LONG-TERM DRAINAGE OF STORED GROUNDWATER RESOLVES THE HYDROLOGIC BUDGET DISCREPANCIES OF ARID CATCHMENTS

The largest basin in the Altiplano-Puna plateau margin, Salar de Atacama is an endorheic watershed encompassing over 17,000 km² of significant topographic relief. It hosts a brine and fresh water aquifer that is subject to substantial and increasing resource demand. Despite this demand, recent work has shown that observed groundwater recharge rates are inadequate to close hydrologic and solute budgets within several topographic catchments. This has led to fundamental questions about the mechanisms controlling these groundwater systems. To address these questions, a detailed characterization of the physical, geochemical and isotopic attributes of waters emerging at perennial springs on the southeastern slope of the basin was coupled to a 3-D finite difference numerical model spanning 2,500 km².

Since this region is extremely arid, receiving very little rainfall except at very high elevation, perennial springs are a primary expression of the groundwater system. Regional δ²H & δ¹⁸O results show waters discharging at these springs have consistent 2 to 3‰ enrichment along the LMWL, ³H values between 0 and 0.1 TU with values in local rainfall averaging 3.2 TU and substantial TDS values. These data suggest the groundwater represented by these springs is quite anomalous with respect to expected source areas inside and outside of the topographic watershed bounds and springs emerging to the north at similar elevation. The distinctive groundwater emerging at these springs precludes employment of a simple model of fluid and solute transport; in which watershed budgets can be resolved at a steady state with “modern” groundwater recharge and discharge within their topographic confines. Modelling simulations integrated with this analysis suggest the draining of groundwater storage recharged thousands of years ago during pluvial periods may be critical to closing the hydrologic budget of arid catchments such as this. The need for a substantial realignment of watershed boundaries and sources of solutes could have important implications for ground water rights and mineral resources and constitute a substantial change in our understanding of how these systems behave.