On the Rio Grande aquifer: Flow relationships, salinization, and environmental problems from El Paso to Fort Quitman, Texas

The Rio Grande floodplain between the City of Fl Paso and Fort Quitman, Tex as (70 mi [113 km] reach) varies from intensively urbanized (first 20 mi [3 2 km]), to intensively irrigated by surface and ground water (next 40 mi [6 5 km]), to sparsely inhabited and little developed (last 10 mi [1.6 km]). W ater-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 re versed the hydraulic gradient between the river and the Rio Grande aquifer. Aquifer flow varies from underflow to gaining stream downstream from the u rbanized zone. Hydrochemistry of the Rio Grande aquifer evolves downstream from a Na-Ca-mi xed anion type water with TDS mostly between 1,000 and 3,500 mg/L to a Na-C a-SO4-Cl type water with TDS mostly between 3,000 to 6,000 mg/L. Plots of H -2 VS O-18 for ground-water samples collected in the Mexican: part of the R io Grande aquifer follow an evaporation trend. The most enriched samples co rrespond to higher salinity and greater distances downstream. Monthly water -quality data from the Rio Grande show similar changes in hydrochemistry. I ncreasing salinity in both the Rio Grande and Rio Grande aquifer generally reflect the tendency for salts to be recycled in irrigation water, to retur n 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 continuo us manner downstream. Other existing and potential water-quality problems in the Rio Grande and R io Grande aquifer include fecal coliform bacteria, nitrate, pesticides, and organic contaminants. These are a concern because of cross-formational now 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 Fl Paso/Ciuda d Juarez corridor.