Contaminant Attenuation Efficacy of Soils in Matakarapa Island - Foxton

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015

PREPARED FOR: Lowe Environmental Impact

CLIENT REPORT No: FW15028

PREPARED BY: Jacqui Horswell and Murray Close

REVIEWED BY: Liping Pang

Manager Peer reviewer Authors

Dr Chris Litten Dr Liping Pang Dr Jacqui Horswell Water and Waste Group Dr Murray Close Manager

DISCLAIMER

The Institute of Environmental Science and Research Limited (ESR) has used all reasonable endeavours to ensure that the information contained in this client report is accurate. However ESR does not give any express or implied warranty as to the completeness of the information contained in this client report or that it will be suitable for any purposes other than those specifically contemplated during the Project or agreed by ESR and the Client.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page i

CONTENTS

EXECUTIVE SUMMARY ...... 1

1. INTRODUCTION ...... 2

2. BOD5 AND NUTRIENTS ...... 3 2.1 CHARACTERISTICS OF NUTRIENTS ...... 3 2.2 NUTRIENT TRANSPORT THROUGH SOILS ...... 3

3. REMOVAL EFFICACY OF E. COLI IN SOILS ...... 6 3.1 MECHANISMS OF MICROBIAL REMOVAL IN SOIL ...... 6 3.2 MOVEMENT OF E. COLI IN SOILS ...... 6 3.3 BIOTIC FACTORS THAT INFLUENCE SURVIVAL OF E. COLI IN SANDY SOILS ...... 7 3.3.1 Soil moisture content ...... 7 3.3.2 Season ...... 8 3.3.3 Soil water flux (saturated vs unsaturated flow; hydraulic load) ...... 9 3.3.4 Nature of the organic matter in the waste effluent solution ...... 9 3.4 ABIOTIC FACTORS THAT INFLUENCE SURVIVAL OF E. COLI IN SANDY SOILS ...... 10 3.5 SUMMARY ...... 10

4. RECOMMENDATIONS ...... 11

REFERENCES ...... 12

LIST OF TABLES TABLE 1. PHYSIOGRAPHIC UNITS IDENTIFIED, AREA AND % OF STUDY AREA...... 3

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page iii

EXECUTIVE SUMMARY

Horowhenua District Council (HDC) through Lowe Environmental Impact have contracted The Institute of Environmental Science and Research (ESR) Ltd to assess the efficacy of contaminant removal though soil on Matakarapa Island – Foxton, as part of the HDC process to gain consent to discharge treated municipal effluent to land on Matakarapa Island. The soil classes identified on Matakarapa Island – Foxton include/cover well drained dune sand, moderately drained sands and poorly drained alluvium over sand. The majority of the site is well to moderately drained. The sandy soils have the potential to effectively filter E. coli provided certain parameters are met. The “alluvium over sand”, soils need to be carefully managed to avoid by-pass flow. If the irrigation rate is high enough to promote irrigation induced drainage, the vast majority of bacteria will travel through soil macropores with limited opportunity for adsorption, which will increase the potential for leaching. Climatic and soil conditions in the winter may increase survivial of effluent derived E. coli and increase the potential risks of leaching due to increased soil moisture. The higher groundwater table and river flows in winter may will also reduce natural attenuation by increasing flow rates through the soils. Summer leaching in the sandy soils such as those on the majority of Matakarapa Island – Foxton is likely to be minimal. Reducing biological oxygen demand in the effleunt to avoid organic matter disrupting adsorption/filtering capacity of the soils will also reduce potential leaching. The sand and sand plain soils have low cation exchange capacity and low P retention and thus, low ability to retain nutrients within the profile. scBOD5 is likely to be metabolised within the soil profile and only low levels of BOD are expected to leach from the soils. Nitrogen compounds will be transformed to nitrate which is very mobile. Removal of N from the system would require harvesting rather than grazing land management. Some removal of N may occur through denitrification in water logged soil conditions. Transport of P is likely to occur due to the low P retention status although at a retarded rate compared to the nitrate. Without proper design of the irrigation system the soil function of filtration will fail and both nutrients and microbial contaminants may pass through soil and into groundwater. Attention needs to be given to ensuring that the loading rates match the soils ability to filter contaminants and nutrients, and that attention is given to maximising matrix flow and avoiding by-pass flow.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 1 1. INTRODUCTION

