Unusually high lead-210 inventory in the sediment column of a tidal embayment

San Diego Bay is a meso-tidal embayment located in southwestern California near US-Mexican border. Atmospheric fallout of Pb-210, which is the dominant source (>90%) of Pb-210 to the coastal sediments, was 0.17 +/- 0.03 dpm/cm²/yr, by direct collection of atmospheric fallout as well as measurement of Pb-210 inventories in two soil cores collected from a nearby wetland. The inventory of Pb-210 in bay sediments was calculated from the Pb-210 profiles of sediment cores collected from 7 stations across San Diego Bay. The enrichment factors of Pb-210, defined as the ratios of sediment inventory of Pb-210 versus the rate of atmospheric fallout, were greater than 10.0 in two stations in the north San Diego Bay, which is connected to the Pacific Ocean. In comparison, the enrichment factors were significantly lower for stations located in the inner San Diego Bay (ranging 1.1-3.7). Enrichment factors of up to 3.5 have been reported in other coastal oceanic environments, and the unusually high inventory of Pb-210 in north San Diego Bay suggested dominant input of particle-reactive Pb-210 from outer sea as well as a highly efficient mechanism that retains Pb-210 in the bay sediments. By measuring Pb-210 in both dissolved and particulate phases in waters across San Diego Bay and nearby outer sea, we determined that tidal exchange between particle-laden bay water with outer sea water containing abundant dissolved Pb-210 (3-5 times higher than that in bay water) should be the cause of this unusually high Pb-210 inventory in north San Diego Bay. We define the process as "Pb-210 stripping", in which dissolved-phase Pb-210 supplied by outer sea through tidal exchange was continuously scavenged by settling particulate matter in bay water and retained in the north San Diego Bay. Heavy shipping traffic, tidal resuspension, as well as high productivity in San Diego Bay should have caused high concentration of particulate matter in bay water.