Maine SeaGrant: Estuarine sediments
produce toxic methylmercury.
July 2008 / Vol. 5
Sediments can be a source of dissolved mercury and organic matter because of the activity of microorganisms.
As the deposited mercury is buried beneath new layers of sediment, organic matter in the sediment is consumed by certain types of bacteria living a few millimeters below the sediment- water interface, beyond the reach of oxygen. These bacteria get their energy through chemical reactions that involve sulfur and iron, and produce toxic methylmercury as a byproduct.
Figure 3 shows a significant production of pore-water methylmercury at a depth of approximately five centimeters, where a concentration as high as 15 ppt is observed. Methylmercury can travel up or down through the sediment.
Figure 3 13 also shows that pore-water methylmercury rapidly disappears between a depth of one and two centimeters, inhibiting the release of this toxic chemical into the water. The exact reason for this is not known. Some bacteria can actually convert methylmercury back into the less toxic form of inorganic mercury. Or, the methylmercury that is diffusing up may get adsorbed by a relatively high concentration of iron hydroxide close to the sediment-water interface.
The lack of methylmercury release into the overlying water from the studied sediments does not mean that the animals in the mud and water are not exposed to this toxic chemical.
Methylmercury maybe directly taken up by deposit-feeding and burrowing organisms, such as worms and clams, and move up the food chain.
In other experiments, we have found that semi-permanent flooding or ponding, as would happen in a salt marsh panne, can create conditions where methylmercury released to the overlying water is greatly enhanced. With sea level rise, many coastal freshwater wetlands will be flooded with seawater, and may act as "hotspots" for methylmercury release.
In mudflats, regular flooding and exposure with changing tides can create conditions that lead to rapid mercury methylation and release into the overlying water. Future research will look at mercury concentrations in marsh plants, sediments, and animals living in the mudflats.
Do mercury levels In the Penobscot Rlver estuary threaten the health of fish, wildlife, or humans?
Mercury in the Penobscot Riyer is currently under a U.S. District Court- ordered investigation by an intemational team of expert scientists.
Phase I of the study was completed and approved bythe court in March 2000. The study team sampled water, sediments, wetlands, benthic invertebrates, fish, shellfish, birds and manmals in the Penobscot Riyerand estuary in 2006 and 2007.The researchers found that mercury concentrations decreased with increasing distance from the Holtrachem site.
The most severe contamination of the Penobscot system is between Brewer on the lower river and about Fort Point or Sears Island in the upper estuary.
They also found that:
* Mercury in the sediments was 20 times more concentrated in the lower river and estuary compared to a reference area in the East Branch Penobscot River.
* Mercury attached to particles suspended in the water was two times higher downstream of the Holtrachem site.
* In some individual lobsters, levels of methymercury in claw and tail muscle exceeded the Maine DEP and U.S. EPA criteria for protection of human health.
* Mercury in mussels was high compared to other sites in Maine and the U.S.
* Mercury levels in songbirds inhabiting wetlands adjacent to the lower Penobscot River in the Frankfort Flats area were very high compared to songbirds in other parts of Maine, and high enough for possible toxic effects on the birds themselves.
* Mercury in cormorant eggs in the upper estuary approached or exceeded levels thought to impair reproduction.
The study's authors concluded,"The Penobscot River and estuary are contaminated with mercury to an extent that poses endangerment to some wildlife species and possibly some limited risk for human consumers of fish and shellfish."
Phase ll of the study will concentrate on understanding where and when mercury is produced in the system and how it is transported and bioaccumulated in the lower river and upper estuary. Data from the study will be used to evaluate the practicalityof possible mitigation measures.
in 2007 (Figures 8,9) showed patterns that were very similar to those found in 2006
(Figures 10-13).
The within site core-to-core variation of these data is typical of sediment mercury data,
and is the reason why we sampled sites repeatedly to adequately characterize mercury
concentrations. For example, for sample period II (Sept. 2006; Figure 11) both the total
Hg and methyl Hg concentrations at sites OB 1 and OB 2 were very high. These
concentrations were not seen for the other five sampling periods at these two stations,
indicating that 2 “hotspots” of mercury had been sampled during period II.
The geographic pattern of methyl Hg concentrations was very similar to the geographic
pattern seen for total Hg concentrations (Figures 8-13). There was a noticeable
increase from East Branch to Old Town – Veazie reach and a much larger increase in
the Brewer – Orrington, Orrington – Bucksport reaches. Concentrations then decreased
with distance into the estuary. This was consistent at all sampling times. Methyl Hg
concentrations were generally lower in the outer part of the estuary, especially at the
most southerly five Estuary sites. Methyl Hg concentrations showed similar levels of
variation at particular sites among sampling times as was evident for total Hg
concentrations. This is because in the Penobscot system microbial production of
methyl Hg is primarily controlled by concentrations of inorganic mercury (see
discussions below).
Also plotted on Figures 8-13 is the percent of total Hg that is methyl Hg. Several other
studies have concluded that the percent of total Hg that is methyl Hg is a good indicator
of the intensity of bacterial methyl Hg production in sediments2
. In the top 3 cm of the
sediment cores, while some sites were higher than others, there was little overall
geographic trend trough out the river and upper estuary (Figures 8-13). This lack of
trend suggests that the efficiency of methylation of inorganic mercury is quite constant,
per unit of total Hg. Efficiencies were somewhat lower in the outer estuary. This may
have been because sulfide concentrations were higher in the waters of the outer
estuary. It is well known that high sulfide concentrations reduce the production of
methyl Hg by binding the inorganic mercury making it unavailable for the mercury
methylating bacteria
Sediments can be a source of dissolved mercury and organic matter because of the activity of microorganisms.
