Determining fine sediment sources from three national estuarine research reserve estuaries using radionuclide budgets and ratios

Accurate accounting of resuspended bed sediment exported from an estuary provides information regarding the estuary's ability to trap and retain sediment, which is needed in the development of BMPs within the watershed and the coastal zone. Sediment mass balances for estuaries that apply the difference between the sediment influxes and effluxes to changes in bed storage provide an estimate of net change in storage but does not suggest the amount of resuspended material that may be exported. Resuspended bed sediment must have a unique signature relative to the sediment delivered in suspension from outside the estuary, in order to determine the amount leaving an estuary.; The relationship between 7Be and 210Pb was used to differentiate resuspended bed sediment and sediment delivered to the estuary from the watershed and the downstream water body in the sediment efflux from three National Estuarine Research Reserves: Old Woman Creek, OH (OWC); Weeks Bay, AL (WB); and South Slough, OR (SS). Bed sediments were characterized by low activities of 7Be and 210Pb and low ratios of 7Be/210Pb relative to the suspended solids delivered from outside the estuaries. Therefore, contributions of 7Be from the estuary bed to the 7Be budget of an estuary were negligible, while contributions of 210Pb to the 210Pb budget were more significant, due to the differences in half-lives. Erosion of and export of resuspended bed sediment over individual runoff events were calculated in the three estuaries with a mixing model using the 7Be/210Pb ratios of the source materials. Erosion rates of the estuary beds during the individual runoff events were 101, 22.9, and 7.60 g/m2/d for the OWC, WB, and SS estuaries, respectively.; The high levels of resuspended bed sediment effluxed from the OWC and WB estuaries resulted in a net export of sediment from the estuaries during the sampled runoff events. At the SS estuary, resuspended bed sediment was exported from the estuary despite the estuary being a net sink for sediment during the runoff event.