Evaluating Long-Term Impacts of Soil-Mixing Source-Zone Treatment Using Cryogenic Core Collection

Dr. Mitchell Olson | Trihydro Corporation

ER-201587

Objectives of the Demonstration

This project was designed to assess the long-term performance of a remediation technology applied to a dense non-aqueous phase liquid (DNAPL) source zone. The remediation technology evaluated, herein referred to as ZVI-Clay Soil Mixing, involves deep-soil mixing with zero valent iron (ZVI) and bentonite (Clay). The overarching goal was to assess post-remediation potential for TCE concentrations to rebound, as well as effects of remediation on natural fate-and-transport processes. To achieve this goal, high-resolution data representing both high-permeability (high-k) and low-permeability (low-k) soil strata was imperative. To satisfy this data need, cryogenic core collection (C3) was implemented. 

The project included four specific performance objectives:

  1. Supplementation of existing remediation performance data with high-resolution data for key parameters.
  2. Assessment of biogeochemical conditions in the treated source-zone.
  3. Generation of data to improve understanding of downgradient and low-k zone processes.
  4. Evaluation of core recovery and production rate achieved by cryogenic coring.

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Technology Description

The project was completed at a trichloroethylene (TCE) DNAPL source zone at Site 17, Naval Support Facility Indian Head, Maryland (Site 17). It involved two technologies: the ZVI-Clay soil mixing remediation technology (completed in 2012) and the C3 characterization technology (implemented in 2016). Although the ZVI-Clay Soil Mixing remedy was completed prior to implementation of the project described herein, description of both technologies is presented below. 

  • ZVI-Clay Soil Mixing. The ZVI-Clay Soil Mixing remediation technology involves admixing ZVI and bentonite into chlorinated-solvent source zones using large-diameter augers. The ZVI-Clay Soil Mixing technology creates a relatively homogeneous distribution of contaminants and reactants, thus overcoming challenges of geologic heterogeneity and incomplete reagent delivery that limit treatment performance of many remediation technologies. 
  • C3 characterization. This project supplemented the existing performance data via a detailed assessment of site conditions four years after remediation was implemented. C3 techniques were used to characterize the contaminants and biogeochemical conditions in the treated body and adjacent plume. The C3 technology involves freezing soil cores in situ and then conducting high-resolution analysis on the frozen cores. The C3 technique provided a natural fit for the characterization needs of this project. 

Cryogenic coring was conducted in June 2016, four years after remediation. Sampling was conducted at six locations, including two within the treated zone and four downgradient (plume) locations. Frozen soil cores were collected following procedures developed under previous work funded by Strategic Environmental Research and Development Program (SERDP; projects ER-1559 and ER-1740). Upon collection, the frozen cores were placed in a cooler on dry ice, and subsequently shipped to a laboratory via overnight delivery. 

While frozen, cores were cut into subsamples while frozen and processed for high-resolution analysis of key parameters including chlorinated ethylenes (TCE, cDCE, and VC), gaseous degradation products (methane, ethane, ethylene, and acetylene), inorganic parameters (chloride, sulfate, and iron), and soil properties (bulk density and clay content). Additional testing, including ZVI content, reactivity, and biological analysis, was conducted on select samples.

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Demonstration Results

The C3 characterization at Site 17, which was conducted under this project, supplements the existing remediation performance data with high-resolution geochemical, biological, and reactivity data. Within the treated source zone, the C3 geochemical data suggest that conditions are generally homogeneous. The highest measured TCE concentration was 0.3 mg per killigram (kg), which is four orders of magnitude lower than the highest pre-mixing TCE concentration (510 mg/kg). Gaseous product concentrations are relatively limited within the treated zone; acetylene was not detected in any sample, and ethylene was non-detect in 29 of 35 samples from within the treated interval. These observations are consistent with the low levels of chlorinated ethylenes remaining in the treated soil zone. ZVI content and reactivity testing confirmed that reactive ZVI remains present and is capable of reacting with chlorinated ethylenes. These results suggest that little contaminant mass remains stored in the source zone, and future releases (i.e., rebound) of chlorinated ethylenes from the treated-soil zone are unlikely. 

In conclusion, source zone remediation appears to have been highly effective at Site 17, and current conditions appear amenable to ongoing assimilation of TCE and related products.  Contaminant concentration rebound appears unlikely within the treated source zone, due to the low contaminant mass remaining, continued apparent reactivity of ZVI toward TCE, and lack of heterogeneity within the treated soil zone. Downgradient of the treated zone, the presence of chlorinated ethylenes in low-k zones, and assimilation processes occurring within these low-k zones, are likely to govern plume longevity. The C3 data may support future modeling efforts to evaluate the back-diffusion potential related to contaminant mass storage in low-k zones associated with the dilute plume, outside of the treated zone at Site 17.

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Implementation Issues

Although the ZVI-Clay Soil Mixing technology was completed four years prior to the ESTCP-funded portion of this project, implementation issues described in the Soil Mixing Completion Report (prepared by CH2M HILL) are presented herein. The primary issue associated with soil mixing involved buried wood, which was encountered over much of the area under Site 17; the buried wood was excavated prior to soil mixing. No other substantial implementation issues were documented in the Soil Mixing Completion Report.

The primary issue with cryogenic coring involved limited core recovery in the mixed-soil zone. The limited recovery was likely related to softness of the bentonite-mixed soils, which restricted soil entry into the core barrel. This issue was not attributed to the cryogenic coring process, i.e., the same issue would likely have occurred using conventional (i.e., unmodified) hollow-stem auger equipment. For future implementation of cryogenic coring in soft soils (possibly including sediments), additional modifications to the sampling apparatus may be required to improve recovery. Other cryogenic coring issues, which resulted in minor delays, included (a) buried wood affecting sample recovery, (b) coring equipment freezing downhole, and (c) freezing or binding of the core sample in barrel. No major changes in implementation are recommended to address these issues, as the issues were readily addressed in the field and solutions did not result in lengthy delays.

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Points of Contact

Principal Investigator

Dr. Mitchell Olson

Trihydro Corporation

Phone: 970-492-6022

Program Manager

Environmental Restoration

SERDP and ESTCP

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