Multi-layered Coastal Aquifer Research Articles

A surrogate based multi-objective management model to control saltwater intrusion in multi-layered coastal aquifer systems

ABSTRACTLinked simulation-optimisation (S–O) models need to simulate the physical processes either by using a rigorous numerical model, or a trained surrogate model approximating the physical processes. A methodology is proposed to evolve Pareto optimal management strategies for a multi-layered coastal aquifer system using a trained and tested Multivariate Adaptive Regression Spline (MARS) surrogate model linked to a multi-objective saltwater intrusion management model. Performance of the developed methodology is evaluated using an illustrative multi-layered coastal aquifer system. Solution results indicate that MARS is capable of approximately replacing the more rigorous numerical simulation model within the linked S–O model to ensure computational efficiency and feasibility in applying such linked S–O models for coastal aquifer management problems. Furthermore, the ability of MARS to recognise the most relevant input variables in predicting the responses as outputs enables the construction of an efficient and robust surrogate model. Integration of parallel processing capabilities within the optimisation model is shown to improve computational efficiency and feasibly of solving such large scale multi-objective problems. Therefore, the developed methodology utilising the MARS based surrogate model is potentially applicable for developing optimal groundwater extraction strategies for sustainable regional scale management of coastal aquifers.

Study on the groundwater sustainable problem by numerical simulation in a multi-layered coastal aquifer system of Zhanjiang, China

Assessing sustainability of coastal groundwater is significant for groundwater management as coastal groundwater is vulnerable to over-exploitation and contamination. To address the issues of serious groundwater level drawdown and potential seawater intrusion risk of a multi-layered coastal aquifer system in Zhanjiang, China, this paper presents a numerical modelling study to research groundwater sustainability of this aquifer system. The transient modelling results show that the groundwater budget was negative (\(-3826\times 10^{4}\) to \(-4502\times 10^{4 }\,\hbox {m}^{3}/\hbox {a}\)) during the years 2008–2011, revealing that this aquifer system was over-exploited. Meanwhile, the groundwater sustainability was assessed by evaluating the negative hydraulic pressure area (NHPA) of the unconfined aquifer and the groundwater level dynamic and flow velocity of the offshore boundaries of the confined aquifers. The results demonstrate that the Nansan Island is most influenced by NHPA and that the local groundwater should not be exploited. The results also suggest that, with the current groundwater exploitation scheme, the sustainable yield should be \(1.784\times 10^{8}\, \hbox {m}^{3}/\hbox {a}\) (i.e., decreased by 20% from the current exploitation amount). To satisfy public water demands, the 20% decrease of the exploitation amount can be offset by the groundwater sourced from the Taiping groundwater resource field. These results provide valuable guidance for groundwater management of Zhanjiang.

Multivariate Adaptive Regression Spline Ensembles for Management of Multilayered Coastal Aquifers

AbstractApplication of multivariate adaptive regression spline ensembles (En-MARS) in a coupled simulation-optimization methodology to derive multiple-objective optimal groundwater extraction strat...

The effectiveness of cutoff walls to control saltwater intrusion in multi-layered coastal aquifers: Experimental and numerical study

The objective of this study was to examine the performance of cutoff walls in controlling saltwater intrusion in stratified heterogeneous coastal aquifers. Numerical and laboratory experiments were completed in laboratory-scale aquifer where the effectiveness of cutoff walls was assessed in three different configurations, including a homogeneous scenario, a stratified aquifer with high K–low K–high K pattern (case HLH) and another stratified aquifer with low K–high K–low K pattern (case LHL). The results show that the cutoff wall was effective in reducing the saltwater wedge in all the investigated cases of layered-aquifers with toe length reduction of up to 43%. The wall exhibited more wedge reduction in shallower than steeper hydraulic gradients. However, the soil stratification appeared to lessen the overall performance of the wall compared to the homogeneous case. The aquifer stratification disrupted the flow dynamics, and thus affected the freshwater velocity at the wall opening to various degrees, depending on the layering pattern. The presence of an interlayer of low k (case HLH) inhibited the downward movement of the freshwater towards the wall opening, and thus decreasing the repulsion ability of the wall. Moreover, the presence of an underlying low permeability layer (case LHL) was found to obstruct the freshwater flow in the lower part of the aquifer, thereby slowing down the velocity through the wall opening. Numerical analysis of other layering patterns of monotonically increasing/decreasing permeability from top to bottom showed that the cutoff wall remained effective in repulsing the seawater wedge.

