Solutions for Improving Water Quality on High-Country Farms in New Zealand

Abstract

Contaminants from agricultural land use have degraded freshwater quality in New Zealand. Several mitigation strategies for agricultural pollutants are effective in New Zealand, however, in the high-country, these strategies are either inappropriate or their efficacy is poorly understood. The objective of this study was to determine the effectiveness of mitigation strategies that are promising yet underused on high-country farms in New Zealand, primarily constructed wetlands and riparian buffer strips. Water quality was monitored monthly on two high-country farms near Wānaka for 12 months, before, during, and after the implementation of a riparian buffer strip, and five years following the implementation of a constructed wetland. The constructed wetland attenuated E. coli and particulate organic matter strongly (60% and 50% reductions), and sulphate, suspended sediment, orthophosphate, and total phosphorus moderately (43%, 42%, 41%, and 36% reductions) during baseflow conditions. Ammonium experienced variable attenuation while nitrate and total nitrogen increased below the wetland (98% and 65% increases), likely due to the intrusion of groundwater, and not a reduction in attenuation. A before-after-control-impact (BACI) study indicated that the attenuation capabilities of the constructed wetland responded inconsistently across variables during a sheep grazing event. Notably, E. coli, total phosphorus, and orthophosphate increased below the wetland following 70 sheep grazing upstream (67%, 62%, and 15% increases). Yet particulate organic matter and suspended sediment concentrations did not increase downstream following the grazing event. The riparian buffer was too immature (six months old) to attenuate contaminants effectively within the study timeframe. Although riparian buffers are likely to be effective mitigation strategies for high-country farms, as the constructed wetland of the study attenuated agricultural contaminants well, if not better than in other environments (with the exception of nitrate), because mitigation features experience less contaminant loading in the high-country. The findings, however, suggest these strategies can experience limitations on high-country farms. Riparian buffers take a long time to mature, particularly in the challenging high-country environment, and constructed wetlands may be more difficult to maintain on high-country farms due to competing land uses. Moreover, large high-country grazing events can undermine the attenuation capabilities of constructed wetlands, by overwhelming the system. Therefore, to ensure good water quality, mitigation strategies must be implemented promptly yet cautiously on high-country farms, and several strategies are needed for any one farm stream.