As the deposited mercury is buried beneath new layers of sediment, organic matter in the sediment is consumed by certain types of bacteria living a few millimeters below the sediment- water interface, beyond the reach of oxygen. These bacteria get their energy through chemical reactions that involve sulfur and iron, and produce toxic methylmercury as a byproduct.
Figure 3 shows a significant production of pore-water methylmercury at a depth of approximately five centimeters, where a concentration as high as 15 ppt is observed. Methylmercury can travel up or down through the sediment.
Figure 3 13 also shows that pore-water methylmercury rapidly disappears between a depth of one and two centimeters, inhibiting the release of this toxic chemical into the water. The exact reason for this is not known. Some bacteria can actually convert methylmercury back into the less toxic form of inorganic mercury. Or, the methylmercury that is diffusing up may get adsorbed by a relatively high concentration of iron hydroxide close to the sediment-water interface.
The lack of methylmercury release into the overlying water from the studied sediments does not mean that the animals in the mud and water are not exposed to this toxic chemical.
Methylmercury maybe directly taken up by deposit-feeding and burrowing organisms, such as worms and clams, and move up the food chain.
In other experiments, we have found that semi-permanent flooding or ponding, as would happen in a salt marsh panne, can create conditions where methylmercury released to the overlying water is greatly enhanced. With sea level rise, many coastal freshwater wetlands will be flooded with seawater, and may act as "hotspots" for methylmercury release.
In mudflats, regular flooding and exposure with changing tides can create conditions that lead to rapid mercury methylation and release into the overlying water. Future research will look at mercury concentrations in marsh plants, sediments, and animals living in the mudflats.
Do mercury levels In the Penobscot Rlver estuary threaten the health of fish, wildlife, or humans?
Mercury in the Penobscot Riyer is currently under a U.S. District Court- ordered investigation by an intemational team of expert scientists.
Phase I of the study was completed and approved bythe court in March 2000. The study team sampled water, sediments, wetlands, benthic invertebrates, fish, shellfish, birds and manmals in the Penobscot Riyerand estuary in 2006 and 2007.The researchers found that mercury concentrations decreased with increasing distance from the Holtrachem site.
The most severe contamination of the Penobscot system is between Brewer on the lower river and about Fort Point or Sears Island in the upper estuary.
They also found that:
* Mercury in the sediments was 20 times more concentrated in the lower river and estuary compared to a reference area in the East Branch Penobscot River.
* Mercury attached to particles suspended in the water was two times higher downstream of the Holtrachem site.
* In some individual lobsters, levels of methymercury in claw and tail muscle exceeded the Maine DEP and U.S. EPA criteria for protection of human health.
* Mercury in mussels was high compared to other sites in Maine and the U.S.
* Mercury levels in songbirds inhabiting wetlands adjacent to the lower Penobscot River in the Frankfort Flats area were very high compared to songbirds in other parts of Maine, and high enough for possible toxic effects on the birds themselves.
* Mercury in cormorant eggs in the upper estuary approached or exceeded levels thought to impair reproduction.
The study's authors concluded,"The Penobscot River and estuary are contaminated with mercury to an extent that poses endangerment to some wildlife species and possibly some limited risk for human consumers of fish and shellfish."
Phase ll of the study will concentrate on understanding where and when mercury is produced in the system and how it is transported and bioaccumulated in the lower river and upper estuary. Data from the study will be used to evaluate the practicalityof possible mitigation measures.
in 2007 (Figures 8,9) showed patterns that were very similar to those found in 2006
(Figures 10-13).
The within site core-to-core variation of these data is typical of sediment mercury data,
and is the reason why we sampled sites repeatedly to adequately characterize mercury
concentrations. For example, for sample period II (Sept. 2006; Figure 11) both the total
Hg and methyl Hg concentrations at sites OB 1 and OB 2 were very high. These
concentrations were not seen for the other five sampling periods at these two stations,
indicating that 2 “hotspots” of mercury had been sampled during period II.
The geographic pattern of methyl Hg concentrations was very similar to the geographic
pattern seen for total Hg concentrations (Figures 8-13). There was a noticeable
increase from East Branch to Old Town – Veazie reach and a much larger increase in
the Brewer – Orrington, Orrington – Bucksport reaches. Concentrations then decreased
with distance into the estuary. This was consistent at all sampling times. Methyl Hg
concentrations were generally lower in the outer part of the estuary, especially at the
most southerly five Estuary sites. Methyl Hg concentrations showed similar levels of
variation at particular sites among sampling times as was evident for total Hg
concentrations. This is because in the Penobscot system microbial production of
methyl Hg is primarily controlled by concentrations of inorganic mercury (see
discussions below).
Also plotted on Figures 8-13 is the percent of total Hg that is methyl Hg. Several other
studies have concluded that the percent of total Hg that is methyl Hg is a good indicator
of the intensity of bacterial methyl Hg production in sediments2
. In the top 3 cm of the
sediment cores, while some sites were higher than others, there was little overall
geographic trend trough out the river and upper estuary (Figures 8-13). This lack of
trend suggests that the efficiency of methylation of inorganic mercury is quite constant,
per unit of total Hg. Efficiencies were somewhat lower in the outer estuary. This may
have been because sulfide concentrations were higher in the waters of the outer
estuary. It is well known that high sulfide concentrations reduce the production of
methyl Hg by binding the inorganic mercury making it unavailable for the mercury
methylating bacteria
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