Effects of different boundary conditions on the simulation of groundwater flow in a multi-layered coastal aquifer system (Taranto Gulf, southern Italy)

The evaluation of the accuracy or reasonableness of numerical models of groundwater flow is a complex task, due to the uncertainties in hydrodynamic properties and boundary conditions and the scarcity of good-quality field data. To assess model reliability, different calibration techniques are joined to evaluate the effects of different kinds of boundary conditions on the groundwater flow in a coastal multi-layered aquifer in southern Italy. In particular, both direct and indirect approaches for inverse modeling were joined through the calibration of one of the most uncertain parameters, namely the hydraulic conductivity of the karst deep hydrostratigraphic unit. The methodology proposed here, and applied to a real case study, confirmed that the selection of boundary conditions is among the most critical and difficult aspects of the characterization of a groundwater system for conceptual analysis or numerical simulation. The practical tests conducted in this study show that incorrect specification of boundary conditions prevents an acceptable match between the model response to the hydraulic stresses and the behavior of the natural system. Such effects have a negative impact on the applicability of numerical modeling to simulate groundwater dynamics in complex hydrogeological situations. This is particularly important for management of the aquifer system investigated in this work, which represents the only available freshwater resource of the study area, and is threatened by overexploitation and saltwater intrusion.

The influence of fish ponds and salinization on groundwater quality in the multi-layer coastal aquifer system in Israel

This study focuses on the impact of surface reservoirs (fish ponds) on a multi aquifer coastal system, and the relation between the aquifer and the sea. The study was conducted in an Israeli Mediterranean coastal aquifer, which includes a sandy phreatic unit and two confined calcareous sandstone units. The geological description is based on 52 wells, from which 33 samples were collected for stable isotope analysis and 25 samples for organic and inorganic parameters. Hydraulic head and chemical measurements suggest that there is an hydraulic connection between the fish ponds above the aquifer and the phreatic unit, whereas the connection with the confined units is very limited. The phreatic unit is characterized by a low concentration of oxygen and high concentrations of ammonium and phosphate, while the confined units are characterized by higher oxygen and much lower ammonium and phosphate concentrations. Organic matter fluorescence was found to be a tool to distinguish the contribution of the pond waters, whereby a pond water signature (characterized by proteinaceous (tryptophan-like) and typical humic-matter fluorescence) was found in the phreatic aquifer. The phreatic unit is also isotopically enriched, similar to pond waters, with δ18O of −1‰ and δD of −4.6‰, indicating enhanced evaporation of the pond water before infiltration, whereas there is a depleted isotopic composition in the confined units (δ18O=−4.3‰, δD=−20.4‰), which are also OM-poor. The Phreeqc model was used for quantitative calculation of the effect of pond losses on the different units.The Dissolved Inorganic Nitrogen (DIN) in the upper unit increases downstream from the ponds toward the sea, probably due to organic matter degradation, suggesting contribution of DIN from shallow groundwater flow to the sea. 87Sr/86Sr and Mg/Ca in the brackish and saline groundwater of the lower confined units increase toward seawater value, suggesting that the salinization process in the region is connected to seawater intrusion and not to old brine from the underlying Cretaceous aquitard.

