Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 384

avoid subsequent solubilization, the precipitated heavy metals should be removed from
the contaminated site.
10.7.1.2 Biostimulation and Bioaugmentation
Chapter 3 introduced the use of biological aids, biostimulation and bioaugmentation , as part
of the available tools for groundwater management. Biostimulation occurs when stimuli
such as nutrients and other growth substrates are introduced into the ground to pro-
mote increased microbial activity of the microorganisms existent in the site. The intent
is to obtain improved capabilities of the microorganisms to more effectively degrade the
organic chemical contaminants in the soil. The addition of nitrates, Fe(III) oxides, Mn(IV)
oxides, sulfates, and CO 2 , for example, will allow for anaerobic degradation to proceed.
Biostimulation is perhaps one of the least intrusive of the methods of enhancement of the
natural attenuation capacity soils.
Bioaugmentation denotes the process whereby exogenous microorganisms are intro-
duced in situ to aid the native or indigenous microbial population in degrading the
organic chemicals in the soil. The reason one would use bioaugmentation is presumably
because the microorganisms in the soil are not performing up to expectations. This could
be because the concentrations of microorganisms are insuficient, or maybe because of
inappropriate consortia. The function of the exogenous microorganisms is to augment
the indigenous microbial population such that effective degradative capability can be
obtained. Frequently, biostimulation is used in conjunction with bioaugmentation. There
is the risk that (a) the use of microorganisms grown in uncharacterized consortia, which
include bacteria, fungi, and viruses, can produce toxic metabolites (Strauss, 1991) and (b) the
interaction of chemicals with microorganisms may result in mutations in the microorgan-
isms themselves and/or microbial adaptations.
10.7.1.3 Biochemical and Biogeochemical Aids
Introduction of geochemical aids in situ utilizes the same techniques employed to intro-
duce the various kinds of growth substrate, nutrients, and exogenous microorganisms
for biostimulation and bioaugmentation. Manipulations of pH and pE or Eh using geo-
chemical aids can increase the capability of the soil to mitigate the impact of some toxic
contaminants. A good case in point is the changes in toxicity for chromium and arsenic
because of changes in their oxidation state. Chromium (Cr) as Cr(III) is an essential nutri-
ent that helps the body use sugar, protein, and fat. Meanwhile, chromium as Cr(VI) has
been determined by the World Health Organization (WHO) to be a human carcinogen.
Cr(III) can be oxidized to Cr(VI) by dissolved oxygen and quite possibly with manga-
nese dioxides. If such a possibility exists in a ield situation, management of the potential
impact can take the form of in situ geochemical and/or biogeochemical intervention to
create a reducing environment in the subsurface. A useful procedure would be to deplete
the oxygen in the subsurface to create a reduced condition in the soil. The danger or risk
of manipulation of the Eh of the soil-water regime is incomplete knowledge of all the
elements in the subsoil that are vulnerable to such manipulation. The case of arsenic in
the ground is a good example. It is known that arsenic (As), as As(III) is more toxic than
As(V). If ground conditions show that arsenic is present as As(V), creating a reducing
environment to prevent oxidation of chromium to the more toxic oxidation state would
create the reverse effect on As(V). Reduction of As(V) to As(III) would increase the toxic-
ity of arsenic.