ESTIMATING RECHARGE AND STORAGE COEFFICIENT IN A FRACTURED ROCK AQUIFER

Turkey Creek Basin is located 20 miles west of Denver. This front-range area is experiencing rapid development, thus water quantity and quality is a concern. Data collected from pumped and abandoned wells show an average water-level decline of 0.004 ft per day over the past 8 years, indicating the Turkey Creek Basin is not in a state of equilibrium. Published values of ground-water storage in the basin are poorly constrained, ranging from 1.5 to 150 years at present rates of water use.

Evaluation of the sustainability of the aquifer requires reasonable estimates of water use, recharge rates, transmissivities and storage coefficients. Sustainability requires that the average rate of water use be less than the average recharge rate. During droughts use exceeds recharge and storage controls sustainability.

Ground water recharge rates may differ significantly within the basin due to spatial variability in the physical environment. Because recharge rates are difficult to measure several methods of estimating recharge are being evaluated by comparing the residuals resulting from use of the recharge rates in a ground-water model. The methods of estimating recharge include 1) a PRMS (Precipitation-Runoff Modeling System) model; 2) a SWAT (Soil & Water Assessment Tool) model; 3) a soil-water balance model, and a locally developed function relating recharge to elevation, slope and aspect. Several methods of estimating storage coefficient are evaluated by comparing the rate of simulated water level declines to those observed in the field when utilizing the estimated values in a transient ground-water model. The estimation methods include: 1) a water budget; 2) pump test analysis; 3) residence time based on measured tritium age-dates of ground-water samples; and 4) analysis of well hydrographs.