Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 381

It is not easy to distinguish between abiotic and biotic (biologically mediated) redox
reactions. To a large extent, it is not always possible to eliminate or rule out involvement
of microbial activity in abiotic redox reactions. There does not appear to be a critical need
to distinguish between the two in reactions that concern organic chemical contaminants,
since it is almost certain that with all the microorganisms in the subsoil, some measure
of microbial activity would be involved. In any event, the number of functional groups of
organic chemical contaminants that can be oxidized or reduced under abiotic conditions
is considerably smaller than those under biotic conditions (Schwarzenbach et al., 1993).
The two classes of electron donors of organic chemical contaminants are (1) electron-
rich B-cloud donors, which include alkenes, alkynes, and the aromatics, and (2) lone-pair
electron donors, which include the alcohols, ethers, amines, and alkyl iodides. Similarly,
in the case of electron acceptors, we have (1) electron-deicient π-electron cloud acceptors,
which include the π-acids, and (2) weakly acidic hydrogens such as s -triazine herbicides
and some pesticides.
A measure of the electron activity in the porewater of a soil-water system is the redox
potential Eh. It provides us with a means for determining the potential for redox reactions
in the contaminant-soil-water system under consideration and is given as

2. RT

F

Eh = pE

(10.3)

where E is the electrode potential, R is the gas constant, T is the absolute temperature, and
F is the Faraday constant. The electrode potential E is given in terms of the half reaction

2H + + 2e − ⇔ H 2 (g) (10.4)

When the activity of H + = 1 and the pressure H 2 (gas) = 1 atmosphere, we obtain E = 0.

10.7 Natural Attenuation and Impact Management
The natural attenuation capacity of soils in the substratum has long been recognized and
described by soil scientists as the assimilative capacity of soils. The discussions at the
beginning of the previous section and in the earlier chapters of this topic show that this
is now a tool that can be used as a passive treatment process in the remediation and man-
agement of sites contaminated by organic chemicals. The USEPA has wisely coupled the
requirement for continuous on-site monitoring of contaminant presence whenever natural
attenuation is to be used as a tool for site remediation—as seen in the deinition provided
in the irst part of Section 10.4. The procedure for application of this attenuation process
is called monitored natural attenuation (MNA). Guidelines and protocols for application of
MNA as a treatment procedure in remediation of contaminated sites have been issued.
Since site speciicities differ from site to site, the prudent course of action is to adapt the
guidelines and protocols for site-speciic use. A general protocol, from Yong and Mulligan
(2004) for considering MNA as a remediation tool is shown in Figure 10.13. A very criti-
cal step in the application of MNA as a site remediation tool is to have proper knowledge
of (a) lines of evidence indicating natural or intrinsic remediation, (b) contaminants, soil