IN SITU MEASUREMENTS OF CONTAMINANT TRANSFORMATIONS

For the last ten years our group has been developing the use of single-well, "push-pull" tests to detect and quantify in situ microbial activity in subsurface environments. Tests consist of (1)preparing aqueous solutions of site groundwater containing conservative tracers and various reactive solutes, (2)Injecting these solutions into the saturated zone using existing wells or similar devices, and (3) sampling the composition of the injected test solution over time. Using various combinations of reactive solutes it is possible to detect and quantify microbial activity as the consumption of an injectd substrate, the formation of a diagnostic metabolic product, or both. Reaction rates are obtained by plotting dilution-adjusted concentrations vs time. Because the tests are conducted in situ and interrogate a relatively large volume of subsurface amterial (~ 1 m3), they provide the best quantitative descriptions of microbial activity currently available. Several case histories are presented to illustrate the use of push-pull tests to detect and quantify contaminant transformations including: Aerobic and anaerobic respiratory processes; anaerobic transformations of chlorinated solvents, petroleum hydrocarbons, heavy metals, and radionuclides; and aerobic cometabolism of chlorianted solvents. Applications include site characterization, remediation pilot testing, monitoring remediation performance, and monitored natural attenuation