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Pesticide Fate and Transport throughout Unsaturated Zones in Five Agricultural Settings, USA

^a U.S. Geological Survey, 1730 East Parham Road, Richmond, VA 23228
^b U.S. Geological Survey, P.O. Box 25046, MS 407, Lakewood, CO 80225-0046
^c U.S. Geological Survey, 5231 South 19 Street, Lincoln, NE 68512
^d U.S. Geological Survey, 5957 Lakeside Boulevard, Indianapolis, IN 46278
^e U.S. Geological Survey, 934 Broadway, Suite 300, Tacoma, WA 98402

^* Corresponding author (thancock{at}usgs.gov)

Received for publication January 12, 2007. Pesticide transport through the unsaturated zone is a function of chemical and soil characteristics, application, and water recharge rate. The fate and transport of 82 pesticides and degradates were investigated at five different agricultural sites. Atrazine and metolachlor, as well as several of the degradates of atrazine, metolachlor, acetochlor, and alachlor, were frequently detected in soil water during the 2004 growing season, and degradates were generally more abundant than parent compounds. Metolachlor and atrazine were applied at a Nebraska site the same year as sampling, and focused recharge coupled with the short time since application resulted in their movement in the unsaturated zone 9 m below the surface. At other sites where the herbicides were applied 1 to 2 yr before sampling, only degradates were found in soil water. Transformations of herbicides were evident with depth and during the 4-mo sampling time and reflected the faster degradation of metolachlor oxanilic acid and persistence of metolachor ethanesulfonic acid. The fraction of metolachlor ethanesulfonic acid relative to metolachlor and metolachlor oxanilic acid increased from 0.3 to >0.9 at a site in Maryland where the unsaturated zone was 5 m deep and from 0.3 to 0.5 at the shallowest depth. The flux of pesticide degradates from the deepest sites to the shallow ground water was greatest (3.0–4.9 µmol m^–2 yr^–1) where upland recharge or focused flow moved the most water through the unsaturated zone. Flux estimates based on estimated recharge rates and measured concentrations were in agreement with fluxes estimated using an unsaturated-zone computer model (LEACHM).

Abbreviations: BSFOD, branched, serial, first-order decay • DEA, deethylatrazine • ESA, ethanesulfonic acid • f_oc, mass fraction of organic carbon in the soil • K_d, soil–water distribution coefficient • K_oc, organic carbon-normalized distribution coefficient • LEACHM, Leaching Estimation And CHemistry Model • OXA, oxanilic acid • RZWQM, Root Zone Water Quality Model

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