Starting at the ground surface,soils consist of locally derived fill, underlain by fluvio-glacial deposits including a glacial till, a thinly bedded silty fine sand, lower glacial till and a weathered shaly-siltstone of the Waterville Formation. Soil thickness ranges from 17.7 to 41.5 feet in the immediate area of the Mill. Underlying the heavily weathered bedrock is moderately competent to competent fractured bedrock. Areal recharge from infiltrating precipitation
within upland areas replenishes upland bedrock groundwater, which then flows downward and laterally toward the river valley, where ultimately it discharges to overburden groundwater and surface water. Based on numerical simulation
results, most of the areal recharge falling between the groundwater divides, represented by the hill tops, eventually discharges to the East Branch of the Sebasticook River or is removed by residential wells. Very little of this recharge contributes to lateral flow moving out of the local
hydrologic system through the overburden
and bedrock aquifers. Bedrock groundwater moves through a network of fractures composed of eastward trending bedding plane and axial plane fractures and north to northwesterly trending
joints. As bedrock groundwater approaches the river channel and begins to pass underneath, vertical head differentials rapidly turn upwards.
The vertical extent of capture appears to be the entire active thickness of the fractured bedrock aquifer.
Targeted Environmental Media:
 Dense Non-aqueous Phase Liquids (DNAPLs) - Fractured Bedrock
A plume with high concentrations of chlorinated benzene compounds exists in
bedrock groundwater to a depth in excess of 300 feet bgs. The bedrock plume extended 1,200 feet laterally along a southwestnortheast axis and approximately 400 feet downgradient from the source area. DNAPL is present in the bedrock.
Major Contaminants and Maximum Concentrations:
- Benzene (40 µg/L)
- Chlorobenzene (19,000 µg/L)
- 1,2-Dichlorobenzene (6,000 µg/L)
- 1,3-Dichlorobenzene (380 µg/L)
- 1,4-Dichlorobenzene (2,900 µg/L)
- 1,2,4-Trichlorobenzene (7,400 µg/L)
- Pumping Tests
- Tracer (dye) Test
- Chemical Oxidation (In Situ)
- Other (Ex-situ Thermal Persulfate and Chelated Iron)
Comments:
2001 relocate the East Branch of the Sebasticook River to allow for DNAPL recovery in sediments of old channel.
2002 to October 2003 ex situ thermal desorption with steam of 75,000 cubic yards of contaminated soil.
2005 to 2007 Deep soil and bedrock treatment with sodium persulfate
Injection of persulfate solution continued through 2009.
Turning off local groundwater pumping wells helped in arresting the spread of the plume.
MCLs: Benzene (5 ug/L), Chlorobenzene (100 ug/L), 1,2-dichlorobenzene (600 ug/L), 1,4-dichlorobenzene (75 ug/L), 1,2,4-Trichlorobenzene (70 ug/L)
The plume has shrunk 40%.
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