Abstract
The quantification of submarine and intertidal groundwater discharge (SiGD) or purely submarine groundwater discharge (SGD) from coastal karst aquifers presents a major challenge, as neither is directly measurable. In addition, the expected heterogeneity and intrinsic structure of such karst aquifers must be considered when quantifying SGD or SiGD. This study applies a set of methods for the coastal karst aquifer of Bell Harbour in western Ireland, using long-term onshore and offshore time series from a high-resolution monitoring network, to link catchment groundwater flow dynamics to groundwater discharge as SiGD. The SiGD is estimated using the “pollution flushing model”, i.e. a mass-balance approach, while catchment dynamics are quantified using borehole hydrograph analysis, single-borehole dilution tests, a water balance calculation, and cross-correlation analysis. The results of these analyses are then synthesised, describing a multi-level conduit-dominated coastal aquifer with a highly fluctuating overflow regime draining as SiGD, which is in part highly correlated with the overall piezometric level in the aquifer. This concept was simulated using a hydraulic pipe network model built in InfoWorks ICM [Integrated Catchment Modeling]® version 7.0 software (Innovyze). The model is capable of representing the overall highly variable discharge dynamics, predicting SiGD from the catchment to range from almost 0 to 4.3 m3/s. The study emphasises the need for long-term monitoring as the basis for any discharge studies of coastal karst aquifers. It further highlights the fact that multiple discharge locations may drain the aquifer, and therefore must be taken into consideration in the assessment of coastal karst aquifers.
Highlights
IntroductionIrish karst aquifers consisting of Carboniferous limestones, which are characterised by a low-lying topography and exposition to the coast, present a number of relevant research challenges, including groundwater flooding dynamics and the interaction of temporary flood lakes, i.e. turloughs (Naughton et al 2012; Gill et al 2013), saltwater intrusion (Perriquet et al.2014), pipe network modelling in conduit-dominated catchments (Gill et al 2013) and associated nutrient input into the aquatic coastal ecosystems (McCormack et al 2014), and Department of Civil, Structural and Environmental Engineering, University of Dublin Trinity College, Dublin 2, Ireland
The recession consists of multiple convex and concave sections, whereas the total hydrograph is split into two main recessions: above 20.2 masl
The present study introduces a novel technique for estimating submarine and intertidal groundwater discharge (SiGD) in the form of a pollution flushing model (Barber 2003; Barber and Wearing 2004), which was supported by an extensive monitoring network that allows times series analysis, water budget estimations, and single-borehole dilution tests, to link onshore catchment dynamics to the offshore discharge pattern
Summary
Irish karst aquifers consisting of Carboniferous limestones, which are characterised by a low-lying topography and exposition to the coast, present a number of relevant research challenges, including groundwater flooding dynamics and the interaction of temporary flood lakes, i.e. turloughs (Naughton et al 2012; Gill et al 2013), saltwater intrusion (Perriquet et al.2014), pipe network modelling in conduit-dominated catchments (Gill et al 2013) and associated nutrient input into the aquatic coastal ecosystems (McCormack et al 2014), and Department of Civil, Structural and Environmental Engineering, University of Dublin Trinity College, Dublin 2, Ireland. Strong heterogeneity must be expected (Burnett et al 2006), which creates challenges in the application of methods to quantify dynamics. Given the nature of the discharge locations, direct physical measurement of SiGD or purely SGD is not possible with the types of gauging methods used at onshore springs. Studies have applied direct and indirect methods for measuring the mass transfer of groundwater across the sea floor (Zektser et al 2007) or towards a surface water body, including (1) measuring the seepage flow rate using seepage meters (Carr and Winter 1980; Corbett et al 2003) or multilevel onshore piezometers (Freeze and Cherry 1979; Taniguchi and Fukuo 1996), (2) applying mass-balance approaches using natural geochemical tracers such as electrical conductivity (EC), short-lived radium and radon isotopes, i.e
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