Thirty one wells in the area were selected for sampling. The locations and well top of casing water level measurement reference points were surveyed to determine accurate New Zealand map grid coordinates and elevations, respectively. Additionally, up and downstream sampling locations were determined on both the Hokitika River and Kokatahi River.
Three sets of water level measurements were obtained during this project (i.e., during February, April, and July-August 2007). Water elevations were calculated from these data and then contoured to produce groundwater potentiometric surfaces. The resultant potentiometric surfaces were all similar. They indicated that groundwater on the south end of the Kowhitirangi Plain is recharged by the Hokitika River and flows in from there generally northward. Groundwater from this system enters the Hokitika River on the northwest side of the Plain. Some of this flow may also enter the Kokatahi River; however, the direction of groundwater flow along that river appears to generally parallel it. Groundwater underlying the Kokatahi Plain flows to the northwest and enters the Hokitika River.
Groundwater and associated surface water samples were obtained during relatively dry and relatively wet periods (April and July-August 2007, respectively) and were analyzed for conductivity, pH, major ions, and nitrate-nitrogen. Results indicate that water quality in the vicinity of the Kowhitirangi and Kokatahi Plains is generally good and, with the exception of nitrate-nitrogen and sulfate does not change significantly between dry and wet seasons. In contrast, groundwater nitrate-nitrogen and sulfate concentrations were higher during the wet season sampling in July-August 2007. These seasonal changes potentially reflect precipitation-facilitated downward migration of nitrate-nitrogen and sulfate from anthropogenic surface sources such as farming operations.
The overall geographic distribution pattern for nitrate-nitrogen groundwater concentrations indicates higher concentrations in the centre of both Plains with lower concentrations along the river boundaries. This also probably reflects inputs from anthropogenic surface sources such as farming operations. Nevertheless, in most cases results for nitrate-nitrogen were low (i.e., on the order of 1 mg/L or less). This probably reflects dilution from low concentration river water and rainfall recharging the aquifer and moving rapidly through the highly permeable gravels involved.
Click here to download the Kowhitirangi and Kokatahi Plains Groundwater report 2009 (PDF, 8.5MB)