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

Short-term (approximately 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 7Be, 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 approximately 0.9 pCi cm(-3). Inventories of 7Be, 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 7Be 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 approximately 0.5 cm of sediment depth, and approximately 6-10 kg of polychlorinated biphenyls (PCBs) over the whole deposit. Short-term accumulation rates were up to approximately 130 times higher than the long-term rates calculated from 137Cs profiles, suggesting an extremely dynamic sediment transport environment, even within an impounded river system.

Sediment-Water Chemical Exchange in the Coastal Zone Traced by in situ Radon-222 Flux Measurements.

In situ radon-222 flux experiments conducted in benthic chambers in Cape Lookout Bight, a small marine basin on the North Carolina coast, reveal that enhanced chemical transport across the sediment-water interface during summer months is caused by abiogenic bubble tube structures. Transport rates for dissolved radon, methane, and ammonium more than three times greater than those predicted on the basis of molecular diffusion occur when open tubes are maintained by semi-diurnal low-tide bubbling.