National Water Quality Assessment Program: Eastern Iowa Basins

Organochlorine pesticides in a sediment core from Coralville Reservoir

by Kalkhoff, S.J., VanMetre, P.C., and E. Callender

Proceedings of the Geological Society of America North-Central Section meeting, May 1-2, 1997, Madison, Wisconsin

A sediment core was collected from the Coralville Reservoir on the Iowa River near Iowa City, Iowa during November 1993 to define historical water-quality trends in the Iowa River watershed as part of the U.S. Geological Survey's National Water Quality Assessment Program. The core was collected about three kilometers upstream of the dam where most sediment is deposited uniformly from suspension. Discrete sections of the core were dated using physical markers and Cesium-137 concentrations and were analyzed for organochlorine pesticides and polychlorinated biphenyls (PCB's).

Results of analyses of 10 core sections indicate that sediment composition changed from sand and silt to primarily clay-sized material upon closure of the dam in 1958. Nine organochlorine compounds were found in trace or quantifiable concentrations in at least one core interval. These compounds include aldrin, three chlordane compounds, dieldrin, total PCB's, and three DDT- degradation compounds. Concentrations of four of the five most commonly detected organochlorine compounds were largest in sediments deposited in the late 1960's and early 1970's. This period corresponds to the greatest usage of organochlorine compounds in the watershed.

The maximum concentration of dieldrin in the core positively correlates to historical peak dieldrin concentration in the Iowa River at Iowa City during the period 1968-1982. The median dieldrin concentration from the Iowa River peaked at 0.016 micrograms per liter in 1970 and a peak dieldrin concentration of 1.6 ug/kg (micrograms per kilogram) occurred in sediments deposited in the early 1970's.

The largest dieldrin concentration, 2.3 ug/kg, occurred in reservoir sediments deposited in 1993 during major basin-wide flooding. This suggests that, even though banned in the 1970's, dieldrin was still present in the Iowa River watershed and that heavy flooding transported the dieldrin to Coralville Reservoir. In contrast, concentrations of DDE, chlordane, and PCB's in sediments deposited in 1993 were not substantially different than concentrations in sediments deposited in 1990.

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