Understanding and mitigating the water quality impact from soils receiving dairy factory wastewater : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Long-term irrigation with dairy factory wastewater can lead to high concentrations of phosphorus (P) in the soil and increase the risk of P losses via surface runoff and leaching. These P losses can impair surface water quality via accelerated eutrophication, restricting its use for drinking, recreation and fishing. The aim of this thesis was to better understand the nature and dynamics of soil P in soils that received inputs of dairy factory wastewater, and to evaluate mitigation strategies to minimize P losses. The hypothesis was that irrigation of dairy factory wastewater substantially increased soil profile P and the potential P loss, which can be mitigated by soil amendment. Soil samples (0-7.5, 7.5-15 and 15-30 cm depths) were collected from irrigated and non-irrigated soils at seven sites covering four soil orders: Brown, Recent, Pumice and Allophanic. Soils were analysed for pH, and different P analysis, including environmental and agronomic tests as well as sequential extractions (P fractionation) to characterise the availability of different P pools. Using existing data, only the concentration of P in the Pumice Soil was analysed from deeper layers (2.2 m and 15 m below ground level) together with analysis of P in surface water and groundwater. Additionally, seven Pumice Soil samples that covered a range of pH in water (5.3-6.9) were incubated with iron (Fe), aluminium (Al) or calcium (Ca) amendments designed to reduce potential P loss, at three different rates. The amendments selected were lime, gypsum, hydrotalcite, alum, iron sulphate and iron chloride. Results from my study showed that wastewater irrigated soils have a greater pH, water extractable P (WEP) and calcium chloride extractable P (CaCl2-P) concentration to 30 cm than non-irrigated soils. In terms of management, the risk of P losses can be assessed by a combination of thresholds in anion storage capacity (ASC) and Olsen P against WEP or CaCl2-P. A hypothesis that increasing pH would bind P in Ca-P forms not available to runoff or leaching was debunked. Instead, a clear change point in ASC was found for each soil, indicating the increased likelihood of P leaching (measured as CaCl2-P) to deeper layers. Analysis from deeper Pumice Soil samples at one site revealed that P has substantially leached to deeper layers to at least 4.5 m depth. High concentrations of P (1.9 mg L-1) were found in a groundwater well and in surface water locations at this site. Results from the incubation experiment indicated that most of the amendments decreased WEP in proportion to the rate applied. Alum was found to be the most cost-effective amendment to mitigate P loss, followed by iron sulphate, iron chloride and gypsum. In conclusion, my hypothesis was proven in that dairy factory irrigated soils have substantially increased P losses, but they could be mitigated by amendments in the short term. However, there is a need to reduce soil P concentrations to decrease the risk of losses in the long term. This will require better information of where the risk is greatest within the farm or catchment and further work on ways to extract the high soil P faster by altering these farms systems with different crops or pastures.