IMPROVED METHODS FOR ASSESSING VULNERABILITY IN KARST AND OTHER HIGHLY HETEROGENEOUS AQUIFERS

Groundwater resources for 25 percent of the United States population are estimated to be derived from aquifers with well-developed karst features. Water-supply wells in karst aquifers are particularly vulnerable to contamination because of the relative ease with which contaminants can be introduced into the subsurface and the rapid transport of contaminants through conduits. Commonly used methods for designating wellhead and spring protection areas are based on flow and transport models developed for porous media. When applied to karst, or other aquifers with highly channelized flow, these conventional methods are likely to underestimate travel times and, hence, the immediacy of an impact to the consumer in the event of an intentional or unintentional release of contaminants. Analysis based on the combined use of a newly developed karst/fractured aquifer modeling tool and a risk-informed methodology will provide increased security for karst groundwater resources. A newly developed flow module (DCM for dual conductivity model) for the MODFLOW system extends the industry-standard MODFLOW capabilities to include rapid flow commonly observed in fractured and karst aquifers. Because characterization of highly heterogeneous aquifers will likely not be sufficient, determination of the bounds of the protected area will need to be risk informed. The risk-informed approach will use the newly developed karst modeling tool to identify the zones which are, for example, predicted to be along upgradient flow paths with probabilities of 90%, 75%, 50%, etc. This guidance can lead to assigning development or security levels to wellhead protection areas. Zones with the highest probability of being on rapid upgradient flow paths would have the greatest restrictions on development and would be awarded the highest level of security. If appropriate, rapid-response, early warning systems can be implemented to monitor groundwater, particularly in karst regions immediately upgradient from major water supply wellfields.