Hydrochemistry and Isotope Hydrology for Groundwater Sustainability of the Coastal Multilayered Aquifer System (Zhanjiang, China)

Groundwater sustainability has become a critical issue for Zhanjiang (China) because of serious groundwater level drawdown induced by overexploitation of its coastal multilayered aquifer system. It is necessary to understand the origins, material sources, hydrochemical processes, and dynamics of the coastal groundwater in Zhanjiang to support its sustainable management. To this end, an integrated analysis of hydrochemical and isotopic data of 95 groundwater samples was conducted. Hydrochemical analysis shows that coastal groundwater is fresh; however, relatively high levels of Cl−, Mg2+, and total dissolved solid (TDS) imply slight seawater mixing with coastal unconfined groundwater. Stable isotopes (δ18O andδ2H) values reveal the recharge sources of groundwater in the multilayered aquifer system. The unconfined groundwater originates from local modern precipitation; the confined groundwater in mainland originates from modern precipitation in northwestern mountain area, and the confined groundwater in Donghai and Leizhou is sourced from rainfall recharge during an older period with a colder climate. Ionic relations demonstrate that silicate weathering, carbonate dissolutions, and cation exchange are the primary processes controlling the groundwater chemical composition. Declining trends of groundwater level and increasing trends of TDS of the confined groundwater in islands reveal the landward extending tendency of the freshwater-seawater mixing zone.

Conceptualization and characterization of a coastal multi-layered aquifer system in the Taranto Gulf (southern Italy)

In some Mediterranean karst areas, groundwater is often the only available supply for fresh water. Besides the contamination induced by human activities, coastal aquifers often suffer from the saltwater intrusion phenomenon, which can be enhanced by both extensive withdrawals and climatic changes. Establishing an effective set of regulatory and management measures to ensure the sustainability of coastal aquifers requires a deep knowledge about natural and anthropic stresses involved in groundwater dynamics. In this regard, a prior conceptualization of aquifer systems and a deeper characterization of balance terms through mathematical modeling are of paramount importance. In this paper, previous works on the three-dimensional hydrostratigraphic characterization and two-dimensional groundwater flow modeling at the regional scale for the Salento peninsula (southeastern Italy) are shown to provide a fundamental basis for an overall conceptualization of the multi-layered aquifer system of the Taranto Gulf (northwestern Salento). This is done by identifying source terms and providing a preliminary estimate of different balance terms, based on available data and information. Then, a further characterization will be provided using three-dimensional groundwater flow models and calibration techniques based on the solution of an inverse problem. As a result the hydraulic features of the deep karst aquifer can be mapped; the stresses due to human water-demanding activities and the occurrence of the saltwater intrusion phenomenon were analyzed and possible weaknesses of the conceptual and of the preliminary numerical model pointed out.

Marine water from mid-Holocene sea level highstand trapped in a coastal aquifer: Evidence from groundwater isotopes, and environmental significance

A multi-layered coastal aquifer in southeast Australia was assessed using environmental isotopes, to identify the origins of salinity and its links to palaeo-environmental setting. Spatial distribution of groundwater salinity (electrical conductivity values ranging from 0.395 to 56.1mS/cm) was examined along the coastline along with geological, isotopic and chemical data. This allowed assessment of different salinity sources and emplacement mechanisms. Molar chloride/bromide ratios range from 619 to 1070 (621 to 705 in samples with EC >15mS/cm), indicating salts are predominantly marine. Two distinct vertical salinity profiles were observed, one with increasing salinity with depth and another with saline shallow water overlying fresh groundwater. The saline shallow groundwater (EC=45.4 to 55.7mS/cm) has somewhat marine-like stable isotope ratios (δ18O=−2.4 to −1.9 ‰) and radiocarbon activities indicative of middle Holocene emplacement (47.4 to 60.4pMC). This overlies fresher groundwater with late Pleistocene radiocarbon ages and meteoric stable isotopes (δ18O=−5.5 to −4.6‰). The configuration suggests surface inundation of the upper sediments by marine water during the mid-Holocene (c. 2–8kyr BP), when sea level was 1–2m above today's level. Profiles of chloride, stable isotopes, and radiocarbon indicate mixing between this pre-modern marine water and fresh meteoric groundwater to varying degrees around the coastline. Mixing calculations using chloride and stable isotopes show that in addition to fresh-marine water mixing, some salinity is derived from transpiration by halophytic vegetation (e.g. mangroves). The δ13C ratios in saline water (−17.6 to −18.4‰) also have vegetation/organic matter signatures, consistent with emplacement by surface inundation and extensive interaction between vegetation and recharging groundwater. Saline shallow groundwater is preserved only in areas where low permeability sediments have slowed subsequent downwards propagation. The configuration is unlikely to be stable long-term due to fluid density; this may be exacerbated by pumping the underlying aquifer.

