Environmental Engineering Reference
In-Depth Information
9.5 Conclusions
The Murray et al. (2004) study of metals in soil represents an initial effort to character-
ize the metal concentrations in surface and near-surface soils in an urban environment
in southeast Michigan. The results of this study have (1) demonstrated that metal con-
centrations in an urban environment are the greatest at the surface, (2) characterized a
directional increase of metal concentrations in a west-to-east trend across the watershed
commensurate with a general west-to-east increase in urbanization and industrial activ-
ity, and (3) identified lead as the heavy metal with the highest surface concentration with
mean levels present at more than 16 times background at residential sites and 15.5 times
greater than background at industrial sites. More importantly, lead concentrations have
been detected in surface soil at specific industrial sites and adjacent residential neighbor-
hoods at concentrations hundreds of times the level that may occur naturally in the soil.
Similarly, arsenic concentrations in the Rouge River watershed for certain residential areas
are 10 times greater than the background concentrations, due most likely to their proxim-
ity to Zug Island.
Heavy metal concentrations beneath the surface (at depths between 0.5 and 10 m),
although elevated, were in general agreement with those of previous studies conducted
in Michigan and the United States. Heavy metal concentrations in soils at depths greater
than 10 m are in complete agreement with previous studies of heavy metal concentrations
in rural areas of Michigan and the United States. This suggests that soils at depths greater
than 10 m are generally only affected to a minor extent from anthropogenic heavy metal
sources if certain conditions are satisfied.
The results of the Murray et al. (2004) soil metals study have important implications for
land use planning and for future site investigations in any urban area when heavy metal
contamination is suspected. First, the evaluation of human and ecological risks can be
achieved by concentrating investigation efforts on heavy metals in the surface soils. The
results of the study have demonstrated that heavy metal concentrations are the highest at
the surface and quickly decrease with increasing depth. Therefore, concentrating investi-
gative and remedial efforts at the surface will efficiently identify the most elevated heavy
metal concentrations derived from anthropogenic sources. Second, innovative regional
strategies for the cleanup of sites with metal contamination need to be developed. Third,
the identification of potential hydrologic and ecological impacts of metals in the near-sur-
face zone is essential for the protection of groundwater. Since this study has identified that
the occurrence of heavy metals in an urban environment is the highest at the surface, the
results can be integrated into evaluations of interflow, baseflow, increased leaching from
acid rain, infiltration/inflow to public water supplies, runoff to surface water systems, and
the potential revegetation capacity at redeveloped sites. Fourth, the mapping of the surface
geology in urban areas can pinpoint soils particularly sensitive and prone to contamina-
tion. Finally, future studies related to the occurrence of heavy metals in other urban areas
should consider the effects of historic land use activities and the potential human health
issues and risks associated with heavy metals in the surface soils.
With respect to dissolved metals in groundwater, results from the 2006 Murray et al.
study indicate urbanization and its accompanying industrial activities have contributed to
heavy metal impacts across all land use categories (industrial, commercial, and residen-
tial) and throughout the varied surface geology of the Rouge River watershed. Chromium
is prevalent at industrial sites, and the high levels of chromium in groundwater strongly
suggest the chromium is in a hexavalent form—which is much more mobile and soluble
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