Hg-Contaminated Industrial and Mining Sites in N. America
107
4
Summary and Conclusions
Investigation and remediation of Hg-contaminated sites in North America has
been a continuing and iterative process in spite of the fact that many sites have
been known for decades. Among numerous reasons for these delays in
restoration, two figure prominently: (1) the unique properties of Hg, which have
complicated identification of appropriate restorative efforts, and (2) the slow
evolution of environmental regulations, which has led to only recent reconsideration of relative risks. Presently, many sites in North America are under
renewed examination to determine the need for and nature of any corrective
action. Information available to the authors from these sites suggests the
following general conclusions.
1. The occurrence and environmental significance of small continuing releases of
mercury from facilities no longer using mercury, or long closed, has been
either not considered or underestimated. Major sites in North America which
illustrate this issue include Onondaga Lake, Lavaca Bay, and Clear Lake.
Contaminated sediments in all three of these systems were initially considered
to be the root cause of slow recoveries. While sediments have not been
eliminated as significant sources at these sites, they now at least share the
spotlight with continuing releases from the facilities of mercury origin.
2. Partitioning of mercury between dissolved and particulate phases (e.g., in
groundwater) can be radically altered in industrial and mining settings
resulting in enhanced mobility. Chlorinated effluents (e.g., cooling water) and
highly alkaline leachates (spills of caustic, mining calcine) can effectively
reverse the strong tendency of mercury to affiliate with solids (e.g., clay
minerals, organic matter) and lead to greater relative solubility and
unexpected environmental mobility.
3. Mercury concentrations in biota (especially fish, crustacea, and shellfish) have
been observed in some cases to decrease rapidly following reductions in
waterborne releases of mercury from point sources. Relationships between
concentrations of waterborne inorganic mercury and bioaccumulation are
poorly understood at contaminated sites, but appear to be highly site-specific
in nature. Further reductions may be limited by continuing inputs from
facilities as discussed above, by residual contamination which persists in
sediments, or by water-quality conditions which are especially favorable to
production of methylmercury even when sediment concentrations are
approaching background.
4. Spatial and temporal patterns in mercury in biota and environmental media
often reveal unexpected or enigmatic trends: apparent temporal increases in
mercury in fish as general water quality improves (Oak Ridge, Onondaga Lake,
Saltville), lower methylmercury concentrations and lower fractions of total Hg
to metlIylmercury near point sources but increasing downstream (Oak Ridge,
Wabigoon River).
107
4
Summary and Conclusions
Investigation and remediation of Hg-contaminated sites in North America has
been a continuing and iterative process in spite of the fact that many sites have
been known for decades. Among numerous reasons for these delays in
restoration, two figure prominently: (1) the unique properties of Hg, which have
complicated identification of appropriate restorative efforts, and (2) the slow
evolution of environmental regulations, which has led to only recent reconsideration of relative risks. Presently, many sites in North America are under
renewed examination to determine the need for and nature of any corrective
action. Information available to the authors from these sites suggests the
following general conclusions.
1. The occurrence and environmental significance of small continuing releases of
mercury from facilities no longer using mercury, or long closed, has been
either not considered or underestimated. Major sites in North America which
illustrate this issue include Onondaga Lake, Lavaca Bay, and Clear Lake.
Contaminated sediments in all three of these systems were initially considered
to be the root cause of slow recoveries. While sediments have not been
eliminated as significant sources at these sites, they now at least share the
spotlight with continuing releases from the facilities of mercury origin.
2. Partitioning of mercury between dissolved and particulate phases (e.g., in
groundwater) can be radically altered in industrial and mining settings
resulting in enhanced mobility. Chlorinated effluents (e.g., cooling water) and
highly alkaline leachates (spills of caustic, mining calcine) can effectively
reverse the strong tendency of mercury to affiliate with solids (e.g., clay
minerals, organic matter) and lead to greater relative solubility and
unexpected environmental mobility.
3. Mercury concentrations in biota (especially fish, crustacea, and shellfish) have
been observed in some cases to decrease rapidly following reductions in
waterborne releases of mercury from point sources. Relationships between
concentrations of waterborne inorganic mercury and bioaccumulation are
poorly understood at contaminated sites, but appear to be highly site-specific
in nature. Further reductions may be limited by continuing inputs from
facilities as discussed above, by residual contamination which persists in
sediments, or by water-quality conditions which are especially favorable to
production of methylmercury even when sediment concentrations are
approaching background.
4. Spatial and temporal patterns in mercury in biota and environmental media
often reveal unexpected or enigmatic trends: apparent temporal increases in
mercury in fish as general water quality improves (Oak Ridge, Onondaga Lake,
Saltville), lower methylmercury concentrations and lower fractions of total Hg
to metlIylmercury near point sources but increasing downstream (Oak Ridge,
Wabigoon River).