Why conceptual groundwater flow models matter: a trans-boundary example from the arid Great Basin, western USA

Spring and Snake valleys, western USA, are scheduled for development and groundwater export to Las Vegas, Nevada (USA). New work, compared to published studies, illustrates the critical role of conceptual models to underpin water withdrawals in arid regions. Interbasin flow studies suggest that 30-55 % of recharge to Snake Valley arrives from adjacent Spring Valley. This study, however, suggest little or no interbasin flow; rather, Spring and Snake valleys comprise separate systems. Contrary to expectation, delta D and delta O-18 contours are perpendicular to proposed interbasin flow paths. C-14 age gradients up to 10 ka along interbasin flow paths indicate that old waters are not displaced by such fluxes. C-14 and H-3 patterns indicate local recharge occurs in adjacent mountain ranges and is transferred to basin-fill by losing streams, mountain front recharge, and upward leakage from carbonate bedrock beneath basins. The choice of conceptual models is critical for groundwater development. Simple analyses of water withdrawals indicate that monitoring discharges at desert springs is an inadequate protective measure. Once flows decline, recovery is lengthy even if pumping is stopped. The conceptual framework behind quantitative evaluations of sustainable yield is critical to determine the ability of a groundwater system to deliver sustained withdrawals.