The Toxic Substances Hydrology Program studied the runoff of pesticides in Midwestern agricultural fields such as this one as part of the Midcontinent Herbicide Reconnaissance.

The USGS sampled 52 sites on Midwestern streams during post-application runoff in 1989, 1990, 1994, 1995, and 1998 as part of a reconnaissance of herbicide concentrations in streams.

Outflow from a tile line that drains a northwestern Iowa agricultural field in the study area for the Midcontinent Herbicide Reconnaissance.

USGS technicians collecting water samples from a bridge for a reconnaissance of herbicide concentrations in streams.

USGS scientist lowering a water-quality sampler into the Iowa River near Marengo, IA (05453100), during the 1993 flood of the upper Mississippi River Watershed. The samples were analyzed for nutrients and pesticides.

Collecting a water sample (grab sample) for the reconnaissance of herbicide concentrations in streams.

Analyzing water samples for herbicide concentrations using an immunoassay test. Immunoassay tests are an economical way of screening a large number of samples for pesticides.

Concentrations of herbicides in flood waters from the 1998 flood of the Nishnabotna River in southwestern Iowa. The highest herbicide concentration was 5.06 micrograms per liter (µg/L) for atrazine (Kolpin and others, 2000).

Damage caused by the historic 1993 floods of the Upper Mississippi River Basin near a well sampled for a reconnaissance of herbicide concentrations in Midwest groundwater.

A large municipal well near Spencer, IA, that was part of the well network for the reconnaissance of herbicide concentrations in Midwest groundwater.

Agricultural land use near a well that was sampled for the reconnaissance of herbicide concentrations in Midwest groundwater.

Diagram of an atrazine molecule showing replacement of its chlorine atom with a hydroxyl group. Hydroxyatrazine is a metabolite of atrazine (a herbicide) commonly found in groundwater.