Many in situ groundwater treatment technologies for chlorinated solvents, as well as many other chemicals of concern, rely upon the formation, dissolution, and/or transformation of target solid-phase minerals, either to degrade or sequester groundwater chemicals of concern. However, evaluation of these solid phase minerals and/or processes is often only inferred from aqueous phase conditions (i.e., groundwater sampling) because of the significant challenges and costs associated with the collection of solid-phase samples. Traditional approaches for the collection of solid-phase samples include the use of high cost drilling/coring techniques, and sub-sampling of discrete zones within the core material for analysis. In addition to the high costs and health and safety risks, drilling/soil sampling examines discrete points/depths within the subsurface, often requiring a relatively large sample number to characterize the subsurface area of interest and account for the high heterogeneity observed across very small scales at most sites.

The objective of this project was to field-validate the Min-Trap® technology, a new in situ monitoring tool for collecting mineralogical data to evaluate and manage in situ groundwater remediation programs. The Min-Trap® is an adaptable tool that may be integrated into a broad range of remediation programs in diverse geologic environments.

The Min-Trap is a sampling device consisting of a solid medium (e.g., silica sand, iron oxide sand, site soil) contained within a water permeable mesh that is deployed inside a monitoring well and allowed to incubate over time. The media serves as a carrier substrate upon which target minerals can form passively. Analysis of the medium through chemical, microscopic, or spectroscopic means gives direct evidence of the formation of target minerals in situ. The degradation of chlorinated volatile organic compounds via the reducing power stored in reactive minerals (e.g., iron sulfides; FeS_x) is recognized as a very important process, and cost-effective tools to support field applications are needed.

The demonstration occurred at Sites SS003 and SD015 at Vandenberg Space Force Base in Central California. Both sites are undergoing active remediation via enhanced reductive dechlorination and Min-Traps® were deployed in multiple wells, many where reactive FeS_x minerals were expected to be actively forming. Min-Trap® samples recovered after various incubation times ranging from 2 to 9 months were analyzed by a variety of analytical techniques, including total iron, Aqueous and Mineral Intrinsic Bioremediation Assessment, Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy, ¹⁴C-assay techniques, and microbiological characterization via QuantArray-Microbially Influenced Corrosion. In summary, Min-Traps® samples were utilized to conclusively verify the formation of FeS_x, measure reactivity of these minerals, and verify the presence of target microbiological communities.

The success criteria for nine quantitative performance objectives were achieved which support the following summary statements:

1. The design allows results in representative groundwater flow rates through the Min-Trap®.
2. The Min-Trap® results correspond with expected results based on geochemical conditions.
3. The minimum deployment time for Min-Traps® was determined to be two months.
4. Min-Trap® samples are stable for at least 2 weeks.
5. The data from Min-Traps® and traditional soil core collection methods are comparable.
6. The spatial variability in Min-Trap® results correspond with expected results based on geochemical conditions.
7. Min-Traps® are applicable for moderate-to-high flux unconsolidated geologic settings.
8. Targeted microbial community characterization can be performed from Min-Trap® samples.
9. Abiotic reaction rates can be measured from Min-Trap® samples.

Costs to utilize Min-Traps® are scalable and highly predictable and it is anticipated that Min-Trap® sampling costs will be relatively small compared to overall performance monitoring costs. For example, Min-Traps® are commercially available for about $300 and laboratory analyses similar to those used in this study cost between about and $1,000 and $4,000 (depending on the specific methods selected), and deployment and retrieval costs are approximately $1,000 to $2,000 per sampled location. Min-Traps® offer the potential for significant overall lifecycle cost savings because they eliminate the need for expensive drilling to collect solid-phase samples from soil cores. Because they can provide remedy performance data that can be used for assessment, they can aid in optimization decisions and improved remedy performance, and therefore ultimately support earlier transition from active to passive remedy phases.

No significant implementation challenges or limitations were identified and Min-Traps® are commercially available from Microbial Insights, Inc. Alternate support media types, including polymer beads, may be considered in future applications and may permit the use of x-ray diffraction techniques. Based on the recent work, it is recommended that samples be stored at temperatures no colder than -18 degrees Celsius. (Project Completion - 2023)

Divine, C., S. Justicia‐León, J.M. Tilton, D. Liles, E. Carter, E. Zardouzian, K. Clark, D. Taggart, D. Taggart, D. Freedman, S. LaRaia, F. Perrell, and K. Gerber. 2023. Min-Trap® Samplers to Passively Monitor In-Situ Iron Sulfide Mineral Formation for Chlorinated Solvent Treatment. Groundwater Monitoring and Remediation. doi.org/10.1111/gwmr.12595.

Divine, C., S. Justicia‐León, J.M. Tilton, E. Carter, E. Zardouzian, K. Clark, and D. Taggart. 2023. Field Methods and Example Applications for the Min‐Trap® Mineral Sampler. Remediation, 33:209–216. doi.org/10.1002/rem.21752.

Horst, J., C. Divine, J. Tilton, S. Ulrich, and S. Justicia-Leone. 2019. New Tools for Assessing Reactive Mineral-Mediated Abiotic Contaminant Transformation. Groundwater Monitoring and Remediation, 39(2): 12-21. doi.org/10.1111/gwmr.12326.

Ulrich, S., J. Martin Tilton, S. Justicia‐Leon, D. Liles, R. Prigge, E. Carter, C. Divine, D. Taggart, and K. Clark. 2021. Laboratory and Initial Field Testing of the Min‐Trap™ for Tracking Reactive Iron Sulfide Mineral Formation During in situ Remediation. Remediation Journal., 31(3): 35-48. doi.org/10.1002/rem.21681

Patents

Ulrich, S., J. Tilton, J. Ford, D. Liles, C. Divine, S. Justicia-Leone, and J. Gillo. 2021. In-Situ Device for Collecting Minerals. US Patent No US 11002643 B1.