30 times the force of gravity, forcing the soil solution out of the
soil, and through the basal hole to be collected around the marble in
the bottom of the larger tube. The extruded solution was retrieved and
the 3H20 activity was measured. The soil sample was dried at 105 C
for 48 h and weighed. The adsorbed 3H20 was determined by subtraction
of the 3H20 activity in the extruded solution from the initial 3H20 in
the injected solution after accounting for the original water content
of the samples. Adsorption isotherms were constructed by plotting
adsorbed versus solution 3H20 activity. Linear-adsorption coefficients
were calculated using linear regression forced through the origin. An
overall soil-column retardation factor, R, was calculated using
weighted mean adsorption coefficients of the gravel and fine-fraction
samples from each horizon.
Results and Discussion
Description of Model Parameters
Information input into the non-linear, least-squares curve-
fitting program that optimizes dimensionless parameters for the CD and
MIM models consists of the observed tritium breakthrough curve (BTC),
which is composed of data pairs consisting of the pore volumes of
solution and the radioactivity of that solution relative to the
activity of the tritiated-pulse solution. The curve-fitting program is
capable of estimating the retardation factor, (R), Peclet number (P),
fraction of solutes in the mobile water region (f), dimensionless mass-
transfer coefficient (w), and tritiated-pulse volume (T). Confidence