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

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)21, and the flux of water pumped across the beach face was 1.0 6 0.3 m3 d21 (m shoreline)21. 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 6 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 6 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. The hydraulic gradient between sea level and the beach water table will fluctuate throughout a tidal cycle. During high water, sea level may be higher than the beach water table, driving surf-zone water into the beach. Conversely, during low water, sea level may be lower than the beach water table, and water in the beach aquifer will flow into the surf zone. This ‘‘tidal pumping’’ fills and drains a wedge of sediments during each tidal cycle, the tidal wedge. Tidal pumping will carry contaminants and nutrients, washed into the surf zone from rivers, marshes, storm drains, or offshore, through the beach. Organisms that live in this interstitial environment take advantage of tidal pumping to supply organic matter and oxygen (Jiao and Li 2004) and remove wastes, creating vertical and horizontal gradients of bioactive solutes (McLachlan 1989; McLachlan and Dorvlo 2005). Water draining out of the beach can be enriched in nutrients from the remineralization of organic matter that occurred within the beach (Hays and Ullman 2007). To predict the location of solute concentration gradients requires understanding the dynamics of flow within the beach. Mathematical models to predict water table fluctuations and circulation in the beach have been developed and generally assume horizontal flow to satisfy the Boussinesq equation (Nielsen 1990; Raubenheimer et al. 1999; Li and Jiao 2003). This limits the volume of sediments affected by tidal pumping to those filled and drained during a tidal cycle. However, modeling studies have shown evidence for the downward flow of water out of the tidal wedge (Boufadel et al. 2006) and the upconing of water at the shoreline (Raubenheimer et al. 1999; Boufadel 2000). Upconing represents water draining out of the beach that was not immediately within the tidal wedge and necessitates more complicated flow paths by adding a vertical component. Although the hydraulic head may not be affected by small vertical flows, advective as well as diffusive exchange with water below the tidal wedge may significantly affect the distribution of solutes. The residence time of water provides information on the bulk flow of water in the beach. Previous residence time estimates calculated on the basis of the wave-driven flux of water into the beach neglected tidal pumping (McLachlan 1982; McLachlan 1989; McLachlan et al. 1985). This is a fair assumption at beaches with a steep slope and small tidal range, where the flux of water driven through the beach by wave pumping is significantly greater than the flux generated by tidal pumping. At a tidal beach with little wave activity, this method predicts very long residence times of water in the beach (McLachlan 1989). However, the residence time of water in beaches with little wave activity may instead be controlled by the tidally induced changes in hydraulic gradient between the shoreline and the beach water table. In this paper, new limits on the flow of water in a beach are identified on the basis of the naturally occurring radioactive noble gas, 222Rn (radon). Radon has a mean life of 5.5 d and is produced by the decay of 226Ra, primarily found in the sediments. As a noble gas, radon behaves conservatively. As low-radon coastal water flows into the beach, radon begins to accumulate in pore waters because of production from 226Ra decay. The longer water 1 To whom correspondence should be addressed. Present address: Department of Geology, University of Puget Sound, Tacoma, Washington 98416 ([email protected]). Acknowledgments We thank Tony Michaels, Liz Capporelli, Kevin Flanagan, Julian Herndon, and Doug Conlin, who made the fieldwork possible. Kim Aldrich, Kathy Cummins, Ida-Maja Karle, and Reni Schimmoeller helped with the fieldwork. We also thank two anonymous reviewers for their useful comments. This study would not have taken place without the financial and logistical support of the Wrigley Institute for Environmental Studies and Catalina Marine Science Center. This research was conducted with support from the University of Southern California Sea Grant Program, part of the National Sea Grant College Program, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, under grants NA86RG0054 and NA16RG2256. Limnol. Oceanogr., 53(2), 2008, 659–665 E 2008, by the American Society of Limnology and Oceanography, Inc.