Hydrogeology of the Hickory Sandstone Aquifer, Upper Cambrian, Riley Formation, Mason and McCulloch Counties, Texas

The Hickory Aquifer is a primary ground water source for Mason, McCulloch, Concho and San Saba counties, Texas. Significant water level declines have been recorded in the Hickory outcrop where large volumes of water are used for irrigation. Additionally, municipal remands on the confined portions of the aquifer are increasing, potentially by an order of magnitude, from wells already drilled but not yet in production. The hydrogeology of the Hickory aquifer is significantly more complex than indicated In previous assessments. Rather than simple radial flow outward from the outcrop, fault-impeded flow through a significantly reduced area into the subsurface is indicated by water-level, geochemical, isotopic-age and structural data. Deep portions of the aquifer are nearly stagnant, indicating poor circulation and minimal interaction with the active, outcrop portions of the aquifer. Radiocarbon isotopic ages were determined for 12 ground-water samples collected from the Hickory Sandstone aquifer as part of this investigation funded by the Hickory Underground Water Conservation District. Elapsed time since infiltration based on C-14 activity varied from at least 28,000 years for wells of significant depth (1,300 meters In Eden, Texas) to less than 30 years on the outcrop of the sandstone. Recharge enhancement activities such as the placement of small impoundments located in the beds of creeks which cross the outcrop demonstrate little potential to raise water levels in the aquifer because of low topographic position of the streams. Katemcy Creek is a single exception to this rule where the uppermost reaches could deliver water to the aquifer during periods of extreme drawdown in the adjacent aquifer. Examination of several reaches along the course of the San Saba River demonstrated points of recharge and discharge from the aquifer. Flow into the aquifer of several hundred cubic meters per day was measured across one fault in particular. Generally, reaches were found to be gaining flow, particularly at the lower reaches of tributaries to the San Saba River. Recommendations are included for a new investigatory well north of the San Saba River, permanent stream-flow measuring stations along the tributary creeks which cross the outcrop, and increased water level collection activities. Additional monitoring of the chemical quality of the aquifer is suggested, especially in the recharge area of the aquifer. Public Information activities regarding pesticide use, fertilizer use, confined animal feeding operations and domestic septic system maintenance are encouraged to provide protection for water quality in the recharge area and ultimately all aquifer users.