Environmental Engineering Reference
In-Depth Information
most cases, they lead to destruction of the target contaminant; however, these
processes are often very slow and thus may be impractical due to risk of
exposure to the public from the potential of contaminants in groundwater
reaching water wells or surface water. The selection and implementation of
both bioremediation and natural attenuation also suffer from a lack of famil-
iarization by remedial managers, typically civil and environmental engi-
neers, and, to a degree, the uncertainties as to long-term performance, par-
ticularly in the case of natural attenuation.
Beginning in the 1990s, a number of successful demonstrations of biore-
mediation technologies were reported for chlorinated solvent sites. These
included active reductive dechlorination systems (Beeman Sewell/Pinellas),
cometabolic systems (Savanah River, Moffet), and, later in the 1990s, a variety
of natural attenuation demonstrations (St. Joe, Dover). While these demon-
strations fulfilled the proof of concept and led to the development of defined
protocols for chlorinated solvent sites, such as the Technical Protocol for
Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water
(USEPA, 1998) and the Reductive Anaerobic Biological In Situ Treatment
Technology (ESTCP, 1998), a number of limitations on the use of biological
treatment approaches became clear. Some of these limitations were applica-
ble to all in situ bioremediation systems, including the challenges associated
with delivery and mixing of nutrients, and rates of cleanup were often
limited by subsurface transport rates rather than biological reaction rates.
However, some of the limitations were unique to DNAPL and chlorinated
solvent sites. Active and passive bioremediation technologies for oxidizable
contaminants such as fuels and petroleum proved to be metabolically robust
and redundant. This means that the genetic capability for the degradation
of a variety of compounds was found to be present at almost all sites where
toxicity was not a major issue. This catabolic capacity was found to exist for
multiple populations and redox conditions, such that a variety of electron
acceptors could be used in sequence to support degradation (Suflita and
Sewell, 1991). However, for chlorinated solvents, the conditions that sup-
ported transformation were more limited and the capacity, at least for com-
plete dechlorination, does not appear to be universal. These limitations
resulted in much slower rates of activity and potential instability in active
systems.
Another potential limitation is that the technologies are focused on dis-
solved phase contaminants and have not yet been shown to remediate areas
contaminated with separate phase contaminants. It has been believed that
the highly toxic nature of the separate phase residual contaminants found
in source zone environments suppresses bioactivity. Extensive research dem-
onstrating the biological transformations of dissolved phase chloroethenes
has been conducted (Fathepure and Boyd, 1988; DiStefano et al., 1991;
DeBruin et al., 1992; Tandoi et al., 1994; Magnuson et al., 1998). Recent
research has shown that complete dechlorination can occur even with initial
PCE concentrations as high as 91 mg/l (DiStefano, et al., 1991; Yang and
McCarty, 2000). The slow transformation rates coupled to the low dissolution
Search WWH ::




Custom Search