Analysis of river water for rice pesticides in eastern Arkansas from 2002 to 2008

Pesticides used in rice production have the potential to harm nontarget species. The objective of this study was to measure concentrations of selected rice pesticides in four rivers ill eastern Arkansas from spring through mid-August for 2002 through 2008 to determine if changes over time might be leading to environmental water quality problems above a threshold of 2 mu g L-1 (2 ppb). Most of the pesticide detections were in May, June, and early July. The probability of detecting a given compound in any sample ranged from 0.0042 on the St. Francis River to 0.0387 on the Cache River. After 2002, clomazone and quinclorac accounted for 55% to 96% of detections each year. Of the samples collected, 60% to 86% that contained a pesticide contained only one pesticide, and 12% to 34% of all samples collected contained two pesticides. Large concentrations were more common in samples collected upstream than those taken downstream. Clomazone and quinclorac were often detected on consecutive sampling dates, indicating the possibility that concentrations greater than 2 mu g L-1 may be present over an extended period of time. Compared to ecotoxicity data, observed levels of these pesticides do not appear to be causing an environmental problem unless there is strong synergism between clomazone and quinclorac over a period of several weeks.