Biotracer tests have been proposed as a means by which to characterize the in situ biodegradation potential for field-scale systems. In this study, field experiments were conducted at two sites to evaluate the utility of the biotracer method for characterizing the spatial variability of microbial activity. The first site is a mixed waste-contaminated surficial aquifer in Utah, and the second site is a chlorinated solvent-contaminated regional aquifer in Tucson, Arizona. Mass recovery of the biotracer decreased approximately linearly with increasing residence time for the Tucson site. Similar behavior was observed at the Utah site, except in the region adjacent to the injection zone, where percent recoveries were much lower than those predicted using a correlation determined using data collected downgradient of the injection zone. First-order biodegradation rate coefficients obtained from model calibration of the tracer data varied between 0.2 and 0.5/day for the Tucson site. For the Utah site, the values varied between 0.1 and 0.6/day downgradient of the injection wells, and between 0.7 and 2.6/day near the injection wells. Considering the large range over which biodegradation rate coefficients can vary, the rate coefficient exhibited relatively minimal spatial variability (factor of 2.5) for the Tucson site. Conversely, the spatial variability of the rate coefficient was an order of magnitude greater for the Utah site. These differences in variability are consistent with conditions associated with the respective sites. For example, the greater microbial activity observed in the vicinity of the injection wells for the Utah site is consistent with the biomass distribution determined from analysis of core samples, which shows larger bacterial cell densities for the region near the injection wells. These results illustrate the utility of biotracer tests for in situ characterization of microbial activity (e.g., biodegradation potential), including evaluation of potential spatial variability.

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[1] The impact of microbial system variability on the biodegradation and transport behavior of a model solute, salicylate, was investigated with a series of miscible displacement experiments. Four systems of increasing complexity were employed: a sterilized, well-sorted sand inoculated with a single bacterial isolate, a sterilized soil inoculated with the same isolate, and two soils, each of which contained an indigenous multiple-population community of bacteria. The experiments were conducted in replicate (three or four experiments per set) and with paired controls. The biodegradation and transport behavior of salicylate exhibited a small degree of variability among the replicates for the two inoculated systems and a relatively large degree of variability for the two indigenous systems. The greater variability observed for the two indigenous systems is attributed primarily to greater variability of microbial system properties, such as initial cell density, metabolic status, and community composition. Values for maximum specific growth rate coefficient, mean lag time, and lag time variance were determined by model calibration to the measured breakthrough curves and compared to values obtained from batch experiments. Reasonable correspondence was observed between the two sets of values for both the inoculated and indigenous systems. The maximum specific growth rate coefficient exhibited a relatively small degree of uncertainty for all four systems, whereas greater uncertainty was associated with the lag time mean and variance. The variability in calibrated parameters among each set of replicate experiments was significantly greater than the uncertainty associated with the individual experiment calibrations and the measured input parameters. These results illustrate that variability inherent to natural microbial systems can cause variability in transport behavior even under controlled laboratory conditions and concomitantly enhance the uncertainty of biokinetic parameters obtained from laboratory studies.