Are groundwater inputs into river-dominated areas important? The Chao Phraya River—Gulf of Thailand

We used the natural geochemical tracers radon-222 and radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) 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 m3 s21) and in July (wet season, 430 m3 s21). 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 222Rn, 223Ra, and 224Ra activities and 224Ra : 223Ra 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 m3 s21 for January and July, respectively. An independent estimate of groundwater discharge in July using a mass balance of excess 226Ra together with our estimated water exchange rates based on 224Ra : 223Ra ratios resulted in a range of 14–19 m3 s21, 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 m3 m21 d21 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. Groundwater discharge is recognized as a significant, but poorly quantified, source of nutrients and dissolved species to coastal waters and estuaries. It may also be a pathway for land-derived contaminants of anthropogenic origin. One of the geographic areas where groundwater transport to the marine environment has been scarcely studied but could be particularly important is in Southeast Asia. The area has a wet climate and large regions are covered by limestone terrains that are particularly conducive to underground flow. An understanding of submarine groundwater discharge (SGD) is needed especially because many countries in the region are undergoing rapid agricultural and industrial development. Such activities may have a significant effect on surface and groundwater chemistry with potential influence on the water quality and ecology of the coastal zone. In recent years, progress has been made in the number of SGD investigations on different types of coastal zones. However, the importance of groundwater inputs in riverdominated margins is still an open question. Identification of SGD in a region with large riverine inputs is challenging since the geochemical signals may be similar. Moore (1997), Krest et al. (1999), and Moore and Krest (2004) have identified groundwater discharge in the Mississippi, Atchafalaya, and Ganges–Brahmaputra estuaries using radium