Hydrology, Salinity, and Salinity Control Possibilities of the Middle Pecos River: A Reconnaissance Report
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The Middle Pecos River between Malaga, New Mexico, and Girvin, Texas, is known for high salinity. Streamflow salinity during the last decade (1991-2000), for example, averaged 3,500 and 6,150 mg L-1 at Malaga and at the Red Bluff release, and upwards of 12,000 mg L-1 at Girvin. These high levels of streamflow salinity not only reduce the economic uses of the water, but also limit the biodiversity of aquatic and riparian species along the river. This report outlines the hydrology, geochemistry, and water management practices of the Middle Pecos River in order to explain the reasons for the high salinity, and to discuss the potential for salinity control. The main causes of high salinity between Malaga and Red Bluff are brine intrusion at Malaga Bend and Bottomless Lakes and a drastic reduction in flow since the late 1930s that does not adequately dilute the intrusion. The amount of salts entering the Pecos River from these two sites is estimated at 450,000 tons/year while freshwater flow at Malaga has decreased from 260 Mm3 (210,000 acre-ft) per year from 1929 through 1937 to 81 Mm3 (66,000 acre-ft) per year from 1959 through 2001. The causes of high salinity between Red Bluff and Girvin are saline water intrusion from both surface and subsurface sources, low runoff into the river, and the evaporative concentration of the stream. The amount of salts entering this reach is estimated at 250,000 tons/year, primarily from Salt Creek, Salt Draw, Toyah Creek, and shallow saline groundwater. The sources of the shallow saline groundwater which enters the Pecos River between Coyanosa and Girvin are suspected to be groundwater flow from adjacent areas, but details are yet to be investigated. Diversion for irrigation, high seepage loss above Pecos, and low runoff resulted in inadequate flow to prevent intrusion or to dilute saline water entering the Pecos below Coyanosa. The annual flow at Coyanosa decreases below 30 Mm3 (24,000 acre-ft) per year. There are interests to lower the salinity of the Middle Pecos River for preserving its biodiversity, protecting groundwater quality, and encouraging the regrowth of native riparian species after ongoing saltcedar control activities, besides increasing the economic value of this water for irrigation. A regional level of concern is its impact on Amistad International Reservoir, located downstream along the Texas/Mexico border. The salinity of this huge reservoir (6.8 billion m3 or 5.5 million acre-ft) has increased from 560 mg L-1 to about 1000 mg L-1, the upper limit of the Texas drinking water standard. The Pecos River accounts for nearly 30 percent of the salt loading into Amistad International Reservoir while providing about 10 percent of the flow, thus raising the background salinity of the reservoir. In addition, historical records from 1941 and 1942 indicate that a high precipitation event between Roswell and Red Bluff can cause the Pecos River to send enough saline water to Amistad to raise the salinity level of the reservoir well above the Texas drinking water standard. Since the potential for additional freshwater inflow from runoff appears to be limited, salinity management strategies must incorporate ways to reduce saline water intrusion and percolation losses from reservoirs and river beds. Streamflow salinity can be restored closer to the original level by reducing saline water intrusion roughly in proportion to the reduction in fresh water flow caused by diversion and percolation losses. Potential control options include saline water intrusion control upstream at Malaga Bend and Bottomless Lakes, and possibly in the segment between Pecos and Girvin. The methods of salt source control at each of these sites are yet to be addressed. Preliminary estimates show that salt source control at Malaga Bend and/or Bottomless Lakes will result in a significant reduction of salinity of Red Bluff Reservoir. The control of brine intrusion at Malaga Bend alone can lower salinity of the Red Bluff release from 6150 to 4800 mg/L, the level comparable to the level that existed shortly after the construction of Red Bluff Reservoir in 1936. However, its impact on Amistad International Reservoir is yet to be analyzed, and it requires good understanding of the hydrologic connection between the middle and the lower reaches. If the connection is weak, salt sources below Pecos should be evaluated for control as a part of the salinity control plan for Amistad International Reservoir. Streamflow salinity below Coyanosa can be lowered simply by reducing the percolation losses from the reservoir and river beds above Pecos, provided that the water saved is left in the river. However, this option will increase salt transport to the Lower Pecos River unless implemented in conjunction with salt source control. Impacts of water management and salt source control options on monthly or daily salinity of the middle and the lower reaches are yet to be evaluated.