Reconnaissance Survey of Salt Sources and Loading into the Pecos River
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The Pecos River of southeastern New Mexico and west Texas is among the saltiest rivers in North America with streamflow salinity regularly exceeding 7,000 mg L-1 at the New Mexico and Texas border, and eventually exceeding 12,000 mg L-1 at Girvin, Texas. It originates in northeastern New Mexico, flows through the semi-arid part of New Mexico and west Texas, and merges into the Rio Grande just below the historical town of Langtry (Fig. 1). The diversion from this river, estimated at 224 million m3 (180,000 acre-ft.) per year, is mostly above Red Bluff, mainly for irrigating an estimated 25,000 ha (60,000 acre) of crop lands in New Mexico. The water stored in Red Bluff is also used for irrigating comparatively small areas (less than 4,000 ha) in west Texas. The area irrigated with ground water is much greater. High salinity of the river has adversely affected stability and diversity of the riparian and aquatic ecosystem (e.g., Hart, 2004; El-Hage and Moulton, 1998; Davis, 1987) as well as the economic use of this water resource, especially in the reach below Red Bluff Reservoir. Degradation of ground water quality along the streamflow is also a concern, as the saline stream percolates through highly permeable alluvium (Boghici, 1999). In addition, one study shows that the flow of this river accounts for nearly one-third of salts entering Amistad International Reservoir located approximately 64 km (40 miles) south of Langtry (Miyamoto, 1996). Salinity of the Amistad Reservoir reached 1,000 mg L-1 (the upper limit of secondary drinking water standard) in February 1988, and there is a concern that such an incident may occur with greater frequency unless salinity control measures are implemented at some point. Several measures to lower salinity have been proposed. Pumping of saline seepage below Malaga, which otherwise enters the Pecos, is among the options tried (e.g., Hale et al., 1954). The saline water is saturated brine consisting of common salts (NaCl). The brine once pumped into a nearby depression was evaporated, but the leakage from the ponding area made this option ineffective or unattractive (Havens and Wilkins, 1979). Deep well injection was also considered, but was found to be costly and probably not sustainable (Cox and Kunkler, 1962). The latest effort has been to evaporate the brine and to harvest salts (Personal Communication with the Red Bluff District). Another attempt which has been implemented in recent years is the eradication of salt cedar which invaded the bank of the Pecos River (Hart, 2004). The idea is to maintain the flow by reducing the evapotranspiration, and hopefully streamflow salinity as well by lowering evaporative concentration of salts (Weeks, et al.,1987; Hart, 2004). This study was conducted for identifying additional sources of salts and river reaches where saline water sources are entering the Pecos River, and was proposed as Subtask 1.5. Identification of saline tributaries is covered under Subtask 1.4.