigraphic layers in the study area. The slices are exported using Rockworks tools to different formats supported in Groundwater Vistas. Hydraulic properties, initial and boundary conditions

The different hydraulic properties of lithologic types in the study area were gathered from the available literature (RIGW, 1980; Barber and Carr, 1981; Attia et al, 1983; Attia, 1985; Abdel Moneim, 1988; MPWWR, 1988; Abdel Moneim and Abu El Ella, 1996; Abd El Bassier, 1997; RIGW, 1997; Shamrukh et al 2001; Warner et al 2001; Ahmed, 2003). Studying the different values determined for the hydraulic parameters, approximate averages were determined for the different zones of the aquifer to be used as initial parameters for the groundwater flow model. The hydraulic properties of each lithologic material was assigned a hydraulic value (hydraulic conductivity, specific storage, specific yield) of the equivalent hydrostratigraphic unit in the model grid. The initial conditions used for the present model are the average known heads in the study area based on the available data and hydrogeologic maps. Boundary conditions for study area include constant heads, River (River Nile, irrigation canals), drains, recharge, lower and lateral boundaries of the model domain (Fig. 11). The south and north boundaries are assigned a constant head boundary to represent the flow of groundwater from south to north in the modeled area. The average hydraulic gradient in the study area (0.0016) was used for calculating the specified heads for the south and north boundaries for nodes where hydraulic heads are unknown. The lower boundary of the aquifer may be considered impervious due to the presence of extensive and thick


deposits of the Pliocene clays of very low permeability (Fig. 4a and Fig. 4b). The east and west boundaries are assigned a no-flow boundary as flow from these boundaries is almost negligible due to the presence of impervious layers.

Figure 11: Boundary conditions, observation targets and pumping wells in the study area.

River Nile, irrigation canals and drains

Hydraulic parameters of River Nile and irrigation canals are used from different sources (Abdel Moneim 1992; Sohag Ministry of Irrigation 1999) (Table 1). Due to lack of data about the thickness of the river bed, it is assumed as 1 meter. Drains are set as 4 m width and drain stage was set as 4 m below ground surface. Water levels at River Nile and irrigation canals at start and end of each reach in the study area are plotted on Fig. 12a and Fig. 12b.

368 Ayman A. Ahmed Table 1: Base level and width of River Nile and irrigation canals at start and end of reaches.

Parameter River Nile West Nag Hammadi

East Nag Hammadi

El-Kasra El- Baliana Tahtawia El-

Gergawia

Location Start End Start End Start End Start End Start End Start End Start EndBase level (m asl)

53.03 41.07 60.09 51.56 61.42 58.86 60.25 58.5562.27 58.61 56.41 54.63 52.49 50.6

Width (m) 700 700 35.7 35.7 24.6 24.6 18.3 18.3 22.5 22.5 22.5 22.5 14.9 14.9

Fig. 12a Monthly water levels in irrigation canals at the start of reach in the study area.


Fig. 12b Monthly water levels in irrigation canals at the end of reach in the study area. Recharge In the study area, the aquifer recharge is mainly due to infiltration from applied irrigation water. Due to lack of information about distributed recharge in the study area, where no records are available about the geographical distribution of different crops with appropriate mapping for the study area, the crop water requirements are analyzed to estimate the approximate recharge rates. Using the mathematical relations between crop water requirements, applied water, leaching requirements, and recharge rates, the latter were estimated. Recharge rates in the study area were calculated using evapotranspiration (ET) using the following relation (Allen et al 1998):

SWSFCRDPROPIET ∆±∆±+−−+= Where;

I: Irrigation or applied water, P: rainfall, RO: surface runoff, DP: deep percolation, CR: capillary rise, ∆SF: change in subsurface flow over the time period, ∆SW: change in soil water content over the time period

Neglecting capillary rise (CR), and change in soil water content (∆SW), the above equation can be more simplified as follows:

DPIET −=

Deep percolation or recharge (DP) can then be estimated as follows: ETIDP −=

Leaching requirement: In general, leaching requirement can be represented as follows:

ETLR %)2010( −=

370 Ayman A. Ahmed

Irrigation efficiency: Irrigation efficiency can be represented as follows:

ILRETIef

+=

where; 8.04.0 << efI

Consequently

ETI

LRETETIDPef

−+

=−=

Due to efficiency in crop types and actual fields for different seasons, the average values of crop water requirements are used to calculate the average recharge rates for the study area. The selected values for the leaching requirement and irrigation efficiency are 10 and 70 % respectively.

