Abstract
Management of karst aquifers is often limited by a lack of understanding of recharge and flow dynamics. This article presents the identification of conduit networks and the connectivity, dynamic nature of recharge and inherent uncertainties in recharge assessment in karstic settings. The study was carried out at two large sinkholes located in the Poocher Swamp fresh water lens, south east of South Australia. Point recharge to the sinkholes was calculated using stream flow data at gauging stations and water balance of the swamp. Conduit system and their interconnectivity in the vicinity of sinkholes were characterized by the use of transient electromagnetic survey (TEM) to identify high potential porosity zones of the aquifer. Resistivity data were used to estimate aquifer porosities using Archie’s law. Recharge response to the karstic aquifer was monitored using four monitoring wells located at various distances from sinkholes. Measurements were taken during recharge and recession phases. Four dynamic stages of water level rise and fall were observed in response to filling of conduit zones, transmission and possible effects of entrapped air pressure within conduits. Electrical conductivity (EC) profiles were obtained at two stages. These confirmed interconnectivity of conduits, and re-adjustment to ambient groundwater quality following the recharge event. The lower EC water was found in the monitoring well furthest from the sinkholes indicating the complexity of conduit connection and the nature of mixing with ambient groundwater.
Highlights
The flow behaviour of karst systems is characterised by recharge, storage and transmission (White, 1988; Ford & Williams, 1989)
In order to make a further contribution to karst aquifer assessment, this study investigates potential conduit zones in the vicinity of a sinkhole by use of downhole geophysical techniques and transient electromagnetic (TEM) survey, quantifying point recharge volume to the sinkhole, and analysing the dynamic behaviour of recharge by the use of hydrograph response during recharge and recession phases
The wells could not be accessed for downhole geophysical logging but electrical conductivity (EC) profiles were taken and are discussed in detail later in the paper under bore hydrograph response to recharge
Summary
The flow behaviour of karst systems is characterised by recharge, storage and transmission (White, 1988; Ford & Williams, 1989). Highly heterogeneous subsurface conduit systems cause groundwater flow magnitude and direction to vary along preferential flow paths. The architecture of conduit networks and their connectivity is generally unknown and difficult to assess This is due to the complexity of the subsurface karstification process and the difficulty in tracing them at depth into the ground (Shaban & Darwich, 2011). Features and characteristics of the karst aquifer processes outlined here are well known and are provided here as a stepping-stone for assessing conduit networks, connectivity and the study of recharge dynamics. The range of porosity and permeability influences many aspects of karst aquifer characteristics like recharge, flow path and velocity, storage and retention capacity (Eiche et al, 2016). Groundwater flow primarily takes place in the conduit system while matrix porosity is mainly responsible for storage (Worthington et al, 2000). Somaratne (2015b) highlighted the view of Taylor and Greene (2001) and Bakalowicz (2005) that conventional study methods used in classical hydrogeology are generally invalid and unsuccessful in karst aquifers and that recharge estimation using the conventional methods do not accurately enrr.ccsenet.org
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