CONCEPTUAL GROUNDWATER FLOW MODELS MATTER: TRANS-BOUNDARY FLOW IN THE ARID GREAT BASIN

Spring and Snake Valleys, Utah and Nevada, provide interesting case studies for large-scale water development in arid regions. Groundwater in these valleys has been targeted for export to the Las Vegas, Nevada area to sustain a growing urban population. Beyond case studies, however, our new work compared to prior investigations illustrates the critical nature of conceptual models to underpin water withdrawals in arid and semi-arid regions.

Previous work, largely based on water budget approaches, suggest that 30% of recharge to Snake Valley arrives by interbasin flow from Spring Valley. By contrast, our investigations, based largely on geochemical criteria, suggest little or no interbasin flow occurs and that Spring and Snake Valleys comprise separate groundwater flow systems. For example, contours of δD and δ¹⁸O values are often perpendicular to proposed flow paths, whereas interbasin flow should produce isotopic contours parallel to flow. Large ¹⁴C age gradients (up to 10 ka) also exist along proposed interbasin flow paths, yet older water should have been displaced by large fluxes of younger water at those locations. Rather, ¹⁴C and ³H patterns indicate local recharge in adjacent mountain ranges that is transferred to basin-fill sediments by losing streams, mountain front recharge, and leakage from carbonate bedrock without interbasin transfers.

Although we favor the local recharge model, the more critical observation is that the choice of conceptual models is critical in a water development plan. A simple analysis of water withdrawal indicates that a monitoring program of discharges at desert springs and wetlands is inadequate to protect these resources. Once flows begin to decline significantly, recovery will take long time intervals (hundreds of years or more) even if pumping is stopped. Futhermore, a lack of interbasin flow indicates that Snake Valley is much more susceptible to overdraft than adjacent Spring Valley. In summary, the conceptual framework upon which any quantitative evaluation of sustainable yield is based is critical to the apparent ability of a groundwater system to deliver flows that will not deplete the resource.