foxton wwtp a1 land assim capacity-final · 2019-11-19 · this report has been prepared for...

Foxton Wastewater Treatment Plant

Assimilative Capacity of Land

Near Foxton and Foxton Beach

(LEI, 2014:A1)

Prepared for

Horowhenua District Council

Prepared by

November 2014

Assimilative Capacity of Land Near Foxton and Foxton Beach (LEI, 2014:A1) This report has been prepared for Horowhenua District Council by Lowe Environmental Impact (LEI). No liability is accepted by this company or any employee or sub-consultant of this company with respect to its use by any other parties.

Quality Assurance Statement

Task Responsibility Signature

Project Manager: Hamish Lowe

Prepared by: Katie Beecroft, Philip Lake

Reviewed by: Hamish Lowe

Approved for Issue by: Hamish Lowe

Status: Final

Prepared by:

Lowe Environmental Impact P O Box 4467 Palmerston North 4462

Ref: Foxton_WWTP_A1_Land_assim_capacity-FINAL

| T | [+64] 6 359 3099

Job No.: 10172

| E | [email protected] | W| www.lei.co.nz

Date: November 2014

TABLE OF CONTENTS

1 EXECUTIVE SUMMARY ............................................................................ 1

2 INTRODUCTION ...................................................................................... 3

2.1 Background ............................................................................................................. 3

2.2 Scope ..................................................................................................................... 3

3 DEFINITIONS .......................................................................................... 4

3.1 Assimilative Capacity ................................................................................................ 4

3.2 Principles of Land Treatment .................................................................................... 4

4 WASTEWATER COMPOSITION ................................................................ 5

4.1 Wastewater Constituents to be Assimilated ................................................................ 5

5 APPLICATION PROCESSES – PRE-SOIL CONTACT .................................. 6

5.1 General ................................................................................................................... 6

5.2 Droplets and Aerosols .............................................................................................. 6

5.3 Evaporative Losses ................................................................................................... 6

5.4 Volatilisation ............................................................................................................ 7

5.5 Application Methods ................................................................................................. 7

5.6 Summary of Application System Losses ...................................................................... 8

6 PLANT UPTAKE OR REMOVAL ................................................................. 9

6.1 General ................................................................................................................... 9

6.2 Land Use Management ............................................................................................. 9

6.3 Crop Water Use ..................................................................................................... 11

6.4 Summary of Plant System Assimilative Capacity ........................................................ 11

7 SOIL ....................................................................................................... 14

7.1 General ................................................................................................................. 14

7.2 Microbiological Parameters ..................................................................................... 14

7.3 Heavy Metals ......................................................................................................... 14

7.4 Organic Matter and Suspended Solids ...................................................................... 15

7.5 Accumulation of Cationic Salts................................................................................. 15

7.6 Organic Contaminants ............................................................................................ 15

7.7 Nitrogen and Phosphorus ....................................................................................... 15

7.8 Water ................................................................................................................... 18

7.9 Summary of Soil Assimilative Capacity ..................................................................... 18

8 GROUNDWATER ATTENUATION ............................................................ 19

8.1 General ................................................................................................................. 19

8.2 Gaseous Loss ........................................................................................................ 19

8.3 Dispersion and Dilution ........................................................................................... 19

8.4 Summary of Groundwater Attenuation ..................................................................... 19

9 UN-ASSIMILATED FRACTION ................................................................ 20

10 STANDARDS, GUIDELINES AND STATUTORY CONSIDERATIONS ........ 21

10.1 National Standards and Guidelines .......................................................................... 21

10.2 Industry Considerations .......................................................................................... 21

10.3 Regional One Plan .................................................................................................. 21

11 LAND TREATMENT ASSIMILATIVE CAPACITY ...................................... 23

12 REFERENCES ......................................................................................... 25

| Foxton WWTP Land Application Assimilative Capacity Assessment | P a g e | 1 |

1 EXECUTIVE SUMMARY

Horowhenua District Council (HDC) is responsible for the provision and management of wastewater treatment for the Horowhenua District. In 2012 HDC reviewed the wastewater treatment and discharge systems for a number of their communities. This included Foxton. Treated wastewater is currently discharged to surface water from Foxton. This direct and continuous discharge is increasingly considered unacceptable by consenting authorities and the wider community. An alternative to surface water discharge is discharge to land. The land is capable of further treating applied wastewater, with the applied nutrients and water being utilised by crops to improve their growth and yield. This report provides an evaluation of the assimilative capacity of land in the vicinity of Foxton and Foxton Beach. The area that could be used for land treatment may extend 10 km or more from each of the communities, and in this report is referred to as the evaluation area. Wastewater parameters to be assimilated include:

• Macronutrients (N, P); • Other nutrients (Ca, Mg, K, Na, S); • Organic matter (as measured by carbonaceous biochemical oxygen demand); • Total suspended solids; • Trace elements such as heavy metals; • Organic contaminants such as persistent organic pollutants (POPs), personal care

products, and pharmaceuticals (typically synthetic compounds);

• Microbiological components; and • Water.

Within the land treatment system, comprising of soil, plant and hydrogeological systems, a number of different processes occur with each playing a role in the assimilative capacity for different contaminants. The assimilative capacity for land treatment in the evaluation area is dependent on the unique properties of the site selected, and its design (land management and irrigation management). A summary table which outlines the relative assimilative potential in the evaluation area of various constituents in wastewater after being subject to a range of process is given below. The table indicates the parts of a land treatment system which are active in assimilating the applied wastewater constituents. The higher the number given, the higher the potential to assimilate the listed constituent. The potential to assimilate applied constituents indicates whether a land treatment system is effective in assimilating the listed constituent.


