The use of aquifer-media characteristics to model vulnerability to contamination, Manawatu region, New Zealand

The Manawatu region of New Zealand consists largely of alluvial deposits which form extensive unconfined aquifers. Intensification of land use, with consequent increases in contamination potential, places the groundwater at serious risk. Aquifer-vulnerability assessment is the first step in protecting this vital resource. Characteristics of the aquifer medium affect vulnerability because the concentration of contaminants is controlled partly by the ability of the aquifer to dilute them. Static water levels in over 1,100 bores were used as point estimates to generate the phreatic surface for the study area. Well-drillers' descriptions of the aquifer medium below this surface were then assigned to grain-size classes. These descriptions were treated as point estimates from which variations in aquifer-media characteristics were interpolated. The point estimates were used to build a network of Theissen polygons which, in turn. were used to estimate the spatial distribution of the aquifer-media characteristics. Analysis of bores drilled subsequently has shown that the regional trends in characteristics predicted from the point data were realistic. Aquifer vulnerability was then assessed on the basis of grain size. Larger grain sizes indicate higher hydraulic conductivity, thus promoting greater dilution. This results in lower vulnerability to contamination. Nitrate concentrations. the major contaminant in the area, show a hi h correlation with the assigned vulnerability classes. Regional assessment of aquifer-medium vulnerability is therefore an effective tool in groundwater resource management and protection.