Project and Client  Horowhenua District Council (HDC) through Lowe Environmental Impact have contracted The Institute of Environmental Science and Research (ESR) Ltd to assess the efficacy of contaminant removal though soil on Matakarapa Island – Foxton, as part of the HDC process to gain consent to discharge treated municipal effluent to land on Matakarapa Island. Objectives  The Foxton wastewater treatment system requires re-consenting and a possible upgrade. The preferred options for the location of the Waste Water Treatment Plant and its associated discharge locations needs to assess the potential impact of the discharge on the receiving environment, and in particular groundwater.  This report will assess key contaminants of concern listed in HDC’s One Plan (Escherichia coli, Soluble Carbonaceous Biological Oxygen Demand, Dissolved Reactive Phosphorus, Total Phosphorus, Ammoniacal-nitrogen, Inorgnaic Nirtogen, and Total Nitrogen) for protecting groundwater and ultimately surface water. It will also: o identify time of year and temporal variations that may influence the potential for an adverse impact, including the significance of high river flows and high groundwater levels or surface ponding and o identify the processes and efficiency of soils on proposed sites for removing contaminants, including consideration of effluent contaminant concentration and application rates. Methods A review of the most relevant literature as well as the personal experience of the researchers has been used to collate this report.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 2 2. BOD5 AND NUTRIENTS

2.1 CHARACTERISTICS OF NUTRIENTS

The nutrients under consideration have different charges and sorption characteristics. BOD5 (5 day biochemical oxygen demand) in effluent is normally dominated by particulate carbon but the sc prefix indicates the soluble carbonaceous BOD fraction. Dissolved reactive phosphorus (DRP), ammoniacal-nitrogen (NH4-N) and soluble inorganic nitrogen (SIN) which is the sum of nitrate, nitrite and ammonia, are all soluble (defined as passing through a 0.45 um filter. Total phosphorus (TP) and total nitrogen (TN) include the dissolved fractions but also include the particulate and organic fractions. Ammonia is a positively charged compound and sorbs to soil and clay minerals whereas nitrate, nitrite and DRP are negatively charged compounds and will sorb much less strongly. These differences affect the way that the species are transported through a soil. Particulates will tend to be filtered out by fine textured soils whereas dissolved compounds will be attenuated by sorption. Compounds can also be removed by breakdown and transformation to other, generally simpler, compounds or they may be taken up by vegetation. Breakdown and transformation will occur mostly in the topsoil layer where the concentrations of organic carbon and the levels of microbial activity are greatest.

2.2 NUTRIENT TRANSPORT THROUGH SOILS Matakarapa Island is formed by the current path of the Manawatu River cutting off a large loop in the river. Table 1 shows that sand dunes make up ca.30% of the mapped area excluding ponds while inter dunes, sand plains and alluvium over sand make up ca. 31%, 16% and 23% respectively (McLeod 2015). The sand dunes are generally well drained without mottles in the top 1 m whereas inter-dune and sand plain areas contain moderately well drained soils with ochreous (rust coloured) mottles at less than 90 cm from the soil surface (McLeod 2015). Ochreous mottles indicate oxygen-poor conditions for part of the year, generally caused by water logging of the soil.

