Infiltration processes and flow rates in developed karst vadose zone using tracers in cave drips

摘要

The purpose of this research was to identify groundwater recharge mechanisms and measure flow velocities through the epikarst and the vadose zone in a typical site of Mediterranean carbonate karst in natural rain conditions. By avoiding artificial flushing more realistic travel times could be measured, and the effective resultant vadose flow velocity could be inferred. Additionally, detailed monitoring enabled better description and quantification of infiltration and percolation processes.Nine different cave drips were monitored and sampled for three hydrological years in fractured karst lithology on Mount Carmel, Israel. At three drips, discharge was measured continuously by tipping buckets. Rainfall and soil water content were recorded above the cave. An artificial tracer experiment was conducted using uranine placed in a joint and at the soil-rock interface at soil pockets, both 27-m above the cave.Four hydrological types of drips: post-storm, seasonal, perennial, and overflow, were identified; each demonstrating different characteristics in terms of discharge and chemistry.The maximum effective flow velocities (uranine dye arrival times) were 41-76-cm/h; tracer mass flux was computed from the uranine breakthrough curves and drip hydrographs. The dominant flow velocities, derived from the peak of tracer mass flux, were 0.35-0.41-cm/h from post-storm drips, and 22-24-cm/h from the other drips.'Pulse-through' (intra-event piston flow) was indicated by the time lags between the increase in drip rate and the time of drop in salinity and appearance of the tracer in most of the measured drips. Chloride concentrations were used to distinguish between 'old/matrix water' and 'preferential-flow water' in the perennial drips, demonstrating a two-component mixing model. In 2005-2006 'preferential-flow water' reached up to 20-25% from annual discharge of these drips. This study promoted a method to compute dominant effective flow rates in the vadose zone in natural rain conditions.