Aquifer/aquitard interfaces: mixing zones that enhance biogeochemical reactions

Several important biogeochemical reactions are known to occur near the inte rface between aquifer and aquitard sediments. These reactions include O-2 r eduction; denitrification; and Fe3+, SO42-, and CO2 (methanogenesis) reduct ion. In some settings, these reactions occur on the aquitard side of the in terface as electron accepters move from the aquifer into the electron-donor -enriched aquitard. In other settings, these reactions occur on the aquifer side of the interface as electron donors move from the aquitard into the e lectron-acceptor-enriched, or microorganism-enriched, aquifer. Thus, the aq uifer/aquitard interface represents a mixing zone capable of supporting gre ater microbial activity than either hydrogeologic unit alone. The extent to which biogeochemical reactions proceed in the mixing zone and the width of the mixing zone depend on several factors, including the abundance and sol ubility of electron accepters and donors on either side of the interface an d the rate at which electron accepters and donors react and move across the interface. Biogeochemical reactions near the aquifer/aquitard interface ca n have a substantial influence on the chemistry of water in aquifers and on the chemistry of sediments near the interface.