The Watervliet Arsenal is located on the west bank of the Hudson River, a regional discharge area. The major bedrock unit consists of brown or dark grey silty sand with angular gravel. Competent bedrock is encountered at depths ranging from 15 to 20 feet below ground surface (bgs). The bedrock underlying the site is black, medium-hard laminated shale, showing some characteristics of minor metamorphism. Veins of calcite and pyrite are commonly present along fracture and bedding planes.
Ground-water in the bedrock aquifer flows along discrete, generally interconnected, fracture pathways. There is a highly transmissive fracture or series of fractures in the vicinity of Building 40 that connects several wells. The average hydraulic conductivity of the shale bedrock matrix is 1x10-7 feet per day (ft/d) indicating that advective ground-water transport in the bedrock is controlled by fractures. The average porosity of the shale is 2.3 percent, as compared to the typical range of 5 to 25 percent for sedimentary rocks. The low porosity is due to the low-grade metamorphism to which the rock has been exposed. The average matrix diffusion coefficient of the shale was 7.5x10-7 square centimeters per second (cm2/s).
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
Ground-water contaminants include chlorinated VOCs, predominantly tetrachloroethene (PCE) and cis-1,2-dichloroethene (cis-1,2-DCE), with a lesser percentage of trichloroethene (TCE) and vinyl chloride. The highest concentrations of chlorinated VOCs have been detected in the bedrock 20 to 150 feet bgs. The affected portion of the bedrock aquifer is not used as a source of potable water. To date no indications of VOCs have been discovered in the overburden materials above the bedrock.
Although fractures provide the only pathway for advective transport of ground-water and VOCs through the bedrock aquifer, the fracture porosity is several orders of magnitude less than the matrix porosity of the rock itself. This means that the capacity of the rock matrix to store VOCs is orders of magnitude greater than the storage capacity in the fractures. The difference in capacity between the fractures and the rock has caused the VOC mass to reside in the rock matrix rather than the bedrock fractures. Given these data and the lack of any surficial sources, it is presumed that the shale bedrock itself is the continuing source of the VOCs in the ground-water.
Major Contaminants and Maximum Concentrations:
- Tetrachloroethene (170,000 µg/L)
- cis-1,2-Dichloroethene (var)
- Trichloroethene (0 µg/L)
- trans-1,2-Dichloroethene (0 µg/L)
- Vinyl chloride (0 µg/L)
- Vertical Chemical Profiling
- Pumping Tests
- Coring
Comments:
The vertical VOC distributions in the rock matrix were obtained from two continuous-cored holes from which small rock samples were collected at many depths between 18 and 150 feet bgs. These samples were then analyzed to determine VOC concentrations.
The rock core data confirm the presence of high concentration ground-water at all depths in the boreholes and that the flow system is comprised of an interconnected network of many hydraulically active fractures. There data support the concept that much of the mass that initially migrated preferentially along the interconnected network presently resides as dissolved and sorbed mass in the low permeability (1x10-10 centimeters per second [cm/s]) matrix blocks.
The results of the site characterization indicate that an effective site investigation strategy in fractured shale must include characterization of both the fracture and matrix contaminant distribution.
- Chemical Oxidation (In Situ)
Comments:
In situ chemical oxidation via potassium permanganate (KMnO4) was selected as a pilot remedy for the shale because unreacted MnO4 in solution is chemically stable, allowing it to diffuse into low permeability media over time. As contaminant concentrations decrease in the fractures following KMnO4 application, the concentration gradient leads matrix contamination to diffuse out of the rock matrix into the fracture. Application of excess KMnO4 allows for diffusion of MnO4- into the matrix at the same time as contamination is diffusing out of the matrix speeding the treatment of contamination sorbed to the rock matrix.
A pilot study consisting of two phases of KMnO4 application was performed. During Phase I (March, 2002), 8,000 gallons of a 2.5 percent KMnO4 solution were injected into a highly transmissive zone. MnO4- concentrations were monitored throughout Phase I to estimate the time of arrival of KMnO4. KMnO4 was detected in each of the monitoring wells 2 days after the original injection. KMnO4 was detected in all of the wells within one hour of detection in the first well. This phase of the pilot demonstrated that the fracture network is highly interconnected and that low injection pressures were sufficient to successfully distribute the permanganate.
