3    PREDICTIVE MODELS FOR VOC PARTITIONS IN SOILS
Ju-Chang Huang, Professor and Director
Environmental Research Center
University of Missouri-Rolla
Rolla, Missouri 65401
Hossein Ganjidoost, Assistant Professor
Civil Engineering Department
University of Tehran
Tehran, Iran
INTRODUCTION
Chloroorganic solvents, such as 1,1,1-trichloroethane (TCA), trichloroethylene (TCE) and tetra-
chloroethylene (or perchloroethylene, PCE), have been found in many contaminated sites and
groundwaters. The outward migration of these chemicals in saturated soils is largely dependent upon
their interactions with the soil particles. In general, the adsorption of these volatile organic compounds (VOC) by most soils are only slight to moderate since plumes of the VOC's have been detected
quite far away from the initial sites of contamination.1,2
In today's engineering practice, it is often necessary to estimate the rate of transport of these VOC's
in the subsurface environment. Many engineers attempt to solve the problem by taking numerous soil
cores and water samples surrounding the contaminated site. After sample analyses, iso-concentration
curves (contour) for both water and soil are "roughly" sketched. The results are then used for
modelling the movement of these VOC's in their future outward migration. Although this approach of
solving the problem seems to be quite logic, it is expensive and tedious because of the need for making
numerous soil borings. In addition, the organic solvents are extremely volatile. As such, they can be
easily lost into the atmosphere during the boring, handling and chemical extraction of the water and
soil samples unless extreme precautions are taken. Therefore, many iso-concentration curves which
are plotted from "less than satisfactory" data are not truly representative of those existing in the field.
If the future transport were estimated according to such inaccurate iso-concentration curves, significant errors would undoubtedly be introduced.
There is an alternative to solving such a problem. It is known that in an aquifer system containing a
homogeneous distribution of soils, the mobility of a VOC is related to its chemical partition in the
solid and the liquid phases according to the following equation:
/ Kg Solid/L of Aquifer Vol. \ / B \
R   =  l/vr = 1 + K    —     = 1 + K    - (1)
\ Kg Water/L of Aquifer Vol. I \ e /
where Rf =  retardation factor, or the ratio of the VOC residence time in a segment
of the aquifer to the residence time of water;
Vr =  relative velocity of the VOC movement to water;
K  =  partition coefficient of the VOC in the aquifer;
B  =  bulk density of the aquifer material; and e = porosity of the aquifer.
Equation 1 can also be rewritten as follows:
Rf = 1 + Kp(l - e)/e (2)
where p  =  density of the aquifer material.
In the application of Eq. 1 or Eq. 2, the terms of B, e, and p are only related to the physical
characteristics of the aquifer system, and their values are easily determinable. It is the term K that
44th Purdue Industrial Waste Conference Proceedings, © 1990 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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