PRELIMINARY ANALYSIS OF GROUNDWATER RESIDENCE AND AQUIFER SEGMENTATION IN THE CENTRAL GREAT BASIN

A geochemical study of major springs and wells in Spring and Snake Valleys [UT & NV] was initiated in response to a Southern Nevada Water Authority (SNWA) proposal to export up to 200,000 acre-ft/yr of water to Las Vegas. Preliminary results indicate that Spring Valley, on average, is relatively depleted in d18O and dD values compared to Snake Valley, although with significant variation and overlap. Small quantities of 3H (usually < 1 TU) were found in most springs, and range from below detection to 4.3 TU in Snake Valley and from below detection to 5.3 TU in Spring Valley. Mountain springs in the Snake Range [surrogates for net modern recharge] exhibit 3H values as high as 7.3 TU. 14C groundwater model ages indicate that there is also an older component of water in some springs. Waters discharging in Spring Valley range from modern to 5,000 yrs, and in Snake Valley from modern to 6,000 yrs. Most are calcium-bicarbonate waters, although some contain considerable magnesium and sulfate. TDS values range from 250 to 550 mg/L in Snake Valley and from 125 to 390 mg/L in Spring Valley.

While 3H values indicate a component of modern recharge, 14C model ages suggest a large component of older water in the system. The presence of old water may have implications to the sustainability of the system to pumping, and also may indicate the long time scales at which the system is naturally recharged.