Environmental Engineering Reference
In-Depth Information
Within groundwater, the concentrations of heavy metals are most significantly influ-
enced by (1) presence of heavy metals in soil that has come into contact and reacts with
groundwater and (2) the chemical nature of the groundwater itself (Fetter 1993). Metals
have increased mobility in groundwater if the groundwater chemistry exhibits a low
cation-exchange capacity or if they are attached to a mobile colloid (Bedient et al. 1994).
Conditions that promote metal mobility would, therefore, include a hydrogeologic envi-
ronment that consists of a subsurface saturated environment composed of an acidic, sandy
soil with low-organic and clay content (Dowdy and Volk 1983).
9.4.1 Study Objectives, Study Area, and Methods
This study (Murray et al. 2006) measured dissolved heavy metal concentrations in shallow
groundwater within the same region—the Rouge River watershed. The primary objectives
of this study were (1) to characterize the distribution of heavy metals in groundwater in
different geologic and land use environments within the watershed, (2) identify the most
mobile metals and the locations associated with their mobility, (3) determine the amount
of contamination represented by the measured mean concentrations of heavy metals in
the shallow groundwater, and (4) assess the potential public health and ecosystem impacts
from any dissolved metal contamination found. The evaluation of heavy metal mobility
was enabled through the direct comparison of the mean concentrations of heavy metals
measured in the soil by the Murray et al. (2004) study and groundwater.
The discharge of near-surface shallow groundwater to the surface waters of the Rouge
River is significantly influenced by the presence of the lower clay unit, which has effec-
tively limited the vertical migration of groundwater to lower aquifers because (1) it is thick
(80 m to greater than 300 m thick), (2) vertical groundwater flow in the clay is insignificant,
(3) the unit completely underlies the watershed, and (4) has a relatively uniform hydrau-
lic conductivity, which has been measured to be less than 1 × 10
−7
cm/s (Rogers 1997b).
Glacial deposits similar to the lower clay unit have also been described as occurring with
a similar hydraulic conductivity at other locations of northern North America (Freeze and
Cherry 1979; Keller at al. 1989, 1991). Because of the lower clay unit, near-surface shallow
groundwater within the watershed has no effective migratory outlet other than surface
water. Therefore, any dissolved contaminants in shallow groundwater will eventually be
discharged to the surface waters of the Rouge River and subsequently flow into the Detroit
River and eventually into the lower Great Lakes.
The most significant hydrogeologic units (other than the lower clay unit) within the
watershed are the Moraine Unit, Outwash Unit, and the Sand Unit (Figure 5.20). These
three units significantly influence surface water because they (1) contain a large volume
of groundwater, (2) account for the majority of the baseflow in the Rouge River, (3) contain
the majority of tributaries of the river, and (4) are the source of groundwater recharge
from precipitation and surface water infiltration (Rogers 1997b; Rogers and Murray 1997;
Murray and Rogers 1999). In addition, these geologic units are also significant because
shallow groundwater within these geologic units is also used as a source of potable water
(Mozola 1954, 1969; Rogers 1996), creating a potential pathway for human ingestion of shal-
low groundwater. Inspection of the groundwater low direction at each site revealed a
good correspondence with the direction of surface water flow (Figure 6.2).
As with the soil metal study discussed earlier, the data used in the study of dissolved
metals in the groundwater were also derived primarily from project files compiled by
the MDEQ. Also, as discussed in Section 9.3, each site selected was carefully evaluated
and screened to eliminate obvious data bias, including the same sites excluded during
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