Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 344

are unknown. Figure 10.3c shows that the use of the Standard Industrial Classification (SIC)
code or the equivalent for each establishment within a general land use category permits
a greatly improved parcel-level spatial resolution of the relative risks of contamination.
The SIC code is a four-digit code defined as follows (United States Office of Management
and Budget 1987)*:
• The first two digits identify a major group, such as agriculture, retail trade, and
manufacturing.
• The third digit denotes industry groups within each major category, such as agri-
cultural crop production.
• The fourth digit identifies a specific industry code, such as metal plating.
A normalized measure of risk between different establishment types is achieved through
the use of contamination incident rates (Kaufman 1997). Incidence rates are obtained by
1. Assigning an SIC code to each source of contamination appearing on a known list
of contaminated sites. Lists of known contaminated sites are available through
private companies or are publicly available through either local municipalities
or state or federal environmental agencies, such as the Michigan Department of
Environmental Quality (MDEQ 2008).
2. Obtaining the total number of establishments for each SIC code within the study
area (United States Bureau of the Census 1997).
3. Dividing the number of contaminated sites with a specific SIC code by the total
number of establishments with the same SIC code in the study area.
To scale the scores equivalently to the other risk factors discussed in the following sec-
tions, these rates are multiplied by 10 and converted to scores between 0 and 10. These
scores are then summed for a circular area encompassing each water well within the study
area.
Figure 10.4 shows an example surface risk calculation for a brownfield site situated
above an area where the subsurface geology is composed of sand. The computed risk
includes only those lighter-shaded establishments contained within the geological unit
composed of sand, because the bounding geological units (sandy clay, sandy, and silty
clay) have much lower hydraulic conductivities yielding much lower contaminant migra-
tion potentials.
The legend box on Figure 10.4 shows the SIC and risk scores for three of the many estab-
lishments within the radius (Kaufman et al. 2003). For example, SIC 2822 represents an
establishment producing synthetic rubber with a risk score of 4.00. This particular risk
score was calculated by dividing the two SIC 2822 establishments known to be sites of
contamination by the five SIC 2822 establishments in the region and then multiplying the
result by 10. The risk scores for the other establishment types shown in Figure 10.4 are
computed similarly.
* The four-digit SIC codes were replaced by the six-digit North American Industry Classification System
(NAICS) in 1997. The NAICS was developed to allow for a higher level of comparability in business statistics
among Mexico, Canada, and the United States. Some governmental agencies still use the SIC code, and the
authors prefer them because the descriptions are more complete than those provided by the NAICS.