The Watershed Approach - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 11

What type of
material (e.g.,
sand or clay) is
the contaminant
flowing through?

How long do specific
contaminants last in
certain geologic
environments before
they physically
degrade?

From the release
point, what is the
direction of
groundwater flow?

In humid climates, groundwater flows into surface water, and this process creates the
need to know more about the local geography of the water resources, particularly the
locations of surface streams, lakes, and wetlands. The locations of streams and their flow
patterns are required to determine the potential for contaminant transport and the larger-
scale environmental problems that may result. And, since the water within lakes and wet-
lands consists of groundwater above the water table, knowledge of their flow patterns is
also necessary to obtain a complete picture of the contamination potential.
The three maps in Figure 1.5 show the outcome for generalized categories of contami-
nants spilled from selected surface locations within the Rouge River watershed in the Great
Lakes region. There are five contaminant categories that have been selectively investigated.
These categories include dense nonaqueous phase liquids (DNAPLs—chlorinated sol-
vents), light nonaqueous phase liquids (LNAPLs—gasoline compounds), polycyclic aro-
matic hydrocarbons (PAHs—oil compounds), polychlorinated biphenyls (PCBs) and a
group of several heavy metals, including chromium and lead. Only those locations where
the near-surface geologic environments have a high migration potential are shown (sand
and moraine). At the top left, the map shows buffers around some of these locations, with
the buffer sizes representing the average distance these contaminants traveled based upon
measurements taken at their points of release These buffered sites were overlaid (symbol-
ized by the “+”) on the surface stream network map to the right (including all first-order
streams and higher) within the watershed. The composite map shown below the arrow
indicates the areas where there are intersections between the average contaminant extent
and a stream channel. To avoid map clutter, the circles represent a sample of some of the
locations where highly toxic DNAPL compounds contaminated groundwater in the sand
and moraine geological units.
Inspection of the contaminant sites and surface stream maps indicates there are numer-
ous other sites where contamination within groundwater has the potential to migrate
to surface water within a few days. This outcome is likely due to the high higher flow
rates within the sand and moraine units in this watershed and the high drainage den-
sities within these same units. At this geographic scale, the hydrologic cycle flows fol-
low these pathways: soil > groundwater > low order surface streams > higher order surface
streams > Great Lakes. Some persistent contaminants such as tetrachloroethene (a com-
mon dry cleaning chemical) and chromium VI (the compound used in chrome plating)
will travel along with the water. The lesson demonstrated here is that by omitting the
linkage between groundwater and flowing surface water, the consideration of ecological
impacts to the larger region of the Great Lakes may not occur.

1.4.3 Theme #3: Industrial Property Abandonment, Contamination, and Risk

We spoke earlier of the decentralization of the automobile industry that began in the
1950s. This trend has accelerated in recent years, as a geographic shift of production
from Michigan and other parts of the upper Midwest continues southward and to other
countries. In addition, so-called “transplants”—foreign nameplate companies producing