Table A: Relative Assimilative Potential in the Evaluation Area N P Pathogens Heavy

metals

OM and

SS**

Cationic

salts

Organic

contaminants

Pre-soil contact

Droplets and Aerosols

1 0 variable variable variable variable 1

Evaporative

Losses 1 0 0 0 0 0 1

Volatilisation variable <15 %

0 0 0 0 0 1

Application Methods

Plant Uptake or

removal

3 300

kg/ha/y

3 40 kg/ha/y 1 2 1 3 1

Land Use Management

variable variable variable 1 variable 1 variable

Soil

Physical and chemical

removal

3* 4* >30

kg/ha/y

5 > 95 %

4 5 4 5

Biological incorporation

3 3 4 3 5 3 5

Natural die off 0 0 4 0 0 0 0

Gaseous loss 3* 5 %

15 kg N/ha

0 0 0 0 0 1

Groundwater

Gaseous loss variable 0 0 0 0 0 1

Dispersion and

dilution variable variable Variable 0 0 variable variable

Potential to

assimilate

applied constituents

Good 315 kg

N/ha/y

Good 40 kg

P/ha/y

Excellent >95 %

Excellent Excellent Good Excellent

Notes: 0 – none; 1 - Very low; 2 – Low; 3 – Moderate; 4 – High; 5 – Very high; variable – Highly variable * depends on chemical form

** Organic Matter and Suspended solids

The table above illustrates that for the listed constituents land treatment in the evaluation area has the potential to assimilate wastewater constituents. For each of the listed constituents it can be seen from the table that a land treatment system function is active in assimilating applied wastewater constituents and therefore a reduction or complete removal of most wastewater constituents can be expected from land treatment. Unfortunately limited quantification of assimilation rates can be provided without site specific information. Despite this, key planning targets of 315 kg N/ha/y and 40 kg P/ha/y can be reasonably expected to be suitable for the vast majority of sites in the evaluation area. It is known that the efficacy of the assimilation will be dependent on design and management. Assuming appropriate design and management are used there is scope for a sustainable land treatment system to be developed without adverse off-site environmental effects within the evaluation area.


2 INTRODUCTION

2.1 Background

Horowhenua District Council (HDC) is responsible for the provision and management of wastewater treatment for the Horowhenua District. In 2012 HDC reviewed the wastewater treatment and discharge systems at a number of the communities. This included Foxton. Treated wastewater from Foxton is discharged to the lower Manawatu River. However, direct and continuous discharges are increasingly considered unacceptable by consenting authorities and the wider community. An alternative to surface water discharge is discharge to land. The land is capable of further treating applied wastewater, with the applied nutrients and water being utilised by crops to improve their growth and yield. HDC has engaged LEI to assist with the evaluation of land in the vicinity of Foxton and Foxton Beach for suitability to establish land based discharges of wastewater. As part of the wider project this report evaluates the capacity for land in the area to assimilate applied wastewater. The assimilative capacity indicates how much wastewater can be applied to land without causing adverse effects on the soil, groundwater or surface water environment.

2.2 Scope

This report provides a summary of assimilative components that need to be considered in a land application system. An assessment of the assimilation by the different parts of the land application system is outlined as follows:

• Section 3 defines land treatment as described in this report; • Section 4 identifies the wastewater parameters to be assimilated; • Section 5 summarises how the application process influences assimilation; • Section 6 discusses plant uptake and removal; • Section 7 discusses the soil transformation processes for applied wastewater; • Section 8 looks at how wastewater parameters are attenuated in the groundwater system;

• Section 9 provides a discussion on the fate of any unassimilated component; and • Section 10 summarises the assimilative capacity and provide recommendations for the

adoption of assimilative capacity for the land treatment system. No specific land parcel has been evaluated. This report represents a generalised assimilative capacity for land in the lower Manawatu area. A site specific assessment should be undertaken following the determination of preferred land options.


3 DEFINITIONS

3.1 Assimilative Capacity

Assimilation is the ability to consume and incorporate material, being nutrients and contaminants. Assimilative capacity in a land treatment system can be defined as the potential for the removal of contaminants through various means, including the application method, plant uptake, soil retention, leaching and groundwater attenuation, so that the resulting surplus of contaminants does not create an adverse environmental effect. In order to assess a site’s suitability for land treatment, and its assimilative capacity potential, a number of factors need to be considered. These factors are discussed in this report.

3.2 Principles of Land Treatment

Land treatment of waste is the utilisation of the biological, chemical and physical properties of the terrestrial environment to further treat solid and liquid wastes beyond the wastewater treatment plant. There are numerous biological and chemical processes within the terrestrial environment that are capable of using, adsorbing, binding, attenuating or otherwise renovating the various chemical and biological components of wastes. Land treatment aims to beneficially use the applied ‘waste’ material for productive use, while using the environment to provide further treatment of the effluent/solid material, through nutrient sequestration and removal, evapotranspirative uptake and atmospheric loss, and pathogen reduction through attrition in the soil environment. The development of a land treatment system involves assessing potential sites, soils, crops or plants, and key processes to determine the effectiveness of the further treatment of the wastes by the land. The land area and system’s effectiveness is then considered by assessing the qualities and quantities of the waste product. It is important to understand that all of these factors are intricately interlinked, and should not be considered in isolation. The sustainability of the treatment and renovation of the waste by the environment is the paramount goal of a land treatment system. This ensures the sustainability of the activity, by ensuring the enduring health of the ecosystem on which treatment performance is based. Land treatment is philosophically and practically distinct from land disposal. Land treatment seeks to utilise the environment to its maximum extent treat the waste, and in so doing may also seek to improve the environment through the characteristics of the waste. This can include using the waste on productive land, improving crop yields through nutrient and water addition. Its design is based around identifying a limiting design parameter to ensure a nominated minimal level of treatment is achieved. Land disposal seeks only to dispose of the waste, using the land as a mechanism to allow the waste to enter the environment with limited or no treatment, with design based around the principal of how quickly the discharged material can be dispersed. Land disposal is not considered further in this report since assimilation is not considered to be a function of a land disposal system. This does not preclude the possibility of HDC adopting a land disposal strategy. Land disposal should still be considered further in the wider land evaluation programme.


