Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 824

heavy metals for three soil textures (the total number of elements reported by
Thibault et al. ( 1990 ) was 49). It is evident that the range of values encountered
for various soil types varies significantly and over several orders of magnitude. In
general, K d values for sand are lower than for silt or clay for the elements shown.
Although not shown in Table 18.3 , Thibault et al. ( 1990 ) noted that elements such
as silver, nickel, americium and thallium showed higher mean K d values for organic
soils than for clay.
In yet another study, Jacques et al. ( 1999 ) reported the spatial variability of the
Freundlich K f and n parameters for atrazine and their correlation with soil textural
variables, cation exchange capacity and organic carbon content of a stagnic podzolu-
visol based on 93 samples. The CV values (of non-transformed variables) for sand,
loam, clay and organic carbon content were 82.6, 37.2, 32.4, and 78.1%, respec-
tively. The CV values for CEC, K f and n were reported to be 35.5, 101.2, and 8.06%
respectively. These properties therefore have CV values that vary from moderate to
high, with K f showing the highest variation.
Dynamic Properties
Jury ( 1985 ) noted that, in general, the CV values of dynamic properties are higher
than those of the static properties. For instance, based on thirteen studies report-
ing replicated measurements of the saturated K s , the CV was found to be between
48 and 320%. Interestingly, both the lowest (48%) and the second highest (190%)
CV's were measured on finer-textured soils. Thus, Jury ( 1982 ) commented that there
appears to be no apparent relationship between the variability of K s and soil type,
at least within the soil textural classes considered in their compilation. His findings
have been contradicted by others (e.g. Cosby et al. 1984 ).
As regards the contaminant transport properties, observations of contaminant
concentration variability have also been compiled by Jury ( 1985 ). The results based
on four field scale studies in which enough replicates were taken to allow a mean-
ingful determination of sample concentration variance revealed a range of 61-127%
(Table 18.2 ). He also reported the log variance of contaminant velocity as another
transport property of interest. Based on six studies, a relatively low CV of 36% and
a high CV of 194% were reported.
18.3.4.2 Stream Tube Models
The downward movement of contaminants from the soil surface to an underly-
ing aquifer may be described stochastically by viewing the field as a series of
independent vertical columns, often referred to as “stream tubes” (Fig. 18.15 ),
while contaminant mixing between the stream tubes is assumed to be negligi-
ble. Transport in each tube may be described deterministically with the standard
advection-dispersion equation (ADE), or modifications thereof to include additional
geochemical and microbiological processes. Transport at the field scale is then
implemented by considering the column parameters as realizations of a stochastic
process, having a random distribution (Toride et al. 1995 ).