In early November 2015, a team of scientists sampled sediment in the Narramissic River. Their goal was to measure mercury to see whether or not it had been carried by tidal waters over the Orland Dam. Mercury is a toxic element present in high concentrations in the lower Penobscot Riverand tidal tributaries. People are exposed to mercury primarily through eating contaminated fish.
Mercury
in Maine
Mercury is a concern throughout Maine, and statewide fish consumption advisories recommend limits on eating fish from inland and coastal waters.
Mercury comes from Midwestern coal-burning power plants and other industrial sources,
and travels east on prevailing winds. Mercury eventually falls to the landscape
with rain and snow and washes into lakes, wetlands, and rivers, where it is processed
by aquatic bacteria in the sediment and magnifies up the food chain into fish.
Mercury
in the Penobscot and Orland watersheds
The lower Penobscot River is contaminated by an additional,
local source of mercury: the former HoltraChem chemical factory in Orrington,
which operated from 1967 until 2000. Over this period of time, the plant discharged
(legally and illegally) 6 to 12 tons of mercury to the Penobscot River. Mercury
pollution in the Penobscot is the subject of major court-ordered studies to
assess the contamination and come up with a clean-up plan. To date, the studies
have found high concentrations of mercury in parts of the Penobscot estuary
where sediment naturally accumulates, including the mudflats from Orrington
south to Fort Point, Mendall Marsh in Frankfort, the East Channel of the
Penobscot, and the Orland River.
HoltraChem mercury is not just in the river. A unique metal
that is liquid at room temperature, mercury readily evaporates to the air. In
contrast to air pollution that comes from a smokestack, mercury emissions are diffuse
and difficult to measure. In the 1990s, the Maine Department of Environmental
Protection sampled lakes in the area downwind (east) of the HoltraChem factory and found concentrations of mercury in fish
(perch) and lake sediments to be higher than lakes outside of the area. Swetts
Pond, Fields Pond, Brewer Lake, Mud Pond, Long Pond, Thurston Pond, and
Williams Pond were predicted to receive the most mercury deposited from the
air; both Alamoosook and Toddy ponds were in the range of
HoltraChem's air emissions (Source: Courtemanch, D.L., J. Hopeck, and K. Ostrowski. 1997. Mercury
contamination in lakes downwind of HoltraChem Manufacturing Co., Appendix 5 in
Initial Evaluation & Recommendations on Mercury in Maine, Appendix A to the
Land & Water Resources Council 1997 Annual Report, Submitted to the Joint
Standing Committee on Natural Resources, January 28, 1998).
Sampling
for Mercury in Orland
In the 2015 study, the Orland Dam Committee, NOAA, and The
Nature Conservancy sought to address the following questions:
- What are
the concentrations of mercury in the Narramissic River above the Orland Village
Dam?
- Is the dam preventing mercury from spreading, or, because the river overtops the dam during monthly high tides, has contaminated sediment already entered the Narramissic?
The scientists collected surface sediment from the river
bed, a total of 36 samples from 12 sites, six sites along the channel of the
Narramissic River from the Orland Dam upstream to the Upper Falls Road Bridge, and
three sites in each of the wetlands at Duck Cove and Wight’s Brook.
They found mercury concentrations greater than regional
background levels in the surface sediments of the Narramissic River. Mercury
concentrations were greatest directly upstream of the Orland Dam (305 nanogram
per gram dry weight), and decreased to a low of 69 ng/g dw near the Upper Falls
Road Bridge. (Compared to below the dam, where concentrations in the surface
sediment average 1,000 ng/g dw and range from 375 ng/g dw to over 1,800 ng/g dw).
According to the report, “In the river, the decline in mercury
concentrations with distance from the dam was not altered by variations in
total organic carbon or sediment grain size. This finding supports the
hypothesis that the mercury in the surface sediments of the Narramissic River
came from contaminated sediments present in the Orland River that moved over or
through the Orland Dam, carried by storm surges or tidal flows, and into the
Narramissic River. If tidal flows are restored to the Narramissic it is likely
that some portion of the toxic sediments
currently in the Orland River will be carried by incoming tides into the
Narramissic. More information is needed on the range of the tidal influence
predicted for the Narramissic River before the geographic extent and the change
in mercury concentrations can be estimated.”
At the concentrations found in the surface sediments of the
Narramissic, toxicity to aquatic life is not expected, but is possible at the site
closest to the Orland Dam. Mercury in the Penobscot and Orland rivers does not
pose a threat to people who are boating, swimming, or using the water for
household purposes. The mercury is a concern for people who eat contaminated fish,
shellfish, or waterfowl from the lower Penobscot River region; for birds and
mammals that eat fish, such as osprey,
eagles, otter and mink; and marsh songbirds that eat insects with high mercury
concentrations.
Read the full report (PDF)
