Acetate retention and metabolism in the hyporheic zone of a mountain stream

An in situ acetate injection was used to determine the influence of labile dissolved organic carbon (DOC) availability on microbial respiration in the hyporheic zone of a headwater stream. We added bromide as a conservative t racer and acetate as an organic substrate to the hyporheic zone of Rio Cala veras, New Mexico, via an injection well. Tracer was observed in four of ei ght capture wells. Three of the four wells showed increases in bromide with out concurrent increases in acetate concentration, suggesting 100% acetate retention. One well had 38% acetate retention. Pore velocity and acetate re tention were negatively correlated, suggesting hydrologic control of acetat e retention. Acetate did not significantly sorb to the sandy hyporheic sedi ments at this site, indicating biological consumption of acetate. Acetate a ddition stimulated total CO2 production along monitored flowpaths and led t o changes in solutes associated with microbial terminal electron-accepting processes (TEAPs). Dissolved oxygen (DO), nitrate, and sulfate significantl y decreased, and ferrous iron and methane significantly increased compared to background concentrations in most wells. These results support the hypot hesis that microbial respiration in the hyporheic zone is limited by labile DOC availability. Furthermore, we have shown that a suite of metabolic pro cesses, from aerobic respiration to methanogenesis, cooccur and that anaero bic processes dominate heterotrophic metabolism in the hyporheic zone of Ri o Calaveras.