A SITE-SPECIFIC FARM-SCALE GIS APPROACH FOR REDUCING GROUNDWATER CONTAMINATION BY PESTICIDES

It has been proposed to vary pesticide applications by patterns in sur face organic C to reduce the potential for contamination of groundwate r. To evaluate the feasibility of this ''precision farming'' approach, data for carbofuran concentrations from 57 locations sampled to a dep th of 180 cm were fit to the convective-dispersive equation. Fitted va lues for pore water velocity (v) ranged from 0.17 to 1.92 cm d(-1), wi th a mean of 0.68 cm d(-1) Values for dispersion (D) ranged from 0.29 to 13.35 cm(2) d(-1), with a mean of 3.39 cm(2) d(-1). Values for the retardation factor (R) ranged from 1.28 to 6.83, with a mean of 2.57. With this data, risks of pesticide leaching were estimated at each loc ation using the attenuation factor (AF) model, and a dispersion based leached factor (LF) model. Using the AF model gave two locations with a very high pesticide leaching risk, 32 with a high pesticide leaching risk, 15 with a medium risk, 6 with a low risk, and 2 with no risk. U sing the LF model, 6 had a high risk, 13 had a medium risk, 18 had a l ow risk, and 20 had no risk. Pesticide leaching risks were not correla ted with any measured surface soil properties. Much of the variability in leaching risk was because of velocity variations, so it would be i ncorrect to assume that surface organic C content controls the leachin g risk.