A ground water vulnerability assessment for the Nelson region

Abstract

Ground water is a highly valued resource for the Nelson region. It is the largest source of drinking water relied on by 70 to 75% of the rural district. Over the last fifty years, a rapid intensification and expansion of agricultural activities, has placed the quality of the ground water resource at risk to contamination, from the infiltration of pesticides and fertilisers. The potential for ground water contamination at any geographic location is dependant upon a wide range of variables, including soil type, depth to ground water, aquifer size and contaminant type and quantity. This study focuses on the intrinsic physical and environmental characteristics of aquifers to show which areas in the Nelson region are more vulnerable to contamination. The information generated can be used in ground water management programmes, to help prevent contamination before it occurs. Simple, computer-based model approaches have been developed for regionally based vulnerability assessments. In this study, the DRASTIC methodology was incorporated into a geographic information system. This study of the Nelson region does not produce an absolute measure of contamination potential, but rather a relative indicative measure. The results showed alluvial gravel systems, particularly the Motueka and Waimea Plains, and coastal areas, are highly vulnerability to contamination, karst areas are moderately vulnerable and clay-bound areas are of low vulnerability. Uncertainty and associated limitations are central themes of the findings in this thesis. Identifying the magnitude of errors was outside the scope of the study and could only be described in qualitative terms. The study would be enhanced with the inclusion of risk based assessment information.