Preliminary observations on bacterial transport in a coastal plain aquifer

A multidisciplinary research team, funded by the U.S. Department of Energy (DOE) Subsurface Science Program, initiated a field-scale bacterial transport study in a sandy aquifer on the coastal plain of Virginia in 1994. The purpose of the study was to evaluate the relative importance of hydrogeological and geochemical heterogeneity in controlling bacterial transport. Extensive geophysical and geochemical characterization of the site was accomplished using intact cores obtained during the construction of the flow field and in a nearby sand pit exposure of the sedimentary facies found in the flow field. Geophysical techniques, including ground penetrating radar and cross borehole tomography, were used to relate the depositional environment of the sand pit to the flow field as well as to produce a 3-dimensional depiction of the flow field to be used in modeling the site and the results of the injection experiments. The 30 m long flow cell consists of ground water production and injection wells, a tracer injection well, and 10 multilevel samplers screened every half meter from 4.0 to 10.5 m below ground surface, The organization that owns the field site required that only native microorganisms be introduced at the site, therefore, the injected bacterial strain was isolated from the indigenous community in the aquifer. Candidate strains were selected by a protocol that enriched for phenotypes of low adhesion and non-clinical antibiotic resistance which could be used to detect the organism on selective media. The bacteria were selected for low adhesion to site sediments so that they might be readily transported through the aquifer. For the field injection experiment detection and quantitation of the strain chosen by this screening process, PL2W31, was accomplished by isotopically enriching the cells with [C-13]glucose. Forced gradient conservative (Br-) tracer tests were performed immediately prior to the bacterial injection experiment to provide a measure of non-reactive transport through the aquifer. The non-reactive tracer test indicated the presence of hydrogeological heterogeneities at the site that caused differential breakthrough of the tracer. Results from the bacterial transport experiment indicate that bacteria traveled the length of the Row field (4 m), but that the majority of the biomass injected was retained in the sediments between the injection well and the first multilevel sampler at 0.5 m. Preliminary bacterial transport models indicate that the observed behavior could be accounted for by the presence of two subpopulations within a single bacterial strain with differing transport properties.