ETETETerechDP −+

=7.0

1.0)arg(

The estimated recharge rates for old and new agricultural lands in the study area are listed in Table 2. Table 2. Estimated recharge rates (m/day) for the old and newly reclaimed lands in the study area. Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Old lands 0.0011 0.0010 0.0014 0.0015 0.0016 0.0021 0.0024 0.0023 0.0022 0.0019 0.0017 0.0018

Newly reclaimed lands

0.0022 0.0021 0.0027 0.0030 0.0032 0.0041 0.0047 0.0046 0.0044 0.0037 0.0034 0.0035

Groundwater extraction

In the study area, the main discharge from the aquifer system is represented groundwater extraction by pumping wells. 73 pumping wells were identified and traced from the hydrogeological maps of Sohag (RIGW 1997). Due to shortage of information about exact pumping rates and duration where pumping rates are listed as a range in the hydrogeological maps of Sohag area (<1000, 1000 - 5000, and 5000 – 25000 m3/day), assumptions are applied to estimate the pumping rates. Assuming that the pumping rates are comparable to the applied water and recharge, the pumping rates are assigned a relative percentage. The higher the recharge, the higher pumping rates. Where the highest recharge occurs in July (assuming that the highest applied water is equivalent to the highest water requirement and in turn the pumping rates), the pumping rates are assumed as 100 % (the maximum pumping rates), and the other percentages for different months are estimated accordingly (Table 3).


Table 3. Estimated groundwater pumping rates in the study area (m3/day). Jun Well Type Jan Feb Mar Apr May

[A] < 1000 461.3 397.3 582.5 622.9 683.5 848.5 [B] 1000 - 5000 2306.4 1986.5 2912.5 3114.5 3417.5 4242.4[C] 5000 - 25000 11532.0 9932.7 14562.3 15572.4 17087.5 21212.1Well Type Jul Aug Sep Oct Nov Dec [A] < 1000 1000.0 966.3 909.1 791.2 707.1 750.8 [B] 1000 - 5000 5000.0 4831.6 4545.5 3956.2 3535.4 3754.2[C] 5000 - 25000 25000.0 24158.2 22727.3 19781.1 17676.8 18771.0 Stress periods, time steps, and simulation time

n for this study as a conceptual

roundwater flow model calibration librated using a combination of

manua

ydraulic conductivities Kh including Kx, Ky for all lithologic

b) nductivities K for all lithologic types.

ed Kz for River Nile. nals.

The able 4a and Table 4b.

A transient two-year simulation is chosemodel for further applications. The model was run as a transient due to transient data used and difficulties of establishing a steady state condition for the model. To establish an appropriate initial condition, the first stress period was assigned a steady-state type. The model is divided into 24 stress periods (each of 30.42 days length), with 30 time steps with a multiplier of 1.2 and using the (PCG2) solver package. The first stress period is assigned a steady-state condition to establish a suitable condition for the rest of the stress periods. Results of the last 12 stress periods are considered in the present study. G

The groundwater flow model was cal and automated parameter estimation techniques. Initial hydraulic

parameters are used from the available literature. Due to scarcity of information about hydraulic parameters of lithologic type in the study area, each unit is assumed to be of unique hydraulic value through out the model domain. Also, riverbed hydraulic conductivities of River Nile, irrigation canals and drains were assumed to be unique for a given reach. The hydraulic parameters were then manually adjusted in an iterative process until good agreement was obtained between modeled and observed heads at 29 observation targets (Fig. 13). Final calibration was performed using an automated parameter estimation program, PEST (Doherty et al. 1994). Due to lack of certain hydraulic values for the different parameters, all hydraulic parameters are calibrated based on the minimum and maximum values as constrains. The calibration of parameters is done using the automated techniques described in Groundwater Vistas (Environmental Simulations Inc., 1996). The parameters are grouped into seven groups as follows:

a) Horizontal htypes in the model (clay, clay and sand, fine sand, coarse sand, sand and gravel, and gravel). Vertical hydraulic co z

c) Specific storage Ss for all lithologic types. d) Specific yield Sy for all lithologic types. e) Vertical hydraulic conductivities of riverbf) Vertical hydraulic conductivities of riverbed Kz for irrigation cag) Vertical hydraulic conductivities of drainrbed Kz for drains. calibrated parameters used in the present model are listed in T

372 Ayman A. Ahmed

Fig. 13. Observed and computed heads at observation wells in the study area.

able 4a. Initial and calibrated hydraulic parameters for the study area.

