GEOCHEMICAL EVOLUTION OF THE GALLINAS RIVER, LAS VEGAS, NM

The Gallinas River in northeastern New Mexico provides almost all of the water supply for the city of Las Vegas. Although the watershed is small (98.4 mi^2), its water chemistry changes markedly as it passes first through crystalline Precambrian rocks, over a range-bounding fault with hot springs, then continuing across the Paleozoic and Mesozoic strata of the Las Vegas basin. Using test strips (later confirmed by ICP analysis) and handheld probes during an undergraduate field camp, we sampled geochemistry and streamflow at 20 sites along the river during severe drought conditions in May 2006. Waters begin as calcium-carbonate type with very low TDS across the crystalline basement. TDS doubles at Montezuma Hot Springs, and the waters become sodium-bicarbonate type. Waters immediately enter the Mesozoic sedimentary sequence, TDS doubles again just above Las Vegas, and the waters steadily migrate to sodium-sulfate type. Below Las Vegas, evaporation at two lakes and baseflow from Mesozoic sediments further increases TDS to near 1,000 ppm by river mile 33. Comparison to a winter study under moderate conditions by NMHU (Lindline, this volume) in 2004-5 shows similar trends but less impact of hot springs discharge at Las Vegas, although greater salinity below Las Vegas. In both cases reduced snowmelt contribution yielded significant salinity increases downstream from the Precambrian basement (including hot springs discharge), and especially in shallow lakes. These increases are likely to become problematic in extended droughts, and may eventually require significant alteration of current storage approaches in this watershed.