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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.

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The hydraulic and transfer response of karst aquifers is complex and often highly nonlinear: due to their high transmissivity and connection with the surface, such systems are very sensitive to modifications of their boundary conditions. The aim of this study was to assess the variation of the response depending on both upstream and downstream parameters, and propose a methodology to simulate the response of the karst system depending on those parameters. The impact of the variations of multiple environmental parameters on the response of a karstic system submitted to tidal variations (Normandy, France) was investigated after a campaign of artificial tracer tests acquired in very different hydrologic conditions (rainfall events, low tide, high tide, low/high piezometric level, and low/high waters). Principal components analysis and hierarchical clustering were applied on both environmental variables and karstic system response variables (parameters of the residence time distribution [RTD] curves). Equations between the RTD parameters and the most relevant variables were established using a symbolic regression algorithm. It appeared that the variations of the RTD parameters depend mainly on the cumulated rainfall preceding the injection, the piezometric level of the aquifer, and on the tide parameters. The hydraulic conditions downstream of the aquifer have a strong influence on the hydraulic and transfer response of the aquifer. The response of the aquifer in various and extreme conditions has been simulated using the equations resulting from the symbolic regression algorithm. Such relationships can be useful for management of water resources in karst media, and support decision making.

Karstified carbonate aquifers may receive significant recharge contributions from losing streams, hence, the knowledge about surface water-groundwater (SW-GW) interactions is crucial with regard to water management (e.g., source protection zone delineation). The dynamics of SW-GW interactions may depend on factors such as the relative water levels between streams and aquifers, resulting in a temporal variation of exchange, which imposes complexity to the understanding of such dynamics. This study highlights the use of high-resolution time series and multiresolution analysis to help to gain insights into such complex dynamics. Wavelet coherence is applied on hourly time series of rainfall, stream, and spring discharges of a low-lying karstified spring catchment to yield a correlation in the time-frequency domain. This analysis provides comprehensive information on the overall impact of the river on the spring, which is supported by the cross-correlation function, as well as by more detailed information, including time-variant influences such as a threshold level of influence. Field observations of turbidity sampling at the spring appear to support this interpretation. This innovative approach relies on basic hydrological parameters, water level, or discharge, and is therefore applicable to many other systems with such existing time series. Article impact statement: Wavelet coherence yields highresolution information on surface water-groundwater interaction.

Karstified carbonate aquifers are highly heterogeneous systems characterized by multiple recharge, flow, and discharge components. The quantification of the relative contribution of these components, as well as their numerical representation, remains a challenge. This paper identifies three recharge components in the time and frequency domain. While the analysis in the time domain follows traditional approaches, the analysis of the power spectrum allows frequencies associated with specific spectral coefficients and noise types to be distinguished more objectively. The analysis follows the presented hypothesis that the different frequency-noise components are the result of aquifer heterogeneity transforming the random rainfall input into a sequence of non-Gaussian signals. The distinct signals are then numerically represented in the context of a semidistributed pipe network model in order to simulate recharge, flow, and discharge of an Irish karst catchment more realistically. By linking the power spectra of the modeled recharge components with the spectra of the spring discharge, the information usually gained by classical performance indicators is significantly widened. The modeled spring discharge is well matched in the time and frequency domain, yet the different recharge dynamics explain the signal of the aquifer outlet in different noise domains across the spectrum. This study demonstrates the conjunctive use of frequency analysis in conceptualization of a hydrological system together with modeling and evaluation.

Several different approaches have been developed to model the specific characteristics of karst aquifers, taking account of their inherent complex spatial and temporal heterogeneities. This paper sets out the development of a semidistributed modelling approach for applications in an Irish karst context using urban drainage software. The models have proven to be very useful for different studies, with examples given for the ecohydrology of ephemeral karst lakes, extreme groundwater-flood alleviation, karst network investigation, submarine groundwater discharge, and quantification of different recharge and flow components. The limitations of the approach are also highlighted, in particular not being able to simulate diffuse infiltration and flow paths explicitly across the groundwater catchment. Hence, a more distributed, finite-difference modelling approach using MODFLOW Unstructured Grid (USG) with the newly developed Connected Linear Network (CLN) process is then compared against the semidistributed approach on the same karst catchment. Whilst it has proven difficult to achieve the same levels of model performance in simulating the spring flows in the distributed model compared to the semidistributed model, the ability to interrogate the flow paths at any point on the three-dimensional aquifer is demonstrated, which can give new insights into flows (and potential contaminant transport) through such complex systems. The influence of the proximity of highly transmissive conduits on the flow dynamics through the much-lower transmissive matrix cells in which the network is embedded has been particularly investigated.

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