Phase II of the study (April-May 2002) consisted of a series of four injection events during which a total of 7,400 gallons of 2.5 percent KMnO4 solution were injected into two wells. The purpose of this Phase was to flood the contaminated areas and achieve KMnO4 diffusion into the rock matrix. The injection frequency was set at two weeks in order to maintain a KMnO4 concentration of at least 1 percent for the duration of the test.
A monitoring program was also conducted during the pilot study. Three synoptic sampling rounds were performed during which samples from all monitoring locations and depths in the pilot study area were collected and analyzed for VOCs, inorganic parameters, and C12/C13 isotopes. A baseline sampling round was performed in February 2002, prior to Phase I. Two other full sampling rounds were conducted in March 2002 and July 2002 immediately after each of the Phases. An Additional sampling event was conducted in January 2003.
The objectives of the pilot study were:
1) Evaluate whether KMnO4 could be effectively delivered and distributed through the bedrock treatment area;
2) Confirm that VOCs in the bedrock ground-water could be oxidized by the MnO4-;
3) Assess the persistence of the MnO4- in the subsurface; and
4) Estimate the degree and rate of diffusion of MnO4- into the shale bedrock matrix.
The purpose of Phase I of the pilot test was to test the delivery of KMnO4 solution in a major transmissive zone and monitor horizontal and vertical distribution in the contaminated area. The primary purpose of Phase II was to evaluate the residence time of permanganate in the bedrock fractures. The main objective of the monitoring program was to confirm that the KMnO4 being distributed in the fractured rock was in fact destroying chlorinated solvents and to assess the geochemical impacts on the system.
The March and July 2002 sampling results show that in the treatment area VOC concentrations diminished significantly, often to non-detectable levels.
Carbon isotope analyses were performed to verify that decreases in VOC concentrations were the result of chemical oxidation, not displacement or other mechanisms. Significant isotopic shifts were observed as soon as KMnO4 was first detected. These data provided evidence of chlorinated VOC destruction.
It was anticipated that after Phase II injections ceases, KMnO4 concentrations would gradually diminish and VOC concentrations would gradually rise. Rebound monitoring showed that permanganate persisted in the majority of monitoring zones throughout the monitoring period. At many monitoring stations there was a sharp decline in the KMnO4 concentration within the first 1 to 3 months of monitoring followed by a slower decline as monitoring continued. The January 2003 monitoring event showed that VOC concentrations in ground-water rebounded from nearly non-detectable to their pre-injection concentrations six months after injections were halted.
Laboratory studies and numerical modeling are being performed to enhance the understanding of the field observations. These include rock oxidant demand tests, permanganate invasion tests, and diffusion rate modeling. The results of these studies, combined with the pilot study results will be used to design a full-scale remedial system involving the injection of sodium permanganate (NaKMnO4) over a period of five years.
References:
Navon, Daria; Andrew R. Vitolins; Kenneth J. Goldstein; Beth Parker; John Cherry; Grant A. Anderson; Stephen P. Wood. 2004. In Situ Chemical Oxidation Using Permanganate for Remediation of Chlorinated VOCs in Fractured Shale. The Fourth International Remediation of Chlorinated and Recalcitrant Compounds Conference, Monterey, California. May 24-27.
Navon, Daria; Ken Goldstein; Andy Vitolins; Beth Parker; John Cherry; Grant A. Anderson; Stephen P. Wood. 2004. In Situ Chemical Oxidation Using Permanganate for Remediation of Chlorinated VOCs in Fractured Shale. The Fourth International Remediation of Chlorinated and Recalcitrant Compounds Conference, Monterey, California. May 24-27.
Parker, Beth; John Cherry; Kenneth J. Goldstein; Andrew R. Vitolins; Daria Navon; Grant A. Anderson; Stephen P. Wood. 2004. Matrix Influence on Contaminant Mass Distribution in a Fractured Shale. The Fourth International Remediation of Chlorinated and Recalcitrant Compounds Conference, Monterey, California. May 24-27.
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