The study described herein determined background levels of four fluorescent dyes (optical brighteners, sodium fluorescein, eosine Y and rhodamine WT) in Guam's coastal waters. The primary objectives were to: (1) provide a baseline for future dye trace surveys in tropical karst environments; (2) make recommendations with respect to dye and sampling site selection, positive detection criteria and background correction; and (3) re-examine previous dye trace studies on Guam based on the results of this study.

Precipitation data from five rain gauges around Guam were used for correlation analysis with sampling data. Preliminary experiments were conducted to determine sampling substrate performance and optimize sampling frequency and extraction techniques. As a result, thirteen sampling sites (subtidal and intertidal springs, dissolution fractures, and perched aquifer discharge) on Guam were monitored biweekly over a 13-month period, beginning in March 2006. In addition, seawater from four nearshore coastal locations on Guam, as well as two additional springs on Saipan, was sampled for comparison. Samples were extracted using a caustic eluent composed of aqueous ammonia, potassium hydroxide, water and 2-isopropanol. This formula worked well for all dyes of interest, but produced the highest yield for fluorescein extractions.

Guam sample data revealed that optical brightener concentrations were consistently two orders of magnitude greater than either fluorescein or rhodamine. Eosine was rarely detected. Background levels in seawater accounted for nearly 40%, 90% and 25% of optical brightener, fluorescein and rhodamine levels, respectively, detected at the thirteen sampling locations. Statistical analysis showed that background levels of the four dyes varied significantly within and between sites over time. Sample data tended to correlate most strongly with data from the nearest rain gauge. Seven of eight monitoring sites exhibited an inverse correlation between rainfall and optical brightener concentrations. Fluorescein and rhodamine concentrations, on the other hand, remained remarkably stable once the wet season began. These findings suggest that surface runoff rather than submarine groundwater discharge exerts the greatest influence on background levels of fluorescence. Accurate detection of dyes is hampered during the dry season, and by background levels in the surrounding seawater. Recommendations for future dye trace studies are presented and discussed.