Abstract:
Dairy farming in the West Coast region has undergone substantial intensification over the last decade. Associated with this intensification has been an increased use of nitrogen fertiliser and an increasing number of farms applying dairy shed effluent (DSE) to land. The aim of this project was to assess the current and potential effects of these practices. Groundwater quality in the most intensive dairying areas on the West Coast was assessed through the monitoring of twenty-two bores on four occasions in 2001. Elevated (>1 g/m3) nitrate-nitrogen concentrations were found in nineteen of these bores (54 of 74 samples, 72%). The only long-term data available, from 3 bores, showed significant increasing trends in nitrate-nitrogen (e.g. 0.41 g/m3/yr in the Bertacco bore) and chloride over the period September 1998 – June 2003. Spatially isolated occurrences of microbial contamination were also recorded: in 7 bores and 12% of all samples analysed. Overall, groundwater quality of the shallow unconfined groundwater of the Kowhitirangi area was of better quality, and less affected by landuse practices, than the deeper unconfined aquifers of the Grey and Inangahua Valleys. Nitrogen isotope analysis indicated the source of the nitrate-nitrogen was likely to be from a fertiliser or soil organic nitrogen source (average d15N of 3.5‰); however, the significant increasing trends of chloride (e.g. 0.50 g/m3/yr in the Bertacco bore, 0.22 g/m3/yr in the Coleman/Lyndale bore) suggest an effluent-based source is affecting groundwater quality in the Kowhitirangi and Grey Valley areas. Groundwater dating, using chloroflurocarbons, suggested that the groundwater in the river recharge dominated shallow aquifer of the Kowhitirangi area is approximately 7 years old. In the deeper unconfined aquifers of the Grey Valley it is approximately 12 years old. These results suggest that the full effects of landuse intensification over the past decade are yet to been seen in terms of changes in groundwater quality. Land application of DSE is currently managed through a regional permitted activities rule that sets a maximum DSE nitrogen-loading rate of 275 kgN/ha/yr. An evaluation of this rate through the use of the NLE and OVERSEER nitrate leaching models suggested that this rate may be sustainable in the future. However, the lack of control over nitrogen fertiliser application rates and detailed (West Coast-specific) data on the rates of complex nitrogen transformations that occur following DSE and nitrogen fertiliser application warrant further research.
