Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 341

10
Contamin antRiskFactors
10.1 Introduction
What are the risks posed by contaminants to humans once they are released? The first step
to answering this question involves combining the following three factors related to the
physical chemistry of the contaminant: (1) toxicity or potency, (2) mobility, and (3) persis-
tence. Next, the composite physical chemistry attributes are considered within the context
of a region's geological vulnerability. This framework provides a powerful tool for assess-
ing the environmental risk of an urban region, but it fails to consider the known risks of
release related to specific surface activities. In this chapter, we introduce the concept of
surface risk, one component of the environmental risk picture within any urban region.
The risks posed by contaminants are not equal. Although it sounds like a paradox, an
extremely toxic contaminant may not pose as much risk as a moderately toxic contaminant.
This outcome can occur if the extremely toxic contaminant does not migrate and degrades
quickly, and the moderately toxic contaminant exhibits higher mobility and lasts for decades
before degrading. The release location is also a factor in determining risk. If the extremely
toxic contaminant is released at a certain location and under certain conditions, it may inflict
significant harm before it degrades. On the other hand, the moderately toxic contaminant
may have much more opportunity to inflict harm because it lasts longer and is mobile.
Contaminants also behave differently in soil, water, and air. It is logical, therefore, to
assess contaminant risk as a function of each environmental media. We begin the evalu-
ation process by assessing the probability that a release will occur given certain land use
criteria. This is a critical step in the evaluation process, because not only must a contami-
nant be present for there to be risk, it must also be released for there to be potential expo-
sure. After examining the potential risks of a release, we will then examine how the media
releases target and develop contaminant risk factors for groundwater, soil, and air. When
these factors are combined with surface risk and geological vulnerability, the total envi-
ronmental risk can be estimated.
This is where the predictive power lies, knowledge of the physical attributes of the
chemicals used combined with the geology of a region and the probability of a release
occurring. Together, these pieces form the basis of a scientifically grounded environmental
assessment process that can lead to the successful development or redevelopment of any
urban region.
10.2 Surface Risk Factor
Surface risk, as employed here, is the probability of any given site contaminating the envi-
ronment given the best available data from public sources. Surface risk evaluations have