Fluorescent properties of organic carbon in cave dripwaters: Effects of filtration, temperature and pH

For the first time the specific fluorescent characteristics of organic carbon (OC) in sequentially filtered cave dripwater samples have been studied and the proportions of organic carbon in each size fraction quantified We examined the effects of pH, temperature and filtration on the fluorescent properties of OC sampled from four drip points in different seasons Dripwaters were sampled from both normal (pH 7 5-8 5) and hyper-alkaline (pH 9-13) drip points in Poole's Cavern, Buxton, UK, which provides a model system for understanding the effects of pH on the chemical properties of OC At high-pH values, charge stabilisation of OC is greatly enhanced, resulting in 10-20 times mole coarse colloidal and particulate (>100 nm) organic carbon than in lower pH dripwaters, indicating that destabilisation (e g charge shielding) of colloidal OC is an important process control on the transmission of OC in cave dripwaters at near-neutral pH OC fluorescence in high-pH dripwaters exhibited a high degree of pH sensitivity between pH 10 and 12, consistent with substantial changes in the coordination or neighbouring environment of fluorescent acidic functional groups Inner-filter effects (IFE) associated with the coarse colloidal and particulate fraction of OM mask the true fluorescent signal, so that size fractionation is necessary to obtain a signal which is correlated with the concentration of organic carbon Fluorescence intensities in the samples studied were best correlated with organic carbon with a dimension <100 nm These results have important implications for the use of fluorescence as a tracer in hydrogeological studies (C) 2010 Elsevier B V All rights reserved