Ambient intraborehole flow in a highly productive aquifer in Ljubljana, Slovenia

Ljubljansko polje aquifer in the central part of Slovenia. Impeller flowmeter was used together with the water quality probe to determine the flow characteristics of water under ambient conditions (without pumping) in 13 boreholes in a highly productive alluvial aquifer. The study focused on analysing the characteristics and variability of intraborehole flows in relation to the technical features of the boreholes, as well as the geometry and geological composition of the aquifer. The study has shown that vertical hydraulic gradients are not limited to complex multiple aquifer systems but can also occur in uniform alluvial aquifers without typical aquitard layers if hydraulic head differences exist between the upper and lower parts of the aquifer. The intraborehole flows are mainly caused by pressure-driven convection in deeper boreholes with long screens. The direction and velocity of these flows are strongly influenced by the morphology of the bedrock. Downward water flows are closely related to the local deepening of the bedrock and upward flows to the local rise of the bedrock. In some boreholes, intraborehole flow rates exceed the typical pumping rates for groundwater sampling, raising concerns about the representativeness of samples collected using conventional sampling methods. Hydrochemical logs proved to be a valuable complementary method to determine the locations of the main inflows. • Borehole logging was used to identify intraborehole flows in 13 boreholes. • Intraborehole flows were analysed in relation to borehole design and aquifer geology. • Pressure-driven convection causes significant intraborehole flows. • The morphology of the bedrock influences the direction and velocity of the flow. • Intraborehole flows can affect the representativeness of the samples.