Environmental Engineering Reference
In-Depth Information
action, or the effects of metabolism. Nevertheless, Petry et al. (1996) found
that the BaP equivalents developed by Nisbet and LaGoy (1992) were valid
markers for PAH health risk assessment.
Environmental risk assessment, in slight contrast to human health risk,
looks at the PAHs that usually occur in contaminated environmental systems
and that have the highest TEFs (by the Nisbet and LaGoy (1992) system).
The seven PAHs listed in Table 7.2 have the highest environmental risk:
benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene,
benzo(a)pyrene, indeno(1,2,3-c,d)pyrene, and dibenzo(a,h)anthracene.
7.1.1.3 PAH bioavailability
As indicated earlier, contaminants react chemically and physically with dif-
ferent kinds of soil particles, which change the physical and chemical natures
of both components. Biological availability, or bioavailability, is used to
describe both the amount of toxin available in soil to harm organisms
(humans, other animals, plants) and, in the case of bioremediation, the
amount of toxin available to be metabolized by microorganisms after con-
taminant-soil interactions.
In situ
bioremediation is a managed or spontane-
ous process in which microbiological processes are used to degrade or trans-
form contaminants to less toxic or nontoxic forms, thereby remedying or
eliminating environmental contamination. Although these microbiological
processes may decrease contaminant concentrations to levels that no longer
pose an unacceptable risk to the environment or human health, the contam-
inants that remain in treated soils still might not meet stringent regulatory
levels, even if they represent site-specific, environmentally acceptable end-
points (National Research Council, 1997). PAHs in soils may be biodegraded
by microorganisms to a residual concentration that no longer decreases with
time or that decreases slowly over years with continued treatment (Thoma,
1994; Luthy et al., 1994; Loehr and Webster, 1997). Further reductions are
limited by the availability of the PAHs to microorganisms (Bosma et al., 1997;
Erickson et al., 1993). Additionally, as contaminants age they become less
available than freshly contaminated material. The adherence and slow
release of PAHs from soils are other obstacles to remediation (National
Research Council, 1994; Moore et al., 1989). Because they bind with soils and
suffer subsequent slow-release rates, residual PAHs may be significantly less
leachable by water and less toxic as measured by uptake tests (Gas Research
Institute, 1995; Alexander, 1995; Kelsey et al., 1997). Generally, contaminants
can only be degraded when they exist in the aqueous phase and in contact
with the cell membrane of a microorganism (Fletcher, 1991). The contaminant
serves as a growth substrate for the microorganism and is incorporated into
the cell through membrane transport and utilized as an energy source in the
cell's principal metabolic pathways. However, physical or chemical phenom-
ena can limit the bulk solution concentration of the contaminant and thus
significantly reduce the ability of the microorganism to assimilate the con-
taminant. Therefore, the availability of the contaminant can control the over-
all biodegradation of these compounds.
