Environmental Engineering Reference
In-Depth Information
rather than basing decisions solely on total concentration of contaminant in
the soil or sediment.
It is also necessary to improve assessment of the toxicity and risk from
residual hydrocarbons. Mechanisms that affect release rates and exposure
need to be better defined. The potential benefits of such work could include
reduced treatment costs, improved evaluation and design for cleanup tech-
nologies, greater regulatory and public acceptance of biotechnology,
increases in the reuse/recovery opportunities for treated contaminated sed-
iments, and potential application for
in situ
capping of contaminated soils
and sediments.
1.3
Biodegradation and bioavailability
Recalcitrant compounds in soils and sediments 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
believed to be limited by the availability of the recalcitrant compounds to
microorganisms (Bosma et al., 1997; Erickson, 1993). Attempts have been
made to increase this availability through the use of surfactants, but results
have varied (Putcha and Domach, 1993; Bury and Miller, 1993; Chung et al.,
1993; Edwards et al., 1994; Auger et al., 1995). Additionally, as contaminants
age, they become less available compared to freshly contaminated material.
As a consequence of binding with soils and sediments and subsequent slow
release rates, residual recalcitrant compounds may be significantly less leach-
able by water and less toxic as measured by uptake tests (Alexander, 1995;
GRI, 1995; Kelsey et al., 1997).
Generally, contaminants can be degraded only when they exist in the
aqueous phase and come into contact with the cell membrane of a microor-
ganism. In this way, the contaminant serves as a substrate for the microor-
ganism and is incorporated through membrane transport into the cell and
utilized as an energy source in the cell's principal metabolic pathways.
However, physical or chemical phenomena can limit the bulk solution con-
centration of the contaminant and thus significantly reduce the ability of the
microorganisms to assimilate the contaminant. Therefore, the bioavailability
of the contaminant can control the overall biodegradation of these com-
pounds.
Another important factor relevant to biodegradation and bioavailability
is the location and density of microorganisms. The majority of bacteria in
the environment are attached to surfaces, and their distribution in and on
soils/sediments is very patchy. The majority of these bacteria range in size
from 0.5 to 1.0
m, whereas micropores present in soils and sediments
measure far less than 1
μ
m. It is generally believed that bacteria are attached
predominantly to the surfaces of soils and sediments and not to the interior
surfaces of the micropores. It has been estimated that more than 90 of the
microorganism types present in geologic matrices accumulate on the surfaces
μ
