Sustainable Water Management - Sustainable Practices in Geoenvironmental Engineering - page 94

destroying contaminants while the contaminants are being released into the groundwater.
A mass balance analysis can be used to estimate the long-term destruction or immobiliza-
tion rates (NRC, 2000). For hydrocarbons, the availability of electron acceptors or donors
may be evaluated to determine the sustainability of remediation techniques such as natu-
ral attenuation for hydrocarbons. However, in the case of metals and metalloids such as
arsenic, this approach is only applicable if the attenuation is biologically driven.
Monitored natural attenuation (MNA), because of its adherence to “remedy by natural
processes” necessitates a proper understanding of the many principles involved in the nat-
ural processes that contribute to the end result. Monitoring of the contaminant plume at
various positions away from the source is a key element of the use of MNA. Remembering
that this (MNA) is a contaminant and soil-speciic phenomenon, one generally tracks a
very limited number of contaminants, and speciically the ones considered to be the most
noxious. Historically, more attention has been paid to documenting the properties and
characteristics of the contaminants. By and large, the contaminants tracked have primarily
been the organic chemicals including, for example, chlorinated solvents (PCE, TCE, and
DCE), and hydrocarbons such as benzene, toluene, ethyl benzene, and xylene (BTEX).
When active controls or agents are introduced into the soil to render attenuation more
effective, this is called enhanced natural attenuation (ENA). This is to be distinguished from
engineered natural attenuation (EngNA), which is probably best illustrated by the permeable
reactive barrier (PRB) shown in Figure 3.11 and the barrier-liner system shown in Figure
3.12. ENA refers to the situation where, for example, nutrient packages are added to the soil

Nest of monitoring wells

Nests of treatment wells

Groundwater table
is near surface

Monitoring
wells

Contaminant plume

Direction of
groundwater flow In situ reactive region (IRR)

(treatment zone)

FIGURE 3.11
Enhancement of natural attenuation using treatment wells. Treatments for enhancement can be any or all of the
following: geochemical intervention, biostimulation, and bioaugmentation. Treatment occurs in the contami-
nant plume and down-gradient from the plume. (From Yong, R.N. and Mulligan, C.N., Natural Attenuation of
Contaminants in Soils , CRC Press, Boca Raton, 310 pp., 2004.)