Much of the information on the geochemical evolution of groundwater has been conducted in relatively deep aquifer systems. Many of the biogeochemical reactions that influence nutrient cycling, concentrations of greenhouse gases, and degradation of contaminants, however, are redox reactions that occur in near- surface groundwaters. Redox boundaries can be either diffuse and extremely sharp under various situations. The nature of these redox boundaries are influenced by sediment chemistry, the kinetics of the redox reaction (including mediation of the reactions by bacteria), groundwater flow paths and mixing of groundwater, or combinations of these factors. In most studies of nitrate concentrations or other solutes involved in redox reactions, the possible physical and chemical systems have not been examined simultaneously or in sufficient detail to identify major influences on groundwater oxidation-reduction reactions. We propose to use extensive field monitoring to physical and chemical characteristics of the aquifers and water at sites with different physical characteristics and sediment chemistry. Nested piezometers and tensiometers will be used to monitor groundwater flow paths, multi-level samplers will be used to determine water chemistry. Groundwater ages will be determined by measuring concentrations of chlorofluorocarbons (CFC's) dissolved in the water. Measurements of nitrogen and other biologically fractionated isotopes will be made to determine whether reductions in solute concentrations are due to biologically mediated processes or due to dilution via groundwater mixing.
