Occurrence and distribution of pesticides in streams of the Eastern Iowa Basins, 1996-98

The U.S. Geological Survey began collection of water samples in streams of the Eastern Iowa Basins in 1996 for the analysis of pesticides and pesticide degradates as part of the National Water Quality Assessment Program (NAWQA). This study provides some of the first large scale monitoring data on pesticides and pesticide degradates in Eastern Iowa. Three hundred and forty-four samples were collected from 1996-98 to document the occurrence, distribution, and transport of pesticide compounds. Pesticide analysis included 80 pesticide compounds and 10 pesticide degradates. The Eastern Iowa Basins study encompasses about 50,500 square kilometers (19,500 square miles) and is drained by four major rivers--the Wapsipinicon, Cedar, Iowa, and Skunk. Agriculture accounts for approximately 93 percent of the land use in the study area.

The most commonly detected pesticides were those most heavily used on crops. The triazine (atrazine and cyanazine) and chloroacetanilide (alachlor, acetochlor, and metolachlor) pesticides are some of the most heavily used (by weight) historically and during the period of data collection 1996-98. Atrazine and metolachlor were detected in all samples. Acetochlor, alachlor, and cyanazine were detected in more than 70 percent of all surface-water samples. Few non-agricultural herbicides were detected. One exception, prometon was detected in more than 80 percent of the samples at very low concentrations (less than 0.1 micrograms per liter).

Pesticide degradates were some of the most frequently detected pesticide compounds in the study. Four pesticide degradates--metolachlor ethane sulfonic acid (metolachlor ESA), alachlor ethane sulfonic acid (alachlor ESA), metolachlor oxanilic acid (metolachlor OA), and acetochlor ethane sulfonic acid (acetochlor ESA) were detected in more than 75 percent of the samples.

A few insecticides that may pose potential risk to aquatic invertebrates were detected in streams from May through September, the months when most application normally occurs. Carbofuran was the most commonly detected insecticide (16 percent of all samples). Although detected in less than 20 percent of all samples, carbofuran was detected in 68 percent of the samples in June. When present, carbofuran concentrations were generally less than 0.80 micrograms per liter. Chloropyrifos was detected in about seven percent of the samples. As with other insecticides, chlorpyrifos was detected most frequently in June (30 percent). The highest concentration was 0.06 micrograms per liter. Diazinon, a common urban insecticide found in other NAWQA studies throughout the Nation, was detected in only 2 percent of the samples in the Eastern Iowa Basins study.

Pesticides were found to occur in mixtures with several compounds rather than individually. Four or more parent pesticide compounds were detected in 91 percent of the water samples and seven or more parent compounds were detected in 46 percent of the water samples. Four or more pesticide degradates were detected in 93 percent of the water samples and seven or more pesticide degradates were detected in 46 percent of the water samples.

Commonly applied parent pesticide compounds (acetochlor, alachlor, atrazine, cyanazine, and metolachlor) were generally detected at low concentrations with median concentrations ranging from 0.01 to 0.22 micrograms per liter. The median concentrations for the pesticide degradates were larger than their parent compounds. Median concentrations for the pesticide degradates ranged from 0.07 to 3.7 micrograms per liter. Acetochlor, alachlor, atrazine, cyanazine and metolachlor pesticides compounds were present at least an order of magnitude or higher in the late spring and summer than at other times of the year. The maximum measured concentrations for acetochlor, atrazine, cyanazine and metolachor were approximately 11 to 48 micrograms per liter (the maximum for alachlor was 0.56 micrograms per liter). In contrast, maximum measured concentrations for the total pesticide degradates were lower than their parent compounds and ranged from approximately 0.7 to 12 micrograms per liter. The maximum measured concentration of a single pesticide compound was for atrazine at 48 micrograms per liter.

Seasonal patterns of atrazine, acetochlor, alachlor, cyanazine, and metolachlor generally show peak concentrations following application in May and June and decreasing during remainder of the growing season. In addition, a small secondary peak in atrazine, acetochlor, alachlor, cyanazine, and metolachlor concentrations occurred at all sites in late winter. This secondary peak may be attributed to early "winter thaw" that can release pesticide residue from soil, making pesticides available to be transported to surface water by snowmelt and early spring rains.

Pesticide degradates account for a significant portion of the total pesticide load at all sites. Eighty-one percent of the total pesticide load in samples from Iowa River near Rowan, Wolf Creek near Dysart, and the Iowa River at Wapello were as pesticide degradates. The pesticide degradates for the triazine compounds tended to follow the load pattern of the parent pesticide compounds closely throughout the year. In general, the degradate loads calculated for the triazine compounds were smaller than loads calculated for their parent pesticides. The loads for the chloroacetanilide degradate compounds were larger than those for the parent pesticides. The loads for alachlor were found to be small or nonexistent. Alachlor has been heavily used in the past, but since 1995 has been largely replaced by acetochlor or other herbicides. The loads for all degradates were higher than the parent compounds during the winter months. Overland flow may be diminished during the winter months, but shallow sub-soil drainage and ground-water flow may be a source of many pesticide degradates during the late fall and winter.

Occurrence of pesticide compounds varied by landform region. The triazine herbicides, atrazine and cyanazine and their degradates were present in significantly greater concentrations in the Southern Iowa Drift Plain (predominantly loess soils) than either the Des Moines Lobe or the Iowan Surface (predominantly till soils). Less atrazine and cyanazine are applied to till soils because of pH and organic carbon content. Alachlor, metolachlor, and acetochlor have often been used to offset triazine pesticide reductions in area with till soils.