IMPACT OF THE 1993 FLOOD ON THE DISTRIBUTION OF ORGANIC CONTAMINANTS IN BED SEDIMENTS OF THE UPPER MISSISSIPPI RIVER

The 1500 km Upper Mississippi River (UMR) consists of 29 navigation pe als and can be divided into the upper reach (pools 1-4), the middle re ach (pools 5-13), and the lower reach (pools 14-26). Comparison of com posite bed sediment samples collected from the downstream third of 24 pools before and after the 1993 UMR flood provides field-scale data on the effect of the flood on sediment organic compound distributions. T he sediments were analyzed for organic carbon, coprostanol, polynuclea r aromatic hydrocarbons including pyrene, linear alkylbenzenesulfonate s, polychlorinated biphenyls (PCBs), and organochlorine pesticides. Mo st of the target compounds were detected in all of the sediment sample s, although concentrations were generally <1 mg/kg. The highest concen trations typically occurred in the upper reach, an urbanized area on a relatively small river. Pool 4 (Lake Pepin) is an efficient sediment trap, and concentrations of the compounds below pool 4 were substantia lly lower than those in pools 2-4. Differences in concentrations befor e and after the 1993 flood also were greatest in the upper reach. In p ools 1-4, concentrations of pyrene and PCBs decreased after the flood whereas coprostanol increased. These results suggest that bed sediment s stored in the pools were diluted or buried by sediments with differe nt organic compound compositions washed in from urban and agricultural portions of the watershed.