Occurrence of Sulfonylurea, Sulfonamide, Imidazolinone, and other Herbicides in Midwestern Rivers, Reservoirs, and Ground Water, 1998

Sulfonylurea (SU), sulfonamide (SA), and imidazolinone (IMI) herbicides are relatively new classes of chemical compounds that function by inhibiting the action of a plant enzyme, stopping plant growth, and eventually killing the plant. These compounds generally have low mammalian toxicity, but plants demon-strate a wide range in sensitivity to SUs, SAs, and IMIs with over a 10,000-fold difference in observed tox-icity levels for some compounds. SUs, SAs, and IMIs are applied either pre- or post-emergence to crops commonly at 1/50th or less of the rate of other herbicides. Little is known about their occurrence, fate, or transport in surface water or ground water in the United States. To obtain information on the occurrence of SU, SA, and IMI herbicides in the Midwestern United States, 212 water samples were collected from 75 surface-water and 25 ground-water sites in 1998. These samples were analyzed for 16 SU, SA, and IMI herbicides by USGS Methods Research and Development Program staff using high-performance liquid chromatography/mass spectrometry. Samples were also ana-lyzed for 47 pesticides or pesticide degradation products. At least one of the 16 SUs, SAs or IMIs was detected above the method reporting limit (MRL) of 0.01 micrograms per liter (µg/L) in 83 % of 130 stream samples. Imazethapyr was detected most frequently (71 % of samples) followed by flumetsulam (63 % of samples) and nicosulfuron (52 % of samples). The sum of SU, SA, and IMI concentrations exceeded 0.5 µg/ L in less than 10 % of stream samples. At least one SU, SA, or IMI herbicide was detected above the MRL in 24 % of 25 ground-water samples and 86 % of 7 reservoir samples