Completed Study Field Studies to Estimate Groundwater Recharge in the Central Carrizo Wilcox Aquifer Bridget Scanlon, principal investigator; Robert C. Reedy Recharge was estimated using Cl concentrations in the unsaturated zone and groundwater and using 3H/3He concentrations in groundwater in 7 boreholes in the outcrop area of the Simsboro Formation in the central part of the Carrizo Wilcox aquifer (Fig. 1). Long-term (50 yr) mean annual precipitation in the central part of the Carrizo-Wilcox aquifer ranges from 29 inches in the southwest to 48 inches in the northeast of the area. Average chloride concentrations in the unsaturated zone ranged from 23 to 519 mg/L. Variability in mean chloride concentrations was high locally. Chloride concentrations were also highly variable within each profile. There was no systematic variation in chloride concentrations with depth. Recharge rates were calculated for the portion of the profiles that generally represented the last 50 yr where possible. In some cases recharge rates were so low that a 50 yr section corresponded to a very narrow depth interval. Recharge rates generally ranged from 0.2 to 1.4 in/yr. The time required to accumulate chloride in each profile ranged from 110 to 2815 yr. Groundwater chloride concentrations were generally lower than those in the unsaturated zone (5 - 180 mg/L). Representative recharge rates based on groundwater chloride concentrations range from 1 to 1.5 in/yr and are higher than those based on unsaturated zone chloride. The generally higher recharge rates based on groundwater chloride relative to unsaturated zone chloride are considered more representative of the regional system whereas the unsaturated zone data indicate that locally recharge rates are lower. Groundwater tritium concentrations ranged from 0.76 to 3.57 TU. These tritium levels were much greater than the detection limit for tritium (~ 0.2 TU) and indicate that a component of the water was recharged in the last 50 yr. Tritium/helium was also used to date the water in 3 of the 7 wells (CW3, CW4, and CW6). There were problems with analysis of 3He in water samples from CW6. 3He concentrations were low in well CW3 and much higher in well CW4. The low 3He concentrations in CW3 indicate a short residence time of the water of 2.2 - 6.1 yr (replicate analyses) whereas the much higher 3He concentrations in CW4 indicate a residence time of 21.4 yr. The times represent the time of 3He accumulation since it was isolated from the unsaturated zone. Water velocities were calculated by dividing the distance between the water table and the center of the well screen by the age of the water and resulted in velocities of 0.4 (CW4) to 1.4 - 4.0 ft/yr (CW3). Recharge rates of 1.6 (CW4) to 6.0 - 16.7 in/yr (CW3) were calculated by multiplying the velocities by the average porosity of 0.35. The recharge rate for CW4 of 1.6 in/yr is similar to that estimated from the groundwater chloride concentration. The recharge rate for CW3 of 6.0 - 16.7 in/yr is high similar to the high recharge rate calculated the groundwater chloride concentration (6.2 in/yr). Future work should consider using gas diffusion samplers for tritium-helium dating rather than copper tubes used in this study, particularly in low yield wells. This approach would help reduce the uncertainties in the age estimates. Reference Reedy, R. C., Scanlon, B. R., and Dutton, A. R., 2003, Collection and analysis of environmental tracers for estimation of recharge rates in the GAM model of the central Carrizo-Wilcox aquifer, in A. R. Dutton, B. Harden, J. P. Nicot, and D. O'Rourke, editors, Groundwater Availability Model for the Central Part of theCarrizo-Wilcox Aquifer in Texas. Contract Report prepared for the Texas Water Development Board. August 2005