Delineation of hydrostratigraphic units in a carbonate aquifer

Aquifer characterization activities were performed to develop a hydrogeologic understanding of a site west of St. Louis, Missouri, which became contaminated as a result of U and Th processing and disposal activities that took place from the 1940s through the 1960s. Slug and pumping tests were performed as part of the aquifer characterization process. Packer tests, rock quality designations (RQDs), and lithologic information were used to determine zones of contrasting hydrologic properties within the aquifer. RQD, a general indicator of rock competence and variability, was determined for about 640 intervals from 107 coreholes. On the basis of the hydraulic conductivity data obtained from the packer tests and the vertical correlations established between RQD values, hydraulic conductivity and depth, the aquifer was characterized as having two different hydrostratigraphic units. The first unit includes bedrock near the phreatic surface. Because of extensive weathering, the hydraulic conductivity of this unit is high. The second unit is composed of unweathered bedrock that has a small hydraulic conductivity and is located below the weathered unit. A three-dimensional groundwater flow model was used to improve the estimated hydraulic conductivity values for each layer. It was determined that ground-water flow through the system is predominantly controlled bymore » a zone of fracturing and weathering in the upper, weathered portion of the aquifer which is more likely to transport contaminants relative to the lower portion of the limestone aquifer. The results indicate the value of packer tests and RQD data in defining hydrostratigraphic units within a single aquifer and ultimately in supporting decisions regarding remedial-action.« less