Spatial Variability of In Situ Microbial Activity: Biotracer Tests

Abstract

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.

Toward efficiency in heterogeneous multispecies reactive transport modeling: A particle‐tracking solution for first‐order network reactions

“…This typically leads to geological structures with preferential channels [Sanchez-Vila et al, 1996;Gomez-Hernandez and Wen, 1998;Trinchero et al, 2008] and low permeability areas where contaminants can be trapped and slowly released in time [Stroo et al, 2012;de Barros et al, 2013]. Moreover, degradation rates of contaminants in aquifers can vary substantially in space [Allen-King et al, 2006] due to, for instance, changes in the bacteria activity responsible for biodegradation [e.g., Fennell et al, 2001;Sandrin et al, 2004]. The effect of these different types of heterogeneities should not be considered independently.…”

Natural Attenuation of Perchlorate in Groundwater: Processes, Tools and Monitoring Techniques

“…The column design is also similar to that used by Gillham et al (1990), Borden et al (1997a), and Sandrin et al (2004). Each column typically consists of a 1-m long chamber that is pushed into the sediment surface allowing sediment and groundwater to be isolated from the surrounding aquifer for controlled observation.…”

Modified Well-Field Configurations for Improved Performance of Contaminant Elution and Tracer Tests

“…Conservative tracers can be employed as part of these tests to enhance evaluation of mass-transfer processes (Bahr, 1989; Thorbjarnarson and Mackay, 1997; Blue et al, 1998; Brusseau, 1993; Brusseau et al, 1999a, 1999b, 2007; Nelson et al, 2003). And, non-conservative tracers, such as biotracers, can be used to assess attenuation (Brusseau et al, 1999c; Sandrin et al, 2004). …”