The Watershed Approach - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 10

The Rouge River watershed consists of over 200 km of streams, tributaries, wetlands,
lakes, and ponds within a 1200 km 2 area. Stream density is highest in the sand and moraine
units. The glacial topography consisting of outwash plains, eskers, and kames has created
irregular drainage patterns, and the wetlands are often associated with these features.
Most of the inland lakes are kettles or impoundments formed by damming the river at
different locations.
The Rouge floodplain is not uniform in height due to the erratic deposition during floods.
Historically, flooding was common during heavy rains, and floodplains of river silt were
built at a height of several meters above the ordinary stage of the water. Today, flooding
of certain low-lying sections occurs during heavy summer thunderstorms and prolonged
precipitation events yielding over 38 mm (1.5 in.) of rain.
The fan-shaped watershed includes all or part of 47 different municipalities and is cur-
rently the focus of intense scientific study and restoration. It has been identified as an Area
of Concern by the International Joint Commission (Hartig and Zarull 1991) and cited as a
significant source of pollution to the lower Great Lakes (Murray and Bona 1993).

1.4.2 Theme #2: Science and Planning—The Movement of Water

There is a fundamental relationship that exists between basic science and the ability to
make informed environmental planning decisions. Regarding the movement of water on
earth, the relevant basic science is the hydrologic cycle , the solar-initiated and gravity-
sustained renewable flow of water between five major reservoirs: the oceans, atmosphere,
ice caps, surface water, and groundwater. Two of these water reservoirs—groundwater
and atmospheric water vapor—have the unique attribute of being invisible. Being out of
sight often makes groundwater out of mind to policy makers and the general public, and
this creates special management challenges. Problems often arise when basic hydrologic
processes linked to groundwater such as surface water flow are not included, such as the
incorrect identification of the extent of a contamination event.
Let us consider groundwater contamination in the Great Lakes region, where the flows
of many contaminants will be largely dictated by the pathways of the hydrologic cycle
within that given region, as shown below:

Contaminants released from
a site (e.g., septic tank,
leaking underground
storage tank, surface spills,
industry, incinerators, and
metals, ash, and gases from
coal powered electrical
generating facilities)

Contaminants
move into the
environment

Where to next?

Different soil types and the near-surface geologic environment will play key roles
in the migration of the contaminant. Releases are typically mixtures of contaminants
(e.g., gasoline), which contain over 100 individual chemical compounds. Each specific
compound behaves and migrates in the near surface geologic environment differently.
Geologic environments consisting largely of clay materials tend to inhibit the migra-
tion of contaminants downward, whereas in a sandy geological environment, con-
taminants often reach the water table. As the next sequence of boxes demonstrates, the
surface environment and groundwater flow direction will help determine the extent of
the contamination.