Fluorescence wavelength and intensity variations of cave waters

The fluorescence properties of groundwaters percolating into four cave systems have been monitored over the period 1997–1998. Fluorescence was excited between 220 and 400 nm and the emission measured from 300 to 500 nm using a fluorescence spectrophotometer. Three fluorescence centres were observed; one at the excitation–emission pair of 290–340:395–430 nm, (humic-like, probably fulvic acid), one at 265–280:300–370 nm (protein like) and a less defined region of high fluorescence at 230–280:310–420 nm (humic and/or protein like). The most consistent fluorescence intensity was observed in the excitation–emission pair of 290–340:395–430 nm, attributed to a fulvic acid source. Subtle differences (±5%) in the fluorescence excitation and emission wavelength of this fluorescence peak in the groundwater were observed between the four sites, and the fluorescence intensity varied considerably (×60) between the four sites. Both the wavelength and the intensity variations in fluorescence are caused by the differences in the vegetation cover, soil type and humification. Data from the most intensely monitored site (Brown’s Folly Mine, England; 9 sample stations, 10–20 days frequency sampling) revealed no spatial variability in the 290–340:395–430 nm (fulvic acid) fluorescence; in contrast time-series analysis suggests that the seasonal variations do occur, with a decrease in the emission wavelength correlating with the first (autumn) peak in fluorescence intensity, and a decrease in the excitation wavelength correlating with a second (winter) fluorescence intensity peak. Results demonstrate the potential of utilising fluorescence wavelength variations in sourcing karst groundwaters, and as a possible palaeoenvironmental proxy of the overlying soil conditions if trapped within the cave speleothems