GROUNDWATER-AGE DISTRIBUTIONS AND IMPLICATIONS FOR DRINKING WATER QUALITY IN THE CLAIBORNE AQUIFER, SOUTH-CENTRAL UNITED STATES

The occurrence and distribution of both naturally occurring and anthropogenic constituents in aquifer systems often are correlated with groundwater residence time. Generally, the probability of detecting organic constituents and high concentrations of nitrate is greater if young groundwater is present. Conversely, the occurrence of anthropogenic constituents typically is low in systems with long residence times. However, naturally occurring constituents such as trace metals are often detected more frequently and in greater concentrations when groundwater residence times are long. The Claiborne aquifer is an important regional aquifer in the south-central U.S. and is a source of water for public and domestic supply, as well as industrial and agricultural use. Regional confining units separate the Claiborne aquifer from overlying and underlying aquifers, and the aquifer is under confined conditions throughout much of its extent. As a result, groundwater flow paths and residence times typically are long and much of the water withdrawn from the Claiborne aquifer for public supply is pre-modern (tritium concentrations < 0.5 Tritium Units) in these confined areas. Age-tracer (tritium, sulfur hexafluoride, carbon-14) results from regional well networks and a flow-path study in addition to age-distributions derived from a regional groundwater-flow model were used to characterize the occurrence and distribution of naturally occurring and anthropogenic constituents in water-supply wells. In general, mean ages based on age-date tracers were comparable to groundwater-age estimates derived from particle-track runs using a regional groundwater-flow model. Tracer data indicate that mixtures of young and old water are prevalent, but the presence of both old and young waters was most common where long-term pumping has caused shallow, young water to move into the Claiborne aquifer. The occurrence of anthropogenic contaminants was correlated to the fraction of young water estimated from age-tracer analysis.