HYDROLOGIC AND GEOCHEMICAL INVESTIGATION OF SPRINGS ON THE DEVILS RIVER, VAL VERDE COUNTY, TEXAS, USA

The Devils River flows through the Edwards Plateau to the Amistad Reservoir, the confluence of the Rio Grande, in Val Verde County, Texas. Many springs issue from the Cretaceous Edwards-Trinity Plateau Aquifer (ETPA), contributing approximately 80% of the base flow to the Devils, and giving it one of the largest base flows of Texas’ rivers. Data collected during field research and from the International Boundary Water Commission stream gage at Pafford’s Crossing in 2012 indicates a 40% increase in base flow 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. Drought, increased population, and increased groundwater pumping of the ETPA may be directly affecting reduced spring and river flow in the Devils River and throughout west Texas. Comprehending the origin and nature of springs issuing into the Devils is critical to their protection. Springs and surface water were sampled throughout the study area to identify water-rock interactions, recharge elevation, age of spring water, groundwater flow paths governed by geologic structures, and spring influence on water quality and quantity of the Devils River. Results of gain/loss studies show increases of 4.19 cms (TCEQ, 2006) and 1.57 cms (this study). Geochemical analysis was conducted on surface and groundwater samples using various methods for ion, stable isotope, and radio carbon dating. Percent modern carbon values of springs range from 60.3 +/- 0.2 pmc to 73.2 +/- 0.3 pmc, and δ¹³C values range from -15.2 ‰ to -17.3‰. 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 (8.00), and dissolved oxygen (7.70 mg/L).