National Water Quality Assessment Program: Eastern Iowa Basins


Stephen J. Kalkhoff, Supervisory Hydrologist, U.S. Geological Survey, Iowa City, Iowa
Douglas J. Schnoebelen, Hydrologist, U.S. Geological Survey, Iowa City, Iowa
E. Michael Thurman, Research Hydrologist, U.S. Geological Survey, Lawrence, Kansas

Proceedings from Agriculture and the Environment: State and Federal Initiatives conference at Iowa State University, March 5-7, 2001

The occurrence of chloroacetanilide (acetochlor, alachlor, and metolachlor) and triazine (atrazine, cyanazine, and simazine) herbicides and degradates were investigated from 1996 to 1998 as part of the U.S. Geological Survey's National Water-Quality Assessment Program. More than 300 water samples were collected from 12 stream sites in the eastern Iowa. Monthly samples were collected for one year beginning in March 1996 and during the growing season (April to September) in 1998. Samples were collected weekly or biweekly at three sites in 1997 to determine temporal variability. Samples were analyzed for the chloroacetanilide herbicides, acetochlor, alachlor, and metolachlor, and their sulfonic (ESA) and oxanilic acid (OA) degradates. Samples were also analyzed for the triazine herbicides, atrazine, cyanazine, and three of their common degradates.

Chloroacetanilide and triazine herbicide degradates are substantial components of the pesticide load in eastern Iowa streams. Degradates are detected more frequently and at greater concentrations than the parent compounds. When adjusted to a common detection threshold of 0.20 micrograms per liter, the four most frequently detected compounds were metolachlor ESA (detected in 99.7 percent of the samples), alachlor ESA (99.1 percent), metolachlor OA (94.3 percent), and acetochlor ESA (71.8 percent). Metolachlor, the most commonly occurring parent compound, was detected in 54.1 percent of the samples. On average, degradates accounted for most (93 percent) of the total pesticide mass measured in each sample. Metolachlor compounds account for 63 percent; alachlor for 19 percent; acetochlor for 9.5 percent; atrazine for 6.9 percent and cyanazine for 1.2 percent of the degradates. The monthly median total metabolite concentration ranged from 4.6 micrograms per liter in February to 10.0 micrograms per liter in June. In contrast, the monthly median total pesticide concentration ranged from 0.16 micrograms per liter in October to 3.8 micrograms per liter in June. The data suggest that differing chemical properties, application rates, and precipitation may affect metabolite concentrations in streams.

The largest quantities of pesticides and pesticide degradates are transported from eastern Iowa basins during the late spring and early summer soon after application. Monthly degradates loads in the Iowa River at Wapello were higher than the loads of the parent compounds throughout the year but degradates loads were particularly dominant during the spring and early summer. In the fall and winter months, however, the pesticide degradates accounted for nearly all of the pesticide compounds transported to the Mississippi River. Parent compounds accounted for from 3 percent (December) to 27 percent (May) of all pesticide compounds transported by the Iowa River. During the relatively dry late fall and winter months, much of the water and dissolved pesticide compounds originated from ground-water discharge to streams. The presence of relatively high concentrations of alachlor and metolachlor degradates in the fall and winter, several months after pesticide application, indicates that these compounds are relatively stable.

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