The site is underlain by up to 8.5 feet of fine-grained soils and highly weathered to decomposed saprolitic soils overlying weathered and fractured schist and phyllitic bedrock. The saprolitic metamorphic bedrock contains subvertical fractures and foliation and subhorizontal cross cutting fractures. The strike of the foliation is southwest to northeast and dips 75 to 80 degrees northwest. Ground water flow primarily occurs in the shallow fractured saprolitic aquifer to a depth of over 30 ft. bgs with lesser amounts of flow occuring in the deeper fractured competent bedrock. The decreased ground water flow with increasing depth is caused by fracture apertures that are closed or in-filled with clay. As a result both the fractures and the rock matrix have low hydraulic conductivities.
Targeted Environmental Media:
 Fractured Bedrock
Primarily dissolved PCE with lesser concentrations of degradation by-products. Contaminant transport was determined to be between 0.25 and 0.75 ft/d. Prior to remediation, dissolved phase PCE ranged from the low ug/L to 3.7mg/L in December 1998 and March 1999. The original PCE release entered the soil and subsequently the saprolitic rock matrix via the secondary porosity of the foliation and fracture network and then, over time became sorbed (absorbed and adsorbed ) into the primary porosity of the rock matrix. Thus, in order for the remediation to be successful both the primary and secondary porosity of the matrix had to be addressed.
Major Contaminants and Maximum Concentrations:
- Tetrachloroethene (370 µg/L)
No technologies selected.
- Chemical Oxidation (In Situ)
- Fenton's Reagent
- Permanganate
- Soil Vapor Extraction
- In Unconsolidated Overburden
- In Fractured Bedrock Vadose Zone
- Bioremediation (In Situ)
- Reductive Dechlorination (In Situ Bioremediation)
Comments:
In-situ chemical oxidation using Fenton's reagent was selected because the clean-up period was thought to be very rapid, if successful. This option was successfully demonstrated during a bench scale test and then was further evaluated during a 10-day field scale pilot test where the oxidation results looked promising. A full scale oxidation program involving the use of ten stainless steel treatment wells was initiated.
In the October 2000 sampling data rebound concentrations were observed. Sodium permanganate was selected to address the remaining PCE concentrations. Sodium permanganate was selected over potassium permanganate because it could be delivered to the aquifer at a higher concentration.
The second phase of the remediation program involved injecting a 20% solution of liquid sodium permanganate with a specific gravity between 1.15 and 1.2. The injection of this high density solution (compared to water)would invade the secondary porosity of the fractured aquifer network and potentially follow similar flow paths as occured during the original solvent release into the aquifer. A total of 11,000 lbs of sodium permanganate (4,400 lbs. of sodium)was injected into the aquifer through the use of direct push technology.
The risk base cleanup goal was 253 ug/L of PCE.
February 2004
Based on the NaMnO4 injection pattern, the flow pattern, and the residual NaMnO4 observed in the ground-water samples, a conservative estimate of the contaminant mass removed was greater than 95%between October 2000 and November 2001. Because PCE and other VOCs were absent from most of the wells, the state of Maryland no longer required continued treatments using permanganate. The site then entered a two-year monitoring program, during which time a biological treatment program was implemented. In the past year-and-a-half approximately 5300 lbs of sugar was injected into the aquifer. It was during this period of time, between March 2002 and December 2002 that a secondary source of PCE was identified. This source is currently being addressed through soil vapor extraction in the vadose zone and by anaerobic bioremediation.
Altough Fenton's Reagent likely oxidized PCE within the fractures, it is also possible that the rapid injection and pressure waves could have displaced PCE from the injection location before the oxidation could take place. More-over, successive FR treatments would have continued to treat the same highly permeable fractures since these were the paths of least resistance.
Given the short lifespan of the hydroxyl radical (on the order of seconds), it is unlikely that FR persisted within the fractured network long enough to degrade PCE as it diffused from the matrix. In addition to being highly ineffective, the FR treatment apparently mobilized the contaminants. PCE that had previously been present in the aquifer matrix or sorbed to soil and rock was released into the ground water due to the heat and pressure generated.
The sodium permanganate treatment was more successful than FR despite the fractured dominated flow system. The high concentrations of the permanganate persisted within the fractured network for long periods (between 10 and 19 months). Any PCE that diffused from the matrix into the fractures was redily oxidaized by residual permanganate.
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