4 WASTEWATER COMPOSITION

4.1 Wastewater Constituents to be Assimilated

Wastewater is a complex mixture of chemicals which when discharged to the wider environment may be beneficial, benign or may create adverse effects. In a land treatment system a primary objective is minimising the potential for adverse effects as a result of wastewater entering the receiving environment. Many of the wastewater constituents which are problematic when discharged to water are beneficial or able to be renovated in the soil environment. Wastewater parameters of interest include:

• Macronutrients (N, P); • Other nutrients (Ca, Mg, K, Na, S); • Organic matter (as measured by carbonaceous biochemical oxygen demand); • Total suspended solids; • Trace elements such as heavy metals; • Organic contaminants such as persistent organic pollutants (POPs), personal care

products, and pharmaceuticals (typically synthetic compounds); • Microbiological components;

o Bacteria; o Virus; o Fungi; o Protozoa; o Helminths; and

• Water. Mechanisms to deal with the wastewater constituents differ. Within the land treatment system, comprising soil, plant and hydrogeological systems, a number of different processes occur with each playing a role in the assimilative capacity for different constituents. The parts of the land treatment system and their role in renovating wastewater are described in the following sections.


5 APPLICATION PROCESSES – PRE-SOIL CONTACT

5.1 General

The method of application has an impact on the amount of wastewater and its constituents that reaches the soil. A small loss of wastewater volume may occur during irrigation as a result of evaporation prior to contacting the ground and spray drift. Along with water loss, constituents in the wastewater may also be lost. The wastewater constituents which may be lost are:

• Ammonia by volatilisation; • Pathogens by transport in or on aerosols; and • Salts and other parameters by aerosolisation and drift.

Due to the high amount of dispersion that occurs, especially with aerial application of wastewater, these losses are considered to be included as part of the assimilative capacity of the system. Application system losses are greatest for spray irrigation systems with small droplet size which are elevated from the ground. Losses are enhanced by irrigation during higher temperatures, lower humidity and higher wind speed.

5.2 Droplets and Aerosols

Spray irrigation of wastewater produces droplets. The droplet size produced is a function of the irrigator nozzle size and the pressure of the discharge. Low pressure, large nozzle size systems typically produce larger droplets, with small nozzle, high pressure systems producing small droplet sizes. Most droplets fall to the ground under gravitational settling a short distance from their source. This is typically the case for droplets greater than 200 µm diameter. Droplets less than 100 µm in diameter tend to lose moisture by evaporation and become aerosols. Between 100 and 200 µm diameter wastewater quality and environmental factors influence whether droplets are aerosolised. Depending on the irrigation system, wastewater quality and environmental factors, around 0.1 to 1 % of the droplets produced are aerosolised and can travel beyond the wetted area they are intended to land (Teltsch, et al., 1980). Aerosols are air-bourne particles typically <20 µm in diameter (some authors use <50 µm). The dispersion of aerosols is controlled by the magnitude and direction or air currents, which is distinct from “wind” due to the thermal and multidirectional implications of air currents, and by the size, shape and density of the aerosol. Wastewater constituents that are aerosolised are considered to be lost from the system, and therefore contribute to the overall assimilative capacity.

5.3 Evaporative Losses

Evaporative losses are variable and largely dependent on:

• Climatic conditions: Higher air and soil temperature, low relative humidity and wind speed all increase the rate of evaporation;

• Time available for evaporation: This is the time that the wastewater is in the air and is influenced by the settling speed which is a function or the trajectory of the irrigator discharge and the droplet size; and

• Surface area of the droplet: Smaller droplet sizes result in a higher surface area available for evaporating moisture.


Evaporative losses may be off-set by the reduction in evapotranspiration of the crop underneath for the period of the irrigation.

5.4 Volatilisation

A significant level of ammonia volatilisation (gaseous loss) has been shown to occur as a result of the spraying action of the irrigation nozzles. This can result in the removal of 2 to 5 % and up to 15 % of total N (Myers et al, 1999) depending on its concentration in the wastewater and the relative equilibrium between ammonia (NH3) and ammonium (NH4

+). Volatilisation is enhanced in climatic conditions which favour evaporation.

5.5 Application Methods

There are a number of options for irrigation of treated wastewater onto a site. The suitability of each option is dependent on site factors, such as surrounding land use (particularly with regard to aerosol production and visual amenity), site topography, soil hydraulic conductivity (influences the application depth able to be accepted) and crop type. Therefore it is not possible to specify a system without site specific design. A brief summary with regard to irrigation losses is given below for typically used irrigation methods.

5.5.1 Border Dyke

Border dyke irrigation involves the flooding of a bunded application area which is maintained in a crop. This method gives a high rate of application and relies predominantly on the soil and groundwater for assimilation. No aerosols are produced using this method and evaporation is offset by a reduction in evapotranspiration of the crop for the period of flooding.

5.5.2 Drip Irrigation

Low-rate irrigation systems using drip irrigation use small-diameter tubes placed above or below the soil's surface. This method relies on the soil and plant system for assimilation of wastewater components. No droplets become airborne using drip irrigation.

5.5.3 Fixed Impact Sprayers

Impact sprayers are high or medium pressure systems which use sprinklers or guns mounted overhead on permanently installed risers or couplings at or above ground level. Droplets produced by the lower pressure systems are typically large (> 200 µm) and do not travel far. Higher pressure systems can produce aerosols. In some systems there is a wide range of droplets produced and therefore there is likely to be minor losses by evaporation and aerosolisation. Some losses can be expected primarily through evaporation but these are minor when off-set by reduced evapotranspiration during irrigation.

5.5.4 Small Moveable Irrigators

These sprinkler or small rotating booms systems are a series of lightweight pipeline sections or irrigators with rotating booms that are moved manually for successive irrigation events. As with the non-movable impact sprayers, these irrigators can provide low application depths depending on the length of time they are operated for.


There is a large variation in droplet size produced by these systems, however the height of the discharge is typically low and so the travel distance of droplets and generation of aerosols is less than for boom type irrigators.

5.5.5 Linear Boom and Centre Pivot Irrigators

Boom and centre pivot irrigators consist of several segments of pipe joined together and supported by trusses, mounted on wheeled towers with sprinklers positioned along its length. As above, there is a large variation in droplet size produced by these systems predominantly due to nozzle selection and variation in pressure. It is possible to produce droplets of a consistent size and so the potential for airborne losses is easier to predict. The spray nozzles are typically mounted higher off the ground than for other systems and so they are susceptible to losses in unfavourable climatic conditions, particularly higher winds. Due to above-ground linkages along the length of boom type irrigators the use of these is restricted to crops that grow no higher than the irrigator nozzles i.e. they are not suitable for trees or tree crops.

