On the Rio Grande Aquifer: Flow Relationships, Salinization, and Environmental Problems From El Paso to Fort Quitman, Texas

Abstract

The Rio Grande floodplain between the City of El Paso and Fort Quitman, Texas (70 mi [113 km] reach) varies from intensively urbanized (first 20 mi [32 km]), to intensively irrigated by surface and ground water (next 40 mi [65 km]), to sparsely inhabited and little developed (last 10 mi [16 km]). Water-level contour maps prepared with recent and historical data illustrate losing stream, underflow, and baseflow conditions on different segments of the alluvial floodplain. The condition of losing stream is apparent along the urbanized zone where draw-down cones from municipal well fields have reversed the hydraulic gradient between the river and the Rio Grande aquifer. Aquifer flow varies from underflow to gaining stream downstream from the urbanized zone. Hydrochemistry of the Rio Grande aquifer evolves downstream from a Na-Ca-mixed anion type water with TDS mostly between 1,000 and 3,500 mg/L to a Na-Ca-SO 4 -Cl type water with TDS mostly between 3,000 to 6,000 mg/L. Plots of 2 H vs 18 O for ground-water samples collected in the Mexican part of the Rio Grande aquifer follow an evaporation trend. The most enriched samples correspond to higher salinity and greater distances downstream. Monthly water-quality data from the Rio Grande show similar changes in hydrochemistry. Increasing salinity in both the Rio Grande and Rio Grande aquifer generally reflect the tendency for salts to be recycled in irrigation water, to return to the Rio Grande, and then to be reapplied to crops. By this process the salinities in the Rio Grande and Rio Grande aquifer increase in a continuous manner downstream. Other existing and potential water-quality problems in the Rio Grande and Rio Grande aquifer include fecal coliform bacteria, nitrate, pesticides, and organic contaminants. These are a concern because of cross-formational flow between the Rio Grande aquifer and the Hueco Bolson aquifer, the latter a source of water for more than 1.6 million residents along the El Paso/Ciudad Juarez corridor.