Evolution of a Deep Fluid in a Surficial Environment Using Stable Carbon and Sulfur Isotopes: Case of the Transitional Zone of the Edwards Aquifer in South Central Texas

Abstract

The Edwards Aquifer in south central Texas (USA) features a distinct fault line defining the freshwater and saline water zones. We aim to elucidate the evolutionary behavior of carbon and sulfur emanating from deep environments and flowing in surficial environment so as to better understand the cycling of these metals in water environments. To characterize the evolution of carbon and sulfur species in surficial environment as the fluid emanates from a deep saline aquifer, we monitored the chemical and isotopic (δ13CDIC and δ34SSO4) parameters over a 700-m stretch on a stream-pond system situated on the saline-freshwater line of the Edwards Aquifer’s artesian zone. A three-tier evolution process associated with the interaction of a deep saline fluid with a fresh-ponded water was observed, namely: A deep fluid control zone characterized by decreasing pH values and relatively high total dissolved solids (TDS) concentrations along the stream path; a mixing zone of the deep fluid and fresh-ponded water where the TDS concentrations showed a sharp decline and a continual decrease in pH values; and a fresh-ponded water-controlled zone with a stable pH value and low TDS concentrations. The decreasing pH values were a result of CO2 outgassing due to the relatively high CO2 partial pressures (10–2.53 atm) relative to atmospheric (10–3.50 atm) in the aqueous system, and the high TDS concentrations were attributed to the deep saline fluid emanating at the well-head. The enrichment in the δ13CDIC and δ34SSO4 for the initial stream path reflects the kinetic isotopic fractionations associated with CO2 outgassing and with the microbial conversion. The zonation of the evolutionary process indicates that (i) solutes play a critical role in the evolution of carbon and sulfur in surficial environment, (ii) kinetic and chemical fractionation are the dominant processes controlling carbon evolution, and (iii) microbial metabolism primarily controls sulfate fractionation and evolution through the conversion of sulfate to thiosulfate.