Environmental Engineering Reference
In-Depth Information
Population increases from west to east
250
Residential
Commercial
Industrial
Lead
concentration
(mg/kg)
200
150
100
50
0
Moraine Outwash Sandy and
silty clay
Sand
Silty clay
Clay
FIGURE 9.3
West-to-east concentration (mg/Kg) of lead in surface soil by land use. (From Murray, K. et al.,
J. Environ. Qual.
,
33, 163, 2004. With permission, American Society of Agronomy.)
several industrial sites (and surrounding neighborhoods) exhibiting lead concentrations
hundreds of times the background concentration. In addition, subsurface lead in indus-
trial areas was nearly five times higher than the background level (Figure 9.3), underscor-
ing the impact of industry's role in contributing to soil lead contamination.
Mercury was present in the near-surface soils in commercial areas at levels four times
the background level of 0.09 mg/kg present in the lower clay unit. Throughout most of the
subsurface, mercury concentrations are equivalent to background levels, while mercury
concentrations at commercial and industrial sites are more than twice the background lev-
els. The high incidence of mercury at industrial sites is probably related to the production
of chlorine, caustic soda, and hydrogen. It may also be related to former automotive paint
industries or the production of electrical equipment (Jaagumagi 1993). The high incidence
of mercury at commercial sites is more problematic and may reflect changing land use in
an urban area with an industrial base that is more than 100 years old.
Concentrations of copper and zinc at the surface were significantly lower at residential
and commercial properties compared to industrial properties. This suggests the primary
sources of copper and zinc were industry related. In addition, concentrations of many
other heavy metals, such as cadmium, chromium, nickel, selenium, and silver were also
detected in surface soil at industrial properties at higher concentrations than commercial
and residential properties. This again suggests the elevated concentrations of these met-
als may have an anthropogenic source, presumably, industrial sites. On the other hand,
concentrations of barium, mercury, and arsenic did not vary significantly between the
land use categories; implying that these metals do not have a significant anthropogenic
source.
Concentrations of heavy metals in the Lower Clay Unit differ only slightly from the
heavy metal concentrations observed in previous Michigan studies. In fact, most metals
(arsenic, barium, cadmium, chromium, lead, mercury, nickel, silver, and zinc) were present
at mean concentrations slightly less than the previous Michigan studies. Only copper and
selenium were present at mean concentrations greater than those of previous studies; how-
ever, these elevated concentrations would not be considered statistically significant. These
results support our contention that metal concentrations in the Lower Clay Unit within
the Rouge River watershed can be reasonably interpreted to represent naturally occurring
concentrations of heavy metals and confirm their use as background heavy metal concen-
trations for this study.
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