Handbook of Solvents - George Wypych - ChemTech - Ventech!

1572 Hanadi S. Rifai, Charles J. Newell, Todd H. Wiedemeier
tions, most chlorinated solvents are degraded only under specific ranges of
oxidation-reduction potential. For example, it is currently believed that PCE is biologically
degraded through use as a primary growth substrate only under strongly reducing anaerobic
conditions.
This chapter is focused on the natural attenuation behavior of CS at the field scale. The
first part of the chapter reviews many of the physical, chemical and abiotic natural attenuation
processes that attenuate CS concentrations in ground water. Some of these processes
have been described in more detail in previous chapters in the handbook and are therefore
only reviewed in brief. In the second part of this chapter, we will review the biological processes
that bring about the degradation of the most common chlorinated solvents, present
conceptual models of chlorinated solvent plumes, and summarize data from field studies
with chlorinated solvent contamination.
23.1.2 NATURAL ATTENUATION PROCESSES AFFECTING CHLORINATED
SOLVENT PLUMES
Many abiotic mechanisms affect the fate and transport of organic compounds dissolved in
ground water. Physical processes include advection and dispersion while chemical processes
include sorption, volatilization and hydrolysis. Advection transports chemicals
along ground water flow paths and in general does not cause a reduction in contaminant
mass or concentration. Dispersion or mixing effects, on the other hand, will reduce contaminant
concentrations but will not cause a reduction in the total mass of chemicals in the aquifer.
Sorption or partitioning between the aquifer matrix and the ground water, much like
dispersion, will not cause a reduction in contaminant mass. Volatilization and hydrolysis
both will result in lower concentrations of the contaminant in ground water. The majority of
these processes, with the exception of hydrolysis and dehydrohalogenation chemical reactions,
do not break down or destroy the contaminants in the subsurface.
Chlorinated solvents are advected, dispersed, and sorbed in ground water systems.
They also volatilize although their different components have varying degrees of volatility.
Chlorinated solvents additionally hydrolyze and undergo other chemical reactions such as
dehydrohalogenation or elimination and oxidation and reduction. These abiotic reactions,
as will be seen later in the chapter, are typically not complete and often result in the formation
of an intermediate that may be at least as toxic as the original contaminant.
23.1.2.1 Advection
Advective transport is the transport of solutes by the bulk movement of ground water.
Advection is the most important process driving dissolved contaminant migration in the
subsurface and is given by:
where:
v
x
K dH
=− [23.1.1 ]
n dL
e
Vx seepage velocity [L/T]
K hydraulic conductivity [L/T]
ne effective porosity [L 3 /L 3 ]
dH/dL hydraulic gradient [L/L]