9708487 Long One of the most fundamental issues in aquifer biogeochemistry concerns the mechanisms by which solute transport and geochemical processes combine with microbiological activity to influence spatial and temporal variations in redox zonation. We will examine these mechanisms in a shallow sandy aquifer contaminated with petroleum hydrocarbons, which has exhibited redox zonation that varies on seasonal time scales. The dynamic nature of redox zonation at this site allows us to examine the interaction among hydrogeological, geochemical, and microbiological processes, and test hypotheses regarding the (1) reactions that govern groundwater evolution from zone to zone and (2) factors which control the spatial and temporal dimensions of redox zones. By coupling information gained from the three disciplines, through field and laboratory research, a three-dimensional, transient reactive flow and transport model for the evolution of redox zonation will be constructed. The goals of this interdisciplinary study are to (1) quantitatively assess hydrogeologic, geochemical and microbiological constraints that influence redox processes within a given zone; (2) use these constraints to identify sets of reactions that describe the evolution of groundwater from one zone to another; and (3) integrate the reaction sets with a dynamic flow and transport model to simulate past observations regarding the dimensions of various redox zones at the site, as well as compare the predicted and observed outcome of groundwater hydrologic events such as prolonged periods of high or low water table. These goals will be achieved through field sampling (seasonal and event), laboratory analysis and experimentation, in-situ hydrolgeological, microbiological, and geochemical analyses, and hydrogeochemical modeling. Unique aspects of the methods include: (1) combining geophysical and hydrolgeological data to estimate aquifer properties; (2) using 16S rRNA nucleic acid probe hybridization to determine microbial abu ndance, fatty acid methyl ester profiles and 16S rDNA restriction analysis (ARDRA) to determine community structure; (3) estimating redox state from H2 gas concentrations; (4) assessing minerals-microbial interactions using in-situ samplers and experiments; and (5) combing geochemical and microbiological reaction sets in a reactive flow and transport model to quantitatively account for observed water chemistry at the study site. This approach will benefit each discipline through improved understanding of the interaction among physical, chemical and biological processes that lead to redox zonation in both pristine and contaminated aquifers, and will provide fundamental information valuable to the design of appropriate sampling, monitoring and remediation methodologies. This proposal was submitted in response to the Environmental Geochemistry and Biogeochemistry solicitation NSF 96-152, and is being funded jointly by the Divisions of Earth Sciences and Environmental Biology.