5.6 Summary of Application System Losses

Losses due to evaporation and spray drift are highly variable and dependent on the method of application. Most systems are designed to minimise losses to air from the irrigation to minimise or avoid nuisance odour, reduction in amenity value and potential public health risk (from aerosols containing pathogens). As such, losses due to the method of application are considered to be < 1%.


6 PLANT UPTAKE OR REMOVAL

6.1 General

Plants have the capacity to assimilate nutrients (N, P, Ca, K, Mg, Na, S) and micronutrients such as heavy metals. The main mechanism is through the update of water by plants in such a way as to also take up the nutrients at the same time. Plants are utilised in land treatment systems to take up nutrients and contaminants in the applied wastewater and allow them to be accumulated in the above and below ground biomass of the plants as they grow. With most plant crops only the above ground biomass is harvested, and therefore while the roots take up nutrients and contaminants, it is only the above ground biomass which is considered to assimilate wastewater applied constituents. There are two terms which are typically used:

• Plant uptake: describes the total requirement for, in particular, nutrients of the plant i.e. includes nutrients accumulated in all parts of the plant; and

• Plant removal: describes the amount of nutrients in the harvested portion of the crop i.e. the amount that is permanently removed from the site.

The difference between plant uptake and plant removal is the biomass left on the site which then becomes part of the soil pool of nutrients. The availability of these remaining nutrients is dependent largely on the amount of mineralisation that occurs which is influenced by particle size, soil temperature, soil moisture content and oxidation state of the soil. A portion of the plant below ground biomass (and stubble) is only slowly available (or recalcitrant) and forms part of the soil organic matter. A significant proportion is made re-available for plant uptake to the next crop, and so off-sets or reduces the demand for future applications of wastewater nutrients. Consequently this ongoing re-availability of nutrients must be factored in to the site nutrient balance when developing a land treatment system and identifying a plant’s assimilative capacity contribution. Plants have other roles in the assimilation of wastewater derived materials at a site. The roots assist with maintaining and improving soil structure and therefore water movement in the soil. Roots also support microbial communities which are involved in the breakdown of applied organic compounds and the inactivation of applied pathogens by encouraging competition and predation.

6.2 Land Use Management

6.2.1 General

Specific land management considerations include: • Crop selection and performance; • Cultivation, harvest and fallow management of cropped areas; and

• Animal grazing rotation. These are discussed in more detail below. An additional consideration is the acceptability of the crop to end-users. This discussed further in Section 10.

6.2.2 Crop Selection and Performance

The lower Manawatu area is well suited climatically to a wide range of crops. An example of some crops commonly grown in the area is given in Table 1 (page 13). Soils are often fertile due to both natural inputs (alluvial flood plain material) and a long cultivation history and associated


fertility management. The incorporation of mixed cropping and multiple rotations per year are appropriate based on the climatic conditions and in many cases the soil type. In the evaluation area the extent of areas cropped and crops selected may change from year to year, and be incorporated into rotations with permanent pasture lasting several years at a time. A key factor influencing crop choice is soil moisture. Because water can be supplied to the crop by wastewater irrigation additional cropping opportunities (alternatives) exist, or the current crops can be grown with greater certainty to achieve a higher yield. With regard to wastewater application, the purpose of the crop is:

• To sequester wastewater derived nutrients as biomass; • To dewater the soil by transpiration; • To protect the soil from erosion damage (overland flow) and surface crusting from

droplet impact; and • To assist with maintenance of soil structure by root exploration.

In order to achieve the above, the crop objectives are:

• Selection of crops and species which have high rates of nutrient uptake either due to high nutrient density or rapid biomass accumulation;

• Selection of species with a low sensitivity to higher moist soil conditions; • Achieve year round growth (nutrient removal);

• Have limited periods of high traffic (harvest), or bare ground (cultivation and fallow); and • Limit nutrient return by grazing animals.

Land management decisions will impact on the performance of the crop species used. These are discussed in more detail below.

6.2.3 Cropping Rotation Management

Non-pasture crops are typically grown for one season and followed by another seasonal crop or pasture establishment. Seasonal crops can achieve high growth rates and produce high rates of nutrient removal (see Table 1 below). However, during cultivation prior to establishment of the crop, and then post-harvest prior to replacement crop establishment, there is little capacity for nutrient removal and if poorly managed there can be a net release of nutrients, particularly N. Avoidance of irrigation during these periods will assist to limit leaching of nutrients. To avoid soil damage machinery movement and mechanical tillage should not occur on wet soils. Irrigation should not occur immediately prior to cultivation, sowing and harvesting. If sites are naturally wet due to seasonal high moisture then this will further limit the ability for machinery to operate on a site. Cropping rotation management can impact on the systems assimilative capacity by the simple fact of having no crop to utilise soil nutrients, leading to them being leached from the site. In addition, harvest and crop establishment practices can influence soil conditions (discussed later) which can also reduce the ability of subsequent crops to utilise nutrients. While cropping rotations can influence the assimilative capacity of a land treatment system, its actual impact on assimilative capacity (and its variability) is covered in considerations discussed elsewhere in this report.

6.2.4 Nutrient Uptake

Table 1 on Page 13 below provides a brief summary of crop options and their ability to take up

nutrients, in particular nitrogen and phosphorus uptake. It should be noted that the uptake rates

given in Table 1 are for the rotation period of the crop, and in some cases more than one crop


can be grown per year. Yearly uptake rates will be a result of crop plus replacement crop or pasture, less the amount of nutrients returned by animal excreta if grazing occurs. The Target Rates indicated in the table below are indicative and developed from the likely uptake achieved under a mixed grazing and cropping regime. The rates given are considered to be the plant assimilative capacity. The rates for the grazed pasture do not include the animal return component. For this report a rate of 300 kg N/ha/y and 40 kg P/ha/y has been adopted as a generic value to be used for further analysis. If more intensive cropping regimes are used this rate could be increased. Similarly this rate could be reduced for less intensive cropping system.

