A modified point dilution test for the assessment of groundwater flux in karst aquifers

Abstract

A parametrization of heterogeneous karst aquifers represents one of the main challenges for hydrogeologist. The aim of this research was to design an easily applicable test that can be used for the assessment of groundwater flux (velocity) in highly permeable karst aquifers. The modified point dilution test (MPDT) is based on the constant-rate injection of tracer solution in a borehole (or sub-vertical cave) and continuous measurement of tracer concentration in the same borehole. When the concentration is stabilized then the groundwater inflow in the borehole can be assessed on the basis of the known injection rate and measured concentrations (initial and stabilized). By the obtained inflow and known borehole cross-sectional area, the apparent groundwater velocity under a natural hydraulic gradient can be assessed. Also, Darcy velocity, hydraulic conductivity and an average linear velocity can be derived from the test results if the borehole-distortion factor, hydraulic gradient and porosity are known. The test was applied in Well B1 (Ljesanska Nahija, Montenegro). The obtained apparent groundwater velocity amounts to 1.71 × 10−4 m/s. In addition to wells and piezometers, the MPDT can also be applied to sub-vertical caves that extend below the water table, and as a results groundwater inflow and the volume of a cave filled with water can be obtained. The MPDT provides satisfactory results only in highly permeable aquifers.