Characteristics of Point Recharge in Karst Aquifers

Abstract

Karstic groundwater basins are characterized by both point and diffuse recharge. This paper describes the hydrologic characteristics of point recharge and their influence on recharge estimation for four groundwater basins. Point recharge is highly transient and may occur in relatively short-time periods, yet is capable of recharging a large volume of water, even from a single extreme rainfall event. Preferential groundwater flows are observed in karst aquifers with local fresher water pockets of low salinity that develop around point recharge sources. Measurable fresh water plumes develop only when a large quantity of surface water enters the aquifer as a point recharge. In fresh water plumes, the difference in chloride concentrations in diffuse and point recharge zones decreases as the plumes become enriched through mixing. The relative contributions to total recharge from point sources using the measured gap between groundwater and rainwater chloride in the chloride vs. δ 18 O plot is not necessarily indicative of sinkholes not directly recharging the aquifer. In karst aquifers, recharge estimation methods based on groundwater age distribution; average annual rainfall and basin average chloride in the conventional chloride mass balance (CMB) method are questionable due to theoretical limitations and key assumptions of these methods not being met. In point recharge dominant groundwater basins, application of: watertable fluctuation, numerical groundwater modelling, Darcy flow calculation or water budget methods are more suitable for recharge estimation as they are independent of the particular mode of recharge. The duality of the recharge mechanism in karst aquifers suggests that modification to the CMB method may be required to include both point and diffuse recharge components.

Karst Aquifer Recharge: A Case History of over Simplification from the Uley South Basin, South Australia

“…Since a more detailed description of Uley South basin features is given in Somaratne [1], Somaratne et al [4] and Somaratne and Frizenschaf [5], only a succinct summary is provided here, focusing in features relevant to the points of contention [2]. The basin is topographically closed due to surrounding sand dunes and basement high areas except in the landward direction where dry limestone is found (Figure 1a).…”

“…Each of the above flows differs depending on the level of development of the karst process. Similarly, the input mechanism from sinkhole to groundwater can also be divided into three mechanisms [9]: Shaft flow from predominantly vertical shafts (connected sinkholes, Somaratne [1]); flow through enlarged joints, cavities and fractures of the vadose zone (connected sinkholes, Somaratne [1]); and vadose seepage or slow sinking through small fissures (macro pores in unconnected sinkholes, Somaratne [1]). Point recharge at sinkholes feeds karst conduits to the saturated zone, bypassing the surrounding matrix and thus largely escaping evapotranspiration.…”

“…In order to track the possible pathway of freshwater from point recharge sources, salinity profiles were obtained according to the method described by Somaratne [1]. Salinity profiles are considered to be superior to water sampling as the sonde records EC of water at 0.05 m interval, thus enabling identification of EC changes at short depth intervals.…”

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Abstract:The article "Karst aquifer recharge: Comments on 'Characteristics of Point Recharge in Karst Aquifers', by Adrian D. Werner, 2014, Water 6, doi:10.3390/w6123727" provides misrepresentation in some parts of Somaratne [1]. The description of Uley South Quaternary Limestone (QL) as unconsolidated or poorly consolidated aeolianite sediments with the presence of well-mixed groundwater in Uley South [2] appears unsubstantiated.

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Integrated Use of Geological, Geophysical, Radiocarbon and Stable Isotopes Data for Tracing the Conduit Flow Paths in a Small Karstic Aquifer: Poocher Swamp Freshwater Lens, South Australia

“…The catchment area extends across the South Australian border into western Victoria and previously described by Somaratne (2014). For brevity, a succinct summary of the study freshwater lens is provided here.…”