6.2.5 Temporal Considerations

For any crop the uptake of nutrients does not occur at a constant rate throughout the year, or indeed through the growing season. Instead the requirement for water and nutrients corresponds to the phase of growth of the crop. In general the rate of uptake of water and nutrients is highest in summer, when soil temperatures are warm and daylight hours are long. Through the winter, plant growth slows and water and nutrient requirements are less, even falling to zero where plants become dormant. In planning a land treatment system, the crop assimilative capacity must be temporally varied to ensure water and nutrients are applied when plants are able to utilise them. This may mean alternative discharge options need to be considered in the winter, or storage of wastewater is used to ensure sufficient excessive application rates do not occur in winter months. In the Horowhenua District soil moisture deficit occurs between November and March. From April to October more rainfall than evapotranspiration occurs. Irrigation is likely to be significantly reduced and may even be ceased during the April to October period. It should also be noted that year to year variation in the soil moisture deficit occurs due to climatic variations. An extreme example of this is the El Nino and La Nina cycle. In the longer term predicted climate change may cause a shift in the climatic regime for the evaluation area.

6.3 Crop Water Use

Evapotranspiration is the primary method for assimilation of the applied water. The evaluation area is subject to around 740 mm/year evapotranspiration. Matching plant water requirements is often the limiting parameter for land treatment application rate design. As with nutrient uptake, water uptake is seasonally variable. Rates can range from 0 mm/day in mid-winter to around 6.9 mm/day in mid-summer for a pasture crop. This is equivalent to 0 to 69 m3/ha/day. For planning purposes the plant assimilative capacity for water may be taken as 740 mm/year.

6.4 Summary of Plant System Assimilative Capacity

Plant removal is the primary mechanism for assimilation of nutrients and water in a land application system. The total assimilative capacity of the plant system is dependent on the land area utilised, with the loading rate refined based on the crop type and its management. A minimum of 300 kg N/ha/y and 40 kg P/ha/y should be targeted for nutrient removal. Plant utilisation of wastewater applied water is in the range of 740 mm water/year (7,400 m3/ha/year).


When planning a land treatment system variability in nutrient and water uptake must be accounted for due to:

• Seasonal variability and plant growth phase; and • Crop rotation management including cultivation, harvest and fallow.


Table 1: Crop Nutrient Uptake

Crop / Land use Growth period Crop yield per rotation

N uptake (kg/ha/rotation)

P uptake (kg/ha/rotation)

Reference

Pasture – irrigated, cut

and carry

Year round (with exclusion

periods for cultivation/harvest) 18 T/ha 500-600 130-160 Morton et al. (2000)

Pastoral – irrigated grazed system

Year round (assumes only 48

hour exclusion for grazing) 18 T/ha 300-500 75-135

FLRC (2009), Williams and Haynes (1990)

Maize silage Sept/Oct to Feb/Mar (130-160

days) 20 T/ha 220 40 FAR (2009)

Kale 11 Apr, 25 July harvest, 29 October harvest (110-140 days)

18 T/ha 380 50 Hanson (2001)

Peas 1 October to 29 May (240 days) 16 T/ha 106 16

Beare et al. (2010) Brown et al. (2007)

Squash mid Sept/late Oct to February

(75-90 days) 30 T/ha 107 20

Hortnet (1995)

White & Russell (1999)

Sweetcorn 9 Dec to 30 Mar (110 days) 16 T/ha 62 9

Hortnet (1995) Fandika et al. (2011)

Standard Rotation Forestry - Pine

Year round NA 100 (kg/ha/year) 30 (kg/ha/year) Nicholas (2003)

Standard Rotation Forestry - Eucalypt

Year round NA 50 (kg/ha/year) 10 (kg/ha/year) Myers et al. (1999)

Eucalypt or Willow Coppice Systems

Year round NA 200-300

(kg/ha/year) 75-125 (kg/ha/year) NZLTC (2000)

Target rates (kg/ha/y) 300 40 NA


7 SOIL

7.1 General

The soil, in particular the biologically active top-soil, is the predominant mechanism for assisting assimilation in a land treatment system. A wide range of processes occur in the soil when wastewater constituents are applied, including:

• Filtration of solids, microbes and large organic compounds; • Sorption of microbes, charged molecules such as organic compounds, cations and

anions; • Conversion of chemical species (e.g. nitrification); • Decomposition of organic compounds including microbial biomass and synthetic organic

contaminants;

• Temporary storage in soil water prior to plant uptake or leaching; and • Incorporation of nutrients and trace elements into soil biomass.

A summary of how various wastewater constituents interact with the soil and their means of soil assimilation follows.

7.2 Microbiological Parameters

There has been extensive evaluation of the survival of pathogens from wastewater irrigation to New Zealand soils. Research shows that pathogen survival is strongly correlated with soil texture and structure. A study by McLeod et al. (2001) showed that highly developed clay soils allowed a lot of the applied pathogens to pass through. This compares to moist sandy soils where there was even through-flow and almost complete removal of pathogens. However, McLeod et al. (2001) also showed that soil development features can also have large impact on pathogen removal. Dry sandy soil (such as Waitarere sand which is representative of much of the Manawatu coastal back dune country) has poor pathogen removal rates due to water repellency. In the evaluation area, the older coastal dune sequences from 1 km to almost 15 km inland are expected to have a high capacity for removal of pathogens providing they are kept moist. Non-sandy soils in the evaluation area are likely to have wetness limitations, either due to poor soil structure, low elevation or interdune zones of water accumulation. The finer textured soils i.e. clay and very fine silt loam soils, may exhibit bypass flow properties, which occurs when water travels down cracks in the soil in preference to flowing through the body of the soil. In this case pathogen removal is poor and wastewater should be applied a rates that avoid saturating the soil. For an appropriately managed wastewater application system 92 - 99.9 % of applied pathogens are removed in the top 10 mm of the soil (Crane and Moore, 1984; Gunn, 1997). Pathogens are not considered to be limiting for wastewater application to land in the evaluation area.

7.3 Heavy Metals

The accumulation of heavy metals in soil has: a) a finite capacity; and b) a toxic level for soil biota and plants. Typically the capacity of the soil to retain applied heavy metals is far in excess of the amount applied from wastewater over the lifetime of a land treatment system. This is because the metals are typically retained in sludge at the wastewater treatment plant.


