WATER QUALITY ASSESSMENT IN THE GALLINAS WATERSHED, LAS VEGAS, NEW MEXICO (WINTER 04 RESULTS)

The Gallinas River is the primary source of water (95%) for the city of Las Vegas (pop. 18,000) and is over-allocated to the city, irrigators, and the Storrie Lake Water Project, which includes the Las Vegas National Wildlife Refuge (LVNWR). This project evaluated water quality within the Gallinas Watershed during the Winter 2004 to examine what influences the local geology and water conveyance operations have on water chemistry. Samples were obtained along a 60km reach during moderate flow (21.0 cfs). Seven surface water samples were collected beginning in the upper headwaters underlain by Precambrian crystalline rocks and extending down stream where the river courses over Paleozoic and Mesozoic strata. Four surface water samples were collected from Storrie Lake and McAllister Lake diversions. Gallinas River waters are HCO3-type in the headwaters and transition to SO4-type as the river crosses into sedimentary rocks. Samples collected along the entire reach of Gallinas River are Ca-type, except water discharging from the Montezuma Hot Springs and water stored in McAllister Lake, both of which are Na-type. Indicator parameters show increasing levels from upper to lower river sites (Ca 11.8-142.5; Mg 2.0-33.3; Na 4.5-78.0; Cl 1.3-68.8; SO4 9.9-450; and HCO3 44.5-214 mg/l) demonstrating that the Gallinas River accumulates dissolved constituents along its course. Hardness, alkalinity, conductivity, and TDS showed 5- to 15-fold increases from upper to lower river sites. The observed change in water quality is attributed to chemical loading from limestone and shale formations underlying a large part of the watershed. Most of the water parameters reach the maximum range of acceptable values at middle and lower river sites and at Storrie Lake, a secondary water reservoir for Las Vegas. There is a dramatic change in water quality at McCallister Lake, the largest water body at LVNWR. Data show elevated levels of Ca, Na, Cl, SO4 and conductivity. Evaporation and mineral dissolution are suspected of influencing McAllister Lake quality. U-Texas Dallas conducted a companion study in the Summer 2006 during low flow (3.1cfs) and found similar trends in the data but lower absolute concentrations of most of the water parameters, confirming our hypotheses and highlighting the effect of discharge on cation-anion concentrations.