Table 1. Physiographic units identified, area and % of study area. Soil class Area (ha) % of area Dune, well drained 27.3 30 Inter dune, moderately drained 28.1 31 Sand plain, well drained 0.5 <1 Sand plain, moderately drained 13.9 15 Alluvium over sand, moderately drained 2.8 3 Alluvium over sand, imperfectly drained 2.7 4 Alluvium over sand, poorly drained 14.5 16

McLeod (2015) presents data for similar dune sand and sand plain soils which indicate that both these soils have very thin topsoils with low organic carbon content and low cation exchange capacity. These soils would also have very low phosphate retention capacity (McLeod, May 2015, pers comm.). McLeod (2015) also noted that the process of levelling or

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 3 smoothing the landscape for cultivation would result in some areas having little or no topsoil and other areas having deeper topsoil layers.

It would be expected that most, if not all, of the scBOD5 in applied wastewater would be metabolised by the soil bacteria as the effluent percolated through the soil. There would be additional removal as the organic carbon moved through the groundwater. Unless irrigation was applied in significantly excess amounts, there should be little scBOD5 reaching the surface receiving waters. Over time the addition of this organic carbon and nutrients associated with the effluent application would be expected to increase the organic carbon in the topsoil and to promote and deeper topsoil layer. Both these outcomes would increase the soil quality and production from these areas. The nitrogen species in wastewater when applied would be transformed from organic N through to ammonia and then through to nitrate in the upper soil profile. Some organic N compounds, such as urea, are fairly mobile and will readily leach through the profile, whilse others will be retained. Ammonia will tend to sorb in the soil although all the soils in the area have a low cation exchange capacity. Once the N is in the form of nitrate it will leach with the next application of water if drainage is induced, either from rainfall or effluent irrigation, through into the groundwater. While the compounds are within the root zone (often the top 40 cm for a pasture although some roots may go deeper) they can be taken up by the pasture. If the pasture is harvested as hay or silage then these nutrients are removed from the system. If the pasture is grazed then most of the N will be returned to the pasture via urine patches where the urea will transform to nitrate and if not used by the plants will leach into the underlying groundwater. The mottled appearance of the inter dune and sand plain soils indicate that oxygen-poor (anoxic) conditions are present for parts of the year. These anoxic conditions may also promote denitrification whereby microbes use nitrate as an oxygen source (electron acceptor) and convert it to N2 gas (Korom 1992). The N2 gas then diffuses back to the atmosphere which is approximately 80% N2 gas. The amount of denitrification likely to take place is difficult to estimate and will depend on the length of time that anoxic conditions occur and how much nitrate is transported through the anoxic zones before entering the surface waters. Unfortunately the conditions that lead to anoxic conditions (water logging of the soil) are also associated with poor drainage and the inability to apply more effluent. This means that maximising the amount of effluent able to be applied will probably minimise the amount of denitrification that will take place. Thus the amount of removal for N depends partly on the land management (harvesting versus grazing) and whether significant denitrification takes place. For a grazing land management and maximisation of effluent irrigation there will be little removal of N and it will mostly be recharged to the surface waters, however this depends on .loading rate. The phosphorus compounds will sorb slightly to the sands but the low P retention status of those soils means that P will leach over time into the underlying groundwater. The land management options for N will also apply to P, in that harvesting of the pasture will result in removal of P from the system. Otherwise the transport of P will be slower than for N, but the low P retention status means that P will be transported to the groundwater and eventually to the river. The amount of P leached depends on the loading rate, thus the irrigation scheme needs to be carefully and diligently managed. During the summer there will be active uptake of N and P by the plant and there is generally an excess of evapotranspiration over rainfall resulting in minimal leaching. In the winter evapotranspiration is low along with plant uptake and most of the N and P applied will leach through the profile into the groundwater.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 4 Land management (harvesting vs grazing) and the choice of irrigation management (deficit irrigation, non-deficit irrigation, or disposal) will impact the amounts of nutrients leached and the amounts of effluents applied.