Large Scale Electrical Resistivity Tomography Survey Correlated to Hydrogeological Data for Mapping Groundwater Salinization: A Case Study from a Multilayered Coastal Aquifer in Rhodope, Northeastern Greece

A large scale two dimensional electrical resistivity survey was carried out to investigate groundwater salinization phenomena of a multilayered coastal aquifer in south Rhodope area, northeastern Greece. The electrical resistivity tomography (ERT) profiles, calibrated with lithological, physicochemical and geophysical log data, were used to demarcate the geometry of the aquifer system and characterize the extent of the interface between saline and freshwater in a coastal groundwater system, which has been subject to overexploitation for the last 35 years. In total, 12 deep high resolution ERT profile lines, covering a distance of approximately 15 km were carried out at key locations across the downgraded part of the aquifer system. The resistivity sections were calibrated using a reference dataset of 45 information wells incorporating 28 lithological logs, 31 groundwater quality samples from a variety of screen placements and five gamma ray logs. Results revealed the presence of four major resistivity zones ranging in values between 0.1 and 150 Ωm at various depths and locations. A very low resistivity zone (0.1–5 Ωm) correlated with saline water saturated layers, a medium resistivity zone (5–15 Ωm) attributed to clays and silts, and two high resistivity zones generally representing freshwater saturated sediments of coarse grain size (15–40 Ωm) or alterations of thin marl and fine sand layers (40–150 Ωm). It is demonstrated that extensive use of high resolution deep geoelectrical sections, calibrated with a large number of hydrogeological data can successfully delineate aquifer geometry dimensions, identify hydraulic boundaries and clarify ambiguous field measurements, thus allowing the development of a thorough hydrogeological conceptual model.

Application of multivariate statistical procedures on major ions and trace elements in a multilayered coastal aquifer: the case of the south Rhodope coastal aquifer

Multivariate statistical techniques including cluster analysis and principal components analysis were applied on 22 variables consisted of 3 physicochemical parameters, 8 major ions and 11 trace elements. Samples were collected from the south Rhodope multilayered coastal aquifer in north Greece which is facing saltwater intrusion and anthropogenic contamination over the last 35 years. Cluster analysis grouped the variables into five main groups while principal components analysis revealed four distinct hydrochemical processes in the aquifer system, explaining 84.5 % of the total variance between the variables. The identified processes correspond to, saltwater intrusion and subsequent reverse cation exchange, the presence of deep connate groundwater masses, application of fertilizers in shallow wells and anthropogenic contamination with heavy metals nearby an improperly constructed landfill. The wells categorized with the above techniques were grouped and five constituent ratios Na/Cl, (Mg + Ca)/Cl, Ca/(HCO3 + SO4), Ca/SO4 and Ca/Mg were utilized to identify the ones which enable the more accurate distinction between the group cases. The results of stepwise discriminant analysis showed that the calculated classification function can distinguish almost 80 % of groundwater samples with the Na/Cl ratio being the most statistically significant grouping variable. All the aforementioned statistical models managed to successfully identify numerous hydrochemical processes in a complex multilayered aquifer system and to explicitly attribute them for every investigated well, allowing a deeper insight into groundwater chemical characteristics with the use of an optimized smaller number of variables.

