The bedrock underlying the site is made up of schists, gneiss, and quartzite belonging to the Precambrian to Lower Paleozoic age Blackstone Series. These rocks are exposed in outcrops over an area extending from 1.5 miles northwest of the site to the southern side of Woonsocket Hill, approximately 2 miles to the south. on-site drilling and geophysical work indicated that: the bedrock surface is irregular; the orientation of joints and fractures appear to be generally northeast-southwest and northwest-southeast; the fractures generally dip between 15 and 35 degrees and are parallel to the foliation planes in the rock. These discontinuities in the rock are important because they are the principal areas where groundwater is stored and transmitted. Natural overburden soils encountered on the site consist of thin glacial till, stratified ice contact deposits and local recent fluvial deposits. Glacial deposits found are generally thin, with relatively dense till deposited as a mantle overlying bedrock. Surficial soils have been significantly altered in the course of excavations and construction of structures at the site. The overburden materials vary in thickness from 0 to 20 feet.
Groundwater migrating beneath the site occurs predominantly in the bedrock aquifer and to a lesser extent in the lower few feet of the overburden.
Additional data, indicates that locally across the site the upper 15 feet of bedrock is significantly fractured providing available openings for groundwater flow while below this depth the bedrock exhibits a much tighter structure limiting the groundwater flow. Groundwater elevations indicate that hydraulic gradients at the site are further effected by the local surface hydrology, specifically the forestdale pond which borders the western section of the site.
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
TCE was detected to at least 175 feet bgs.
Major Contaminants and Maximum Concentrations:
- Trichloroethene (850,000 µg/L)
- 1,2-Dichloroethene (31,000 µg/L)
- Toluene (16 µg/L)
- 1,1-Dichloroethene (36 µg/L)
- 1,2,4-Trichlorobenzene (300 µg/L)
- 1,2-Dichlorobenzene (110 µg/L)
- 1,4-Dichlorobenzene (130 µg/L)
- Coring
Comments:
Geophysics was used but none were called out specifically.
- Pump and Treat
- Multi Phase Extraction
Comments:
The GWE system was started in May 2000 and has remained operational since then. Pumping is conducted in cycles, allowing groundwater to recharge almost completely before the pumps restart, and promoting a flushing effect for the most effective practical mass removal. While this mass is being removed in the shallow source zone, deep groundwater is being recovered from well B-3, setting up a hydraulic gradient toward the Site in the aqueous plume, which historically moved offsite during FWAW pumping.
Approximately 44 MG have been treated by the system since startup.
The cleanup levels were MCLs for those chemicals having them.
The overburden aquifer has a multi-phase extraction system. Data show order-of-magnitude decreases in TCE concentrations in several MPE wells since the drop-tube modifications were implemented in 2003. The majority of the remaining wells have maintained stable concentrations (within the same order-of-magnitude) over the monitoring period with several having concentrations greater than 1,000 ug/L TCE and as high as 49,000 ug/L (March 2010).
The bedrock pumping wells show veriable concentrations by quarter. The highest level of contamination in the first quarter of 2010 in well B-3 (150 ft deep with pump set at 144 ft) was 6,700 ug/L TCE. The historic high for this well was 9,200 ug/L.This well is designed to draw water from off-site. While variable the three bedrock extraction wells can have concentration of TCE greater than 1,000 ug/L.
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