Natural Attenuation - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 983

other hand, larger volumes of groundwater become contaminated through dilution.
To overcome this contradiction, besides reduction of concentrations often a reduc-
tion in mass flow rates is required in some countries before accepting MNA. This
can only be achieved when other processes, i.e. degradation, volatilization and sorp-
tion, also contribute to the concentration reduction. Hence, substantial importance
should be given to these processes and their quantification.
The circumstances under which MNA is accepted as the sole, rather than an
accompanying, measure for contaminant site management varies for different coun-
tries. The US EPA ( 1999 ) emphases that 'MNA should not be considered a default or
presumptive remedy at any contaminated site'. Source removal or control techniques
remain an important measure to address the principal threat where applicable, and
MNA is only appropriate when the remediation timeframe is reasonable compared
to other alternatives.
In Germany, the time scale for MNA alone is not decisive, however the pro-
portionality of active remediation measures becomes decisive regarding decisions
on the implementation of MNA. This means, that MNA can only be accepted as a
stand-alone measure if other active remediation technologies are not proportional,
i.e. if they are too expensive in relation to their efficiency (LABO 2009 ).
In the Netherlands Natural Attenuation falls within the responsibility of the
regional authorities. An important boundary condition based on national legisla-
tion, however, is that unacceptable human health risks must be excluded and the
consequences of Natural Attenuation must be monitored.
22.2 Principles of Natural Attenuation
22.2.1 Plume Development and Transport Processes
Release of contaminants into soil and/or groundwater usually leads to formation of
contaminant plumes in groundwater. A contaminant plume is defined here as the
volume of groundwater containing a non-zero quantity of the contaminant. This
plume shows different states of development, i.e., an expanding, a steady-state or a
shrinking phase (see Fig. 22.1 ).
These phases of plume development depend on the mass balance of contaminants
for the groundwater compartment (Fig. 22.2 ). The plume is in steady state when the
input rate of contaminants into groundwater equals the output rate of contaminants
from groundwater (individual rates discussed below). If the input rate exceeds the
output rate the plume will extend, while a higher output than input rate will cause a
shrinking plume.
Inputs (into groundwater) result from dissolution from NAPL, desorption or
a direct release of dissolved contaminants via groundwater recharge or leachate.
Outputs (from the groundwater) are the above mentioned Natural Attenuation pro-
cesses (see Section 22.1.1 ), however (in a strict sense) excluding dispersion and
diffusion (Fig. 22.2 ). While dispersion and diffusion lead to a spreading of contam-
inants and thus to a reduction of contaminant concentrations, they do not lead to a