A DEUTERIUM MASS BALANCE MODEL APPROACH FOR EVALUATING A GROUNDWATER BUDGET OF A REGIONAL FLOW SYSTEM IN SOUTHEASTERN NEVADA

A deuterium-calibrated mass-balance model was constructed for the White River Flow System (WRFS) in southeastern Nevada to evaluate new groundwater recharge and discharge estimates. The model was constructed using deuterium data for mountainous recharge areas and springs discharging from the regional flow system in the valleys. Recharge estimates were assigned to mountain blocks and discharge estimates were assigned to valleys. Discharge in a valley was assigned as the total spring flow measured for regional springs in a valley plus calculated evapotranspiration amounts for areas of phreatophytic vegetation. The model also accounted for mixing of groundwater recharge along the greater than 200 km length of the flow system, which encompasses 13 valleys with 35 mountainous recharge areas and four regional spring discharge areas. Average deuterium values for recharge areas and spring discharge areas were used in the model. The model supports (1) new estimates of groundwater recharge and discharge for the WRFS, (2) proposed recharge sources and mixing for the WRFS, (3) the WRFS acting as one continuous carbonate-rock aquifer from Long Valley in the north to Upper Moapa Valley (Muddy River Springs area) in the south, and (4) 53,000 acre-feet per year (afy) of groundwater flowing out of Coyote Springs Valley to the Muddy River Springs area at the terminus of the WRFS. Of this total discharge, 37,000 afy is being discharged in Upper Moapa Valley and 16,000 afy is flowing out of the WRFS to the south-southeast in the carbonate-rock aquifers. These conclusions are supported by oxygen-18 and geochemical data.