ESTIMATING RECHARGE TO RESIDUUM MANTLED KARST AQUIFERS USING TRITIUM/HELIUM AND CFCS

A study was carried out to compare methods of estimating recharge through residuum derived from weathering of carbonate rocks in southeastern Tennessee. This type of residuum is typically very fine-grained (up to 90% smaller than 2 microns), but can also contain substantial macroporosity due to roots, fractures, chert layers and saprorelicts. In many cases there are also sinkholes or swallets, which can divert runoff directly into the underlying aquifer without passing through the residuum. As a result of the heterogeneity and the potential for bypass flow, there is a great deal of uncertainty about the amount of recharge that occurs through these clay-rich “aquitards” and the degree of protection they provide to aquifers from sources of near-surface contamination. Recharge rates were estimated for a karst aquifer in southeastern Tennessee, which is overlain by 23 to 130 feet of fine-grained residuum. Rates were estimated using conventional Darcy flux calculations and on calculations based on concentrations of the environmental tracers tritium, Helium-3 and CFC-11 and -12 measured on samples collected from 12 wells in the residuum and 3 in the bedrock. The groundwater ages determined from 3H/3He values ranged from 2 to 32 years, while CFC ages were somewhat older, especially for CFC-11, which may have been effected by anaerobic degradation in the residuum. Corresponding recharge velocities of 1 to 8 ft/year were estimated based on the 3H/3He data, with the highest values occurring for wells near the bedrock-residuum interface, where mixing with younger water in the karst bedrock is suspected. Recharge velocity estimates from the Darcy flux calculations were generally higher by a factor of about 2 to 10 times that of the values based on 3H/3He data and had a greater range of variability. The is likely because the hydraulic conductivity values, which were measured using slug tests, are more strongly influenced by small scale heterogeneity and because of scarcity of data on seasonal variations in vertical hydraulic gradient. In summary, the environmental tracers were helpful in constraining estimates of groundwater velocity and recharge through the residuum and are not expected to be as susceptible to local heterogeneity or seasonal gradient variations as Darcy law based calculations.