BACKGROUND FLUORESCENCE IN GROUNDWATER FROM A TROPICAL ISLAND-KARST AQUIFER

摘要

Background levels of four fluorescent compounds were monitored biweekly at thirteen sampling sites on Guam, consisting of subtidal and intertidal springs, dissolution fractures, and internal upland contact spring discharge, over a 13-month period from 2006 to 2007. Samples were compared to local precipitation and to seawater samples from four nearby reef flats. The data revealed that the concentrations of optical brighteners were consistently two orders of magnitude greater than either sodium fluorescein or rhodamine WT, while Eosine Y was rarely detected. Background levels in seawater, by comparison, accounted for 25% or more of the fluorescent compounds detected at the thirteen sampling locations. During the wet season, seven of eight monitoring sites exhibited an inverse correlation between rainfall and optical brightener concentrations, while sodium fluorescein and rhodamine WT concentrations remained remarkably stable. Statistical analysis showed that background levels of the four compounds varied significantly within and between sites over time. These findings suggest that (1) surface runoff rather than submarine groundwater discharge exerts the greatest influence on background levels of fluorescence, and (2) the noisiest background in fresh water occurs during the dry season, and (3) the surrounding seawater exhibits persistent significant background noise.