Topic: Sustainable Yield, Groundwater Modeling, and DFCs
Source: Justin Camp, Brian Hunt, Brian Smith
Groundwater availability from the Edwards and Middle Trinity Aquifers in the study area is generally limited, however, the Lower Trinity Aquifer within the study area is untested because of the aquifer depths, assumed low yields, and poor water quality. This study provides an initial assessment of groundwater availability in a Lower Trinity well field based on aquifer tests, geochemistry, and limited analytical modeling.
The well field contains three wells spaced about 1/3 mile apart that range in total depths from 1,505 to 1,620 ft below ground surface. Depths to static water levels in the wells range from 140 to 150 ft below ground surface. Aquifer testing involved an initial single well test in 2018 followed by a test that pumped the two newer wells drilled in 2019 at 170 gallons per minute (gpm) independently for 96 hrs each, while monitoring the other wells. Maximum drawdown in the pumped wells ranged from 320 to 460 ft. Drawdown in the observation wells (0.2 to 0.4 miles distant) ranged from 8 to 34 ft. To estimate aquifer parameters, continuous water-level data were collected with transducers and fit to Theis and Cooper-Jacob analytical solutions using Aqtesolv software. Results of transmissivity from observation wells averaged about 401 ft2/day and a storativity of 5.57-E05. These parameters are higher than published median values of Lower Trinity wells in the Hill Country to the west of the study area.
Specific conductance of groundwater was monitored throughout the aquifer test and groundwater samples were analyzed for total dissolved solids (TDS) resulting in values of about 560 mg/L, indicating fresh water. Ion geochemistry indicates a calcium bicarbonate (Ca-HCO3) water with a tritium value of -0.05 TU, and a carbon-14 value of 0.006 percent modern carbon, PMC, indicating very old water.
Using parameters from the aquifer test and analytical models we forecasted drawdown from the well field with a combined pumping rate of 380 gpm (200 MGY) over a 30-year period. Preliminary analytical model results indicate an estimated drawdown up to about 130 ft at a distance of 2.0 miles from the well field.
Because the results of the aquifer test indicate that the water quality and yield of these wells are high. the Lower Trinity Aquifer may be a direct alternative groundwater supply for the study area. More groundwater studies and modeling are needed to evaluate long-term drawdown effects for the study area. The nearby Lower Trinity Aquifer of western Travis County offers a cautionary example of groundwater mining. In addition, the aquifer may have potential for aquifer storage and recovery (ASR) activities.
This summary was modified from a presentation and published abstract at the 2020 Geological Society of America South-Central Meeting (Camp et al., 2020).