Submarine and intertidal groundwater discharge through a complex multi-level karst conduit aquifer

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 Bpollution 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 m 3 /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.

An evaluation of semidistributed-pipe-network and distributed-finite-difference models to simulate karst systems

“…The model has allowed the impact of potential flood alleviation channels at strategic locations in the catchment to be quantified with respect to reduction in peak flood levels in the turloughs against a range of different rainfall scenarios, including climate change predictions. The model has also been used to assess potential impacts in Kinvara Bay, where the (Schuler et al 2018) spring discharges, in terms of additional reductions in salinity due to freshwater being diverted overland during large flood events (Fig. 7).…”

A combined-method approach to trace submarine groundwater discharge from a coastal karst aquifer in Ireland

“…The groundwater catchment of Bell Harbour (Fig. 1)-as previously adopted by McCormack et al (2017); Schuler et al (2018)-is located in the north-eastern part of the Burren Limestone Plateau in the west of the Republic of Ireland. The Burren, including the upland catchment of Bell Harbour, is a temperate glaciokarst landscape, which has been subject to repeated glaciation during the Pleistocene, showing features typical of glaciation such as ice-plucked crags, scoured rock surfaces, limestone pavements and erratic boulders (Simms 2014).…”

“…Known locations of SGD are either in conjunction with intertidal springs along the western shore of the Burren Plateau or limited to Bell Harbour and Kinvara Bay (Drew 2003;O'Connell et al 2018) with estimated discharge rates ranging between 0 and 4.3 m 3 /s, and 5-16 m 3 /s respectively (McCormack et al 2014;Schuler et al 2018). On a larger scale, significant areas of sea-surface-temperature anomalies interpreted as SGD were detected on the western side of the Burren Plateau linked to structural geology (Wilson and Rocha 2012).…”

“…Such flow paths will have developed during periods of lower sea levels tied to global eustatic sea level variations (Shennan et al 2018). The occurrence of significant deep groundwater circulation including a bypassing effect of intertidal springs had been proposed based on long-term monitoring, quantification of SiGD, water balance calculations and numerical modelling (Bunce and Drew 2017;Schuler et al 2018). Such deep flow is hypothesised to be linked to preferential flow paths along north/north-north-west-trending Variscan veins (Moore and Walsh 2013); hence, this study supports the relevance of these structural features with regard to regional groundwater flow.…”

Coupling SKS and SWMM to Solve the Inverse Problem Based on Artificial Tracer Tests in Karstic Aquifers

“…Many modeling approaches dealing with solute transport in karstic aquifers have been developed in past decades. As an example, the pipe flow models [22,23] consider the karst aquifer as a horizontal network of pipe connecting points within the aquifer [24,25]. This modeling approach can also be implemented in the Modflow Conduit Flow Processes (CFP) model, allowing the simulation of turbulent and laminar groundwater flow conditions [26] and consideration of lateral exchanges between the conduit and saturated fissured matrix through a dependence between head difference between matrix continuum and discrete conduit [27].…”