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Dissipation of Picloram in Storm Runoff¹

ABSTRACT

The extent of progressive decrease in concentration of the herbicide picloram (4-amino-3,5,6-trichloropicolinic acid) in intermittent streamflow was determined during runoff-producing storms. Maximum concentrations of picloram were 48 and 250 mg/m³ in initial runoff water leaving a sprayed 8-ha area in 1978 and 1979. Concentration decreased with distance from the treated area in proportion to the size of adjacent, untreated watershed subunits, which contributed runoff water to streamflow. Picloram concentrations usually decreased with each successive runoff event; an exception occurred when unusually large quantities of runoff (8.7 cm) diluted concentrations to levels less than those observed in the succeeding event (1.4 cm runoff). About 6% of the applied picloram left the treated area during a period of 1 month when conditions were especially conducive to transport of the herbicice in surface runoff water.

Rapid dilution of picloram was observed after direct introduction of the herbicide near the upper end of a larger stream during rising stage of flow. Maximum concentrations at 90, 1170, and 5400 m downstream from the point of introduction of 1270 g of picloram were 13 720, 470, and 5 mg/m³, respectively. The calculated quantity of picloram which was detected at the 5400-m sampling point in streamflow was only 0.13% of that introduced. Mixing of water during streamflow and additions of uncontaminated water along the channel apparently reduced picloram concentration to below that detectable by gas chromatography.

Key Words: herbicide • residue • dilution • watershed

¹ Cooperative investigations of the USDA-ARS and the Texas Agric. Exp. Stn., Temple, TX 76503 and College Station, TX 77843.

² Range Scientist, Agricultural Engineer, Research Agronomist, Soil Scientist, and Plant Physiologist, respectively, USDA-ARS, Temple and College Station, Tex.

³ Professor, Dep. of Soil and Crop Sci., Texas A & M Univ., College Station, Tex.

Received for publication June 29, 1983.