Factors Controlling In Situ Biogeochemical Transformation of Trichloroethene: Field Survey

Abstract

We have previously defined in situ biogeochemical transformation as the biogenic formation of reactive minerals that are capable of abiotically degrading chlorinated solvents such as trichloroethene without accumulation of degradation products such as vinyl chloride (AFCEE et al. ). This process has been implemented in biowalls used to intercept contaminated groundwater. Abiotic patterns of contaminant degradation were observed at Altus Air Force Base (AFB) and in an associated column study, but not at other sites including Dover AFB. These abiotic patterns were associated with biogenic formation of reactive iron sulfide minerals. Iron sulfides in the form of small individual grains, coatings on magnetite, and sulfur‐deficient pyrite framboids were observed in samples collected from the Altus AFB biowalls and one of the EPA columns. Larger iron sulfide grains coated with oxide layers were observed in samples collected from Dover AFB. Altus AFB and the EPA column differed from Dover AFB in that groundwater flow at Dover AFB was relatively slow and potentially reversing. High volumetric sulfate consumption rates, an abiotic pattern of trichloroethene (TCE) degradation, and the formation of small, high surface area iron sulfide particles were associated with relatively high rates of TCE removal via an abiotic pattern. Geochemical modeling demonstrated that iron monosulfides such as mackinawite were near saturation, and iron disulfides such as pyrite were supersaturated at all sites. This environmental condition can be supportive of nucleation of small particles rather than crystal growth leading to larger particles. When nucleation is dominant, small, high surface area, and reactive particles result. When crystal growth dominates the crystals are larger and have lower specific surface area and reactivity. These results taken together suggest that creation of a dynamic environment can promote biogeochemical transformation based on generation of reactive iron sulfides.

Efficacy of an In‐Well Sonde to Determine Magnetic Susceptibility of Aquifer Sediment

“…Currently, there is no method available to measure the content of green rusts in aquifer sediment. The content of makinawite can be estimated as acid volatile sulfide; however, Whiting et al () found that there was no relationship between the rate constant for abiotic degradation of TCE and the content of acid volatile sulfide in aquifer sediment. Evans et al () showed that FeS can be passivated in aquifer sediment and required ongoing sulfate reduction in the sediment for the FeS to be reactive to TCE.…”

New Tools for Assessing Reactive Mineral‐Mediated Abiotic Contaminant Transformation

“…Previous attempts to correlate observed degradation rates with FeS (determined as AVS in aquifer sediments) were unsuccessful (Whiting et al ). Therefore, BioPIC employs a spreadsheet calculator that uses groundwater parameter inputs (e.g., sulfate, sulfide, pH, and groundwater flow velocity) to predict the potential accumulation of FeS in the aquifer.…”

“…Other field studies have attempted to identify which mineral characteristics lead to more effective or sustained reactive iron mineral treatment of CVOCs. Whiting et al () looked at two sites that displayed different abiotic degradation behavior. One site showed a bimodal distribution of particle sizes with some small and some large pyrite grains.…”

“…The newly nucleated crystals consisted of high‐reactivity mackinawite (FeS), whereas the aging crystals consisted of lower‐reactivity pyrite. Whiting et al () suggest that sites with stagnant/low groundwater flow and reagent flux encourage Ostwald ripening, where it is energetically favorable for tiny grains to dissolve to support “ripening” of larger particles. This is speculated to cause consumption of small, high surface area grains of mackinawite and growth of established large grains of less‐reactive pyrite.…”

Thinking Outside the Boxcar: Effective and Sustainable Combined Remedies Using Single Application of Multifunctional Amendments

“…Degradation trends varied throughout the plume. The equation below, developed by Whiting et al (), was used to differentiate abiotic and biotic degradation patterns. …”