The
sediments of most urban rivers like the Neponset can be
presumed to contain some level of contaminants as a legacy of
past pollution discharges. However, during the course of
studying fish passage alternatives, it has become apparent
that sediments in the
Lower
Neponset
River
are severely contaminated with PCBs
(polychlorinated biphenyls) in the area downstream of
Mother Brook (Hyde Park, Mattapan, Dorchester and Milton).
PCBs
in the river would be cleaned up in conjunction with any dam
removal project. In the absence of dam removal, the state is
not required to implement any cleanup activities and as of
this date has indicated no interest in pursuing a voluntary
cleanup. Combining river restoration with PCB cleanup will
help ensure that PCB cleanup is implemented relatively quickly
and in a manner sensitive to river aesthetics, recreational
uses and habitat considerations.
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.
People
should not be afraid to go boating, walking or cycling along
the Neponset
, but freshwater fish caught in the Neponset
River
between
Walpole
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
June
2008
