FIGURE 3.3 Map for Island of Oahu, Hawaii depicting relative vulnerability to ground water contamination with the pesticide ethylenedibromide (EDB). Vulnerability ratings are based on the values of retardation factor (RF), which is used as an index of pesticide leaching through soils. (Khan and Liang 1989. Reprinted, by permission, from Springer-Verlag, 1989.)

applied pesticide mass that is likely to leach past the depth of interest. Equations used for calculating RF and AF are shown in Table 3.8. The scales used to group contaminants on the basis of RF and AF indices in Figure 3.3 are entirely arbitrary.

Two principal assumptions made in deriving RF and AF indices must be noted: (1) water is assumed to move through the soil under steady, unsaturated conditions, with net ground water recharge rate (q) representing the steady flux, and (2) the soil profile is assumed to be homogeneous, where the soil property values used are depth-weighted averages. Computational schemes to eliminate both restrictions can be waived, as discussed by Rao et al. (1985). Model errors resulting from the use of these simple indices may be partially evaluated by comparing the ranking of the leaching potentials of several pesticides as determined from RF and AF with rankings from more rigorous model simulations. Of particular interest are evaluation of: (1) errors introduced by using annual recharge rate neglecting temporal