HYDROGEOLOGIC INVESTIGATION OF SPRINGS PROVIDING BASEFLOW TO THE DEVILS RIVER, VAL VERDE COUNTY, TEXAS

The Devils River flows through the Edwards Plateau in south-central Texas to the Amistad Reservoir, the confluence of the Rio Grande, in Val Verde County, Texas. Numerous springs issue from the Cretaceous Edwards-Trinity Plateau Aquifer (ETPA), contributing an average of 65% to the baseflow of the Devils, classifying it as having one of the largest baseflows of Texas’ rivers. Data gathered during field research and from the International Boundary Water Commission (IBWC) stream gage at Pafford’s Crossing in 2014 indicates a 66% increase in baseflow within the study area, from Baker’s Crossing to Pafford’s Crossing. The Devils River is considered the standard for surface water quality in Texas due to its pristine nature. However, natural and cultural resources have been impacted due to diminishing flows associated with drought, increased population, and increased groundwater pumping of the ETPA. Understanding the origin and nature of the springs supplying the Devils River is critical to their protection, and to a greater knowledge of groundwater behavior. Spring and surface water samples were collected to identify water-rock interactions, recharge elevation, age of spring water, groundwater flow paths, and spring influence on surface water quality and quantity of the Devils River. Results of gain/loss studies show increases of 4.19 m3/s (TCEQ, 2006) and 1.79 m3/s (this study). Geochemical analysis was conducted on surface and groundwater samples using various methods for ion, stable isotope, and radiocarbon dating. Percent modern carbon values of springs range from 60.3 +/- 0.2 pMC and δ13C values range from -15.2‰ to -17.3‰. Apparent radiocarbon ages of groundwater range from 2510 yr. to 4060 yr. Stable isotope values range from dD ~ -33 to -25‰ and d18O ~ -5 to -3‰. Field measurement averages for springs include values of specific conductivity (491 μS/cm), temperature (22.16°C), pH (7.4), and dissolved oxygen (5.6 mg/L). Surface water average values include specific conductivity (421 μS/cm), temperature (27.06°C), pH (8.0), and dissolved oxygen (7.7 mg/L).