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Variations in Pesticide Leaching Related to Land Use, Pesticide Properties, and Unsaturated Zone Thickness

^a U.S. Geological Survey, Denver, CO
^b U.S. Geological Survey, Baltimore, MD
^c U.S. Geological Survey, Reston, VA
^d U.S. Geological Survey, Richmond, VA
^e U.S. Geological Survey, Tacoma, WA
^f U.S. Geological Survey, Indianapolis, IN

View larger version (92K): [in this window] [in a new window]   Fig. 1. (A) Location of the Morgan Creek watershed on the Delmarva peninsula. Also shown are the locations of the three weather stations in the Chesapeake Bay region that were used to fill in data or to provide observations back to October 1994. (B) Land use for growing season in 2004 determined from field surveys. The soybean field surrounding the intensive study site containing the flow-path wells is located near the outlet. The soils and instruments in the cross-section below X-X' are presented in Fig. 1D. (C) Unsaturated-zone thickness computed as the difference between the land surface elevation and the steady-state potentiometric head simulated with a ground-water model (MODFLOW). (D) Cross-section of Maryland intensive study site showing unsaturated-zone thickness, and the location of lysimeters and peizometers. Volumetric soil moisture was measured with time-domain reflectometers (TDRs) collocated with the pan lysimeters at M21.

View larger version (29K): [in this window] [in a new window]   Fig. 2. Observed versus predicted moisture content (A) and bromide concentration (B) for the calibrated LEACHM model.

View larger version (48K): [in this window] [in a new window]   Fig. 3. (A) Seasonal variations in runoff, infiltration, evapotranspiration, and recharge simulated for a 5-meter thick unsaturated zone. The sum of runoff and infiltration are equal to the total rain plus snowmelt for the same period. A difference between infiltration and the sum of evapotranspiration and recharge will result in a change of moisture content in the unsaturated zone. (B) Seasonal recharge for simulated for 1-m, 5-m, and 10-m thick unsaturated zones. Recharge that peaks in the winter at 1-m lags until the following fall before reaching a peak recharge at 10 m. Astronomical seasons are used: Fall (Oct.-Dec.); Winter (Jan.-Mar.); Spring (Apr.-June); and Summer (July-Sept.).

View larger version (38K): [in this window] [in a new window]   Fig. 4. Variations in mean daily bromide concentrations at 50 cm simulated for 2 yr following a hypothetical application of 208 mg m–2 on 28 April of each year starting in 1995. The dark gray envelope and the black line shows the minimum, maximum, and mean concentrations simulated for ten 2-yr periods following springtime applications from 1995 to 2004. The white lines show daily variations in concentrations during the wettest period (1996–97) and the driest period (2001–02). Astronomical seasons are used: Sp, Spring (Apr.-June); Su, Summer (July-Sept.); F, Fall (Oct.-Dec.); and W, Winter (Jan.-Mar.).

View larger version (23K): [in this window] [in a new window]   Fig. 5. Variations of leached bromide, atrazine, and metabolites simulated for 50 cm and 180 cm, for 2 yr following a hypothetical application of 208 mg m–2 on 28 April of each year starting in 1995. Values are 30-d moving averages. Astronomical seasons are used: Sp, Spring (Apr.-June); Su, Summer (July-Sept.); F, Fall (Oct.-Dec.); and W, Winter (Jan.-Mar.).

View larger version (14K): [in this window] [in a new window]   Fig. 6. Total leached mass of three parent pesticides and seven metabolites simulated by LEACHM for unsaturated zone thickness of 1.0, 2.5, and 5.0 m. Paraquat and glyphosate are not shown since simulations predicted no leaching even for the 1.0-m-thick unsaturated zone.

View larger version (14K): [in this window] [in a new window]   Fig. 7. Mass of five parent pesticides and seven metabolites simulated to leach past 1 m versus compound half-life, organic sorption coefficient, and solubility. [Abbreviations: ATR, Atrazine; HYA, Hydroxyatrazine; DEA, Deethylatrzine; DIA, Deisopropylatrazine; DDA, Didealkylatrazine; MET, Metolachlor; MES, Metolachlor ESA; MOX, Metolachlor OXA; GLY, Glyphosate; AMP, AMPA; SIM, Simazine; and PAQ, Paraquat.]

View larger version (26K): [in this window] [in a new window]   Fig. 8. Percent of applied pesticides and metabolites to leach to ground water simulated by the distributed LEACHM model and by the two-term regression model. [Abbreviations: ATR, Atrazine; HYA, Hydroxyatrazine; DEA, Deethylatrzine; DIA, Deisopropylatrazine; DDA, Didealkylatrazine; MET, Metolachlor; MES, Metolachlor ESA; MOX, Metolachlor OXA; AMP, AMPA; and SIM, Simazine.]

View larger version (62K): [in this window] [in a new window]   Fig. 9. Maps of the Morgan Creek watershed showing the total mass of metolachlor and metabolites applied or produced, transformed, leached, and residual in the unsaturated zone as of October 2004. A total of five corn-soybean rotations from 1995 through 2004 are simulated. All areas with corn–soybean rotation received a loading of 1.36 kg ha–1 of metolachlor preceding the corn crops. This resulted in a total load of 6.8 kg ha–1 for the 10 yr except for the intensive study field which received only four applications for a total of 5.44 kg ha–1. The pattern shows the location of corn and soybean fields, with greater leaching in areas with thinner unsaturated zones, and also greater residuals for areas that were in corn in 2004. Units are kg ha–1. White areas indicate values of zero.

View larger version (35K): [in this window] [in a new window]   Fig. 10. Pesticides and metabolites to leach to ground water as simulated by the distributed LEACHM model compared to that predicted by the two-term regression model. [Abbreviations: ATR, Atrazine; HYA, Hydroxyatrazine; DEA, Deethylatrzine; DIA, Deisopropylatrazine; DDA, Didealkylatrazine; MET, Metolachlor; MES, Metolachlor ESA; MOX, Metolachlor OXA; GLY, Glyphosate; AMP, AMPA; SIM, Simazine; and PAQ, Paraquat.]