Geoscience Reference
In-Depth Information
Fig. 12.35 Distribution of the resident concentration of A terbuthylazine and B bromacil in a
vertical cross section after leaching with (A1, B1) 7.3 cm and (A2, B2) 89.5 cm of applied water.
Contour spacing is 0.1 times the mean concentration (Toiber-Yasur et al.
1999
)
the effects of contaminant dispersion and assumed that transport is controlled by
three mechanisms: advection by water flow, equilibrium sorption, and exponential
mass loss. Transport of conservative contaminants (advection only) was modeled
by averaging the solution over probability density functions for the various model
parameters (including, saturated hydraulic conductivity, water content, and rate of
water application); experimental data for bromide concentrations were used to
constrain parameter value estimates. The transport of two reactive chemicals,
bromacil and terbuthylazine, then was modeled. The calculated and measured
concentrations of bromide, bromacil, and terbuthylazine, 18 and 32 days after
chemical application in the irrigated field, are shown in Fig.
12.37
.
The analysis was limited in part by the scarcity of measurements, and clear
discrepancies between measured and calculated values may be observed. As dis-
which was not accounted for in this model. Interestingly, the field-scale retardation
coefficient values of the reactive contaminants were smaller by an order of mag-
nitude than their laboratory values, obtained in an accompanying experiment.
Another modeling analysis is presented by Russo et al. (
1998
), who examined
field transport of bromacil by application of the classical one-region, advection-
dispersion equation (ADE) model and the two-region, mobile-immobile model
(MIM); recall
Sects. 10.1
and
10.2
. The analysis involved detailed, three-dimen-
sional numerical simulations of flow and transport, using in situ measurements of
hydraulic properties and adsorption-desorption and degradation rate coefficients
obtained from laboratory measurements.