Analysis of environmental isotopes in groundwater to understand the response of a vulnerable coastal aquifer to pumping: Western Port Basin, south-eastern Australia

The response of a multi-layered coastal aquifer in southeast Australia to decades of groundwater pumping, and the groundwater age, flow paths and salinization processes were examined using isotopic tracers. Groundwater radiocarbon and tritium contents decline with distance and depth away from basin margins; however, in the main zone of pumping, radiocarbon activities are generally homogeneous within a given depth horizon. A lack of tritium and low radiocarbon activities (<25 pMC) in groundwater in and around the pumping areas indicate that seasonal recovery of water levels is related to capture of old water with low radioisotope activities, rather than arrival of recently recharged water. Mechanisms facilitating seasonal recovery include release of water from low-permeability layers and horizontal transfer of water from undeveloped parts of the basin. Overall stability in seasonally recovered water levels and salinities for the past three decades indicate that the system has reached a dynamic equilibrium with respect to water balance and salinity, following a major change in flow paths and solute distributions after initial development. Groundwater δ18O, δ2H and chloride contents indicate mixing between fresh meteoric-derived groundwater and marine water at the coast, with the most saline groundwater approximating an 80:20 mixture of fresh to oceanic water.

Locating Nested Monitoring Wells to Reduce Model Uncertainty for Management of a Multilayer Coastal Aquifer

Although the aquifers below the Eastern Shore of Virginia serve as the region’s only sources of fresh water, increased development and intensifying groundwater demands have put this resource at risk. Simulation modeling is used as a tool to evaluate impacts of groundwater withdrawals on the aquifers. Using available monitoring data, a regional three-dimensional variable-density groundwater model (SEAWAT) was previously calibrated to the Eastern Shore’s aquifer system. In general, distributed parameter values were determined by kriging values around specified or calibrated parameter values at 21 pilot point locations spread across the Eastern Shore of Virginia. After calibration, an uncertainty analysis of the model’s performance was completed, and it was concluded that in order to most cost-effectively improve the model accuracy to facilitate management, it is important that any additional data collection be focused on the key modeling objectives, which are predicting hydraulic head and chloride concentrations under pumping conditions. The work described in this paper develops and demonstrates a methodology to identify the most useful locations, with respect to improving model accuracy, for installation of new multilayer nested groundwater monitoring wells. First, critical subregions where additional data may be useful were identified. Then a set of hypothetical new nested observation wells were evenly distributed throughout each critical subregion within the model. For each potential observation well site, the cumulative impact of data collection at that location on the prediction of hydraulic head values was estimated using the cumulative sensitivity of the relevant pilot point parameters to data at the new observation well. This cumulative sensitivity was calculated by summing the dimensionless scaled sensitivities of the new observation over all the relevant parameter values. The impact of collection of new hydraulic head information on the prediction of chloride concentrations was also estimated by determining the cumulative sensitivity of the pilot point parameter most sensitive to chloride concentrations based on the prediction scaled sensitivities. Once cumulative sensitivities were calculated for each potential observation well site, maps of the influence of new observations were created and used to identify the most effective location within each critical subregion to install a new nested observation well.

Using a Monte Carlo approach to evaluate seawater intrusion in the Oristano coastal aquifer: A case study from the AQUAGRID collaborative computing platform

Uncertainties in the physical parameters of a groundwater system, due to the lack of direct access to the subsurface, strongly affect the design of water management policies, so that the risk of mismanagement becomes a critical factor in complex ecological and economic analyses. Stochastic modeling may help provide uncertainty quantification and also add robustness to the analysis by means of probabilistic forecasts. In this study a stochastic approach has been employed to model hydraulic conductivity of a confining formation in a multi-layered coastal aquifer system, under conditions of uncertainty. A Monte Carlo simulation, based on a coupled flow and transport groundwater 3D model, has been carried out to propagate the hydraulic conductivity parameter uncertainty to groundwater model outputs, namely pressure head and salt concentration. The aim of the study is to assess the risk of seawater intrusion into the aquifer by means of probabilistic threshold analysis on the simulated groundwater concentrations for different aquifer exploitation schemes. Maximum difference for nodal concentrations, with reference to homogeneous aquitard configuration, was found equal to 95%, proving how important can be the impact of the spatial variability of the hydraulic conductivity of the confining layer on the simulated salt concentrations. Such analysis enables to take better decisions about the management of the groundwater resource and to make additional field investigations consistent with environmental protection. The application workflow, based on the integration of both in-house developed and public domain software tools with hydrogeological data, has been deployed on a problem solving Grid platform (http://grida3.crs4.it). Further developments will include the planning of cost-effective additional field data acquisition based on the outcome of the stochastic model.

