Using Trends and Geochemical Analysis to Assess Salinity Sources along the Pecos River, Texas

Increasing salinity has been a growing concern for users of waters from the Pecos River and the reservoirs it feeds in the Texas portion of the River's watershed. Irrigation water diverted from the river in the northern reach of this watershed is often only suitable for a limited number of crops, reducing harvesting options for local farmers. In the south, the Pecos feeds into the International Amistad Reservoir along the border with Mexico. During the 1990s, total dissolved solids concentrations in the reservoir rose as much as 10 mg/L per year and often approached the drinking water standard for potable water (1000 mg/L). Since this time, control efforts have focused on reducing the river's salinity, requiring the identification of salt sources. Hydrologic trend analysis and geochemical identification methods were used to determine these sources for the reach of the river between Red Bluff Reservoir and Brotherton Ranch. Between Red Bluff Reservoir and Coyanosa, flow diversions remove much of the flow that carries the salts, resulting in decreased salt loads, but also making the river more sensitive to evapotranspirative concentration. This sensitivity is evident in the river between Coyanosa and Girvin, where salinity begins to increase to the highest levels within the study area. However, salt loads increase here as well, indicating external salt sources as a contributor. The most substantial increase in bromide ions and the Br-/Cl- ratio appears between Grandfalls and Imperial, although no conclusion could be drawn regarding the identity of the source. The ratio continues to increase up to Girvin, where it appears that evapotranspirative concentration again has a significant effect. Here, several points drifted to the right of the groundwater mixing zones, plotting at values that were uncharacteristic of these sources.