The effects of land application of farm dairy effluent on groundwater quality – West Coast 2001

Land application of agricultural effluent is becoming a standard farming practice. The application of farm dairy effluent to land, as opposed to direct discharge to waterways, is the preferred method for disposal in New Zealand as regulatory authorities move to protect and enhance water quality and meet Maori spiritual and cultural values. Land application recognises the nutrient value of dairy effluent; however, it is not without risks. Careful management of land application of the effluent is required because of the potential nutrient and bacterial contamination of groundwater. In 2001, 19 groundwater bores were sampled on four occasions to assess the effects of farm dairy effluent on groundwater quality. Elevated (> 1.6 g m⁻³) nitrate-nitrogen concentrations were found in 14 of these bores (43 of 74 samples). The available long-term data shows statistically significant increasing trends in nitrate-nitrogen and chloride over the period 1998 to 2007. The nitrate-nitrogen and chloride results suggest effluent is the source of the elevated nitrate-nitrogen; however, the nitrogen isotope analysis indicates that the source of the nitrate-nitrogen may be from fertiliser or soil organic matter (average δ¹⁵n value of 3.5‰). Spatially isolated occurrences of bacterial contamination were also recorded: in 7 bores and 12% of all samples analysed. Groundwater dating, using chlorofluorocarbons, suggested that the groundwater in the region was young (8 to 12 years). Overall, the spatial and temporal data suggests human influences are affecting groundwater quality on the West Coast.