Measurement and potential importance of submarine groundwater discharge

Full text: While the major rivers of the world are reasonably well gauged and analyzed, thus allowing comparatively precise estimates of riverine inputs to the ocean, it remains very difficult to evaluate the influence of direct groundwater discharge into the ocean. In spite of the recognition that many land-sea interfaces of the world are characterized by 'leaky' continental margins, it is unclear how important groundwater-derived springs and seeps are in terms of overall marine geochemical budgets. The principal reason that groundwater estimates have not attained the precision base that is typically achieved of other oceanic inputs is that the direct discharge of groundwater into the coastal zone is inherently very difficult to measure. Concerted efforts are required to improve this situation by integrated application of hydrological and oceanographic techniques. Standard hydrological and oceanographic methodological approaches are quite different and have rarely been systematically compared by following a systematic scientific evaluation process. Hydrogeologists and oceanographers are literally approaching the same problem from different ends. In order to develop the scientific and technical knowledge that will enable these issues to be addressed with a higher degree of certainty, the IAEA together with UNESCO (IOC and IHP) is conducting a collaborative program which includes carefully designed intercomparison experiments in different coastal environments in order to provide a standardized methodology for assessment of SGD. Many of the methods being assessed are based on naturally-occurring radioisotopes that act as tracers of SGD. An important aspect of our program is to disseminate the results widely, to coastal managers and other relevant parties, in the hopes that national authorities will encourage the scientific community to investigate these phenomena properly. Thus far, we have conducted intercomparison experiments in Florida (August, 2000; karst hydrogeology); Western Australia (November, 2000; coastal plain); Sicily (volcanic and karst; March, 2002); Eastern Long Island (May; 2002; glacial till); and Ubatuba, Brazil (fractured crystalline rock). Results have generally shown good agreement between direct seepage meter measurements and estimates made via geochemical tracers. The agreement with hydrogeological models, however, has been more spotty. This may be a reflection of the models not including consideration of transient processes, such as tidal pumping, that force water through nearshore sediments. Plans for next year include an SGD assessment intercomparison experiment in the Mauritius Islands, Indian Ocean. It is becoming recognized that oceanic islands display disproportionately higher amounts of SGD compared to many continental areas. (author)