Multi‐depth monitoring of electrical conductivity and temperature of groundwater at a multilayered coastal aquifer: Jeju Island, Korea

AbstractTo supplement conventional geophysical log data, this study presents temporal variations in electrical conductivity (EC) and temperature with depth in a multilayered coastal aquifer, on the eastern part of Jeju Island, Korea. One‐month time‐series data obtained at eight points from a multi‐depth monitoring system showed that semidiurnal and semimonthly tidal variations induced dynamic fluctuations in EC and temperature. At some depths, EC ranged from 1483 to 26 822 µS cm−1, while some points showed no significant variations. The results of EC log and time‐series data revealed that a sharp fresh‐saltwater interface occurred at low tide, but the diffusion zone broadened to 20 m at high tide. EC, temperature, and tide level data were used for the cross‐correlation analysis. The response time of EC and temperature to tide appears to range from less than 30 min to 11 h. Using end‐member mixing analysis (EMMA), the fraction of variations of chloride concentration in the multilayered aquifer was explained, and a conceptual model was developed which subdivided the coastal aquifer into four vertical zones. The percentage of water derived from seawater varied from 2 to 48 at specific depth, owing to tidal fluctuations. Continuous observations of EC and temperature at multiple depths are powerful tools for quantifying the transport of saline water by tidal variations in multilayered coastal aquifers. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Tide‐induced groundwater head fluctuation in a coastal aquifer system with a submarine outcrop covered by a thin silt layer

AbstractWe present an analytical solution of groundwater head response to tidal fluctuation in a coastal multilayered aquifer system consisting of an unconfined aquifer, a leaky confined aquifer and a semi‐permeable layer between them. The submarine outcrop of the confined aquifer is covered by a thin silt layer. A mathematical model and the analytical solution of this model are given. The silt layer reduces the amplitude of the hydraulic head fluctuation by a constant factor, and shifts the phase by a positive constant (time lag), both of which depend on the leakances of the silt layer and the semi‐permeable layer. The time lag is less than 1·5 h and 3·0 h for semi‐diurnal and diurnal sea tides respectively. When the leakance of the semi‐permeable layer or the silt layer assumes certain special values, the solution becomes the existing solutions derived by previous researchers. The amplitude of the hydraulic head fluctuation in the confined aquifer increases with the leakance of the silt layer and decreases with the leakance of the semi‐permeable layer, whereas the phase shift of the fluctuation decreases with both of them. A hypothetical example shows that neglecting the silt layer may result in significant parameter estimation discrepancy between the amplitude attenuation and the time‐lag fittings. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Tide-induced groundwater head fluctuation in coastal multi-layered aquifer systems with a submarine outlet-capping

This paper considered the tide-induced head fluctuations in two coastal multi-layered aquifer systems. Model I comprises two semi-permeable layers and a confined aquifer between them. Model II is a four-layered aquifer system including an unconfined aquifer, an upper semi-permeable layer, a confined aquifer and a lower semi-permeable layer. In each model, the submarine outlet of the confined aquifer is covered with a skin layer (“outlet-capping”). Analytical solutions of the two models are derived. In both models, leakages of the semi-permeable layers decrease the tidal head fluctuations. The outlet-capping reduces the aquifer’s head fluctuation by a constant factor and shifts the phase by a positive constant. The solution to Model II explains the inconsistency between the relatively small lag time and the strong amplitude damping effect of the tidal head fluctuations reported by Trefry and Johnston [Ground Water 1998;36:427–33] near the Port Adelaide River, Australia.

