Hydrological characterisation of stalagmite dripwaters at Grotte de Villars, Dordogne, by the analysis of inorganic species and luminescent organic matter

Five stalagmite drip-waters in the Grotte de Villars, Dordogne, have been m onitored From early 1997 to early 1998, for variations in discharge, major inorganic species and dissolved luminescent organic matter. When compared t o surface precipitation, each drip-water has a subtly different response, b oth in terms of discharge variability and lag time between surface precipit ation and drip rate response. Calculated water excess is shown to be import ant in determining drip-water discharge; during periods of soil moisture de ficit, drip-waters either show no response to surface precipitation, or in the case of one sample station, respond only to high intensity and/or high quantity precipitation events. All drip-waters have a large storage component to their flow. Four sample s tations have a similar hydrochemical and luminescence response, although th e precise timing and magnitude of the responses may vary between drip sourc es that are <5 m apart. Drip-water luminescence intensity increases in winter and spring, and incre ases in discharge lag by 2-3 months, suggesting that the water in the risin g limb and peak olt he winter discharge comes from the: stored groundwater component rather than a soil source. Drip-water strontium anti-correlates w ith luminescence and exhibits a strong (<plus/minus>100%) seasonal variatio n, with high-strontium waters derived from stored groundwater and is inferr ed to originate in localised Sr-rich primary components in the limestone. D rip-water conductivity reflects Ca-HCO3 variations and falls during late su mmer to autumn, which is inferred to result from increased calcite precipit ation above the cave with enhanced degassing related to progressive drying of the aquifer. Drip-water magnesium (following removal of the marine aeros ol component) is just above detection limits and does not show strong seaso nal variations. Variations in solution PCO2 occur, With a particularly stro ng increase in early 1997. The various chemical trends are observed at a nu mber of different sites despite a pronounced variation bt tween them in ter ms of total Ca-HCO3 mineralisation and PCO2 One sampling station of the five investigated had a different response to s urface precipitation; drip discharge was more variable, with evidence of no n-linear responses, and luminescence intensity exhibited a dilution respons e to drip rate. For this sire, flow switching occurred at times of high rai nfall, with a rapid discharge response less than 24 hours after rainfall. Luminescence intensity, inorganic chemistry, and discharge characteristics at the site are compared with results published from other cave systems; si gnificant inter-site variability depends on the geology, depth of sample si res and extent of karstification. This suggests that the interpretation of stalagmite luminescence, and variations in Sr, Ca and;Mg must be considered on a site by site basis.