PRELIMINARY-OBSERVATIONS ON BACTERIAL TRANSPORT IN A COASTAL-PLAIN AQUIFER

A multidisciplinary research team, funded by the U.S. Department of En ergy (DOE) Subsurface Science Program, initiated a field-scale bacteri al transport study in a sandy aquifer on the coastal plain of Virginia in 1994. The purpose of the study was to evaluate the relative import ance of hydrogeological and geochemical heterogeneity in controlling b acterial transport. Extensive geophysical and geochemical characteriza tion of the site was accomplished using intact cores obtained during t he construction of the flow field and in a nearby sand pit exposure of the sedimentary facies found in the flow field. Geophysical technique s, including ground penetrating radar and cross borehole tomography, w ere 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 flo w field to be used in modeling the site and the results of the injecti on experiments. The 30 m long flow cell consists of ground water produ ction and injection wells, a tracer injection well, and 10 multilevel samplers screened every half meter from 4.0 to 10.5 m below ground sur face, The organization that owns the field site required that only nat ive microorganisms be introduced at the site, therefore, the injected bacterial strain was isolated from the indigenous community in the aqu ifer. Candidate strains were selected by a protocol that enriched for phenotypes of low adhesion and non-clinical antibiotic resistance whic h could be used to detect the organism on selective media. The bacteri a were selected for low adhesion to site sediments so that they might be readily transported through the aquifer. For the field injection ex periment detection and quantitation of the strain chosen by this scree ning process, PL2W31, was accomplished by isotopically enriching the c ells with [C-13]glucose. Forced gradient conservative (Br-) tracer tes ts were performed immediately prior to the bacterial injection experim ent to provide a measure of non-reactive transport through the aquifer . The non-reactive tracer test indicated the presence of hydrogeologic al heterogeneities at the site that caused differential breakthrough o f the tracer. Results from the bacterial transport experiment indicate that bacteria traveled the length of the Row field (4 m), but that th e majority of the biomass injected was retained in the sediments betwe en the injection well and the first multilevel sampler at 0.5 m. Preli minary 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.