THE IMPORTANCE OF GEOLOGIC MAP SCALE WHEN EVALUATING WATER RESOURCES
Geologic maps are essential for understanding and properly managing water resources. The town scale, at which many water resource decisions are made, lies between the state-wide and regional scales typically investigated by the USGS and the site-specific scales more typical of consulting projects. Synthesizing geologic and hydrologic conditions over a range of scales provides a set of hydrogeologic maps that can act as a foundation for planning and development at the scale of an individual town. The value of the geologic maps can be evaluated by comparing the interpretations using maps of different scales.
An information synthesis effort was performed for the town of Lee, New Hampshire. The area of interest was defined using a combination of town and watershed boundaries and has been included in numerous studies ranging in scale from site-specific remediation investigations to the NAWQA New England Coastal Basin. A database of over 1000 wells was assimilated from private records, USGS reports, and NH Department of Environmental Services files.
Data from the wells were compared to stratified drift aquifer maps based on the state-wide map of surficial geology and the more recent 7.5 minute quadrangle-scale maps. The increased availability of well logs combined with the higher resolution maps of surficial geology result in a significantly different interpretation of the location and quality of the stratified drift aquifers in the study area.
Previous studies have shown that the bedrock chemistry of several lithologic units in northeastern New England is correlated with arsenic contamination in drinking water and that the calcareous metasediments that dominate the study area tend to have higher concentrations of arsenic. However, the ability to determine the relationship between arsenic contamination and local bedrock geology is limited by the resolution of the state-wide bedrock map. Successful mitigation of the high arsenic concentrations in the study area may require mapping the bedrock geology at higher resolution.
The effect of the more refined surficial information on the hydrogeologic interpretation as well as the limiting effect of the course-resolution bedrock maps demonstrates the importance of refining geologic maps and integrating these refinements into the decision making tools.