Summary:  The sand and sand plain soils have low cation exchange capacity and low P retention and thus, low ability to retain nutrients within the profile.

 scBOD5 is likely to be metabolised within the soil profile and only low levels of BOD are expected to leach from the soils.  Nitrogen compounds will be transformed to nitrate which is very mobile. Removal of N from the system would require harvesting rather than grazing land management. Some removal of N may occur through denitrification in water logged soil conditions.  Transport of P is likely to occur due to the low P retention status although at a retarded rate compared to the nitrate.  Careful and diligent management of the site is required (e.g. low loading rates) to avoid transport of nutrients into the ground and surface water.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 5 3. REMOVAL EFFICACY OF E. COLI IN SOILS

3.1 MECHANISMS OF MICROBIAL REMOVAL IN SOIL The soil mechanisms that reduce microbial movement through soils include filtration, adsorption, predation and competition (die-off). Filtration or straining is related to soil pore size and soil water interfaces. Soils with many fine pores (e.g. sandy loam soils) facilitate uniform movement of water through the soil and effective filtration (matrix flow); whereas soils with dense structural units separated by cracks often allow rapid movement of water through soil cracks and continuous large pores (by-pass flow). Numerous field experiments confirm that bypass flow causes rapid and deep chemical and microbial leaching (Germann and Beven 1981; Powlson 1998; Woessner et al., 1998; Geohring et al., 1999; Kladivko et al., 1999; Aislabie et al., 2011). Microbial movement and leaching in soils are mainly influenced by soil type and degree of water saturation. Soil type affects the mechanism of microbial movement, consequently the extent of leaching of microbes through soils varies for different soils (Gagliardi and Karns, 2000, Aislabie et al., 2001; Aislabie et al., 2011).

Once trapped in the soil, the behaviour of pathogens is not easy to predict and some studies have shown pathogenic organisms may survive for relatively long periods in soils under optimal conditions. Thus, the factors influencing the survival of pathogens, both biotic (e.g. soil physical and chemical properties) and abiotic (e.g. predation and competition) also require consideration, as increased survival of microbes in soil will increase the risks of leaching. The Table 1 (section 2.2) details the seven soil classes that have been identified on Matakarapa Island – Foxton, based on physiographic position and drainage class (McLeod, 2015). The majority of the site is well to moderately drained and predominantly sand.

3.2 MOVEMENT OF E. COLI IN SOILS The soils found on the sand dune and inter-dune are well and moderately well drained sandy soils that do not have hydraulically restrictive layers within 1 m of the soil surface; however they show signs of hydrophobicity (water repellence). The sandy soils have the potential to effectively filter E. coli provided certain parameters are met. The mechanics of filtration by sandy soils can be explained by 3 processes. (1) actual filtration by the solid matrix; (2) attachment/detachment of bacteria in the soil pores; (3) “bridging” whereby previously filtered bacteria and organic matter act to reduce pore diameter and increase filtering action of the soil. In a study by Aislabie et al., (2001), no leaching of E. coli was detected in sandy soils when diary shed effluent was applied. Further work by Aislabie et al., (2011) investigated long- term application and leaching of microbial indicators in a Manawatu sandy loam soil, where it was found that microbial indicators (including E. coli) were rarely detected in leachates.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 6 In sandy soils matrix flow is the main mechanism of movement facilitating maximum contact between the soil particles and bacteria, enabling adsorption and/or filtration processes to limit bacterial movement (Aislabie et al., 2001; Aislabie et al., 2011). Sandy soils have a district advantage over heavier clay soils where by-pass flow may occur due to continuous macropores, such as structural cracks and/or large earthworm burrows (Gagliardi and Karns 2000; Aislabie et al., 2001). There is a higher risk of leaching of E. coli for the ‘alluvium over sand soils’ on Matakarapa Island – Foxton that contain clay and very fine polyhedral peds in the top layers of the soil profile. Microbial leaching in these soils can be minimized by soil tillage; where tillage can reduce microbial transport by disturbing preferential flow paths, and thus slow down microbial leaching through the soil profile (Abu-Ashour et al., 1998; McMurry et al., 1998; Gagliardi and Karns, 2000). The high filtration/adsorptive capacity of sandy soils means that bacteria applied to the surface will be mainly harboured within the top layer of the soil (Sorber and Moore 1987; Horswell et al., 2010; Avery et al., 2004; Aislabie et al., 2011). Aislabie et al. (2011) spiked dairy shed effluent with the dye pyranine to investigate leaching mechanisms in a sandy loam soil and a clay loam soil. Dyed soil was observed at depths below 150mm in the clay loam soil but not in the sandy loam soil. The E. coli in the dairy shed effluent followed the same pattern as the dye (Aislabie et al., 2011). Soil adsorbed cells do not remain permanently fixed and release and movement can be expected as physical adsorption is a reversible process. Thus in the top layer of soils, mechanisms such as predation and competition become important as there is greater organic matter and microbial activity. McLeod (2015) reviewed the soils on Matakarapa Island – Foxton and observed that the sandy soils showed signs of hydrophobicity (water repellence). Soil hydrophobicity, a common property of sandy soils, can cause the wetting front in the soil matrix may become unstable and break up into preferential wetting columns, called fingers which create preferential flow paths. Hydrophobicity can allow much faster transport of water and contaminants and a greater risk of leaching of microbes such as E. coli. Thus the dune and inter-dune sandy soils should be classed as a “medium-bypass-flow” risk due to their hydrophobicity tendencies. Once fully wet hydrophobicity disappears (Bauters et al., 2000), thus under regular irrigation hydrophobicity will likely be a minimal issue. Summary:

