Geophysical and Geochemical Delineation of Sites of Saline-Water Inflow to the Canadian River, New Mexico and Texas

Lake Meredith, which supplies water for domestic use to all major Texas cities on the Southern High Plains, exceeds State of Texas limits for chloride and sulfate content. Locating sources of these solutes along the Canadian River, which supplies Lake Meredith, marks the first step toward a remediation effort to reduce river salinity and improve the water quality of Lake Meredith.Our approach was to use surface-water conductivity and flow measurements, geological observations, and previous studies to identify areas where highly saline water enters the Canadian River, then complete detailed ground-conductivity studies in the probable inflow areas to locate discharge points.Our measurements of conductivity and salinity of Canadian River waters indicated that the most important saline ground-water discharge areas are concentrated in two river segments between Ute Reservoir and Rana Canyon, New Mexico: one between the reservoir and a point 14 to 16 km downstream, and the other 32 to 64 km downstream from the reservoir. Chemical analyses of surface water and subsurface log data suggest that saline water in the Canadian River valley evolved by the mixing of fresh water derived from meteoric precipitation and highly saline water derived from dissolution of halite from the Permian San Andres Formation and the Artesia Group. Modern dissolution occurs along a front that lies about 335 m beneath the Canadian River in the Ute Reservoir area and that extends about 16 km south of the river at depths of 305 m.Analysis of near-vertical joints in bedrock along the river valley indicates that the primary joints are oriented east-west and may be dilated and open to ground-water flow, whereas the secondary joints are oriented north-south, commonly terminate against the primary joints, and are not dilated. These observations are consistent with a hypothesis that saline water from depth preferentially flows into Canadian River alluvium through open bedrock joints, either directly beneath the Canadian River valley or indirectly beneath tributary valleys, and subsequently flows into the river.Electromagnetic surveys, consisting of more than 2,200 ground-conductivity measurements along seven segments of the Canadian River and its tributaries, reveal that apparent conductivities in alluvium within the Canadian River valley range from a few to nearly 300 millisiemens per meter. Three broad high conductivity zones, ranging from 1.6 to 4.2 km long, were detected between Ute Reservoir and Revuelto Creek; a fourth high-conductivity zone, 2.7 km long, was located 35 km downstream in the Dunes area. Each zone spans a number of individual conductivity peaks that range from 60 to 320 m across and may represent discrete brine discharge sites in Canadian River alluvium. Many of these peaks are located where tributary drainages enter the main Canadian River valley. Conductivity profiles computed from vertical electromagnetic soundings show increasing conductivity with depth at most sites.