Resumen
Sulfate isotopes (d34S, d18OSO4) interpreted in conjunction with sulfate concentrations show that sulfate of both agricultural and geologic sources is present in groundwater and surface water in the Rio Grande flood plain within the Hueco Bolsón. From previous studies, water isotopes (d2H, d18O) in the study area indicate groundwater age relative to dam construction upstream. Surface water entering the Hueco Bolsón contains a mixture of soil-amendment sulfate and sulfate from deep-basin groundwater seeps at the terminus of Mesilla Valley. In the shallow Rio Grande alluvial aquifer within the Hueco Bolsón, ranges of d34S in pre-dam (+2 to +9?) and post-dam (0 to +6?) groundwater overlap; the range for post-dam water coincides with common high-sulfate soil amendments used in the area. Most post-dam groundwater, including discharge into agricultural drains, has higher sulfate than pre-dam groundwater. In surface water downstream of Fabens, high-d34S (>+10?) sulfate, resembling Middle Permian gypsum, mixes with sulfate from upstream sources and agriculture. The high- d34S sulfate probably represents discharge from the regional Hueco Bolsón aquifer. In surface water downstream of Fort Hancock, soil-amendment sulfate predominates, probably representing discharge from the Rio Grande alluvial aquifer near the basin terminus. The d18OSO4 dataset is consistent with sulfate origins determined from the larger d34S dataset.