DNAPL Removal from Fractured Rock Using Thermal Conductive Heating

ER-200715

Objective

Most of the technologies used to treat dense nonaqueous phase liquids (DNAPL) are not applicable in fractured materials. Unless treatment removes mass from the fractured rock matrix, back-diffusion of contaminants can continue for hundreds of years following treatment. The objective of this project is to demonstrate and validate a DNAPL remediation technique for fractured bedrock sites and to provide Department of Defense (DoD) remedial project managers (RPM) and site owners guidance on (1) When does it make sense to attempt aggressive remediation? and (2) What type of performance is to be expected from an aggressive technology application such as thermal conductive heating (TCH)? Such guidance will help practitioners avoid misperceptions regarding what is attainable in terms of mass removal, reduction of aqueous phase contaminant flux, reduction of aqueous phase concentrations, and reduction in source zone lifespan.

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Technology Description

TCH involves the placement of heater wells that have the capacity of operating at temperatures as high as 800ºC, thereby raising the temperature of the surrounding rock through conductive heating. TCH is uniquely poised for fractured bedrock remediation because of its ability to uniformly distribute heat, insensitivity to heterogeneity and electrical conductivity, insensitivity to the type of soil or rock being heated, and versatility in reaching and maintaining uniform temperatures. In this project, investigators will conduct a treatability study to calculate removal rates and establish necessary treatment temperatures and duration in the field, both of which are necessary for successful transferability to other sites. The field demonstration will validate a heating strategy, achievable heating rates and fluid control, and matrix heating and desaturation. The microbial characterization evaluation at the site before and after TCH application will focus on changes in the population capable of reductive dechlorination. Technical guidance for TCH application in fractured rock settings also will be developed.

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Benefits

A DNAPL source zone survey conducted by the Navy illustrated that 29% of the cases submitted were fractured media sites, and the average site had more than 100,000 cubic feet of impacted material. It is estimated that DoD owns 870 DNAPL source zone fractured bedrock sites and is responsible for the remediation of 3.22 million cubic yards of fractured bedrock material. Implementing TCH could potentially lead to significant cost savings over chemical oxidation and surfactant enhanced removal. Furthermore, since effective bedrock remediation is dependent on removing contaminants from the rock matrix, TCH offers distinct advantages over fluid flushing technologies (e.g., oxidant flushing and surfactant flushing) and other thermal technologies that are adversely affected by geological heterogeneity. (Anticipated Project Completion - 2011)

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Interim Results

Treatability Tests

Laboratory treatability tests are being performed on rock samples (red, black, and grey mudstone) from the Naval Air Warfare Center (NAWC) field site in West Trenton, New Jersey, and on siltstone, limestone, sandstone, and dolostone samples from other sites. Physical characterization of all the rock has been completed. This includes measurement of dry bulk density, fraction of organic carbon, matrix porosity, and intrinsic permeability. For the NAWC samples, large variations in these parameters were observed between the different rock colors and depths. The testing confirmed that the selected rocks represent the range of fractured rock sites most likely to be of interest.

Thermal treatability testing involves the contamination of the rock samples followed by thermal treatment at different temperatures and durations. These tests showed very large reductions in rock trichloroethene (TCE) concentrations when heated to 105oC, with better results the longer the heating took place. Matrix concentrations as high as 500 mg/kg were reduced to levels near 1-5 mg/kg in 72 hours of heating. The thermal testing indicates that TCE removal can be very effective in rock which is heated and treated at temperatures at or near the boiling point of water.

Field Demonstration

The application of TCH at the NAWC site is complete. The field demonstration involved installing 15 heater borings, each equipped with a vapor extraction screen, to 55 ft below grade. Energy was applied to the 15 borings between 5 and 55 ft below grade. The heating system operated for a total of 98 days, delivering a total of 493,000 kWh of electrical energy to the subsurface. In general, all zones from 35 ft and above reached temperatures in the range of 210-230oF (99-110oC), consistent with in-situ boiling temperatures of groundwater. At depths of 40-50 ft, the temperatures were below the boiling point of water. Extraction of fluids was accomplished using vacuum applied to the screens. This resulted in removal of air, steam, and water from the recovery points, under vacuum. It was estimated that approximately 500 lbs of volatile organic compounds (VOCs) were removed in the vapor stream, based on observed PID readings and flow rates. Additionally, it was estimated that approximately 30 lbs of VOCs were removed in the liquid stream, based on laboratory data and flow rates, for a total estimated mass removal of 530 lbs of VOCs.

The temperature monitoring and water and energy balances indicated that unexpectedly high groundwater flows acted to slow and dampen the steam generation and removal in the subsurface, thereby slowing the process in comparison to full-scale TCH projects. Rock concentrations of TCE and other VOCs were collected in three boreholes prior to heating and in adjacent boreholes one week after the heating ended. The data show that most rock concentrations were lowered to around 0-5 mg/kg, but that higher concentrations were maintained at distinct depth intervals (thought to be distinct fracture zones). Substantial flow of contaminated groundwater occurred in these distinct fracture zones during thermal operations, thus leading to cooling and incomplete heating in these fracture zones. Relatively good heating and remediation occurred in the larger matrix blocks where concentrations in thick zones without evident fractures were reduced substantially to levels below 5 mg/kg. The mass reduction in rock indicated by the data at the site reflects a concentration reduction of 69-84%.

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Points of Contact

Principal Investigator

Ms. Carmen Lebr�n

Naval Facilities Engineering Command Engineering Service Center

Phone: 805-982-2907

Fax: 805-982-4304

Program Manager

Environmental Restoration

SERDP and ESTCP

Document Types

  • Fact Sheet - Brief project summary with links to related documents and points of contact.
  • Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
  • Cost & Performance Report - Overview of ESTCP demonstration activities, results, and conclusions, standardized to facilitate implementation decisions.
  • Technical Report - Additional interim reports, laboratory reports, demonstration reports, and technology survey reports.
  • Guidance - Instructional information on technical topics such as protocols and user’s guides.
  • Workshop Report - Summary of workshop discussion and findings.
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