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

Abstract

Knowledge about the hydraulic connections between submarine groundwater discharge (SGD) and its terrestrial coastal catchment is relevant with regard to the management of marine and coastal waters in karst areas. This study applies different methods and monitoring approaches to trace SGD between the Burren Limestone Plateau and Galway Bay in western Ireland, via an excavated sinkhole shaft and deep conduit. Areas of potential SGD were first delineated based on sea surface temperature anomalies using Landsat satellite images. Two fluorescent dyes and solid wood chips were then used as tracers. Solid wood chips were tested as potential means to circumvent the problem of high dispersion in the sea, impacting on the fluorescent dyes to yield readings below the detection limits. Sampling was conducted at 10 different terrestrial locations and in the sea at Galway Bay. Offshore sampling was conducted in transects over a period of four successive days onboard of a vessel using an automated field fluorometer and a conductivity-temperature-depth sensor. No wood chips were recovered in the sea but both fluorescent dyes were successfully sampled. The estimated travel times are in the order of 100 to 354 m/h, and localised tracer readings correlate well in space and time with low conductivity readings. By confirming hydraulic connections between the two karst features and Galway Bay, the study substantiates the hypothesised importance of Variscan veins with regard to regional groundwater flow in the region.

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

“…5;Schuler et al 2018). A tracer test was subsequently carried out between the upland of the Bell Harbour catchment (Burren Plateau) and the shoreline and sea, confirming (deep) groundwater flow and submarine groundwater discharge into the Atlantic Ocean (Schuler et al 2020a).…”

“…8 for the Ballindine spring (see Fig. S3 of the ESM for catchment), which is largely influenced by river-bed exfiltration (Schuler et al 2020a ). This approach exemplifies the conjunctive use of frequency and time domain analysis in conceptualisation of a hydrological system along with modelling and evaluation.…”

Quantitative analysis of Cenozoic faults and fractures and their impact on groundwater flow in the bedrock aquifers of Ireland

“…8), whilst vertical connectivity is promoted by dilations along vertical bends and accentuated dissolution, which may even be promoted by sulphuric acid derived from underlying sulphides, muddy limestones and shales (e.g. Schuler et al 2020a). The importance of accentuated karst along the main Cenozoic faults in the CW Zone (Fig.…”

“…Permeability (k) and porosity (p) were calculated for mechanical units of stratabound joints using the cubic flow law (e.g. NRC 1996; Zimmerman and Bodvarsson 1996;Schwartz and Zhang 2003), assuming horizontal flow through regular spaced fractures with constant aperture (Reiss 1980;Motyka 1998;Müller et al 2010 and refs therein;Fig. 10).…”

“…Variscan (late Carboniferous) NNW-to NNEtrending veins and NE-and NW-trending strike-slip faults are characteristically vein filled and are therefore also intrinsically sealing to groundwater flow (Moore and Walsh 2013). The most important departure from the sealing behaviour of these Carboniferous structures is, however, their susceptibility to the dissolution of adjacent host rock limestones or calcite vein infills (Moore and Walsh 2013;Walsh et al 2019;Schuler et al 2020a). In Ireland this limestone dissolution is usually very recent and uplift-related, leading to the generation of karst-related groundwater flow systems characterised by complex networks of conduits and caves that are strongly controlled by preexisting, and originally sealing, structures and bedding planes (Drew 1990;Moore and Walsh 2013;.…”