Hydrogeologic study of springs in the Lower Canyons of the Rio Grande, Rio Grande Wild and Scenic River, Texas

The Lower Canyons reach of the Rio Grande Wild and Scenic River defines the U.S.-Mexico international border downstream from Big Bend National Park between Maravillas Creek and Dryden, Texas. Numerous springs in this reach issue from the Cretaceous Edwards-Trinity Plateau aquifer (ETPA), a potentially trans-boundary aquifer. These springs have been designated Ecologically Significant by the Far West Texas Water Planning group. International Boundary and Water Commission (IBWC) gage data on the Rio Grande indicate that base flow progressively increases by 60% due to spring inflow in the Lower Canyons. Base flow data from these gages in 2004 suggest spring input of 178,000 acre feet/year (Acft/yr). Data from the winter 2006 and 2011 USGS/NPS seepage run indicate 0.87 km³/yr (94,482 Acft/yr) and 0.94 km³/yr (102,084 Acft/yr) in the Lower Canyons. Discharge into the Rio Grande from these springs is important to many stakeholders, including the ecosystem, agriculture, industry, municipalities, and tourism. Ground-water input is also responsible for dilution of total dissolved solids (TDS) in the Rio Grande. Multiple sections of the Rio Grande upstream of the Lower Canyons are considered impaired water bodies on the Texas 303(d) List, and it is the springs in the Lower Canyons reach which ameliorate this problem. Understanding the origin of the springs within the Lower Canyons is critical for their protection. Therefore, structural and geochemical studies of the springs have been initiated to elucidate recharge areas, ground-water flow paths, the effects of regional structure, and the influence that the springs have on water quality and quantity in the Rio Grande.;Initial mapping of the structures in the Lower Canyons shows northwest-trending joints and fold axes. GIS analysis of the water chemistry reveals that the NDI waters are located south and west of a major, northwest-trending structure represented by the Bullis Fold, and the Ca-HCO3 waters are located north and east of this structure. Variations in the NDI waters could be the result of a longer residence time, mixing of ground-water via structural features, and possible interaction with Paleozoic sedimentary rocks in the Marathon Basin, located ∼30 km to the northwest, beyond the mapped extent of the ETPA. Alternatively, the Ca-HCO3 waters may be associated with recent meteoric recharge directly into the ETPA. Further geochemical investigations will attempt to identify recharge areas and flow paths. This information will assist in protection of the resource and provide critical information to water-planning groups.;Major element variations reveal two groups of springs: one set with an average of 533 mg/L TDS that plots on a piper diagram as a no dominant ion (NDI) type of water and a second set with an average of 282 mg/L TDS that plots as Ca-HCO3 type of water. The two spring groups plot in distinct areas on a graph of d-18O versus d-D. The NDI waters originate near a local meteoric water line (MWL) ranging from dD ∼ -53 to -48 per mil and d18O ∼ -7 to -3 per mil. The Ca-HCO3 waters plot with higher dD and d18O values, approximately on the MWL. Local ground-water northwest of the Ca-HCO3 waters also plots in this location.