Natural radionuclides ²¹⁰Po and ²¹⁰Pb in the Delaware and Chesapeake Estuaries: modeling scavenging rates and residence times

During the spring and summer months of 2012, ²¹⁰Po and ²¹⁰Pb activity were measured in the dissolved and particulate phases from the Delaware and upper Chesapeake estuaries. The upper Delaware estuary, near the freshwater end member, was characterized by high-suspended matter concentrations that scavenged dissolved ²¹⁰Po and ²¹⁰Pb. Box models were applied using mass balance calculations to assess the nuclides residence times in each estuary. Only 60% of the dissolved ²¹⁰Po and 55% of the dissolved ²¹⁰Pb from the Delaware estuary were exported to coastal waters. A large fraction of soluble ²¹⁰Po and ²¹⁰Pb within the estuary was either reversibly adsorbed onto suspended particles, trapped in sediment accumulation zones (such as intertidal marshes), bioaccumulated into phytoplankton and discharged to the coastal ocean. The upper Chesapeake estuary was largely characterized by sub-oxic bottom waters that contained higher concentrations of dissolved ²¹⁰Po and ²¹⁰Pb, hypothesized to be subjected to redox cycling of manganese. The Delaware and Chesapeake estuary mean residence times for ²¹⁰Po differed significantly at 86 ± 7 and 126 ± 10 days respectively, while they were similar for ²¹⁰Pb (67 ± 6–55 ± 5 days). The difference in residence times corresponds to the greater extent of biogeochemical scavenging and regeneration processes within the upper Chesapeake. - Highlights: • Estuarine ²¹⁰Pb and ²¹⁰Po data reveal key biogeochemical processes and rates. • Delaware Bay displays regional differentiation due to dominant particle reactions. •Chesapeake Bay displays vertical differentiation from deep sub-oxic redox cycling. • Parent (²¹⁰Pb) grand-daughter (²¹⁰Po) disequilibria evidence principle processes. • Net scavenging residence times calculated weeks (Delaware) to months (Chesapeake)