Impact of managed aquifer recharge on nitrate concentration changes in an alluvial aquifer : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Water Resource Management at Lincoln University

Abstract

Nitrate leaching poses significant challenges to agricultural environments, endangering water quality, human health, and ecosystems. Managed Aquifer Recharge (MAR) is a technology aimed at improving water quality in contaminated aquifers by dilution with high quality recharging water. This study investigates the effectiveness of MAR techniques, specifically infiltration basins and streambed channel modifications, in mitigating rising nitrate levels in the alluvial aquifers of the Hekeao/Hinds Plains (HHP), New Zealand. The research addresses four key objectives: identifying water sources contributing to groundwater recharge, evaluating biochemical processes affecting groundwater chemistry, comparing nitrate fluxes under different irrigation practices, and assessing MAR's impact on groundwater nitrate concentrations. Using chemical analyses, including isotopic signatures and major ion concentrations, alongside multivariate statistical techniques such as Bayesian mixing models, this study quantifies multiple recharge sources and their impact on nitrate pollution. The findings reveal that land surface recharge contributes 60% of groundwater recharge, with the remainder coming from water race recharge, and river recharge, collectively termed unmanaged aquifer recharge (UMAR). Oxidizing groundwater conditions were found to impede denitrification, significantly influencing groundwater chemistry and limiting natural nitrate attenuation. Areas irrigated with groundwater showed nitrate levels comparable to those irrigated with low-solute water, indicating additional influencing factors beyond irrigation water quality. Even with high-quality water inputs from abundant rain and low-solute irrigation, persistent soil solutes inevitably leach into groundwater. MAR demonstrated significant potential for nitrate dilution as far as 3 km down-gradient from the infiltration basin, with the dilution effect decreasing with distance. In one area, MAR contributed up to 52% of groundwater recharge, effectively decreasing nitrate concentrations from 26.6-30.9 mg NO3/L (start of MAR scheme operation in 2016) to 7-12 mg NO3/L (monitoring 2022). However, in areas without substantial MAR influence, the volume of MAR-influenced water was insufficient to dilute extensive nitrate plumes to below 50 mg NO3/L. This research contributes a transferable methodology for quantifying pollutant inputs in catchments with isotopically similar but chemically diverse sources. It precisely quantifies MAR's role in nitrate dilution, enhancing our understanding of sustainable groundwater management practices. The findings underscore the potential of MAR techniques as viable solutions for mitigating nitrate pollution while highlighting the need for comprehensive management strategies in alluvial aquifers. Given the oxidizing conditions that impede denitrification, effective management practices, including optimized MAR implementation, remain crucial for mitigating nitrate contamination. Keywords: Managed aquifer recharge, unmanaged aquifer recharge, heterogeneous alluvial aquifer, nitrate pollution mitigation, Bayesian mixing model, multivariate statistical techniques, stable water isotope, irrigation, groundwater chemistry, land surface recharge, Hekeao/Hinds Plains.