Decision support tools to reduce the risk of groundwater nitrate contamination at land treatment systems : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Process and Environmental Engineering at School of Engineering and Advanced Technology, Massey University, Palmerston North, New Zealand

Abstract

One of the main environmental concerns of a land treatment system (LTS) for effluent disposal is the risk of groundwater contamination. Although there have been many studies on the leaching of nitrate-nitrogen (NO3-N) from soils, there is still a need for procedures by which engineers can design and operate a LTS in ways that minimise NO3-N leaching into the groundwater. The goal of this study was to develop decision support tools to assist managers reduce groundwater contamination at a LTS. An exploratory field scale investigation was undertaken at a land treatment site belonging to Carterton District Council of Wellington Region, New Zealand. The field scale investigation demonstrated that increasing effluent application rates resulted in increased groundwater NO3-N concentrations. During summer and autumn the NO3-N groundwater concentrations remained below the maximum permissible limit (MPL) of 11.3 mg/L, but during winter, when significant rainfall events caused leaching, the groundwater NO3-N concentrations increased and exceeded the MPL. Based on this finding a simple hydrological model was developed to predict the occurrence of leaching events. This model was capable of predicting the timing and amount of leaching, but it was not able to quantify the nitrogen (N) loss that would occur during these leaching events. The model may be a useful tool to manage effluent application during the drier months of the year to avoid leaching and maximise the plant uptake of effluent nutrients prior to the inevitable onset of leaching during the wetter winter months. A decision support system (DSS) based on LEACHN was developed to predict the movement of N in a LTS. The LEACHN model was parameterized using data from the Carterton site and was then used to explore alternative effluent disposal strategies at the site. The ability of the OVERSEER® model to describe N leaching at the Carterton site was compared with that of the LEACHN model. Finally the parameterized LEACHN model was successfully tested against a separate data set from an effluent disposal area on a commercial dairy farm. The knowledge gained by using this DSS would enable managers to monitor and manage a LTS in a way that minimises the impact of contaminant leachate.