 The “alluvium over sand soils” have limitations for effluent irrigation. due to the potential of bypass flow, a deficit irrigation scheme is recommended..  The sandy soils on Matakarapa Island – Foxton have the potential to effectively filter E. coli.  The sandy soils show signs of hydrophobicity which can create preferential flow paths and an increased risk of leaching of E. coli; however, under regular irrigation hydrophobicity will be minimised.

3.3 BIOTIC FACTORS THAT INFLUENCE SURVIVAL OF E. COLI IN SANDY SOILS 3.3.1 Soil moisture content Increased soil moisture will favour the survival of microbes, but in a sandy soil survival of microbes will be lower with low water holding capacity. In the ‘alluvium over sand soils’ the

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 7 survivial of microbes may be longer as clay particles may protect bacterial cells by creating a barrier against microbial predators and parasites. The initial water content has a variable effect on soil attenuation of pathogens with some unaffected, while in others water content impacts leaching potential (Crane and Moore, 1984; Shelton et al., 2003; Aislabie et al., 2011; 2015). Aislabie et al. (2015) found that a sandy loam Manawatu soil leached E. coli after the application of dairy shed effluent only in late winter when rainfall was frequent and soil was wet. The soil observations on Matakarapa Island – Foxton, undertaken by McLeod (2015) indicate some of the soils may be water logged for part of the year, and hence this could create a risk for leaching.

3.3.2 Season Seasonal changes have a large influence on the die-off of microbes in soils. Several studies have found that microbial survival times are longest during the winter when temperatures are cooler (Sorber and Moore, 1987; Horswell et al., 2007). Seasonal impacts on soils is controlled by temperature and the water balance. Summer rainfall in New Zealand is generally much less than the potential evapotranspiration, thus summer leaching in a sandy soil such as that found on the majority of Matakarapa Island – Foxton, is likely to be minimal (Aislabie et al., 2011) in the summer. In a study looking at die-off of E. coli in applied to land Horswell et al. (2007) found that die-off was faster in the spring/summer biosolids application, compared to an Autumn/Winter application. Thus, survival is generally longer in cooler climates and reduced in warm climates. This may be because as soil temperature rises, increases in the metabolic activity of soil (which compete with the introduced pathogen) may reduce pathogen survival (Hon, 2003). Sunlight also influences pathogen die-off rate with increased die-off with high UV (Straub et al., 1993). In sandy soils such as the majority of those on Matakarapa Island – Foxton, winter may be higher risk in terms of contamination of ground and surface water from E. coli in irrigated effluent due to increased risks of leaching in wet soils and longer survival times of the applied microbes. If soils become saturated (e.g. after periods of heavy rainfall) the potential for surface runoff can be increased. A higher water table during winter may increase the risk of pathogens reaching groundwater as there is less distance between the soil surface and the ground water table for the soil to filter contaminants. In conditions that exceed the soil’s surface infiltration rate (e.g. rainfall or heavy irrigation) overland flow can be generated; on flat land this will result in surface ponding (Needelman et al., 2004), and on sloping land ponded surface water will move downslope creating surface runoff or overland flow (Srinivasen et al., 2002; Needelman et al., 2004) and enhancing risks to surface water contamination by microbes such as E.coli. Some low-lying areas of Matakarapa Island – Foxton may be prone to flooding which can have a potential impact on the attenuation of contaminants from irrigated effluent. Surface flow is the primary transport mechanism by which microorganisms from waste applied to land may reach receiving waters during a flood event. Enhanced sub-surface flow will also occur but the majority of microbes will move via surface flow associated with small particles (e.g. silt). If natural attenuation in the soil is rapid, as in sandy soils, then distribution during subsequent flood events will be significantly limited. For the “alluvium over sand” soils management practices such as resting periods of at least 24 hrs before forecasted heavy

