STRUCTURAL CONTROLS ON GROUND WATER FLOW AND RECHARGE INTO THE SOUTHERN SAN LUIS BASIN NEAR TAOS, NM

Subsurface lithologic and geophysical data from a series of municipal, exploratory, subdivision, and domestic wells are used to delineate variations in thickness and extent of Tertiary through Quaternary sediments and Pliocene basalt flow sequences in the southern San Luis Basin near Taos, New Mexico and are utilized to help constrain the location of buried faults. Two deep wells drilled in 2007 are used to refine previously published stratigraphic analyses of basin fill deposits. Analyses of long-term pumping tests conducted in relatively close proximity to faults are used to evaluate the effect of faults on groundwater flow in shallow and deep basin fill aquifers. Selected wells and surface water sources were sampled and analyzed for major anions and cations, trace metals, ⁸⁷Sr/⁸⁶Sr, ³H, δ²H, and δ¹⁸O. Tritium results indicate that recharge to the shallow aquifer occurs on a time scale of < 5 to 10 years, but that recharge to some deep alluvial wells and wells completed into sediments interbedded with the Servilleta basalts occurs on a time scale of > 50 years. Recharge into the mountain front fractured bedrock aquifers occurs on time scales ranging from < 5 to 10 years to > 50 years. δ²H and δ¹⁸O data suggest that some deep basin-fill aquifer wells have received higher-elevation or older (Pleistocene?) recharge, whereas other deep wells have received lower-elevation or modern/Holocene recharge. ⁸⁷Sr/⁸⁶Sr varies from 0.7074 to 0.7156, indicating water-rock interaction with variable source rocks. Because these waters have relatively enriched ⁸⁷Sr/⁸⁶Sr it is likely that the Pennsylvanian carbonates and granitic basement rocks have contributed significantly to their Sr isotopic compositions. Although faults typically do not act as impermeable boundaries in the shallow alluvial aquifer, the Seco fault and several of the Los Cordovas faults act as impermeable boundaries in the Servilleta Formation and in the deep basin fill aquifer. However, other Los Cordovas faults do not act as significant impermeable boundaries in the deep aquifer, suggesting variable cementation along fault planes at depth.