Contaminant Fate and Reactive Transport in Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 858

Fig. 19.4 Relative concentration as a function of distance in the flow direction, illustrating the
combined effect of advection and hydrodynamic dispersion on the migration of a contaminant
after an instantaneous ( left ) and a continuous release ( right )

Since hydrodynamic dispersion strongly influences the migration of contami-
nants in groundwater, the determination of dispersion coefficients and in particular
of hydromechanical dispersivities turns out to be of key importance for the descrip-
tion of contaminant transport. Research studies conducted primary on longitudinal
dispersion showed a remarkable difference between laboratory (typically 10 − 4 -
10 − 2 m) and field scale dispersivities, which were often found to be orders of
magnitude larger. This has led to extensive research on the role of field scale
macro-dispersion, e.g. by applying stochastic approaches. The reader interested in
stochastic approaches to describe subsurface flow and transport can refer to the work
of Dagan ( 1989 ) and Gelhar ( 1993 ), among others.

19.2.4 Sorption

In aquifer sediments, as in the upper (unsaturated) soil layers, contaminants are
temporarily removed from the groundwater by interaction with the solid matrix by
chemical, physical or electrostatic forces. This process is generally called sorption.
Two sorption phenomena can be typically distinguished: adsorption and absorption.
Adsorption refers to processes in which the contaminant accumulates on the sur-
face of a soil particle. Absorption describes processes in which the contaminant
penetrates into a separate phase e.g.organic matter in soils. In heterogeneous soils
and aquifers the two processes may occur simultaneously. For more comprehensive
information see the review by Allen-King et al. ( 2002 ).
The affinity of a dissolved contaminant for sorption by aquifer sediments
is commonly described by sorption isotherms (Grathwohl 1998 ; Schwarzenbach
et al. 1993 ). In the simplest case, the concentration in the solid, C s [M M − 1 ], is
proportional to the equilibrium contaminant concentration in the aqueous phase
C w [M L − 3 ]:

C s = K d C w (19.12)

where K d [L 3 M − 1 ] is the distribution coefficient. K d is the ratio between the con-
taminant concentration in solid soil particles and groundwater; it represents the
slope of the linear sorption isotherm (see Fig. 19.5 ).