Sulfur hexafluoride as a complementary method for measuring the extent of point-source thermal effluents

The transport and dilution dynamics of power-plant thermal effluent were measured for 10 consecutive days, between 25 June and 4 July 2006, by concurrently mapping the daily distributions of seawater temperature and concentrations of deliberately released sulfur hexafluoride (SF₆) within the tidal Kwangyang Bay on the southern coast of Korea. Estimates of the daily extent of the thermal plume based on temperature and SF₆ data showed distinct differences. These differences were particularly pronounced on sunny days during which solar radiation significantly heated river or bay waters moving across the tidal flats; in these cases, the estimates based on seawater temperature data were consistently greater than those based on SF₆ data, indicating considerable overestimates of the extent of the thermal plume when temperature data were used. The present results indicate that the concurrent use of seawater temperature and SF₆ data is a powerful method in determining the extent of thermal plumes, particularly for shallow areas in which the effects of solar heating lead to large uncertainties in temperature-based estimates.