Empirical assessment of the influence of the unsaturated zone on aquifer vulnerability, Manawatu region, New Zealand

Vulnerability assessments provide essential screening tools to focus efforts on prevention of ground mater contamination. The unsaturated zone, between the base of the soil and the phreatic surface, has an important role on the fate of contaminants, Yet most vulnerability assessments either do not address the unsaturated zone or overly simplify its effect on ground water contamination, In the Manawatu region of New Zealand, four factors have been found to predominantly control vulnerability: the soil, the unsaturated zone, rainfall recharge, and the aquifer medium. This paper focuses on assessing the spatial variability of the unsaturated zone. The model is based on an assessment of the total surface area available for sorption, or total sorption capacity of the unsaturated zone at each borehole, This assessment uses well drillers' logs, which provide the only available database covering the entire unsaturated zone of the study area, and published sorption values of various sediments to estimate vulnerability resulting from sorption capacity These "point estimates" are treated as a spatially distributed parameter that can be contoured as a continuous surface. The entire modeling process was computerized in a geographical information system. Although the quality of drillers' logs varies considerably, high and low sorption areas can be distinguished readily, and the final map of unsaturated zone sorption shows clear regional trends. The sorption index generally decreases with distance from the recharge areas. The methods developed here can be applied and modified when necessary at other hydrogeological settings.