Groundwater flow patterns in the San Luis Valley, Colorado, USA revisited: an evaluation of solute and isotopic data

Groundwater systems in the San Luis Valley, Colorado, USA have been re-evaluated by an analysis of solute and isotopic data. Existing stream, spring, and groundwater samples have been augmented with 154 solute and isotopic samples. Based on geochemical stratification, three groundwater regimes have been identified within 1,200 m of the surface: unconfined, upper active confined, and lower active confined with maximum TDS concentrations of 35,000, 3,500 and 600 mg/L, respectively. The elevated TDS of northern valley unconfined and upper active confined systems result from mineral dissolution, ion exchange and methanogenesis of organic and evaporate lake sediments deposited in an ancient lake, herein designated as Lake Sipapu. Chemical evolutions along flow paths were modeled with NETPATH. Groundwater ages, and delta C-13, delta H-2 and delta O-18 compositions and distributions, suggest that mountain front recharge is the principle recharge mechanism for the upper and lower confined aquifers with travel times in the northern valley of more than 20,000 and 30,000 C-14 years, respectively. Southern valley confined aquifer travel times are 5,000 C-14 years or less. The unconfined aquifer contains appreciable modern recharge water and the contribution of confined aquifer water to the unconfined aquifer does not exceed 20%.