HYDROLOGICAL AND GEOCHEMICAL CHARACTERISTICS IN THE EDWARDS AND TRINITY HYDROSTRATIGRAPHIC UNITS USING MULTIPORT MONITOR WELLS IN THE BALCONES FAULT ZONE, HAYS COUNTY, CENTRAL TEXAS

This study was conducted to better understand the hydrologic relationships between the Edwards and Trinity hydrostratigraphic units. Permeabilities are one of the defining characteristics of hydrostratigraphic units. Detailed and quantified permeability information from the hydro- and lithostratigraphic units composing the Edwards and Trinity groups is lacking. To better manage water resources, the hydraulic parameters of the aquifers must be better characterized and quantified.

This study presents detailed hydraulic permeability test results combined with geochemistry data (total dissolved solids, TDS) from zones within two Westbay® multiport monitor wells. The multiport wells are 1,125 and 1,375 feet deep and contain 14 and 21 zones, respectively. Zones range from 20 to 197 feet thick and are hydrologically isolated from one another by inflatable packers allowing for discrete permeability testing and groundwater sampling. Rising or falling head tests were performed for each zone. Hydraulic conductivity (K) of each zone was calculated from data with either Bouwer-Rice or Butler analytical solutions using Aqtesolv®. Major ions were analyzed from groundwater samples collected from each zone using a specialized wireline sampling tool.

Results reveal three groups of hydrostratigraphic units with distinct permeabilities and geochemistry. These include (from stratigraphically highest to lowest): 1) Edwards Aquifer (Georgetown, Edwards and uppermost Upper Glen Rose) with K values ranging from 0.3 to 66 ft/day (avg. 26 ft/day) and 313 mg/L average TDS; 2) the Upper Trinity Aquitard (Upper Glen Rose and uppermost Lower Glen Rose) with K ranging from 0.01 to 0.7 ft/day (avg. 0.25) and average TDS 3,200 mg/L; 3) the Middle Trinity Aquifer (Lower Glen Rose, Hensel, and Cow Creek) from 0.2 to 37 ft/day (avg. 13.6 ft/day) and 750 mg/L average TDS.

What has historically been called the Upper Trinity Aquifer is best described as an aquitard in the study area based on its low average hydraulic conductivity of <1 ft/day and >2,000 mg/L TDS. These values are two orders of magnitude less in hydraulic conductivity and 5 to 10 times higher TDS than the overlying and underlying aquifers. The Upper Trinity Aquitard prevents any significant vertical interaction between the Edwards and Middle Trinity Aquifers.