USING MULTIPLE LINES OF EVIDENCE TO EVALUATE AQUITARD INTEGRITY FOR A CONTAMINATED SEDIMENTARY ROCK AQUIFER SYSTEM

Quantitative characterization of aquitards and assessment of their integrity is fundamental to management of groundwater resources. Integrity assessments for bedrock aquitards traditionally focus on lithology and associated hydraulic conductivity. In this study, we integrate a diverse set of data to evaluate the integrity of 7 aquitards within a sedimentary rock aquifer system to attenuating migration of dissolved and dense non-aqueous phase liquid (DNAPL) organic contaminants. Here, an aquitard is defined by an interval of rock that produces a distinct increase in the vertical component of hydraulic gradient in a high-resolution (3 zones/10 m) head profile. The study area bedrock units were put into a sequence stratigraphic context using cores and natural gamma logs and their lateral continuity was assessed. A 3-D numerical groundwater flow model constructed and calibrated with emphasis on matching the observed high-resolution head profiles provided robust estimates of bulk vertical hydraulic conductivity (K_v). Evaluation of the contaminant distribution from detailed sampling of continuous cores and groundwater sampling of high-resolution multilevel wells characterized the vertical extent of contaminant migration. The geologic, hydraulic, and contaminant evidence were combined in a ranking scheme to provide an overall assessment of aquitard integrity. A laterally extensive, 6 m thick, maximum flooding interval with the highest bulk K_v (2 x 10^-7 m/s) but with demonstrated integrity with respect to DNAPL had the highest overall integrity ranking. A contact between two units with poorly connected fracture networks had the second highest integrity rating based on the large measured vertical gradient, a dramatic shift from high dissolved phase contaminant concentrations to non-detects, and resistance to DNAPL penetration. Overall, the study indicated 4 high integrity aquitards, 3 that are maximum flooding intervals and 1 that is a contact associated with poor fracture connectivity between units. This study highlights the importance of multiple, high-resolution data sets for aquitard integrity assessments and demonstrates the potential for poor fracture connectivity across a contact to function as an aquitard, influencing groundwater pathways and impeding downward contaminant migration.