U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems, Water-Resources Investigation Report 99-4018B

Ratios of Metolachlor to its Metabolites in Ground Water, Tile-Drain Discharge, and Surface Water in Selected Areas of New York State

By Patrick J. Phillips, David A. Eckhardt, E. Michael Thurman, and Stephen A. Terracciano

This report is available in pdf format: Phillips .pdf

ABSTRACT

The transport and fate of the herbicide metolachlor and its principal metabolites - metolachlor ESA (ethanasulfonic acid) and metolachlor OA (oxanilic acid) - in ground water from Suffolk County, on Long Island, from tile drains in an agricultural area in central New York and in surface water from five streams in central and western New York, was evaluated in 1997-98. Comparison of (1) the ratio of metolachlor ESA to metolachlor (SAM ratio) and (2) the ratio of metolachlor ESA to metolachlor OA (SAO ratio) in well water from Suffolk County with these ratios in tile-drain runoff and in the five streams indicates that these ratios vary according to soil properties, hydrologic factors, and time since metolachlor application. Both ratios were lower in well-water samples from Suffolk County (which is dominated by coarse-grained soils), than in the tile-drain samples from central New York fields (which have finer grained soils). The lower SAM ratios for the Suffolk County well-water samples than for the tile-drain samples probably result from the more rapid transport and lesser degradation of metolachlor in the coarse soils than in the fine-grained soils. The lower SAO ratios in Suffolk County well water also result from rapid infiltration of metolachlor OA through the coarse grained soils. SAM ratios for the surface-water samples were lower than those for the well-water and tile-drain samples, probably because surface water has a shorter residence time in the soil than ground water; the shorter contact time with soil microbes and enzymes than in ground water allows less degradation of metolachlor to metolachlor ESA.

SAM and SAO ratios for samples from one of the tile drains decreased after rainstorms that followed metolachlor application. SAM ratios for this tile drain decreased from more than 500 to less than 1 during the storm, and SAO ratios decreased from about 3 to less than 2. These decreases are attributed to preferential flow that accelerated the movement of metolachlor and metolachlor OA into the subsurface during the storm. SAM ratios in the stream that receives discharge from the tile drains were lowest during stormflows in June 1998 that followed metolachlor application. These low postapplication SAM ratios are attributed to the water's short contact time with soil and, hence, minimal transformation of metolachlor to metolachlor ESA.