Given the lack of heavy industry in the WWTP catchments, heavy metals are unlikely to be a limiting parameter in any land treatment system for Shannon, Foxton or Foxton Beach, and therefore their impact on assimilative capacity does not need to be considered further.

7.4 Organic Matter and Suspended Solids

The main mechanism for removal of organic matter (as measured by BOD) and suspended soils is filtration. Wastewater applied suspended solids and BOD is unlikely to travel below the topsoil of an irrigation area. If elevated suspended solids is found in groundwater coming from a land treatment site it is typically the result of either excess application of wastewater causing illuviation, that is; migration of dispersed clay material in the soil, or high rates causing a portion of the applied water passing through large cracks and having minimal soil contact. Both these processes are managed by applying either a lower application volume or a lower strength wastewater. A healthy soil environment can assimilate up to 600 kg BOD/ha/day (NZLTC, 2000). A typical application of a pond treated wastewater (at around 40 g BOD/m3) would result in less than 3 kg BOD/ha/d being applied. The BOD applied is substantially lower than the soils capacity to assimilate it and it is not considered to be limiting for land application of municipal wastewater in the evaluation area.

7.5 Accumulation of Cationic Salts

A commonly expressed concern with irrigation, both clean water and wastewater irrigation, is the risk of increased salinity in the soil and resultant damage to soil structure. Salinity is often associated with an accumulation of sodium in the soil. Municipal wastewater typically has a sodium concentration of around 100 g/m3 (Metcalf and Eddy, 2003); and at this rate the risk of sodium accumulation would be minimal as rainfall would be sufficient to flush it through the soil. It is considered that salt accumulation in the soils of the evaluation area is unlikely to occur and would not pose a restriction on land treatment.

7.6 Organic Contaminants

The soil is the primary mechanism for treatment of any organic contaminants that may be present in the wastewater. The main source of these chemicals is pharmaceuticals and personal care products, if they are not broken down in the WWTP. These compounds are emerging as a concern due to the limited understanding of their cumulative impact in the environment and specifically ecosystem functions. Soil organic matter and the soil biota is the primary mechanism for assimilating organic contaminants. Occlusion by chelation (i.e. constituents become strongly bonded at multiple points to complex organic molecules in the soil) and decomposition by microbial action of these compounds occur in the soil environment. There is a potential for acute or cumulative effects to the soil from these compounds however there is limited understanding of these effects.

7.7 Nitrogen and Phosphorus

Nitrogen (N) and phosphorus (P) are the elements that are typically limiting for land treatment systems. Soil retention of the applied nutrients is a significant mechanism for assimilation. There are two main ways in which nutrients may be retained by the soil being, incorporation into soil


organic matter and sorption to clay minerals and organic complexes. Other processes such as gaseous losses also occur in the soil.

7.7.1 Soil Organic Matter

Incorporation of both N and P into soil organic matter (SOM) occurs as a result of soil microbial processes. Both elements are essential for microbial activity and are cycled within the soil through growth and death of microbes. Consequent portions of any applied N and P are taken up and used by soil microbes. The process of growth and death of microbes also makes N and P more readily available for uptake by plants. The amount of N and P incorporated into the SOM is dependent on the rate of accumulation of organic matter. The methods adopted for cultivation and management of any crop grown in a land application system will influence the accumulation of SOM, and consequently the potential for retention of N and P within SOM. Quantification of SOM contribution to nutrient assimilation should be considered based on the design and management of the cropping regime. In particular it should be noted that in some cases there may be a maximum level of potential retention, or a release following cultivation. Compared to other parts of the land treatment system, SOM incorporation of N and P is relatively minor and may account for up to 5 % of the material applied. For P, SOM incorporation may be important especially on sites with low amounts of the clays discussed in Section 7.7.2. This portion may also rapidly disappear from the soil depending on management and therefore should not be relied on in a land treatment system for assimilation.

7.7.2 Sorption

Nitrogen (N) changes form readily depending on its chemical composition. When applied in wastewater organic and ammoniacal N can be readily bound to the soil, however various processes result in its transformation to other more mobile forms, such as nitrate N which can be up taken by plants or leached in drainage water. Sorption of N in a land treatment system should not be relied upon. A significant mechanism for P assimilation in a land treatment system is absorption (into) and adsorption (onto) to soil particles. The rate of sorption is enhanced in clay soils and in soils where there are greater portions of iron and aluminium minerals. The amount of these minerals will determine the soils capacity for P storage, often referred to as P retention, and is an indicator of the soils capacity for retaining P on-site. Soils in the evaluation area are understood to be comparatively low in these minerals, but there is still some capacity for P storage. The P retention capacity of soils in the evaluation area is shown in Figure 2 below. The prevalence of areas with a low (green) or very low (purple) P retention capacity highlights the limitation to P retention in the evaluation area. It should be noted that peat soils, which occur in the Opiki Basin can contribute to and have a moderate P retention potential. Phosphorus retention is a critical design component in a land treatment system, however like SOM, there is a maximum potential for retention. This maximum retention is an equilibrium point after which additional P is likely to be retained at a lesser rate, being more readily used by plants or able to be leached in drainage. The extent of P retention under a nominated P application rate helps to determine the site life, being a duration after which greater P losses will be seen in drainage water. Low P retention top soils are able to retain 950 -1,900 kg/ha. Municipal wastewater applications are typically in the order of 40 kg P/yr. Therefore there is scope for more than 30 years application before retention potential decreases.


Figure 2: P Retention Map for the Lower Manawatu

7.7.3 Gaseous Loses

Phosphorus is not lost in gaseous forms. Gaseous N is lost through a number of processes including denitrification and volatilisation. Denitrification causes nitrate N to be converted to nitrogen gas or nitrous oxides. Volatilisation causes ammoniacal N to be converted to ammonia. Both denitrification and volatilisation occur in un-irrigated soils, but the rates can be significantly greater with wastewater irrigated soils depending on the resulting soil moisture content, the duration of elevated moisture conditions and the composition of nitrogen in the wastewater. Design guidance documents for wastewater land application indicate losses of 15-20 % of applied N can be achieved by denitrification (Mercer et al., 2001). However, literature values vary widely (0-238 kg N/ha/y) indicating the need to apply site specific conditions to a determination of denitrification losses. Denitrification loss has been measured in Manawatu pasture soils receiving no N fertiliser or irrigation at 4.5 kg N/ha/y, and it was identified that this low value was due to low soil moisture in summer (Luo et al., 2001). It can be expected that significantly higher rates could be achieved by an irrigated system with a consistent supply of N and water.


