EVALUATING WATER QUALITY STANDARDS, SHIFTING BASELINES, AND WHAT THE DATA SAYS: A CASE STUDY FROM THE RIO GRANDE WILD AND SCENIC RIVER

The Big Bend Reach of the lower Rio Grande is one of the most remote stream segments in Texas and is therefore one of the least studied segments. At 318 miles, with significant geochemical, hydrological and ecological gradients, this segment can be described as two different stream reaches. The Parks Reach (Rio Conchos to La Linda, MX) flows through a small agricultural area near Redford, Texas and Mulato, Chihuahua, Mexico, and through a network of state and federally protected areas, including 4 units protected by Mexico encompassing nearly 3.3 million acres. This reach is highly dependent on dam releases, is vulnerable to periods of very low flow and sections that go dry under drought conditions. The Lower Canyons Reach (La Linda, MX to the headwaters of Amistad Reservoir) has stable base flows provided by groundwater inputs and increased flood pulses due to larger watershed inputs.

The Big Bend reach flows through 3.5 million acres (1.4 million hectares) of protected lands including: Big Bend National Park, the Rio Grande Wild and Scenic River, two state administered protected areas, three Mexican protected areas, and the Monumento Natural Rio Bravo del Norte. It is located in Texas Commission on Environmental Quality (TCEQ) segment 2306 (from the confluence of the Rio Conchos to Val Verde County). Increasing salinity concentrations (chloride, sulfate, and total dissolved solids (TDS)) caused the TCEQ to place the entire segment on the state’s list of impaired water bodies in 2010. The TECQ has proposed to split the segment and alter standards to reflect current conditions using roughly the same reaches as described above with the new Segment 2315 beginning at the Rio Grande confluence with Cow Canyon, or 15 miles upstream of La Linda, MX. The current proposal would raise the water quality standards in this new segment from 1550 mg/L TDS to 2100 mg/L TDS, chloride from 300 to 450 mg/L, and sulfate from 570 to 750 mg/L. The minerals criteria for the lower reach would decrease from 1550 to 1400 mg/L TDS, chloride from 300 to 200 mg/L, and sulfate from 570 to 450 mg/L. Here we present a variety of water quality and biologic data to illustrate ongoing problems with declining water quality, possible sources of pollutants, cascading impacts on ecological systems and the role of groundwater in mitigating the poor conditions.