Beryllium-7 as a tracer of short-term sediment deposition and resuspension in the Fox River, Wisconsin

Short-term (similar to monthly) sediment deposition and resuspension rates of surficial bed sediments in two PCB-laden impoundments on the Fox River, WI, were determined in the summer and fall of 1998 using Be-7, a naturally occurring radioisotope produced in the atmosphere. Decay-corrected activities and inventories of 7Be were:measured in bed sediment and in suspended particles. Beryllium-7 activities generally decreased with depth in the top 5-10 cm of sediments and ranged from undetectable to similar to0.9 pCi cm(-3). Inventories of Be-7, calculated from the sum of activities from all depths, ranged from 0.87 to 3.74 pCi cm(-2) and the values covaried between sites likely reflecting a common atmospheric input signal. Activities of 7Be did not correlate directly with rainfall. Partitioning the Be-7 flux into "new" and "residual" components indicated that net deposition was occurring most of the time during the summer. Net erosion, however, was observed at the upstream site from the final collection in the fall. This erosion event was estimated to have removed 0.10 g (cm of sediment)(-2), corresponding to similar to0.5 cm of sediment depth, and similar to6-10 kg of polychlorinated biphenyls (PCBs) over the whole deposit. Short-term accumulation rates were up to similar to 130 times higher than the long-term rates calculated from Cs-137 profiles, suggesting an extremely dynamic sediment transport environment, even within an impounded river system.