GROUNDWATER - Water Quality Engineering in Natural Systems - page 193

(a) (b)

Figure 5.27. (a) Pump-and-treat system; (b) disposal. Source : U.S. Environmental Protection Agency (2005b).

within a reasonable time period. In fact, contaminant
levels can readily return to higher values if pumping is
stopped. Pump-and-treat systems for dissolved contami-
nants work best when the rate of cleanup is controlled
by advection. If rate-limiting processes become control-
ling, such as nAPL dissolution rates or diffusion rates,
pump-and-treat systems will not be efficient at mass
removal. In these cases, agents that enhance the reme-
diation, such as surfactants and nutrients for microbial
activity, can be added to the water injected into the
aquifer; however, such materials should be chosen so as
not to cause other types of contamination. In many
cases, the cleanup of aquifers using the pump-and-treat
strategy to remediate groundwater to drinking-water
standards is not practical.

Figure 5.28. Bioremediation system. Source : Pacific north-
west national Laboratory (2005).

5.8.2.7 Bioremediation. Bioremediation is usually
carried out in situ and involves stimulating the indige-
nous subsurface microorganisms by the addition of
both nutrients and an electron acceptor. A typical bio-
remediation system is illustrated in Figure 5.28, where
upstream injection wells add nutrients and/or oxygen to
groundwater flowing toward the contaminated zone,
and a downstream extraction well withdraws water that
has passed through the contaminated zone. Typical
nutrients are nitrogen, potassium, and phosphorus, and
typical electron acceptors are oxygen, nitrate, sulfate,
and carbon dioxide. Oxygen is the most popular elec-
tron acceptor in aerobic biodegradation and is usually
added to the groundwater by sparging air, pure oxygen,
hydrogen peroxide, or ozone. Care should be taken
in using hydrogen peroxide, which is toxic to some
microbes. In many instances, the shallow subsurface
has sufficient nutrient material so that the only ingredi-
ent lacking for successful bioremediation is oxygen. A
wide variety of contaminants are amenable to bioreme-
diation, including gasoline hydrocarbons, jet fuels, oils,

aromatics, phenols, creosote, chlorinated phenols, nitro-
toluenes, and PCBs. Bioremediation of chlorinated
organic compounds, such as tetrachloroethene (PCE)
and trichloroethene (TCE), is most effective under
anaerobic conditions, where nitrate, sulfate, and carbon
dioxide can be used as electron acceptors.
For bioremediation to be a feasible remediation
option, the hydraulic conductivity must be sufficiently
high to allow the transport of the electron acceptor
and nutrients through the aquifer, and microorganisms
must be present in sufficient numbers and types to
degrade the contaminants of interest. Hydraulic con-
ductivities exceeding 1 m/d (3 ft/d) are considered suf-
ficient for transporting nutrients and oxygen in the
subsurface; however, microbial growth in aquifer mate-
rial can cause permeability to decrease by a factor of
1000. Bioremediation projects are generally preceded
by laboratory experiments of microbial stimulation and
modeling studies of nutrient delivery and transport to