HR: 15:15h
AN: OS23G-06    [Abstracts]
TI: Fresh Submarine Groundwater Discharge from a Contaminated Beach Aquifer is Enhanced During Neap Tide
AU: * de Sieyes, N R
EM: desieyes@stanford.edu
AF: Stanford University, Environmental Engineering and Science Program Department of Civil and Environmental Engineering Stanford, Stanford, CA 94305-4020, United States
AU: Yamahara, K M
EM: kevany@stanford.edu
AF: Stanford University, Environmental Engineering and Science Program Department of Civil and Environmental Engineering Stanford, Stanford, CA 94305-4020, United States
AU: Boehm, A B
EM: aboehm@stanford.edu
AF: Stanford University, Environmental Engineering and Science Program Department of Civil and Environmental Engineering Stanford, Stanford, CA 94305-4020, United States
AB: Experiments were carried out to assess the fortnightly effects of mixed semi-diurnal tides and tidally-induced aquifer overheight on the timing, magnitude, and quality of submarine groundwater discharge (SGD) from an unconfined beach aquifer impacted by septic tank effluent at Stinson Beach, California. Groundwater- and ocean elevations, salinity, and nutrient concentrations were monitored throughout a 14-day neap/spring cycle. A freshening of the surf zone coupled with an increase in nutrient concentrations was observed at neap tide and attributed to discharge of fresh, nutrient-rich groundwater during that part of the fortnight. Nutrient concentrations in the surf zone returned to near-offshore levels during the spring tide. Estimates of SGD were made with chemically- and physically-based methods. Fresh SGD was maximal during the neap tide although total SGD was maximal during the spring tide. An overheight of time-averaged piezometric head was observed in the aquifer near the beach face throughout the 14-day experiment. This overheight varied significantly with tidal range, thereby controlling the seaward hydraulic gradient across the fresh part of the aquifer and, thus, the flow of nutrient-rich fresh groundwater to the coastal ocean. We discuss the role of aquifer overheight in controlling submarine discharge of fresh groundwater and related non-point source pollution from unconfined aquifers in similar environments.
DE: 0496 Water quality
DE: 1830 Groundwater/surface water interaction
DE: 1871 Surface water quality
DE: 4217 Coastal processes
DE: 4251 Marine pollution (0345, 0478)
SC: Ocean Sciences [OS]
MN: 2007 Joint Assembly