Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 371

of its high solubility, we would expect chromium VI to have the lowest soil risk factor; and
it does as a result of its low sorptive potential. Examining the contaminant risk factors for
many compounds helps explain their fate and transport, especially where contaminants
are found after being released into the environment (their sinks) and why they have time
to migrate or degrade after being released. For instance, mercury and PCBs tend to accu-
mulate in river and lake sediments and soil, but are rarely detected in water. When they
are detected in water, it is at typically low concentrations. This fate and transport is pre-
dicted by the contaminant risk factors scores for mercury and PCBs—they score very high
for soil, and very low for groundwater (Figure 10.7).
Two more examples of the predictive power of the risk scores are seen with the DNAPL
compounds and chromium VI in groundwater. As shown in Table 10.7, groundwater con-
taminant plumes for DNAPL compounds and chromium VI are far longer than any of the
other compounds evaluated. When the CRF GW and the geological vulnerability assessment
described in Chapter 6 are combined, it results in a synergistic effect. This outcome is
likely when chromium VI or DNAPLs are released (often continuously over a long dura-
tion) into a vulnerable geological environment (e.g., soils composed of sand with shallow
groundwater used as a source of potable water). In these cases, significant adverse envi-
ronmental and human health effects may occur (Rogers et al. 2006).
Another view of the data is provided by the pie charts shown in Figure 10.8. For each
type of contaminant risk factor, certain contaminants favor certain environmental media.
Table 10.15 provides an interpretation of the data presented by the pie charts above, and
can be used as a summary reference for CRFs by major contaminant groups and com-
pounds of selected interest.
Additional support for the validity of the risk scores would be their ability to explain a
high amount of the variation in remediation costs, and to also explain a high amount of the
variation in remedial costs across a wide range of geological environments. When the air,
groundwater, soil, and surface risk factors are combined, a total risk characterization of a
chemical's environmental risks is achieved. Does more risk result in higher remediation cost?
To answer this question, a stepwise regression was used to search for the variables
that could explain the most variance in remediation costs at 79 sites in an urbanized

100,000

10,000

1,000

Chromium VI
DNAPL
Arsenic
Mercury
Lead
LNAPL
PAHs
PCBs
Chlordane

100

10

1

0.1

0.01

0.001

0.0001

CRF (GW) CRF (SOIL) CRF (AIR)

FIGURE 10.7
Groundwater, soil, and air CRF distribution.