Tidal effects on variations of fresh–saltwater interface and groundwater flow in a multilayered coastal aquifer on a volcanic island (Jeju Island, Korea)

Summary We conducted various field studies at the seawater intrusion monitoring wells located in the eastern part of Jeju Island, Korea, to observe the tidal effect on groundwater–seawater flow in the coastal aquifer. Studies included monitoring the fluctuations of groundwater and tide levels, electrical and temperature logging, and 2-D heat-pulse flowmeter tests. According to time-series analysis, tidal effects on groundwater level reached up to 3 km inland from the coastline. Water-level variation was more sensitive to tidal fluctuations near the coast, and more related to rainfall toward inland areas. Temporal and spatial variations in the shape and location of the freshwater–saltwater interface were analyzed using data from nine monitoring wells. The results indicated that the interface toe is located at a distance of 6–8 km from the coastline and its location was related to geological layers present. Long-term seasonal variations revealed no major changes in the interface; minor variations were due to moving boundary conditions induced by tidal fluctuations. Using the two-dimensional heat-pulse flowmeter, groundwater flow directions and velocities at four tidal stages were measured on three monitoring wells drilled into the multilayered aquifers. This direct measurement enabled us to relate the differences of flow velocities and directions with geology and tidal fluctuations. Combining the results of EC logging and flowmeter tests, we found a zone where freshwater and saltwater moved alternately in opposite directions, as influenced by the tidal fluctuations. Integrating various physical logging and flowmeter data with water-level fluctuations improved our understanding of the behavior of fresh and seawater flow in the coastal aquifers.

Saline intrusion and refreshening in a multilayer coastal aquifer in the Catania Plain (Sicily, Southern Italy): dynamics of degradation processes according to the hydrochemical characteristics of groundwaters

The Catania Plain is a large alluvial depression which represents one of the most important hydrogeological units in eastern Sicily. A detailed hydrochemical and hydrogeological investigation focussed on its north-eastern corner has provided new insights into the past and present relationship between freshwater and seawater in shallow and deep aquifers. According to hydrochemical data, over the last centuries the area has been affected by local variations of the sea level, giving rise to sequential processes of re-equilibration of the exchangeable complex, involving freshwater and seawater in both surficial and deep aquifers. These ongoing processes consist in the refreshening of both deep aquifers >500 m from the shoreline and, to a lesser extent due to lower freshwater influx, shallow aquifers. In the former case, there is a shift of the groundwater compositions towards the Na–HCO 3 type and a significant drop in the salt contents, while in the latter case, the salt contents do not drop significantly. At a distance <500 m from the shoreline the exchangeable complex of deep aquifers is presently subject to salinization. According to the temporal variabilities in the hydrochemical data over the period 1995/96, the apparent dualism can be attributed to a general worsening of the current freshwater/seawater relationship in deep aquifers. This new tendency appears to be related to a sharp increase in water consumption by the local industrial plant, which has reversed the natural refreshening tendency, giving rise to a new saline intrusion that is particularly severe close to the shoreline.

Tide-induced seawater–groundwater circulation in a multi-layered coastal leaky aquifer system

Mean groundwater levels of a multi-layered coastal leaky aquifer system are considered. The system consists of an unconfined aquifer, a confined aquifer and a semi-permeable layer between them. Both exact asymptotic solutions and approximate perturbation solutions are derived for multi-sinusoidal-component sea tide. At inland places far from the coastline, the perturbation solutions show a good agreement with the exact asymptotic solutions. Due to the watertable-dependent transmissivity of the unconfined aquifer, the mean groundwater levels of the aquifer system stand considerably above the mean sea level even in the absence of net inland recharge of groundwater and rainfall. These lead to landward positive gradients of both the mean watertable and mean head in the region near the coastline, which consequently results in a seawater–groundwater cycle. Seawater is pumped into the unconfined aquifer by the sea tide and divided into two parts. One part returns to the sea driven by the mean watertable gradient. The rest part leaks into the confined aquifer through the semipermeable layer, and returns to the sea through the confined aquifer driven by the mean head gradient. The total discharge through the confined aquifer is significant for coastal leaky aquifer system with typical parameter values. This seawater–groundwater cycle has impacts on better understanding of submarine groundwater discharge and exchange of various chemicals such as nutrients and contaminants in coastal areas. If the observed mean water levels in coastal areas are used for estimating the net inland recharge, the enhancing processes of sea tide on the mean groundwater levels should be taken into account. Otherwise, the net inland recharge will be overestimated.