Environmental Engineering Reference
In-Depth Information
site in Ashland, Massachusetts (Weiner and Shields, 2000).
Hg was used as a catalyst in the production of dyes and
approximately 2.3 metric tons of Hg were used per year at
the site from 1940 to 1970. The disposed chemical wastes
at the Nyanza site migrated via overland fl ow into nearby
wetlands and eventually into the Sudbury River, about 330 m
from the industrial site. The facility was closed in 1978,
and in 1982 the site was placed on the National Priorities
List. The EPA completed remediation, including excava-
tion and capping of the contaminated soils at the site, in
1991. The EPA has since removed contaminated sedi-
ments from wetlands and drainage streams near the site,
however Sudbury River sediments remain signifi cantly
contaminated with inorganic Hg for many miles down-
stream of the site.
The fate of Hg discharged at the Nyanza site has been
complex both spatially and temporally (Beckvar et al., 2000;
Frazier et al., 2000; Weiner and Shields, 2000; Waldron
et al., 2000; Haines et al., 2003). Studies in 1989-1995
revealed that Hg sediment concentrations were highest
in impoundments and slow-fl owing reaches within a few
miles downstream of the site, decreased with distance from
the source, yet remained somewhat elevated above regional
background concentrations even in Fairhaven Bay 30 km
downstream (Figure 9.8). For study purposes, the river was
divided into 10 distinct reaches, representing impound-
ments, fl owing reaches, and reaches with wide bordering
wetlands. Included in the 10 reaches of the river affected by
the contamination are two former drinking-water reservoirs
and the Great Meadow National Wildlife Refuge (GMNWR),
named for the extensive wet meadow and scrub-shrub
wetlands bordering the river channel in this reach. The
reservoirs are located a short distance downstream of the
source, and the sediment in these reservoirs has high con-
centrations of ionic Hg (average, 15 mg/kg dw). Within the
GMNWR, located 25 km downstream of the source, Hg
2+
in sediment is found in relatively low concentrations (aver-
age, ~1 mg kg
-1
dw), but is broadly distributed across the
bordering wetlands as a result of downstream fl ow from the
site at seasonal high water when the wetlands are fl ooded.
The exported Hg
2+
has fueled methylation processes in the
contaminated wetland 25 km downstream, where the Hg
2+
is transformed into MeHg and bio-accumulated in the food
web. In contrast, Hg
2+
in the reservoirs that are closer to the
original source appears to be far less available to methyla-
tion because of natural burial processes.
In a study of stream discharge and concentration measure-
ments (Waldron et al., 2000), Hg budgets were calculated and
the annual mean load for total Hg increased sixfold in the
river reaches just below the point source while the MeHg load
did not increase. In the two reservoirs directly downstream
of the source, net MeHg production was similar to levels in
natural lakes. However, in the riparian wetland reach 25 km
downstream, the calculated net MeHg production was 15
times greater than that reported in other studies in which
there were no point sources of Hg. Since the total Hg loads
in the reach immediately upstream of the wetland reach do
not appear elevated, it appears that the increases in the wet-
lands were not related to current releases from the source
or to mobilization from the contaminated sediments in the
upstream reservoirs, but rather from Hg previously deposited
in the wetland from the Nyanza site (Weiner and Shields,
2000). The differences in MeHg transport to the water column
from the reservoirs as compared with the wetlands may be
due to the diffusive processes in the reservoirs in comparison
with periodic fl ooding in the wetlands (Waldron et al., 2000).
The spatial heterogeneity of Hg and MeHg fl ux from sedi-
ments and water in the Sudbury River system also resulted
in spatial discontinuities in MeHg bio-accumulation in
biotic compartments. Haines et al. (2003) investigated Hg
and MeHg in fi sh and their prey from four sites, including
two reference sites upstream (Whitehall Reservoir and Cedar
Street Bridge) and two downstream sites (Reservoir 2 and
Sherman Bridge within the GMNWR) (see Figure 9.8). As
expected, the THg concentrations in largemouth bass fi llets
were higher in Reservoir 2 than from Whitehall, and whole-
body concentrations were higher at Sherman Bridge and
Reservoir 2 than the two reference sites. However, the THg
concentrations in large mouth bass in Reservoir 2 were lower
than those measured in earlier studies from 1989-1990.
Whole-body Hg concentrations in yellow perch were also
highest in Reservoir 2 and lowest at Whitehall (Figure 9.9),
as expected, but in the benthic feeding brown bullhead, Hg
concentrations were not different between Whitehall and
Reservoir 2. In smaller prey fi sh species, MeHg concentra-
tions were signifi cantly related to total Hg concentration in
large mouth bass, although concentrations in prey and bass
were not higher than those in other studies in uncontami-
nated sites, suggesting that the bio-availability of Hg to the
aquatic food webs has declined (Haines et al., 2003).
The relative MeHg bio-availability in reservoirs versus the
downstream wetlands was tested in earlier studies using may-
fl y nymphs in laboratory exposures and freshwater mussels in
situ as bio-indicators of accumulation. The fi nal concentra-
tions of MeHg were the highest in nymphs exposed to wetland
sediments, intermediate in reservoir sediments, and lowest
in reference sediments (Naimo et al., 2000), again suggest-
ing greater bio-accumulation potential in the contaminated
wetlands. Mussel-tissue Hg concentrations were greatest in
stations closest to the Nyanza site and decreased downstream
(Beckvar et al., 2000). These contrasting results may be due to
the difference between benthic and pelagic exposures. More
SPATIAL DISCONTINUITY
The Sudbury River system provides an interesting example
of the source of Hg
2
being spatially disconnected from
the major site of methylation, thus representing two of the
mechanisms responsible for the transformation and fate
of MeHg (see Figure 9.2). As a result, the concentrations of
Hg
2
are not spatially correlated with the concentrations
of MeHg in sediments (US Geological Survey [USGS], 2001).
