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

摘要

Five stalagmite drip-waters in the Grotte de Villars, Dordogne, have been monitoredfrom early 1997 to early 1998, for variations in discharge, major inorganicspecies and dissolved luminescent organic matter. When compared to surfaceprecipitation, each drip-water has a subtly different response, both in terms ofdischarge variability and lag time between surface precipitation and drip rateresponse. Calculated water excess is shown to be important in determiningdrip-water discharge; during periods of soil moisture deficit, drip-waterseither show no response to surface precipitation, or in the case of one samplestation, respond only to high intensity and/or high quantity precipitationevents. All drip-waters havea large storage component to their flow. Four sample stations have a similarhydrochemical and luminescence response, although the precise timing andmagnitude of the responses may vary between drip sources that are <5 m apart.Drip-water luminescence intensity increases in winter and spring, and increasesin discharge lag by 2 – 3 months, suggesting that the water in the rising limband peak of the winter discharge comes from the stored groundwater componentrather than a soil source. Drip-water strontium anti-correlates withluminescence and exhibits a strong (±100%) seasonal variation, withhigh-strontium waters derived from stored groundwater and is inferred tooriginate in localised Sr-rich primary components in the limestone. Drip-waterconductivity reflects Ca-HCO₃ variations and falls during late summer to autumn,which is inferred to result from increased calcite precipitation above the cavewith enhanced degassing related to progressive drying of the aquifer. Drip-watermagnesium (following removal of the marine aerosol component) is just abovedetection limits and does not show strong seasonal variations. Variations insolution Pco₂ occur, with a particularly strong increase in early 1997. Thevarious chemical trends are observed at a number of different sites despite apronounced variation between them in terms of total Ca-HCO₃ mineralisation andPco₂.One sampling station of the five investigated had a different response to surface precipitation; dripdischarge was more variable, with evidence of non-linear responses, andluminescence intensity exhibited a dilution response to drip rate. For thissite, flow switching occurred at times of high rainfall, with a rapid dischargeresponse less than 24 hours after rainfall. Luminescenceintensity, inorganic chemistry, and discharge characteristics at the site arecompared with results published from other cave systems; significant inter-sitevariability depends on the geology, depth of sample sites and extent ofkarstification. This suggests that the interpretation of stalagmiteluminescence, and variations in Sr, Ca and Mg must be considered on a site bysite basis. style="line-height: 20px;">Keywords: stalagmite; dripwaters; luminescence; discharge; major ion chemistry