Characterisation of hydrogeological connections in a lowland karst network using time series analysis of water levels in ephemeral groundwater-fed lakes (turloughs)

Abstract

This research has used continuous water level measurements five groundwater-fed lakes (or turloughs) in a linked lowland karst network of south Galway in Ireland over a 3 year period in order to elucidate the hydrogeological controls and conduit configurations forming the flooded karstic hydraulic system beneath the ground. The main spring outflow from this network discharges below mean sea level making it difficult to determine the hydraulic nature of the network using traditional rainfall–spring flow cross analysis, as has been done in many other studies on karst systems. However, the localised groundwater–surface water interactions (the turloughs) in this flooded lowland karst system can yield information about the nature of the hydraulic connections beneath the ground. Various different analytical techniques have been applied to the fluctuating turlough water level time series data in order to determine the nature of the linkage between them as well as hydraulic pipe configurations at key points in order to improve the conceptual model of the overall karst network. Initially, simple cross correlations between the different turlough water levels were carried out applying different time lags. Frequency analysis of the signals was then carried out using Fast Fourier transform analysis and then both discrete and continuous wavelet analyses have been applied to the data sets to characterise these inherently non-stationary time-series of fluctuating water levels. The analysis has indicated which turloughs are on the main line conduit system and which are somewhat off-line, the relative size of the main conduit in the network including evidence of localised constrictions, as well as clearly showing the tidal influence on the water levels in the three lower turloughs at shallow depths ∼8 km from the main spring outfall at the sea. It has also indicated that the timing of high rainfall events coincident with maximum spring tide levels may promote more consistent, long duration flooding of the turloughs throughout the winter.