Drip flow variations under a stalactite of the Pere Noel cave (Belgium). Evidence of seasonal variations and air pressure constraints

The study of drip rate and seepage water electrical conductivity (hereafter called conductivity) under one stalactite in the Pere Noel cave (Belgium), with data produced from an automatic station since 1991, demonstrates seve ral previously unobserved features: (1) measurement of drop volume shows th at, for 94% of the time series, drop volume is constant (= 0.14 ml), but wh en discharge exceeds 48.2 drips min(-1), drop volume decreases, probably be cause of secondary drop formation; (2) the interannual drip rate variation is correlated to the annual water excess and its correlant, rainfall (R-2 = 0.98; exponential model); this result introduces a new improvement in the understanding of the previously investigated relationships between stalagmi te annual laminae thickness and mean annual rainfall; (3) the drip rate sho ws a well marked seasonality: it increases abruptly in late fall or early w inter and decreases slowly during spring, summer and fall. Increased discha rge is accompanied by an increase in conductivity, which suggests that the flushed water is more mineralized and was stored in the karst aquifer for s everal months; (4) superimposed on these seasonal variations, there are two kinds of flow regimes which are driven by the atmospheric pressure: (i) a "wiggles regime", whose duration is 1-7 days in length and which is inverse ly proportional to the air pressure wiggles; it is explained by either a '' shut-off faucet'' process due to the rock formation stress, or to a change in the two-phases flow component proportions (air/water); (ii) an "unstable regime" characterized by abrupt switches (<2 h) or oscillations with varia ble periodicities, from a few minutes to a few hours. These occur when the drip rate reaches a threshold (i.e. 240 drops 10 min(-1)); the chaotic beha viour of this phenomenon is discussed. (C) 1998 Elsevier Science B.V. All r ights reserved.