Geologic Vulnerability - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 204

Rouge River sediments have shown a significant presence of heavy metals and polychlo-
rinated biphenyls (PCBs) (Michigan Department of Natural Resources 1988; Murray 1996;
Murray et al. 1999).
Shallow groundwater within the Rouge River discharges to surface water and accounts
for most of the baseflow (Rogers and Murray 1997). Average annual precipitation amounts
and the geology of the watershed determine this flow pattern. Climatically, the Rouge
watershed is situated within the humid microthermal zone of the Midwestern United
States, meaning the Rouge River is effluent and fed by groundwater entering as baseflow.
With respect to its geology, the lower clay unit (ground moraine or lodgment till) that com-
pletely underlies the watershed is very thick (ranging from 9 m to more than 55 m thick),
has a very low hydraulic conductivity (less than 1 × 10 −8 cm/s), and does not show signs
of unconformities or features suggesting any significant secondary porosity (i.e., verti-
cal fractures or root fragments). Therefore, the lower clay unit is a very effective confin-
ing layer. This type of formation is not uncommon. Most other areas in North America
glaciated during the Pleistocene have similar ground moraine or lodgment till deposits
of similarly low hydraulic conductivity (Keller et al. 1989). This is a significant finding
because any contamination that does not degrade and reaches groundwater will eventu-
ally migrate and discharge to the surface waters of the Rouge River. From there the con-
tamination subsequently discharges into the lower Great Lakes (Rogers and Murray 1997;
Rogers 1997a). Figure 6.2 shows this discharge sequence based on the flow patterns of the
watershed's surface water and groundwater.

Outlet at the
Detroit River;
then on to
Lake Erie

Flow direction
Groundwater

Surface water

0 10 20 30 40

Kilometers

FIGURE 6.2
Surface water and groundwater flow patterns in the Rouge River watershed. (Murray, K. et al., Dissolved heavy
metals in shallow ground water in a southeastern Michigan urban watershed, Journal of the American Water
Resources Association , 2006, 42(3), 777-792. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with
permission.)