State law requires landowners to cleanup hazardous waste in “soils” on their property. However, there is no comparable requirement to cleanup underwater sediments. This is because sediments move around, making it difficult to determine the source of the contaminants (and therefore cleanup responsibility) once they enter a river system.

The US Geological Survey has been evaluating the extent, severity and potential sources of PCB concentrations in sediments of the Neponset River. Some of these results have been published and some are still pending publication. The USGS has collected “surface grab” samples that reflect pollutants in the first few inches of sediment, as well as “core” samples that reflect concentrations in the first 36 inches of sediment. Samples were also collected from the water.

The concentration of most pollutants in sediments of the Neponset River is similar to, or lower than, the same pollutants in other urban rivers. However, PCB concentrations are much higher than “normal” urban river background levels. In fact, PCB concentrations in Neponset River sediments are up to 120 times higher than the state reporting standard for soils and are generally two to 12 times higher than the level at which a property owner would be required to immediately put up a fence around similarly contaminated soils.

PCB concentrations are highest in Hyde Park behind the T&H Dam. Concentrations are lower, but still quite high, behind the Baker Dam and in the area known as the “braided channel” between the two dams. Concentrations are also lower in the top few inches of sediment, and higher below the surface. Finally, PCBs tend to be found in fine-grained, “muddy” areas where the river is slow moving (such as behind the dams) rather than in sandy, gravelly or rocky areas where the river is faster moving. See a map of PCB concentrations in Lower Neponset River sediments.

The PCBs in the river are not safely contained at the bottom of the river or behind the dams. Rather, they are being continually stirred up during storm events and flushed down into the Neponset Estuary and Boston Harbor. During the spring of 2007 an estimated 200 to 300 lbs. of PCBs were flushed into the harbor from behind the Baker Dam. Even in the absence of storms, the PCBs are constantly diffusing into the river water at low levels.

It is unhealthy for children or adults to handle or consume these heavily contaminated sediments. People should not be afraid to go boating, walking or cycling along the Lower Neponset, but they should take sensible precautions to minimize their contact with river mud. In addition, PCBs are also found in fish tissue at three times the levels safe for human consumption. As a precaution, freshwater fish caught in the Neponset River between Norwood and Lower Mills should not be eaten.

The PCBs in the river would have to be cleaned up at the same time that any dam removal project is implemented. In the absence of a dam removal project, the state (which owns most of the river-bottom) currently has no plans to do anything to cleanup the PCBs, and the state would be under no obligation to undertake a cleanup.

There is an ongoing effort to identify the original source of PCBs in the river. If that source is associated with a property that also has PCB-contaminated soils, the owner of the property potentially could be required to cleanup the river. However there are more than 70 known PCB discharge sites in the Neponset River Watershed, and experts agree that it is unlikely that one or two property owners would be held liable for the cleanup.

Even if a private party were required to cleanup PCBs in the river, it would likely involve decades of legal wrangling, and the private party would be under no obligation to consider aesthetics, recreational uses or habitat considerations in designing their cleanup plan.

Where hazardous waste cleanups have been implemented in area waterways in the past, they have been designed using the least expensive method adequate to eliminate public health risks. In such situations, sediments are generally excavated and then the river and its banks are filled-in or covered-over with angular stone (rip-rap) or interlocking concrete pavers. After remediation, these areas are generally unattractive, devoid of habitat value and undesirable as destinations for boating or riverside recreation.

Aside from its other benefits, the Watershed Association believes that a comprehensive restoration program for the Lower Neponset River that involves dam removal, habitat restoration, recreational enhancements and PCB cleanup is the only way that PCB cleanup will be accomplished in the near term and in a manner sensitive to aesthetic, recreational and habitat concerns.

During the same 1960s-era flood control project that involved building the Baker and T&H Dams, the state dredged the river and used the dredged sediments to fill-in riparian wetlands in a number of areas. It is therefore possible that these filled-in riverbank areas may have high levels of PCBs. These bank areas would be considered soils (rather than sediments) under the state hazardous waste law, and the landowners (primarily the state) would be required to implement cleanup actions.

In December 2007, the Mass. Department of Environmental Protection was scheduled to collect soil samples for PCBs in areas where dredge spoils are believed to have been placed on the banks of the river. The preliminary results of that sampling are expected in February 2008.

Additional Sediment Quality Links

NepRWA Map of PCB levels in the Neponset

“1st” USGS Sediment Quality Study (hyperlink to come)

Under Construction: Additional information on PCBs (hyperlink to come)

January 2008