It would not be unreasonable to expect combined gaseous nitrogen losses to account for 5 % of the applied nitrogen.

7.8 Water

The impact of water in a land treatment system is often overlooked. It has a critical impact on nutrient transformation and the potential for nutrient leaching. The rate of application of water influences the soil’s ability to retain it for plant uptake, drain to underlying groundwater or the potential for saturated conditions in the soil rooting zone. Water retention and the rate it passes through the soil is also important to enable many of the reactions described above to be maximised. There is no direct assimilative capacity of soil for water. Instead the soil holds water so that it can be accessed by plants as needed. Therefore in planning a land treatment system soils which have adequate capacity to retain water, without causing water logging or excessive drainage should be selected.

7.9 Summary of Soil Assimilative Capacity

The soil is the predominant assimilation environment for many wastewater constituents including:

• Pathogens; • Organic matter; • Suspended solids; • Heavy metals; and • Organic contaminants.

Soil is an efficient treatment system for these constituents and under irrigation of a typical strength treated municipal wastewater each of these constituents are capable of being assimilated so that effects beyond the application area are minimal. This tells us that if the application is designed to meet the nutrient needs of a crop then accumulation of the above constituents should not occur to unacceptable levels in the soil. For the determination of the soils contribution to assimilation of nutrients, in particular N and P, site specific information is required. In reality, the design of the land treatment system will be undertaken to ensure that nutrients are assimilated by alteration of the loading rate to match the crop grown and soil properties.


8 GROUNDWATER ATTENUATION

8.1 General

The off-site receiving environment is likely to be either groundwater that has passed under the site or surface water that receives the groundwater that has passed under the site. There is considerable potential for constituent assimilation within this groundwater system; with two potential assimilation mechanisms being gaseous loss and dispersion/dilution.

8.2 Gaseous Loss

Nitrogen is the only constituent for which gaseous losses can occur close to the groundwater under a land treatment system. The mechanism of assimilation is denitrification, being conversion of nitrate N to gaseous forms of N. The rate of denitrification is highly variable and dependent on properties of the groundwater environment. Recent investigations into N losses in several catchments with soils similar to the evaluation area have been undertaken (Stenger et al, 2012) and show considerable potential for denitrification. In these situations groundwater denitrification in the order of 50 % of the mass of nitrogen entering groundwater is believed to have occurred.

8.3 Dispersion and Dilution

Dispersion is the gradual change in concentration from areas of high concentration to low concentration (move along a concentration gradient) which allows concentrations to equalise when drainage water reaches groundwater. Dilution is the physical mixing of two different water sources, with the concentration in one of the sources being reduced when mixed with a second source with a lower concentration. All constituents applied in wastewater that pass through the soil and drain to groundwater will be subject to both dispersion and dilution. This will usually result in the concentration of the drainage water being reduced in the groundwater system, and the elevation of concentration in the groundwater system. The extent of concentration increase will be variable and subject to a number of groundwater parameters, including background concentration and the speed of flow in the underlying groundwater system. The type of strata that the groundwater flows through will also influence dispersion and dilution, as the extent of both horizontal and vertical mixing may be variable. At some stage the groundwater is either utilised or discharges to surface water. At this point its effect needs to be considered as it may influence the suitability of that water; whether it is for stock watering, irrigation, domestic water supply or contact recreation. There are many variables that influence the rate of dispersion and dilution, and consequently assimilative capacity is difficult to assess in the absence of site specific information. For planning purposes dispersion and dilutions should not be relied upon.

8.4 Summary of Groundwater Attenuation

Groundwater attenuation of applied wastewater constituents is possible. However, the determination of groundwater flow properties is needed to enable accurate predictions of assimilation. Consequently, at a planning and feasibility stage conservatism should be used and groundwater assimilation should not be considered.


9 UN-ASSIMILATED FRACTION

While it may be ideal to develop a land application system that does not generate an effect beyond the application areas, there may be instances when elevated concentrations of some constituents are acceptable. Discussions with Horizons Regional Council have indicated that there is no standard methodology adopted by them for determining acceptable levels of wastewater parameters entering groundwater. For the upper Manawatu catchment a Landuse Capability approach was taken to determine acceptable N losses. However, this does not apply to the lower Manawatu area, which includes the evaluation area. At an early planning stage it is ideal to develop a system that results in assimilation of all constituents applied. However, if there was a surplus and down-gradient groundwater concentrations were elevated, then acceptable concentrations could be based on:

• Groundwater abstraction: Concentrations of extracted water should not exceed ANZECC guideline concentrations for the water use (irrigation, stock water, domestic supply); or

• Surface water used and effects: Acceptable concentrations entering surface water should be calculated using a methodology to be determined by HDC’s water quality science contributor.


10 STANDARDS, GUIDELINES AND STATUTORY CONSIDERATIONS

The consideration of guideline values, standards and statutory considerations is not directly related to assimilative capacity of a land treatment system. Never-the-less these documents typically utilise a relationship between the land assimilative capacity and the potential for loss of wastewater derived constituents to the wider environment. Typically these documents assist with enabling a design of a sustainable system. The intention is to minimise impacts to:

• Public health; • Animal health; • Environmental quality; • System longevity; and • Protection of markets for produce (milk, crops, meat, etc).

10.1 National Standards and Guidelines

There is no national standard which relates to wastewater irrigation to land. A number of New Zealand and international guidelines exist which are intended for use to manage land treatment design. A list of these documents and a summary of their content is beyond the scope of this report. However, the support of available guides should be used at the design stage for a land treatment system.

