HR: 0800h
AN: H21B-1013    [Abstracts]
TI: Radium and Radon as Tracers of Ground Water Flow Into Upper Newport Bay, CA
AU: * Worsnopp, M B
EM: worsnopp@usc.edu
AF: University of Southern California, Dept. of Earth Sciences. 3651 Trousdale Pkwy ZHS 117, Los Angeles, CA 90089 United States
AU: Hammond, D E
EM: dhammond@usc.edu
AF: University of Southern California, Dept. of Earth Sciences. 3651 Trousdale Pkwy ZHS 117, Los Angeles, CA 90089 United States
AU: Cable, J E
EM: jcable@lsu.edu
AF: Louisiana State University, Department of Oceanography & Coastal Sciences/Coastal Ecology Institute 208 Coastal Studies Building, Baton Rouge, LA 70803 United States
AB: Subsurface waters are highly enriched in isotopes of radon and radium, relative to surface waters. Thus, the measured isotope distribution and the decay constants of Ra-223 (half-life 11.4d), Ra-224 (half-life 3.6d), and Rn-222 (half-life 3.82 d) can provide constraints on transport within an aqueous system, making them potential tracers of groundwater flow and boundary exchange. Boundary exchange mechanisms within Upper Newport Bay estuary include molecular diffusion through pore waters, macrofaunal irrigation of sediments, tidal pumping, river inputs, tidal exchange with the Lower Bay, radioactive decay, and (for radon) gas exchange with the atmosphere. Groundwater flow is also a potential source of these isotopes, but because it is likely to be localized, it is difficult to measure. By developing budgets for Rn and Ra, and placing constraints on other processes, an estimate for groundwater flow was calculated. Sampling in February, April, and August of 2004 included analysis of water column and pore water samples for radium and radon. Concentrations of both Rn and Ra in the water column were smallest in February (71+/-11 dpm/m3 Ra-223, 595+/- 51 dpm/m 3 Ra-224) but were about twice as large in April (188+/-17 dpm/m3 Ra-223, 1080+/-dpm,m3 Ra-224) and August (128+/-21 dpm/m3 Ra-223, 977+/-48 dpm/m3 Ra-224). This reflects the influence of rapid flushing with storm runoff in February, when salinity was also greatly reduced. Pore water concentrations were 1800 dpm/m3 for Ra-223, 20,000 dpm/m3 for Ra-224, and 300 dpm/l for Rn-222. Both core incubations and benthic chamber deployments were used to directly measure radium and radon fluxes from sediments. Fluxes measured (in atoms/m2-sec) from in situ chambers were 13 (Ra-223), 36 (Ra-224), and 450 (Rn-222) in August. These exceeded diffusive fluxes from pore waters by factors of 6, 3.5, and 3 for the 3 isotopes, respectively. This high in situ flux indicates that irrigation by macrofauna must enhance transport, and the different enhancements for each isotope indicates the effect occurs on a variety of length scales. A mass balance for Upper Newport Bay indicates the sediment flux is responsible for ~20-30% of the isotope input. River runoff supplies 10% of the Ra-223 and 20% of the Ra-224 and the Rn-222 inputs. Approximately 30-40% of the supply is brought in by the flood tide. The remaining input must be supplied by a combination of tidal pumping and groundwater flow. An upper limit for groundwater flow can be calculated if it alone balances the isotope budgets equivalent to a distributed flow of 2-4 cm/day.
DE: 4850 Organic marine chemistry
DE: 4235 Estuarine processes
DE: 1829 Groundwater hydrology
DE: 1836 Hydrologic budget (1655)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting