Environmental Engineering Reference
In-Depth Information
mixed solids, (6) as secondary minerals, and (7) in the structure of primary minerals. The
metals that have been introduced into the environment through human (anthropogenic)
activities are associated with the first five (Shuman 1991).
Migration of metals in soil is influenced by physical and chemical characteristics of each
specific metal and by several environmental factors. The most significant environmental
factors appear to be (1) soil type, (2) total organic content, (3) redox potential, and (4) pH
(McLean and Bledsoe 1992; Jaagumagi 1993; Murray et al. 1999). Metals in soil solution also
migrate through mass transfer by leaching to groundwater, plant uptake, and volatization,
which are important migration mechanisms when considering the mobility of arsenic,
mercury, and selenium (Mattigod et al. 1981).
With respect to soil type and organic content, clay-rich soils generally have a higher
retention capacity than soils with little or no clay (e.g., soils composed primarily of sand).
Soils with a high organic content also have a higher retention capacity than soils with a
lower relative organic content (Stevenson 1991). Oxidizing conditions generally increase
the retention capacity of metals in soil, while reducing conditions will generally reduce
the retention capacity of metals (McLean and Bledsoe 1992). In terms of pH, cationic met-
als, which include lead, copper, nickel, and zinc, have a higher retention capacity in soil
with a pH greater than 7 compared to soils with a pH less than 7 (Lindsay 1979; Harter and
Lehmann 1983). However, oxyanion metals, which include arsenic, selenium, and hexava-
lent chromium, have a higher retention capacity with a pH of less than 7 compared to
soil with a pH greater than 7 (Lindsay 1979; Neal et al. 1987). Through the interaction of
these factors, lead, copper, and silver demonstrate the highest capacity for retention in soil.
Conversely, arsenic, chromium, and mercury are mobile if concentrations are high enough
and favorable soil conditions are present (McLean and Bledsoe 1992).
9.3 Case Study: Metals in Soil, Rouge River Watershed
The contamination of urban soils can pose a significant threat to human health if cer-
tain contaminants are present at sufficient concentrations or at locations where human
or ecological exposure can occur. Since the majority of the U.S. population lives in these
areas, this contamination is a significant concern. Yet, the characterization of these soils
has always posed a difficult challenge. Urban soils have the greatest potential to be dis-
turbed by human activity, complicating site investigations; more importantly, background
information is frequently limited, and, as pointed out by Alkhatib and O'Connor (1998),
the sheer cost of analyzing soil samples discourages investigators from collecting addi-
tional background samples.
9.3.1 Study Objectives, Study Area, and Methods
Because of its industrial base, the Rouge River watershed in southeastern Michigan con-
tains thousands of sites of environmental concern. Environmental site investigations have
been conducted at locations with known or suspected impacts to evaluate the presence
of heavy metals and other compounds (MDEQ 1998). In 2004, Murray et al. adopted a
watershed-level approach to characterize the background levels of metals in both the near-
surface and subsurface soils in southeast Michigan. The watershed approach is particu-
larly important in these types of studies because many of the processes that contribute
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