Redox Processes and Release of Organic Matter after Thermal Treatment of a TCE-Contaminated Aquifer

Abstract

Redox conditions in heated and unheated microcosm experiments were studied to evaluate the effect of thermal remediation treatment on biogeochemical processes in subsurface environments. The results were compared to field-scale observations from thermal treatments of contaminated sites. Trichloroethene-contaminated aquifer material and groundwater from Ft. Lewis, WA were incubated for 200 days at ambient temperature (i.e., 10 degrees C) or heated to 100 degrees C for 10 days and cooled slowly over a period of 150 days to mimic a thermal treatment. Increases of up to 14 mM dissolved organic carbon were observed in the aqueous phase after heating. Redox conditions did generally not change during heating in the laboratory experiment, and only minor changes occurred as an effect of heat treatment in the field. The conditions were slightly manganese/iron-reducing in two sediments and possibly sulfate-reducing in the third sediment based on production of up to 0.20 mM dissolved iron and 0.15 mM dissolved manganese and consumption of 0.08 mM sulfate. The calculated energy gain of less than -20 kJ/mol H2 for iron and sulfate reduction as well as methane production indicated that these processes were thermodynamically favorable. Sulfate reduction and methane production occurred in the unheated microcosms upon lactate amendment. Little or no reduction of the redox level was identified in heated lactate-amended microcosms, possibly because of limited microbial activity. Because the redox conditions, pH, and alkalinity remained within normal aquifer levels upon heating, bioaugmentation may be feasible for stimulating anaerobic dechlorination in heated samples or in future field applications.

Identification of abiotic and biotic reductive dechlorination in a chlorinated ethene plume after thermal source remediation by means of isotopic and molecular biology tools

“…Such a release of sediment-bound organic matter due to thermal treatment has been previously reported near treated source areas (Newmark andAines, 1995, Friis et al, 2005). Friis et al confirmed via experiments performed with field material that up to 8% of sedimentbound organic carbon could be released in temperature conditions usually achieved with thermal treatments (Friis et al, 2005). Releasing organic matter can be expected to affect redox conditions.…”

“…Among the various developed remediation methods, thermal treatment by steam injection is particularly adapted for source treatment in subsurface sediments of relatively high permeability such as sandy aquifers (von Schnakenburg, 2013).This remediation strategy is known to release dissolved organic carbon (DOC) (Friis et al, 2005) the increase of which may trigger a chain of microbiallymediated redox processes. When natural attenuation has been observed prior to active source remediation, steam injection might thus influence the naturally occurring degradation.…”

Microcosm evaluation of bioaugmentation after field-scale thermal treatment of a TCE-contaminated aquifer

“…Sediment was transferred into diffusion-proof aluminum bags coated inside with Teflon (Friis et al 2005). Groundwater used for all microcosms was sampled at one location (B09) between 6.3 to 8.0 mbs using pre-sterilized and Ar-flushed serum bottles, as previously described (Friis et al 2006b).…”

“…After thermal treatments are performed, elevated temperatures and unfavorable geochemical conditions (e.g., more oxic conditions), (Friis et al 2005) which have been reached during heating could limit the survival and activity of microorganisms (Friis et al 2006b). Therefore, there is a need for investigations on the sensitivity of dechlorinating microorganisms in order to establish the optimal time and conditions for bioaugmentation.…”

“…Previous work focused on redox processes and anaerobic dechlorination in samples heated under controlled conditions in the laboratory (Friis et al 2005(Friis et al , 2007a, and field samples heated by ERH (Friis et al 2006b). In addition, the temperature optima of dechlorination were determined in the diluted mixed culture KB-1 TM (Friis et al 2007b).…”