Sy

T

Kh (m/day) Kz (m/day) Ss (m-1) logic Calibrated

Lithotype Initial

(Ky) Initial Calibrated

Initial Calibrated

(Ss) Initial

Calibrated (Sy) (Kh) (Kx) (Kz) (Kz) (Ss) (Sy)

Clay 5 2.375 1.575 5 0.33 0.015 0.000625 0.02 0.15

Sand 15 7.75 5.665 15 2.825 0.01 0.00018 0.04 0.2

Fine sand 25 22.75 5 16.8 25 5.815 0.001 0.0001709 0.12 0.25Coarse 50 82.5 65.45 50 36.77 0.0001 0.000156 0.2 0.32 sand Sand aGravel

nd 5 75 125 76.7 75 57.68 0.00001 0.000125 0.22 0.36

Gravel 150 175 175 150 96.4 0.00001 0.00001 0.24 0.4

Clay and 7


Table 4b. Hydraulic conductivities of riverbed for River Nile, irrigation canals and rains.

(m/day) (m/day)

d Reach type Reach name Initial (Kz) Calibrated (Kz)

River Nile River Nile 15 0.425 East Nag Hammadi canal

15 0.2243

West Nag Hammadi canal

15 0.2142

El-Kasra canal 15 0.1254 El-Baliana canal 15 0.1739 El-Gergawia canal 15 0.1535

rigation canals Ir

Tahtawia canal 15 0.1434 Main Girga Drain 15 0.377 El-Kasra Drain 15 0.347 Main Sohag Drain 15 0.417 Tahta Drain 15 0.467 Kom Badr Drain 15 0.333

Major Drains

n

Akhmim Main Drai 15 0.325

RESULTS Aquifer characterization

Results of lithologic models indicate that the Quaternary aquifer system in the study area is mainly compos ologic categories, which are clay,

, coarse sand, sand and gravel, and gravel. These

nd the generated three-dimensional nce diagrams revealed that there is a wide range of hydraulic conductivities in

ge of hydraulic conductivities and their patial variation in the study area, sandy materials tend to be connected to form

ed of six lithclay and sand, fine sandcategories are represented as spatially repeated sequences that have significant spatial changes in terms of their occurrence, thickness of individual categories, and elevation of top and bottom of each layer. Interfingering and presence of lenses is a main characteristic of the sedimentary basin represented in the study area. Due to these characteristics, heterogeneity of the aquifer system are represented by a spatial variation in hydraulic conductivities ranging between clay and gravel (Fig. 6, Fig. 7 and Fig. 9). Aquifer heterogeneity

Results of the lithologic models afethe modeled area which vary spatially and controls the groundwater flow regime and will have a great importance if these models will be used in contaminant transport studies. Heterogeneity of aquifer system is spatially represented in the study area where different hydraulic conductivity fields are found in the different directions (Fig. 7 and Fig. 9). Hydraulic continuity of sandy materials

Due to presence of a wide ransa certain continuation which is more important in groundwater flow as well as in case of contaminant transport. Hydraulic continuity is represented by clusters of sandy materials within the aquifer system as illustrated in the fence diagrams of sandy layers (Fig. 8).

374 Ayman A. Ahmed

ow model results showed that the general groundwater flow in the study area is from south to north and is affected by the River Nile

discharging line for the aquifer. Highest contour are encoun

a drain for the

in the River Nile.

canals when water levels in the

4)

5) outflow from the northern

Groundwa

The groundwater balance for the study area was calculated as different lements with daily rates such as constant head, storage, flux of river and canals,

e to constant head is about 8.12E+05, storage is abo

Groundwater flow

Groundwater fl

where it acts as atered at the outer parts of the valley (63 m at the eastern side and 61 m at

the western side), whereas lower contour are encountered adjacent to the River Nile (51 m). Groundwater flow model results showed that there is a hydraulic interaction between the River Nile, irrigation canals, and the aquifer system. Recharge, River Nile, and irrigation canals are the main boundaries controlling the aquifer system. In arid and semi-arid regions, the surface water streams are generally recharging the aquifer system. The Nile Valley is a special case where irrigation is the main recharge and groundwater levels are generally higher than the water levels in the River Nile. Exceptions are found at the locations where dams or barrages are present where the accumulated water behind the dams or barrages is higher than the groundwater levels and river is recharging the aquifer system. The groundwater flow regime in the modeled area revealed different flow components that could be summarized as follows (Fig. 14):

1) Downward flow from the upper active layers due to infiltration from irrigated lands.

