Geochemistry and deposition of 7Be in river‐estuarine and coastal waters

Abstract

The atmospheric flux of cosmogenic 7Be (53.3‐day half‐life) and the mode of 7Be deposition in river‐estuarine and coastal environments have been examined. The atmospheric flux of 7Be commonly supports inventories ranging from 1.0 to 2.0 pCi/cm2 (1 pCi = 0.037 Bq). Beryllium 7 concentrations in water phase samples, collected across salinity gradients in several estuaries along the eastern coastline of the United States, range from 0.03 to 0.53 pCi/L and primarily reflect variations in 7Be supply and sorption kinetics. The major process controlling the concentration of 7Be on estuarine suspended particles appears to be the length of time that these particles remain in the water column. Field particle‐to‐water distribution coefficients for 7Be have a median value of about 4 × 104 but range over an order of magnitude reflecting short‐term variations in 7Be input, particle dynamics, and particulate iron content rather than equilibrium sorption‐desorption responses to changes in water salinity or particle type. Residence times of 7Be in the water column range from a few days in estuarine areas of rapid fine‐particle deposition, to several weeks in high‐energy environments where pronounced sediment resuspension reintroduces deposited 7Be back into the water column. Inventories of 7Be in sediments range from nondetectable to 3.3 pCi/cm2, with the highest inventories in areas where fine particles are accumulating rapidly. Such sites are also major repositories for other particle‐reactive substances. A 7Be budget for the James estuary indicates that less than 5% of the expected 7Be input is in the water column and that the short‐term estuarine trapping efficiency for atmospherically derived 7Be is somewhere between 50 and 100%.