Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 346

This watershed is a good example because it is heavily urbanized and includes a broad
spectrum of geological units including clay, silty clay, sandy silty clay, and sand (Kaufman
et al. 2003).
The mean surface risk values are highest in the sand for three reasons: (1) there is a high
density of sites with a propensity for releases of contaminants to occur, (2) the larger cap-
ture zone for sand translates into potentially more sites of contamination than the other
geological units, and (3) the subsurface geology (i.e., sand) does not significantly impede
the migration of contaminants.
Industrial location patterns also help to explain other mean surface risk values in this
watershed. For example, the silty clay unit has a mean surface risk value of 33.7 and the
sandy silty clay unit has a surface risk value of 0.42, yet the silty clay unit has a 10 year
capture zone smaller than the sandy silty clay unit. This discrepancy is due to the signifi-
cantly greater number of potential contaminant sources located within the silty clay unit.
The next section examines individual contaminants and their migration potential within
different subsurface geological environments.
10.3 Groundwater Contaminant Risk Factor
The adverse risk posed to groundwater by the contaminants themselves is often over-
looked or underrepresented (Kaufman et al. 2005; Rogers et al. 2007a). Considering specific
types of contamination using vulnerability models is important because each contami-
nant has unique physical chemical properties significantly influencing its behavior when
released into the environment (Chapters 7 and 8).
Contaminant fate and transport evaluations require interdisciplinary analyses
involving chemical, geological, hydrological, and biological factors (USEPA 1989, 1992).
Correspondingly, the development of groundwater vulnerability models for specific con-
taminants requires an interdisciplinary process (Rogers et al. 2007a). The critical physical/
chemical attributes influencing contaminant risk are associated with mobility and persis-
tence and include the following factors (USEPA 1989; USEPA 1996a; Wiedemeier et al. 1999;
USGS 2006; Rogers et al. 2007a):
1. Solubility
2. Vapor pressure
3. Density
4. Chemical stability
5. Persistence
6. Adsorption potential
As noted previously, toxicity is an important factor when examining risk. Mobility and
persistence are also critically important because these two factors dictate a chemical's
ability to migrate from its point of release in the environment to a distant point where
human exposure may occur, such as a drinking water supply or a surface water body. The
environmental risk posed by specific contaminants to contaminant groundwater, termed
Contaminant Risk Factor for Groundwater (CRF GW ) can be developed as a function of
these three factors (Kaufman et al. 2005; Rogers et al. 2007a).