2) Flow toward the River Nile, where the Nile acting as groundwater aquifer in cases where groundwater levels are higher than water levels

3) Flow out of and into irrigation canals where the water levels in these canals are higher than groundwater levels, whereas at some cases flow occurs towards the irrigation canals drop to below groundwater levels. Flow into drains where the groundwater levels are higher than the drain stages. Inflow from the southern boundary and boundary where the general flow in the system occurs from south to north.

ter flow model budget

ewells, and recharge. The inflow du

ut 3.03E+05 m3/day, river and canals is about 2.69E+06 m3/day, and recharge is about 3.56E+06 m3/day. On the other hand, the outflow is calculated as due to constant head, storage, wells, river and canals, and drains. The outflow due to constant head is about 2.47E+05, storage is about 1.20E+05 m3/day, river and canals is about 5.73E+06 m3/day, wells is about 2.04E+05 m3/day, and drains is about 1.09E+06 m3/day. Model results showed that recharge components in the study area include inflow from applied irrigation and seepage from irrigation canals (Fig. 15a and Fig. 15b) whereas discharge from the aquifer system includes groundwater extraction through pumping wells, groundwater seepage into the River Nile and drains (Fig. 15a and Fig. 15b).


Flux into and out of River Nile irrigation canals, and drains is plotted on Fig. 16 a, Fig. 16b, Fig. 16c and Fig. 16d.

Fig. 14. Computed groundwater flow in the study area.

Fig. 15a. Computed groundwater budget for the study area.

376 Ayman A. Ahmed

Fig. 15b. Computed groundwater flux into and out of River Nile, irrigation canals and drains in the study area.

Fig. 16a: Monthly computed groundwater flux into and out of irrigation canals

3/day).

(m


Fig. 16b: Monthly computed groundwater flux into the River Nile (m3/day).

Fig. 16c: Monthly computed groundwater flux into drains (m3/day).

378 Ayman A. Ahmed

Fig. 16d: Monthly computed groundwater flux into main Sohag drain (m3/day).

DISCUSSION

Importance of using lithologic models in groundwater modeling Lithologic models gives a three dimensional representation of the subsurface, it illustrates the spatial relations between boreholes, it indicates the presence of lenses, and it gives a general configuration of the aquifer system. Using lithologic models in groundwater models is a better way of representing aquifer systems. The groundwater flow model based on the solid models provide detailed characterization of the subsurface and clear idea about the pathways or flow regime that could help in developing the contaminant transport models for the study area. Results of the present study revealed that the obtained lithologic models honored the boreholes data and showed the complex sedimentary system in the study area. Because these models depend on interpolation schemes and filling the gap between the boreholes, it is worse mentioning that the expected lithologic representation between boreholes may not reflect the real situation as it depends on the number of boreholes to use. The more boreholes to use, the more accurate resolution the model will be. Due to scarcity of data and lack of detailed configuration of the subsurface, these models are good to be used for aquifer characterization with recommendation to use more boreholes to get a finer resolution. Although borehole data used in this study are not close enough to get a finer resolution of the subsurface, assumptions on number of layers, thickness, elevation of top and bottom surfaces, as well as representation of clayey lenses in groundwater flow model are avoided. Another benefit that gained from this study is representing heterogeneity and clayey lenses with real three-dimensional representation of the subsurface.

Uncertainty, assumptions and limitations

As lithologic models are generated from boreholes and using interpolation scheme, the resolution of the obtained model depends on the number of boreholes and distance between them. The more boreholes to be used, the more resolution of the model obtained. Consequently, using the current


lithologic models for building the groundwater flow model is better than assuming a homogeneous 2 or 3-layer model. Caution should be taken into consideration when using the presented lithologic models in contaminant transport studies, as sandy and clayey materials may be more exaggerated in areas where boreholes are not enough. Lithologic models are built assuming that the used boreholes are representative of the exact sedimentary sequences in the study area. This assumption may work in areas where boreholes are close to each other and may not work in other areas where boreholes are sparse. The resolution of the lithologic models used is 1000 m in the horizontal direction, whereas 5 m in the vertical direction. Consequently, lens features may be exaggerated or over stretched with the model cells and consequences of this stretching is probable errors in the exact pathways of groundwater flow and contaminant transport if this model will be used for studying contaminants. Using the current lithologic and groundwater flow models The obtained lithologic models in the present study are useful in conceptualizing the Quaternary aquifer system in the study area and designing groundwater flow models as a lot of assumptions will be avoided regarding representation of the hydrogeologic setting of the aquifer system and model layering. The present models could be also extended to be used in contaminant transport as heterogeneity of the aquifer system is well represented and continuity between sandy layers could be illustrated. For prediction purposes, it is recommended to implement more boreholes to get a finer resolution and more accurate results. This extended model could be also used for studying groundwater contamination. We defer this study to a forth-coming paper.

ACKNOWLEDGEMENTS

The author wishes to thank anonymous reviewers for their insightful comments and constructive suggestions.

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