Texas State University- San Marcos, San Marcos, TX
Abstract
The Lower Canyons (LC) reach of the Rio Grande Wild and Scenic River defines the U.S.-Mexico international border downstream from Big Bend National Park (BBNP) between La Linda and Dryden, Texas. Numerous springs issue from a trans-boundary aquifer in the area. We have initiated structural and geochemical studies of the springs to elucidate recharge areas, groundwater flow paths to the springs, the effects of regional structure, and the influence that the springs have on water quality and quantity in the Rio Grande. Groundwater in the LC is represented by the Cretaceous Edwards-Trinity Plateau aquifer (ET). International Boundary and Water Commission (IBWC) gage data indicate that base flow progressively increases by as much as 60% due to spring inflow in the LC. Base flow data from these gages in 2004 suggests spring input at 178,000 acre feet/year. Data from the winter 2006 USGS/NPS seepage run indicates 108,595 to 144,793 acre feet/year. Ground water input is also responsible for dilution of total dissolved solids (TDS) in the Rio Grande.
Major element variations reveal two groups of springs, one with an average of 533 mg/L TDS which plots on a Piper diagram as a Na-SO4 type of water and a second set with an average of 282 mg/L TDS that plots as a Ca-HCO3 type water. The two spring groups plot in distinct areas on a graph of d-18O versus d-D. The Na-SO4 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 groundwater northwest of the Ca-HCO3 waters also plot in this location. Our mapping of the structures in the LC show northwest trending joints and fold axes. GIS analysis of the water chemistry reveals that the Na-SO4 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 Na-SO4 waters could be the result of a longer residence time and possible communication with Paleozioc sedimentary rocks in the Marathon Basin, which are located ~30 km to the northwest, beyond the mapped extent of the ET aquifer. Alternatively, the Ca-HCO3 waters may be associated with recent meteoric recharge directly into the ET aquifer.