Environmental Engineering Reference
In-Depth Information
The detection of Cr at such high relative concentrations in groundwater compared to
other metals in this study indicates that the species of Cr present is likely Cr
+6
, hexavalent
chromium. Moreover, Murray et al. (2008) found high levels of Cr
+6
in the shallow ground-
water. Hexavalent chromium has far greater mobility in soil and solubility in water than
other forms of chromium (Fetter 1993). It is also considered the most toxic form of chro-
mium to humans and ecosystems at the higher and lower levels of the food chain (Krishan
1997; Nussey et al. 2000; Proctor et al. 2002).
Compounding the contamination problem within the sand unit are the physical char-
acteristics of its aquifer and surface drainage. The relatively high hydraulic conductivity
of the sand unit, coupled with the high density of streams there (see Figure 5.20) creates a
short residence time for contamination. In the humid microthermal climate of this region,
the stream system is effluent, meaning groundwater recharges streams through baseflow.
Thus, any contaminant entering groundwater within the sand unit will flow in the general
direction of the surface drainage, and would enter a surface stream within a relatively
short time period due to the high drainage density. For example, using a conservative
estimate of the hydraulic conductivity for the sand unit from Murray and Rogers (1999) of
1 * 10
−2
cm/s, the groundwater would flow 315 m in 1 year: (0.01 cm/s * 86,400 s/day * 365
days/year/1000 cm/m = 315 m/year). Given the surface drainage density, there is a good
likelihood that contamination within the groundwater would reach a stream within a rela-
tively short period of time. Once in the surface water, the flow will carry the contamination
within hours to the Detroit River and to Lake Erie.
Another significant finding of this study is the presence of elevated concentrations of
metals (Hg, Se, Cr, and As) within the groundwater of the clay and silty clay geologic units,
as mobility of metals in clay is generally believed to be very low. Several factors could
account for this result, including very high concentrations of these metals within the clay
soil units, cracks within the clay unit which allow for some downward migration of con-
tamination (Murray et al. 1997), and the long duration and continuous deposition of these
metals within these areas due to the development history of the watershed.
From public health and environmental perspectives, the contamination of the regional
groundwater along with potential damage to the hydraulically connected Great Lakes eco-
system poses significant challenges for watershed management efforts. Significant numbers
of residential wells (over 5000) exist within the watershed, and many homeowners do not
test their water since it is not a legal requirement. Moreover, the testing services provided
by Michigan's county public health departments only test for bacteria and anions, such as
chloride and nitrate. If homeowners want to test their wells for metals, they must pay $10.00
per metal, and it is doubtful if most homeowners know about this option. There are also a
large number of public supply (Class II) wells that provide water for over 25 people. Wayne
County, which comprises over 40% of the Rouge River watershed land area does not test for
chromium in its Class II wells. Consequently, there may be a need to reevaluate the well-
head protection guidelines communities follow, given the already existing contamination,
and the ability for metals to reach groundwater through all types of soil.
Surface water may also be directly impacted by metals transported by stormwater run-
off. The presence of some metals at the surface—many of which are sorbed onto small soil
particles—means that surface water bodies will receive these contaminants when they
are entrained by overland flow. Wetland vegetation may be capable of removing some
percentage of the heavy metals; however, most lakes in southeastern Michigan no longer
have this capability due to development. Since copper and chromium VI are highly toxic
to fish, this is another concern. The process of stormwater runoff within urban watersheds
is presented in Chapters 12 and 13.
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