Identifying the regional extent and geochemical evolution of interbasin groundwater flow using geochemical inverse modeling and 87Sr/86Sr ratios in a complex conglomeratic aquifer

Interbasin groundwater flow (IGF) occurs when water that is recharged in one watershed or basin discharges into an adjacent watershed or basin. IGF can directly impact the water and solute mass discharged from a watershed and consequently, skew estimates of watershed-scale weathering rates. It is extremely difficult to determine the impact of IGF on weathering processes in adjacent watersheds because it is difficult to differentiate between solute mass originating within the watershed from solute mass transported into the watershed from sources outside the watershed. We quantify the regional extent of IGF in three watersheds (Canjilon, El Rito, and Vallecitos) draining the Tusas Mountains of northern New Mexico, USA (sites where IGF has been shown to occur) using spatial trends in 87Sr/86Sr in springs and streams and geochemical modeling using PHREEQC. 87Sr/86Sr ratios show a strong connection from the Canjilon watershed to the El Rito watershed and suggest that the IGF connection may extend to the Vallecitos watershed. Geochemical modeling constrained by XRD analysis of aquifer mineralogy indicates that IGF from the Canjilon watershed is the dominant control on perennial flow in springs and streams in the adjacent El Rito watershed. However, the geochemical signature of an IGF connection from El Rito to the Vallecitos watershed is weak to non-existent. The maximum possible IGF contribution from El Rito to Vallecitos is during snowmelt, contributing as much as 20% of the water and solute mass to Vallecitos. However, the maximum IGF contribution is <5% during summer and fall months. In the El Rito watershed, the solute load is controlled by weathering kinetics occurring from within and outside the drainage area of the watershed. This diverges from the traditional watershed weathering model which assumes that the solute mass comes from only within the drainage area of the watershed. In comparison, the Vallecitos watershed conforms more to the traditional watershed weathering model. Our methodology is unique because it accounts for the background geochemical evolution of IGF in addition to mixing with locally-recharged groundwater. The mineral-weathering models, when combined with 87Sr/86Sr ratios, appear to be a robust methodology in sorting out the geochemical kinetics of watersheds connected through IGF and may also offer unique capabilities to investigate the regional extent of IGF in other settings.