Modeling - Field Sampling: Principles and Practices in Environmental Analysis - page 183

model also requires meteorological, soil water partition coefficients, and kinetic rate
constants [6].
Kinetic models are useful when it is important to determine the rate of both production
and loss of a single species or component. Ammonia applied to soil is oxidized to nitrite
and subsequently to nitrate. The amount of nitrite existing at any one time will depend on
both these rates. Assuming a one-time contamination of a field and knowing the removal
rate of the contaminant, suitable sampling intervals can be developed that can reduce the
amount of sampling needed. If the rate of oxidation of nitrite is known, for instance, the
time needed for its level to decrease to acceptable levels or below action levels can be
calculated. In this case sampling and analysis can be done on the basis of the predicted
rate of loss rather than on an arbitrary schedule and consequently decrease the amount of
sampling needed.
Models will also incorporate such factors as vapor pressure and sorption and diffusion
characteristics, which are temperature-dependent, and thus the temperature becomes an
essential component of the equation. The soil characteristics (i.e., sandy or clayey), along
with dry and wet volumetric water contents, are important, along with solar radiation, air
temperature, relative humidity, and wind speed. All of these interact to moderate the
temperature changes in soil and so they also become part of the equation.

7.2.1. Computer Chemical Models
Computer models typically involve the use of numerical modeling discrete versions of
partial differential equations. This is particularly the case for modeling transport
processes. Table 7.2 gives a number of common computer programs used to model
chemicals in soil. In some cases these are used to model a single species and its form
under the conditions specified. In other cases they are intended to be more general. In
both cases the models use many assumed or inferred conditions, constants, and inputs. In
spite of the limitations many of the models can be useful in understanding the position
and movement of a contaminant in soil or in the environment in general.
Model usage involves three steps. The first step is to pick the appropriate model. This
may seem simple, but in some cases the suitability

TABLE 7.2 Common Soil Chemistry Models

Model Source

GEOCHEM Mattigod SV, Sposito G. Chemical Modeling of Trace Metal in Contaminated Soil
Solutions Using Computer Program.
GEOCHEM. In: Chemical Modeling in Aqueous Systems.
ACS symp. ser. 93. Jenne EA, ed. Washington, DC: American Chemical Society,
1979:837-856.
SOILCHEM Sposito G, Groves J. SOILCHEM on the MACINTOSH. In: Chemical Equilibrium
and Reaction Models. Loeppert RH, et al., eds SSSA spec. pub. 42. Madison, WI:
ASA and SSSA, 1990:271-288.