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 8 rain events will reduce risks of contaminant movement. The type of rain event that will cause flooding can be accurately forecasted in advance and management put into place to mitigate risks.

3.3.3 Soil water flux (saturated vs unsaturated flow; hydraulic load) Microbial movement in soils is dependent on the water saturation state, which is in turn influenced by hydraulic loading. In low water potentials, microbial travel is restricted to surface water films on soil particles and smaller pores, increasing the potential for adsorption of the bacteria. At higher water potentials (e.g. saturated conditions, high hydraulic loading) the vast majority of bacteria will travel through soil macropores with limited opportunity for adsorption which will increase the potential for leaching (Smith et al., 1985). Thus, microorganisms move rapidly under saturated conditions. In addition, in saturated conditions, bacteria may become mobile by means of their own locomotion; this has been shown to be a significant means of transport for motile strains of E. coli (Reynolds et al., 1989). It is recommended that management strategies to reduce E. coli contamination of ground and surface waters following application of effluent to soils on Matakarapa Island, must aim to decrease preferential flow by adjusting irrigation schemes (e.g. low hydraulic loading rates) to avoid by-pass flow and excessive drainage.

3.3.4 Nature of the organic matter in the waste effluent solution Soluble organic matter (e.g. soluble nitrogen) can increase survival and in the case of some bacteria such as E. coli favour their re-growth when degradable organic matter is present (Gagliardi and Karns, 2000; Santamria and Toranzos, 2003). As well as increasing survival, some studies have found that large amounts of organic matter in wastes applied to land can inhibit filtering of microbes by the soil as the organic matter blocks the smaller pores and promotes bypass flow and increased leaching of microbes (Gagliardi and Karns, 2000; Horswell et al., 2010). Conversely, increasing organic matter (e.g. bound microbes and soluble nitrogen), can enhance adsorption. Studies have shown that previously filtered bacteria act to reduce pore diameter and increase filtering action of the soil, and also increase adsorption sites (Gagliardi and Karns, 2000). Biological Oxygen Demand (BOD) is a way of measuring the amount of organic compounds in effluent; and keeping BOD low will reduce the potential disruption of soil filtration processes important in sandy soils. Summary:

