Environmental Engineering Reference
In-Depth Information
was consistent with the range of detection reported by Shackette and Boergnen (1984).
A review of the occurrence data suggests that all of these metals are likely the result of
anthropogenic sources rather than from natural occurrence.
9.3.2 Results
The analytical results for the eleven remaining metals are presented in Tables 9.4 and 9.5.
Table 9.4 indicates each metal's horizontal (west-to-east distribution across the six soil
units within the watershed) and vertical (surface, subsurface, and lower clay) distribution
across the watershed. Table 9.5 reports each metal's mean concentration in both surface
and subsurface soils across each of the three land use categories and is split into separate
tables for surface soil (Table 9.5a) and subsurface soil (Table 9.5b).
A few trends are apparent from these results. As shown in Table 9.4, surface concen-
trations are generally greater than subsurface and concentrations generally increase in
a west-to-east direction across the watershed. This latter trend is commensurate with
a west-to-east increase in urbanization and industrialization. The moraine unit shows
consistently lower levels of metals than any of the other soil units, and has mean con-
centrations of metals statistically similar to that found in the lower clay layer, which is
assumed to contain naturally occurring metal concentrations. Although the moraine has
locally high permeability, it generally contains substantially more clay than either the
outwash or the sand units. The low concentration of metals present in this unit is thus
attributed to fewer anthropogenic sources in the western, more rural part of the water-
shed. More interesting, however, is the relatively high concentration of metals found in
the sand unit. The sand unit varies in thickness from less than a meter to more than 10 m
and is highly permeable with a hydraulic conductivity ranging from 10
−4
to 10
−1
cm/s
(Rogers and Murray 1997). Yet, the sand unit contains statistically higher concentrations
of metals than either the moraine or outwash units, which are both exposed at the surface
westward of the sand.
The explanation for this apparent paradox is the location of the sand within the water-
shed. The sand is located in the center of the watershed along the urban fringe. This is an
area that has undergone rapid urbanization and industrialization over the past 20 years
and represents the transition between the rural west and the more heavily industrialized
eastern part of the watershed. Contamination derived from spills, leaking underground
or above ground tanks can quickly pass through the vadose zone within the sand to reach
the water table, typically at a depth of no more than 3 m below the ground surface in
southeast Michigan. Consequently, this part of the watershed has the highest incidence of
groundwater contamination (Murray and Rogers 1999; Kaufman et al. 2003), and the sand
unit, despite its lack of clay and organic material contains a higher concentration of the
metals most often associated with industry: arsenic, barium, cadmium, chromium, cop-
per, nickel, and selenium than even the sandy silty clay unit that is located immediately to
the west. The pivotal position and characteristics of the sand unit is underscored by metal
concentrations that are typically 50% less than the metal concentrations found in the two
easternmost clay-rich units.
The silty clay unit, which is exposed at the surface immediately to the east of the sand
unit contains the highest arsenic concentrations of all the soil units with its highest con-
centrations present in both surface and subsurface soils in industrial areas. Although arse-
nic concentrations were expected to be uniformly high across the entire watershed, as
a function of either the weathering of natural arsenic-bearing minerals associated with
the underlying Marshall Sandstone or from the atmospheric deposition associated with
Search WWH ::
Custom Search