The overburden formation consists of a mixture of clay and silt with variable amounts of sand and gravel and overlies three separate bedrock formations. The bedrock formations overlie the Maquoketa Shale which is considered the regional aquitard (i.e., confining unit). The overburden varies in thickness from one to 90 feet with the thinner portions being in the northern
portion of the site. The Scotch Grove formation is characterized by an upper and lower unit which has been interpreted to reflect the difference between the relatively weathered and porous rock in the upper unit compared to the unweathered and dense rock below. The upper unit of the Hopkinton formation is the Picture Rock formation, which has relatively low porosity and hydraulic conductivity compared to the formations above and below. As a result, this formation
may be retarding the vertical migration of contaminants to the underlying Farmers Creek and Lower Hopkinton formations as well as the Blanding Formation.
Groundwater occurs in both the overburden and underlying bedrock formations. In general, groundwater flows from the north to the south, with an increasing hydraulic gradient in
the southwest and southeast areas near the tributaries. In the vicinity of the tributaries, the flow directions are skewed towards the tributaries, even in the lower bedrock members.
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
- Light Non-aqueous Phase Liquids (LNAPLs)
Lateral extent was approximately 3,200 feet by 1,600 feet (benzene).
Major Contaminants and Maximum Concentrations:
- Benzene (60,000 µg/L)
- Tetrachloroethene (95,000 µg/L)
- Polycyclic aromatic hydrocarbons (PAHs) (15,445 µg/L)
No technologies selected.
- Chemical Oxidation (In Situ)
- Pump and Treat
- Bioremediation (In Situ)
- Reductive Dechlorination (In Situ Bioremediation)
- Other (Monitored Natural Attenuation)
Comments:
Groundwater extraction wells were
installed in strategic areas and screened over the five geologic formations to pump contaminated
groundwater out of the ground and into common lift stations and then to the treatment plant. There are five lift stations and 51 groundwater extraction wells. The extraction wells were located at different depths and have differing flow rates (ranging anywhere from one to 20 gallons per minute). The system began operation in 1994-95.
The site has a technical impracticability finding for the source areas. For non source area groundwater the following cleanup goals were adopted in ug/l:
Benzene 1 Toluene 2000
Ethylbenzene 700
Tetrachloroethylene 0.7
Trichloroethylene 3
1,1-Dichloroethylene 7
1,2-Dichloroethylene 70
Methylene Chloride 5
1,1,2,2-Tetrachlorethane 0.2
1,1,1-Trichloroethane 200
Vinyl Chloride 0.015
Styrene 100
1,2-Dichlorobenzene 600
The pump and treat system was turned off in September 2008. The new approach is MNA with enhanced bioremediation or chemical oxidation at hotspots.
http://cfpub.epa.gov/fiveyear/index.cfm?fuseaction=fyrsearch.showSitePage&id=0700157
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