RISK-BASED AQUIFER VULNERABILITY MAPPING APPLIED TO A MIXED AGRICULTURAL-URBAN COUNTY IN IOWA

Vulnerability assessments are frequently used to guide land use and resource planning efforts in groundwater dependent regions. However, many readily-available methodologies are not particularly useful to assess risks to aquifers in specific hydrogeologic settings. In this study we evaluated aquifer vulnerability in Black Hawk County, Iowa, using an index-based approach conceptually aligned with traditional methodology focused on groundwater resistance through overlying sedimentary layers. Incorporating elements of groundwater recharge, travel time to the uppermost used aquifer, pollution risk and groundwater use, our enhanced, risk-based aquifer vulnerability index (AVI) method revealed spatial variability in vulnerability at local and regional scales within the county. Increased aquifer vulnerability was evident within the Cedar River floodplain and other minor floodplain areas where there is greater recharge, faster vertical groundwater travel times, the presence of point and nonpoint sources risks and increased water use. In upland regions underlain by unoxidized till, bedrock aquifers are largely protected from groundwater risks, whereas in areas where the bedrock surface is relatively shallow, the protection of unoxidized till is missing and aquifer vulnerability to contamination is higher. Groundwater use reflected by incorporating 10-year capture zones into the index model identified zones where water supply aquifers are at greater risk from point and nonpoint source contamination. Overall, the new methodology adapts and greatly expands on the traditional AVI approach and can be adopted in other regions as applicable. Users are able to modify and adapt the index-based groundwater vulnerability schemes to better account for localized patterns and location-specific use and risks.