Radon-222 budget in Catalina Harbor, California: 2. Flow dynamics and residence time in a tidal beach

Abstract

We measured the flux of water through a beach at the head of Catalina Harbor, California, by monitoring the water table in a transect of wells perpendicular to the shoreline. During a 24-h period, the volume of sediments filled and drained, the tidal wedge, was 7.4 m3 (m shoreline) -1, and the flux of water pumped across the beach face was 1.0 ± 0.3 m3 d-1 (m shoreline)-1. 222Rn concentrations measured in four wells were used to calculate the minimum age of water in the beach. Radon samples collected below the tidal wedge were not in equilibrium with the production rate, suggesting vertical mixing across the base of the tidal wedge. The age of water ranged from up to 8.0 d landward of the high water mark to less than 2.0 d in the littoral zone. This latter age agreed with an independent estimate of 1.7 ± 0.5 d on the basis of a mass balance for radon at the head of the harbor. However, a mass balance for water in the tidal wedge was significantly greater, 3.0 ± 0.5 d. The difference was most likely due to evasion of radon from wet, unsaturated sediments. Water draining out of the beach was a mixture of water that infiltrated the beach on the previous high tide and older water from either below or the back of the tidal wedge. © 2008, by the American Society of Limnology and Oceanography, Inc.