 The land treatment system must be carefully and diligently managed, especially in the winter when soils will be wetter, microbes can survive for longer and groundwater table and rivers are higher.  Application rates must be managed to maximise the receiving environment capacity to attenuate microbial contaminants, for example, maximise matrix flow through the soil and avoid preferential flow.  Long-term application of municipal effluent to sandy soils may increase organic matter loading which may influence the leaching potential of the soils by disrupting important soil processes such as adsorption/filtering capacity. It is recommended that the BOD of the effluent be kept low to avoid irrigation of large amounts of organic matter.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 9 3.4 ABIOTIC FACTORS THAT INFLUENCE SURVIVAL OF E. COLI IN SANDY SOILS The presence of other microorganisms in soils greatly impacts the survival of bacteria such as E. coli through predation, competition for nutrients and as a source of nutrients. Studies have shown that survival is longer in sterile soils (Gerba et al., 1975) and that protozoa predation is a significant factor in the control of bacterial populations (Trevisan et al., 2002). A study undertaken by Gagliardi and Karns (2000) investigating leaching of E. coli O157:H7 from cow manure found that in undisturbed soils cores microsites protected the microbe from predation and competition and survival was prolonged in a sandy loam soil (Gagliardi and Karns, 2000). Survival of E. coli in undisturbed soils in the sandy Matakarapa Island may be aided by continual input of nutrients which may provide the microbe with nutrients, however this will likely be balanced by increased nutrients enhancing microbial activity of the soil microflora which may limit E. coli survival as they are outcompeted for food.

Summary:

 Survival of E. coli in soils is not straight forward and impacted by predation and competition for nutrients.

3.5 SUMMARY

The soil classes identified on Matakarapa Island – Foxton include/cover well drained dune sand, moderately drained sands and poorly drained alluvium over sand.The majority of the site is well to moderately drained. The “alluvium over sand” soils contain clay and very fine polyhedral peds in the top layers, by-pass flow and leaching of E. coli may occur and these soils must be carefully managed (e.g. low irrigation rates) to ensure matrix flow and natrual attenaution is maximised.

The sandy soils have the potential to effectively filter E. coli provided certain parameters are met. Matrix flow in sandy soils facilitates maximum contact between the soil particles and bacteria, enabling adsorption and/or filtration processes to limit bacterial movement. The high filtration/adsorptive capacity of sandy soils means that bacteria applied to the surface will be mainly harboured within the top layer of the soil.

If the irrigation rate is high enough to promote irrigation induced drainage, the vast majority of bacteria will travel through soil macropores with limited opportunity for adsorption which will increase the potential for leaching. Climatic and soil conditions in the winter may increase survivial of effluent derived E. coli and increase the potential risks of leaching, thus the irrigation scheme requires careful and diligent management. Summer leaching in the sandy soils such as those on the majority of Matakarapa Island – Foxton is likely to be minimal. High organic matter content in the irrigated effluent may influence the leaching potential of the soil by disrupting important soil processes such as adsorption/filtering capacity.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 10 4. RECOMMENDATIONS

Without proper design of the irrigation system the soil function of filtration will fail and both nutrients and microbial contaminants may pass through soil and into groundwater. Attention needs to be given to ensuring that the loading rates match the soils ability to filter contaminants and nutrients, and that attention is given to maximising matrix flow and avoiding by-pass flow. At certain times of the year (e.g. winter) it is suggested that drainage is minimised where possible and that biological oxygen demand is kept low. The below risk matrix tables summarise the attenuation of biological and chemical contaminants in effluent on the soil types found on Matakarapa Island – Foxton. Soil type – inter-dune/ sand plain/dune sand

Loading E. coli BOD TP & DRP Nitrogen

Deficit Low risk Low risk Low risk Low risk irrigation

Non-Deficit Low risk Low risk Moderate Moderate risk irrigation risk

Disposal Moderate Low risk High risk High risk risk

Soil type 2 - organic

Loading E. coli BOD TP & DRP Nitrogen

Deficit Moderate risk Low risk Low risk Low risk irrigation

Non-Deficit Moderate risk Low risk Moderate Moderate irrigation risk risk

Disposal Moderate risk Moderate High risk High risk risk

*For the inter-dune and sand plain soils there may be some denitrification in water logged conditions. However these will also be associated with drainage issues.

Contaminant Attenuation Efficacy of Soils in Matakarapa Island – Foxton 14 June 2015 INSTITUTE OF ENVIRONMENTAL SCIENCE AND RESEARCH LIMITED Page 11 REFERENCES

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