An assessment of critical nutrient (nitrogen and phosphorus) loss pathways and potential mitigation options for improved water quality outcomes in an intensive pastoral farming area in the lower Manawatu river catchment : a thesis presented in partial fulfilment of the requirements for the degree of Master of Environmental Management at Massey University, Manawatu, New Zealand

Abstract

New Zealand's rapid agricultural intensification has created significant water quality challenges, particularly in catchments where intensive dairy farming has been established on artificially drained former wetlands. The Moutoa-Whirokino catchment (6,000 hectares) in the lower Manawatu River system exemplifies this transformation, with extensive drainage infrastructure supporting intensive pastoral agriculture on organic-rich soils that discharge to the internationally significant Manawatu Estuary (Ramsar Convention Wetland of International Importance). Preliminary monitoring revealed unusual water quality patterns, namely elevated ammonia-nitrogen and dissolved reactive phosphorus alongside unexpectedly low nitrate-nitrogen which differed from typical agricultural drainage in other catchments which are dominated by nitrate losses. This research characterised shallow groundwater and drainage nutrient dynamics through eleven months of intensive monitoring (November 2023-October 2024) across ten groundwater piezometers and twenty-three surface water sites. Groundwater analysis revealed predominantly anoxic conditions (74% of 109 samples) controlling nitrogen speciation, with ammonia-nitrogen concentrations measured 6.79-18.30 g/m³ (median values) at the organic soil groundwater sites, as compared to 1.47-1.54 g/m³ in the sandy soil sites. Soil type exerted strong control on shallow groundwater and drainage nutrient mobilisation, with organic matter decomposition under oxygen-limited conditions driving coupled ammonia-nitrogen and dissolved reactive phosphorus release. However, surface drain water monitoring showed substantial spatial variability, with ten of twenty-three sites exceeding national and regional ammonia-nitrogen standards, while all the sites exceeded regional dissolved reactive phosphorus targets. Evaluation of mitigation strategies demonstrated that controlled drainage successfully manipulated water table elevations by 0.4-0.5 metres, while existing borrow pits in the flood spill way indicated remarkable potential treatment performance with the median ammonia-nitrogen and dissolved reactive phosphorus concentrations in the spill way measured 0.042 g/m³ and 0.097 g/m³ respectively, as compared to 0.43 g/m³ and 0.15 g/m³ measured in the farm drains. These findings establish that ammonium-dominated drainage from anoxic organic soils requires fundamentally different mitigation approaches than conventional nitrate-focused strategies, providing guidance for managing nutrient losses from New Zealand's extensive drained peatland agricultural systems.