ASSESSING THE IMPORTANCE OF INTERBASIN FLOW IN THE GREAT BASIN, WESTERN USA

The Great Basin (GB), USA, is a large endorheic system comprised of both internally and externally drained sub-basins. Interbasin flow through bedrock, in particular carbonate rock, is the conceptual mechanism by which relatively large groundwater fluxes flow through multiple sub-basins and intervening mountains. Interbasin flow has been proposed based on 4 criteria: 1) modeling studies 2) water budget imbalances, 3) potential differences between basins, and 4) stable isotope evidence.

Modeling studies are difficult to assess as the assignment of flow or no-flow boundaries between sub-basins will result in water transfers or a lack thereof. Thus, models are not good indicators of interbasin flow. Water budgets, however, are too imprecise to discern interbasin transfers, and potential differences between basins will exist with or without interbasin fluxes. Potentiometric maps are dependent on their conceptual underpinnings, leading to possible false inferences regarding interbasin transfers. Isotopic evidence is prone to nonunique interpretation and may be confounded by the effects of climate change in regions with waters of contrasting age. Thus, these 4 criteria are not necessarily reliable indicators of interbasin transfers.

Structural and stratigraphic features of the GB should produce compartmentalization where a long history of tectonic and magmatic activity has produced a complex bedrock hydrostratigraphy. For example, a review of the stratigraphy of GB carbonate aquifers indicates that impermeable siliciclastic interbeds are relatively common, and greater dimensions in a candidate aquifer system increases the odds of segmentation along a given flow path.

A review of the hydrogeology of the Snake Valley and Death Valley regions indicates that interbasin flow is relatively unimportant. Thus, we conclude that initial conceptual hypotheses within various sub-basins should first seek to explain flow with local recharge and short flow paths. Where bedrock interbasin flow is still suspected, it is most likely controlled in the GB region by diversion of water into the damage zones of normal faults, with fault cores often acting as barriers. Large-scale bedrock interbasin flow where fluxes must transect multiple basins, ranges, and faults at high angles should be the conceptual model of last resort.