We used the natural geochemical tracers radon-222 and radium isotopes ( 223 Ra, 224 Ra, 226 Ra, 228 Ra) to assess exchange rates between the Chao Phraya River and the Gulf of Thailand, and the magnitude of groundwater discharge in the estuary. We performed tracer surveys during two periods in 2004, in January (dry season, gauged river discharge 47 m 3 s 21 ) and in July (wet season, 430 m 3 s 21 ). The isotopic data suggested that there are at least three different sources of these tracers in the estuary: river water, seawater, and groundwater. We estimated the extent of each input via a mixing model using 222 Rn, 223 Ra, and 224 Ra activities and 224 Ra : 223 Ra ratios. Our analysis showed that the largest groundwater outflow occurs near the mouth of the river. Our groundwater discharge estimates based on the mixing model are 10 and 16 m 3 s 21 for January and July, respectively. An independent estimate of groundwater discharge in July using a mass balance of excess 226 Ra together with our estimated water exchange rates based on 224 Ra : 223 Ra ratios resulted in a range of 14-19 m 3 s 21 , depending upon the estimated amount of desorbable radium. Our estimated groundwater inputs therefore represent about 20% of the river flow during low flow in January and 4% during high flow conditions in July 2004. The unit shoreline flux (,200 m 3 m 21 d 21 in July) for the area around the river mouth is over one order of magnitude higher than two other areas of the Gulf of Thailand where groundwater fluxes have been evaluated.
Seasonal variation in seabed elevation in the muddy intertidal zone of the Chao Phraya River delta, an area of serious coastal erosion for 40 years, was assessed using information on waves and tides predicted by numerical simulations. The study area is under the infl uence of the Southeast Asian monsoon climate and lies in the innermost part of a sheltered gulf, across which a low-gradient slope has developed. Observations, aimed at evaluating the effectiveness of a prototype breakwater on mitigating coastal erosion, indicated that the seasonal variation in the seabed elevation, typically about 30 cm, was caused primarily by seasonal changes in wave direction and height. The breakwater seems to have contributed to a net rise in the seabed level at sites behind the structure. Seabed erosion was most apparent during the northeast monsoon, when waves are weak. Erosion under this low wave energy state was attributed to the combined effect of wave breaking and the low tidal level. A difference in the observed seabed accretion rate between the transitional intermonsoon period and the succeeding southwest monsoon period was attributed to the direction of the wave energy fl ux; offshore sediments seem to have been supplied effi ciently to the study area by waves during the transitional period. Another potential cause of seabed erosion and accretion during the wet southwest monsoon season was the discharge of water and sediments from local canals associated with intense tropical rainfall; this discharge seems to be linked to land use in the coastal area. The results of this study show the importance of monitoring across-shore sediment transport for better understanding of coastal erosion processes.
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