10.2 Industry Considerations

While, as described in previous sections, a land treatment system has the capacity to assimilate a determinable amount of applied wastewater constituents, the final selection of a crop for a land treatment system will need to consider industry requirements. These requirements provide a method for industry to demonstrate to their markets that products meet high quality specifications. A frequently referenced industry requirement is Fonterra’s policy on the use of municipal wastewater on land that supplies milk to them. The policy requires that where municipal wastewater is applied to feed grown and fed to lactating dairy cows, that wastewater must comply with California Health Law, Title 22. Alternatively, if municipal wastewater not meeting California Health Law, Title 22 requirements is irrigated to land, feed from that land may not be fed to lactating cows or within 30 days of lactation beginning.

10.3 Regional One Plan

The One Plan is the planning device that forms the basis for Horizons Regional Council to manage the sustainable use of the Manawatu region’s resources. The One Plan has no numerical guidelines for the protection of soil quality. Instead the document focuses on the management of land to protect water quality. No rules in the One Plan relate directly to the application of municipal wastewater to land. However, Rule 13-13 imposes requirements on the design and location of human effluent storage and treatment facilities (ponds), including a requirement to seal ponds to restrict seepage and to ensure that discharges comply with minimum separation distances from sensitive receptors. Rule 13-27 is a ‘catch-all’ default rule that classifies all discharges to land that are not described in any other Rules as discretionary activities. Rule 13-4B imposes a limit to the amount of nitrogen that can be leached from discharges of poultry farm litter or pig farm litter, while Rule 13-6 provides constraints on farm animal effluent discharges onto land. These agricultural Rules are considered to be a baseline against which a land treatment system could be measured.


It is recommended that a full planning assessment is undertaken at the time of resource consent preparation for a land treatment system.


11 LAND TREATMENT ASSIMILATIVE CAPACITY

The assimilative capacity for land treatment in the evaluation area is dependent on the unique properties of the land treatment site selected, and on the design of the land treatment system (land management and irrigation management). Table 2 below summarises the relative contribution that a number of processes have to assimilation of wastewater when applied to land. It provides a summary to the discussion in this report. The table indicates the parts of a land treatment system which are active in assimilating the applied wastewater constituents. The higher the number given, the higher the potential to assimilate the listed constituent. The potential to assimilate applied constituents indicates whether a land treatment system is effective in assimilating the listed constituent. Table 2: Relative Assimilative Potential in the Evaluation Area N P Pathogens Heavy

metals OM and SS**

Cationic salts

Organic contaminants

Pre-soil

contact

Droplets and

Aerosols 1 0 variable variable variable variable 1

Evaporative Losses

1 0 0 0 0 0 1

Volatilisation variable

<15 % 0 0 0 0 0 1

Application Methods

Plant Uptake or removal

3 300

kg/ha/y

3 40 kg/ha/y

1 2 1 3 1

Land Use

Management variable variable variable 1 variable 1 variable

Soil

Physical and chemical

removal

3* 4* >30

kg/ha/y

5 > 95 %

4 5 4 5

Biological

incorporation 3 3 4 3 5 3 5

Natural die off 0 0 4 0 0 0 0

Gaseous loss 3* 5 %

15 kg N/ha

0 0 0 0 0 1

Groundwater

Gaseous loss variable 0 0 0 0 0 1

Dispersion and dilution

variable variable Variable 0 0 variable variable

Potential to

assimilate applied

constituents

Good 315 kg

N/ha/y

Good 40 kg

P/ha/y

Excellent >95 %

Excellent Excellent Good Excellent

Notes: 0 – none; 1 - Very low; 2 – Low; 3 – Moderate; 4 – High; 5 – Very high; variable – Highly variable

* depends on chemical form ** Organic Matter and Suspended solids


The table above illustrates that for the listed constituents land treatment in the evaluation area has the potential to assimilate wastewater constituents. For each of the listed constituents it can be seen from the table that a land treatment system function is active in assimilating applied wastewater constituents and therefore a reduction or complete removal of most wastewater constituents can be expected from land treatment. Unfortunately limited quantification of assimilation rates can be provided without site specific information. Despite this, key planning targets of 315 kg N/ha/y and 40 kg P/ha/y can be reasonably expected to be suitable for the vast majority of sites in the evaluation area. It is known that the efficacy of the assimilation will be dependent on design and management. Assuming appropriate design and management are used there is scope for a sustainable land treatment system to be developed without adverse off-site environmental effects within the evaluation area.


12 REFERENCES

H. E. Brown, D. J. Moot & K. M. Pollock (2005). Herbage production, persistence, nutritive characteristics and water use of perennial forages grown over 6 years on a Wakanui silt loam, New Zealand Journal of Agricultural Research, 48:4, 423-439

Crane, S.R. and Moore, J.A. (1984). Bacterial Population of Groundwater: A Review. Water Air &

Soil Pollution, Vol 22 No1, Springer, Netherlands. Frost, K.R. and H.C. Schwalen. (1960). “Evapotranspiration during sprinkler irrigation.”

Transactions of the ASAE 3: 18-20, 24. Gunn, I. (1997). On-site wastewater and bacterial reduction in subsoil disposal areas – a review.

On-site NewZ Special Report – 97/2 a CaRE for the environment project J. Luo*, R.W. Tillman, P.R. Ball. (2000). Nitrogen loss through denitrification in a soil under

pasture in New Zealand. Soil Biology & Biochemistry 32 pp 497-509 Lowe Environmental Impact (2012) Assimilative Capacity of Land near Shannon, Foxton and

Foxton Beach McLeod, M., Aislabie, J., Smith, J., Fraser, R., Roberts, A. and Taylor, M. (2001). Viral and

Chemical Tracer Movement through Contrasting Soils J. Environ. Qual. 30:2134–2140. S. Mercer Meding, Lawrence A. Morris,* Coeli M. Hoover, Wade L. Nutter, and Miguel L. Cabrera.

(2001). Denitrification at a Long-Term Forested Land Treatment System in the Piedmont of Georgia. J. Environ. Qual. 30:1411–1420.

Metcalf and Eddy. Inc. (2003). Wastewater Engineering: treatment and reuse. 4th edition. 1819

pages stenger, R; Barkle, G, Wall, A; Clague, J. (2012). Attenuation of nitrate in the subsurface

environment of a lowland dairying catchment. NZ Hydrological Soc Teltsch, et al., (1980)

foxton wwtp a1 land assim capacity-final · 2019-11-19 · this report has been prepared for...

Documents