Evaluating behavior of oxygen, nitrate, and sulfate during recharge and quantifying reduction rates in a contaminated aquifer

This study evaluates the biogeochemical changes that occur when recharge water comes in contact with a reduced aquifer. It specifically addresses (1) which reactions occur in situ, (2) the order in which these reactions will occur if terminal electron acceptors (TEAs) are introduced simultaneously, (3) the rates of these reactions, and (4) the roles of the aqueous and solid-phase portions of the aquifer. Recharge events of waters containing various combinations of O₂, NO₃, and SO₄ were simulated at a shallow sandy aquifer contaminated with waste fuels and chlorinated solvents using modified push−pull tests to quantify rates. In situ rate constants for aerobic respiration (14.4 day ^-1), denitrification (5.04−7.44 day^-1), and sulfate reduction (4.32−6.48 day^-1) were estimated. Results show that when introduced together, NO₃ and SO₄can be consumed simultaneously at similar rates. To distinguish the role of aqueous phase from that of the solid phase of the aquifer, groundwater was extracted, amended with NO₃ and SO₄, and monitored over time. Results indicate that neither NO₃ nor SO₄ was reduced during the course of the aqueous-phase study, suggesting that NO₃ and SO₄ can behave conservatively in highly reduced water. It is clear that sediments and their associated microbial communities are important in driving redox reactions.