A water quality study of Barkers Creek, South Canterbury.

Diffuse nitrogen and phosphorus pollution from farming practices is a water resource management issue throughout New Zealand. Efficient management of diffuse pollutants requires a conceptual understanding of the relationship between groundwater and surface water in the catchment being investigated. With this knowledge, transfer pathways and “hot-spots” can be identified. Barkers Creek is a small sub-catchment of the Waihi River, in South Canterbury. Diffuse pollution is causing water quality issues within the Barkers Creek catchment that propagate to Waihi River. There were three key components to this study. First, to characterise the hydrology, hydrogeology and hydrochemistry of Barkers Creek catchment. Then, to determine the main transfer pathways that nitrogen, phosphorus and sediment are entering Barkers Creek. Lastly, to understand temporal dynamics of nitrogen, phosphorus and sediment, and in particular the role storm flows have on these dynamics. A field campaign was conducted to intensively monitor the surface water and groundwater regime in Barkers Creek over the year 2016-2017. Data collection occurred at different temporal resolutions, with parameters measured at all sites bimonthly intervals and a subset of sites measured at fortnightly and 5-minute intervals. About of 44% of the flow in Barkers Creek is attributed to groundwater seepage occurring from the lower catchment, between McKeown Road (5.2 km upstream of the confluence) and the confluence with the Waihi River. Flow paths and residence times between the recharge and discharge zones for groundwater appear to be short. There is evidence of anthropogenic influence, particularly on shallow groundwater, with elevated nitrate-nitrogen concentrations observed throughout much of the lower catchment. Nitrate-nitrogen and dissolved reactive phosphorus concentrations are typically higher in groundwater and some of the spring-fed drains than what is observed in Barkers Creek. There is also evidence of a natural phosphorus source in the catchment. Of the nitrate-nitrogen load exported from the Barkers Creek catchment to the Waihi River, 20% is from diffuse groundwater seepage into the creek, 11% is from the Barkers Creek catchment upstream of McKeown Road and the remainder is from drain systems in the lower catchment, most of which are spring-fed. 56% of the total nitrate-nitrogen load is from the 3 (of 10) spring-fed drains in the lower catchment and can be considered the “hot-spots”. Nitrate-nitrogen loads during storm events do not differ significantly from loads during baseflow conditions and the spring-fed drains are a significant transfer pathway under all flow regimes. Minimal DRP load in Barkers Creek comes from diffuse groundwater seepage. Barkers Creek upstream of McKeown Road contributed 13% of the total load export with the remainder attributable to export via spring-fed drains. The hotspots for DRP are 4 (of 10) spring-fed drains in the lower catchment. Export of phosphorus and sediment is sensitive to flow regime, with storm events being the major time of transport from Barkers Creek to the Waihi River