Shallow Groundwater and DNAPL Movement within Slightly Dipping Limestone, Southwestern Kentucky

Recent hydrogeological work, including dye tracing, provides a basis for revising the conceptual site model for groundwater and dense nonaqueous-phase liquid (DNAPL) movement at the site of a historical DNAPL release. This recent work also illustrates some of the problems associated with applying traditional, well-based characterization approaches to karst aquifers. Groundwater and DNAPL movement have been studied at this site for more than 20 years. As the scope of the investigations increased, the conceptual site model for fluid movement through the rock has been periodically updated. Relevant findings based on the latest round of fieldwork include the following: (1) The absence of nearby topographic karst features and large solution voids in boreholes should not be construed to mean that none exist or that conduit flow is not relevant. (2) Pumping tests that produce low yields, homogeneous zones of influence, and drawdown data that fit analytical diffuse-aquifer solutions should not be used to assume diffuse-flow dominance. (3) At the typical site scale, groundwater flow directions and rates estimated using hydraulic head data from monitoring wells can be misleading. (4) DNAPL-migration pathways and the extent of DNAPL in the karst aquifer beneath the site cannot be mapped with the accuracy routinely required by regulators at non-karst sites. (5) Tracer investigations reliably indicate where the groundwater is going, how fast, and in what type of aquifer permeability.