This study provides a twenty-two-year record of in situ degradation of chlorinated organic compounds by a granular iron permeable reactive barrier (PRB). Groundwater concentrations of trichloroethene (TCE) entering the PRB were as high as 10,670 μg/L. Treatment efficiency ranged from 81 to >99% and TCE concentrations from <1 μg/L to 165 μg/L were detected within and hydraulically down-gradient of the PRB. After 18 years, effluent TCE concentrations were above the maximum contaminant level (MCL) along segments of the PRB exhibiting upward trending influent TCE. Degradation products included cis-dichloroethene (cis-DCE), vinyl chloride (VC), ethene, ethane, >C4 compounds, and possibly CO 2(aq) and methane. Abiotic patterns of TCE degradation were indicated by compound-specific stable isotope data and the distribution of degradation products. δ 13 C values of methane within and down-gradient of the PRB varied widely from −94‰ to −16‰; these values cover most of the isotopic range encountered in natural methanogenic systems. Methanogenesis is a sink for inorganic carbon in zero-valent iron PRBs that competes with carbonate mineralization and this process is important for understanding porespace clogging and longevity of iron-based PRBs. The carbon isotope signatures of methane and inorganic carbon were consistent with open-system behavior and 22% molar conversion of CO 2(aq) to methane.

The Mineral Trap, or Min‐Trap™, is a monitoring well‐based sampler designed to collect direct physical evidence of reactive mineral formation in situ without collecting soil or rock core samples. The Min‐Trap consists of a nonreactive granular medium (e.g., silica sand) within water‐permeable mesh pillows that are supported inside a slotted polyvinyl chloride housing that is incubated within a conventional monitoring well. The primary objective of the Min‐Trap in this application is to collect reactive minerals that are forming in the aquifer in a retrievable format that can be submitted for laboratory analysis. To evaluate the capability of Min‐Traps to capture reactive iron minerals, both a laboratory tank test and a field test were conducted. Both tests confirmed that iron sulfide minerals form in the Min‐Trap under sulfate reducing conditions within several weeks. Analysis of the precipitated minerals via the AMIBA analysis suite showed that they almost entirely consisted of weak acid soluble (biogenic, microcrystalline) ferrous iron‐based minerals, and at least two thirds of the sulfur‐containing minerals were monosulfides (i.e., mackinawite) at the end of each test. Scanning electron microscopy confirmed the colocation of iron and sulfur in the mineral masses. The dominance of the ferrous iron and reduced sulfur verifies that little to no oxidation of the captured minerals occurred between sample collection and analysis. A subsurface soil core was collected during the field test next to the Min‐Trap‐containing well. AMIBA results were consistent between the native soil and the Min‐Trap except for much higher strong acid soluble (crystalline) ferric iron in the native soil when compared to the silica sand of the Min‐Trap, as expected. This work shows that Min‐Traps are useful for documenting the formation of reactive iron sulfides (FeSx) that can form during in situ anaerobic biostimulation and can drive complementary abiotic treatment of chlorinated volatile organic compounds. Mineralogical data obtained from Min‐Traps can be applied to assess remedial objectives at several stages of the remedial program, including initial characterization, alternatives evaluation, feasibility testing, remedy optimization, and transition from active treatment to passive remedial methods.

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Important reactive minerals are commonly created during in situ groundwater remediation activities; for example, iron sulfides formed during enhanced reduction approaches can abiotically degrade many chlorinated solvents. However, cost‐effective tools to evaluate these treatment processes in field applications are limited and the collection of samples to evaluate in situ mineral formation is costly due to drilling requirements. The new passive Min‐Trap sampler is a simple and cost‐effective tool that can directly measure the formation of reactive minerals in situ without the need for additional drilling or soil core collection. The methods presented here describe how Min‐Traps deployed in conventional monitoring wells can measure reactive minerals and how these minerals can be identified through commercially available analytical methods. Several examples are presented that show how Min‐Traps can be used to characterize the rate and spatial variability of reactive mineral precipitation and these data may support operation and optimization decisions.