Modern Mercury Contamination from Historic Amalgamation Milling
339
chemical form of Hg in sediments and soils is an important determination, but
there is currently little consensus on the best methodology for determining the
distribution of Hg species in soils or sediments. This contribution reports on the
distribution of Hg and its chemical form in sediments of the Carson River and
Jordan Creek, and soils from the Carson River Superfund Site and nearby areas.
While the quality assurance of speciation methodology is difficult (Wallschlager
et al. 1996), compatibility of the speciation results reported here with theory (EhpH diagrams), other geochemical features of the systems, and systematic,
geochemically logical spatial variations in speciation, impart confidence to the
techniques employed.
2
Methods
2.1
Field Methods
Sediments were collected from the upper 10 to 20 cm of the active channel of
Jordan Creek, the Carson River, and the bed of Lahontan Reservoir. Soils and
tailings were collected as integrated samples from discrete subsurface horizons,
defined by visually determined media heterogeneities such as changes in grain
size, texture, color, and degree of induration. The samples were placed in plastic
bags and refrigerated until analysis. Wet samples were used for all analyses, while
moisture content was determined on a separate sample split; all analytical results
were corrected to a dry weight basis (105 to no QC).
Five sediment samples from the active channel of the Carson River, and one
from the bed of Lahontan Reservoir, were collected from more-or-Iess evenly
spaced sites to investigate spatial variations in the chemical speciation of Hg
during downstream transport (Fig. 1). In the same way, nine samples were
collected every 1.6 km down Jordan Creek from just below Silver City to the base
of the Owyhee Range, 14.5 km downstream (Fig. 2).
The upstream sample site in the Carson River is located above the point at
which mercuriferous tailings are judged to be entering the river and therefore
constitutes a background sample representative of the concentration and
speciation of Hg in uncontaminated sediments in this area. The next four
samples were collected from 8 to 58 km downstream in the river and the final
sample was collected from Lahontan Reservoir, 12 km more distant (Fig. 1).
No such background sample was collected from Jordan Creek because of
logistical difficulties, although the upstream sample below Silver City is
substantially less contaminated than downstream samples. This is compatible
with most of the amalgamation milling of ores being performed below Silver
City (Wells 1983).
Soils in the western Nevada study area are typical semiarid soils characterized
by poor A and B soil-horizon development. Often, a layer of caliche is developed
339
chemical form of Hg in sediments and soils is an important determination, but
there is currently little consensus on the best methodology for determining the
distribution of Hg species in soils or sediments. This contribution reports on the
distribution of Hg and its chemical form in sediments of the Carson River and
Jordan Creek, and soils from the Carson River Superfund Site and nearby areas.
While the quality assurance of speciation methodology is difficult (Wallschlager
et al. 1996), compatibility of the speciation results reported here with theory (EhpH diagrams), other geochemical features of the systems, and systematic,
geochemically logical spatial variations in speciation, impart confidence to the
techniques employed.
2
Methods
2.1
Field Methods
Sediments were collected from the upper 10 to 20 cm of the active channel of
Jordan Creek, the Carson River, and the bed of Lahontan Reservoir. Soils and
tailings were collected as integrated samples from discrete subsurface horizons,
defined by visually determined media heterogeneities such as changes in grain
size, texture, color, and degree of induration. The samples were placed in plastic
bags and refrigerated until analysis. Wet samples were used for all analyses, while
moisture content was determined on a separate sample split; all analytical results
were corrected to a dry weight basis (105 to no QC).
Five sediment samples from the active channel of the Carson River, and one
from the bed of Lahontan Reservoir, were collected from more-or-Iess evenly
spaced sites to investigate spatial variations in the chemical speciation of Hg
during downstream transport (Fig. 1). In the same way, nine samples were
collected every 1.6 km down Jordan Creek from just below Silver City to the base
of the Owyhee Range, 14.5 km downstream (Fig. 2).
The upstream sample site in the Carson River is located above the point at
which mercuriferous tailings are judged to be entering the river and therefore
constitutes a background sample representative of the concentration and
speciation of Hg in uncontaminated sediments in this area. The next four
samples were collected from 8 to 58 km downstream in the river and the final
sample was collected from Lahontan Reservoir, 12 km more distant (Fig. 1).
No such background sample was collected from Jordan Creek because of
logistical difficulties, although the upstream sample below Silver City is
substantially less contaminated than downstream samples. This is compatible
with most of the amalgamation milling of ores being performed below Silver
City (Wells 1983).
Soils in the western Nevada study area are typical semiarid soils characterized
by poor A and B soil-horizon development. Often, a layer of caliche is developed