Western Boundary

The Western Boundary Study Area consists of two distinct geographic sub regions: 1) the Monument Beach Well Field located in the town of Bourne and 2) the southwestern portion of Camp Edwards. The portion of the study area on MMR includes military troop training areas, gun and mortar firing positions, observation points overlooking the Impact Area, and the Former D Range, which was used as an anti-aircraft miniature rifle range.

In 1999, five monitoring well clusters were installed upgradient of the Bourne Water District Monument Beach well field to evaluate groundwater quality at the MMR base boundary. Initial perchlorate sampling of these boundary wells was conducted in August 2000 and all results were non-detect.

Perchlorate was first detected in a monitoring well at the base boundary in August 2001 at a concentration of 1.7 ppb. The EPA Health Advisory is 15 ppb and the MA Maximum Contaminant Level is 2 ppb.

In response to the perchlorate detection at the base boundary, routine sampling for perchlorate at the water-supply wells in the Monument Beach Well Field and nearby monitoring wells was implemented. Sampling results received indicated the presence of perchlorate at low concentrations (i.e., less than 1 ppb) in groundwater samples collected from within the well field. The maximum historic perchlorate detection in groundwater is 2.89 ppb. In response to these detections, an investigation was initiated to characterize the extent of perchlorate in groundwater in the Western Boundary and to identify potential source areas for these detections.

The most likely source of perchlorate is the deposition on soil of unburned particles from the use of perchlorate containing pyrotechnics. The contaminant concentrations in groundwater are relatively low, and relatively uniform for those locations that have detections. The location of detections is somewhat sporadic. Perchlorate concentrations appear to be declining in groundwater and this is consistent with the end of perchlorate-containing pyrotechnic use at MMR in 1997.

Currently there are no wells in the Western Boundary with perchlorate concentrations above the MMCL of 2 ppb. While the model predicts that the plume would have attenuated below 2.0 ppb by now, it is possible that this small plume has moved beyond MW-233M3 but not yet reached the nearest downgradient well MW-268M1 (see figure). Therefore, potential response actions to address this plume have been evaluated in the Remedial Investigation/Feasibility Study report.

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