in the predictive capacity of the model is improved as the number of
parameters which the model is required to predict decreases. The
curve-fitting procedure that estimates the model parameters from the
BTC, bases the parameter-selection process on the goodness of fit of a
model-predicted BTC with the observed effluent data. The model
calculates a 95% confidence interval for each estimated parameter;
however, the confidence interval measures the goodness of fit of the
estimated parameters to the effluent curve and does not involve any
consideration of random experimental error. Therefore, the final
estimation of soil-property parameters requires judicious
interpretation of the model-estimated parameters.
Several of the dimensionless parameters are measurable by
laboratory techniques. The retardation coefficient may be calculated
from an adsorption coefficient, KD, the water content, and the bulk
density. The tritiated-pulse volume may be measured during the
miscible-displacement process. Experimental methods to measure the
other three parameters, P, 8, and w, are generally unavailable.
The dimensionless parameters P and w are specific to the
particular conditions of the experiment from which they are derived.
The Peclet number relates [Eq. 4-14] the column's length and pore-water
velocity to the dispersion coefficient. Dispersion results from
physical mixing of soil water travelling at different velocities or
following different paths. The dispersion coefficient is an indicator
of soil-pore sizes and the pore-size distribution. Since the velocity
of water in a confined capillary is dependent on the capillary radius,
large capillaries can transport water more rapidly than smaller pores