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Dissipation of the Herbicides Atrazine and Alachlor in a Maryland Corn Field¹

ABSTRACT

Atrazine and alachlor are commonly used in Maryland corn fields (Zea mays L.), primarily as pre-emergence herbicides. This research was part of a watershed program designed to monitor nonpoint source pollution due to runoff of chemicals from agricultural lands. Atrazne (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and alachlor (2-chloro-2',6'-diethyl-N-methoxymethyl acetanilide) were sprayed on a corn field at rates of 1.7 kg/ha and 2.3 kg/ha, respectively. Soil cores were taken from sites at four elevations in the field. At the highest elevation, atrazine underwent a decrease in the surface 5-cm layer, but a fraction of atrazine was leached into the deeper soil profile. At the middle altitude, atrazine concentrations in the 2.5- to 30-cm soil profile changed very little during the growing season. At the lowest elevation, where soil moisture was rather high, atrazine had an even distribution in the soil profile. Thus, the data on field soil indicated both downward and lateral movement of atrazine. Alachlor remained in the top 8 cm of the soil. Low teachability of alachlor in field soil is believed to result from a strong affinity of alachlor molecules for soil organic matter. Runoff losses were detected at a weir situated near the bottom of the drainage channel of the watershed. During the 1976 growing season, 17 g/ha of atrazine and 3.6 g/ha of alachlor were discharged. About 60% of the discharged herbicides were in the dissolved phase. Flow-weighted mean herbicide concentrations in the runoff water were 16.9 µg/liter (atrazine) and 0.6 µg/liter (alachlor). Atrazine carryover detected in the corn field soils was generally from 0.09 to 0.56 µg/g. Between 5 and 13% of the atrazine applied to tbe fields was estimated to be carried over to the next growing season.

Key Words: movement in field soils • runoff • carry-over

¹ Contribution from Chesapeake Bay Center for Environmental Studies, Smithsonian Institution, P.O. Box 28, Edgewater, MD 21037.

² Chemist, Smithsonian Institution, Edgewater, Md.

Received for publication July 7, 1979.

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