Chapter 5
An Integrated Approach for Quantifying Pesticide Dissipation under Diverse Conditions II: Field Study Observations 1
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S. A . Cryer , H. E . Dixon-White , C . K. Robb , P . N. Coody , and J. White 1
Dow AgroSciences, 9330 Zionsville Road, Indianapolis,IN46268 Bayer Corporation, 8400 Hawthorn Road, Kansas City,MO64120 Stone Environmental Inc., 58 East State Street, Montpelier,VT05602 2
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A n extensive multi-year runoff study was performed to quantify chlorpyrifos dissipation, leaching, and edge-of-field transport in a commercial corn production watershed. All runoff leaving the 17.29-acre watershed was quantified before entering a 0.6-acre farm. In addition, small scale runoff meso-plots (~0.16 acres) were both nested within the watershed and nearby to address scaling issues in extrapolating meso-plot results to watershed predictions. During 1992, the total chemical edge-of-field transport as quantified by flumes and pond monitoring corresponded to approximately 0.24 % of the seasonal chlorpyrifos applied. Chlorpyrifos edge-of-field transport during 1993, a year of intense precipitation and flooding, was approximately 0.38 % of applied. Even under the extreme precipitation conditions, limited amounts of chlorpyrifos were transported off-site.
© 2002 American Chemical Society
In Pesticide Environmental Fate; Phelps, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
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Introduction
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Details of an extensive field study design to quantify edge-of-field runoff has been documented elsewhere (1). A l l runoff leaving a 17.29-acre watershed was channeled through sampling flumes for quantification and directed into a neighboring farm pond. Observations of hydrology, erosion, chlorpyrifos transport, and dissipation in soil, pond water, pond sediment, and corn from this multi-year runoff study are presented. Data generated from this study is an ideal candidate for use in model validation procedures. Additional information regarding site-specific model validation and regional extrapolation of results can be found in the follow-up documentation for this work (2-3).
Observations In 1992, no significant amount of precipitation occurred until late in the growing season around the time of the third application (Figure 1). Runoff from the relatively small events on days 184 and 187 were not properly sampled by the field equipment due to the position of the Teflon sampling tubes in the flume and the resulting cavitation. This problem was corrected for all later runoff events by installing a mixing chamber below the outflow of the flume from which samples were withdrawn. However, since the pond water and sediment were intensively sampled, the mass of chlorpyrifos leaving the field during these runoff events was estimated by differences in pond matrix concentrations.
Figure 1. Precipitation and runoff events during the 1992 growing season.
In Pesticide Environmental Fate; Phelps, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
59 Excessive precipitation during the 1993 growing season produced severe flooding in a nine-State area in the upper Mississippi River basin (4). Total rainfall in 1993 from planting (Day 139) up to the day before the second application (Day 179) was 7.69 inches. Precipitation within 12 days following the Lorsban* 4E insecticide application (Day 180-192) was 8.38 inches, with 5.46 inches falling on days 185-186. The rainfall received by the watershed for the runoff events on days 184-207 totaled 9.05 inches, or 681,603 ft . A large runoff event occurred on day 186, but the pond had inundated the sampling flumes at that time, and the flow-measuring portion of the study was terminated. Chlorpyrifos loadings from the Julian day 186 runoff event were estimated from residue measurements of the receiving pond. Downloaded by TUFTS UNIV on December 6, 2015 | http://pubs.acs.org Publication Date: March 1, 2002 | doi: 10.1021/bk-2002-0813.ch005
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Figure 2. Precipitation and runoff events during the 1993 growing season.
Water and Eroded Sediment Transport Near drought conditions early in 1992 were followed by massive precipitation amounts having almost a 100-year return frequency between day 184-207 during 1992. Based upon a 500-year weather simulation using the weather generator program C L I G E N (CLImate GENerator (5)) for Grinnell, Iowa, 9.05 inches between Days 184-207 would occur l-in-71 years. A total of
* Trademark of Dow AgroSciences
In Pesticide Environmental Fate; Phelps, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.
60 6 distinct periods of runoff into the pond were quantified in 1992, either by runoff sampling in the flume and/or by mass difference pre/post a runoff event between pond water and pond sediment . Total runoff leaving the watershed through the flumes during the 1992 study interval measured 130,505 ft , or 19.2% of the total rainfall. The 1993 year had the largest monthly precipitation amounts in June and July ever recorded in the 121 years of records, with 16.55 inches falling at the study site from day 139 through day 199. Heavy rainfall between day 179 and 186 (7.38 inches) produced 4 discrete runoff events. Precipitation that occurred between Julian days 179-186 corresponds to a l-in-500 to l-in-833 year return frequency as determined by climatic modeling using the program C L I G E N . During the shorter monitoring interval in 1993, the quantified runoff from the watershed measured 131,152 ft , or 22.5 % of the total rainfall 1
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Chemical Transport Chlorpyrifos transport ranged from 91 mg for small events to 37.5 g for the most significant runoff events captured late in 1993. Most of the chlorpyrifos was transported sorbed to the eroded sediment with values ranging from 63 to 99.5% for distinct runoff events. This distribution between soluble and sediment-bound pesticide is largely described by the soil/water partition coefficient (Kd) for the compound. Although a K
