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