Unconfined Coastal Aquifer Research Articles

Erratum to: Optimal design of BP algorithm by ACOR model for groundwater-level forecasting: A case study on Shabestar plain, Iran

Groundwater always has been considered as one of the major sources of drinking and agricultural water supply, especially in arid and semi-arid zones. Thus, there is a need to simulate (i.e., forecast) groundwater levels with an acceptable accuracy. In this paper, we present two applications of intelligent optimization algorithms for simulations of monthly groundwater levels in an unconfined coastal aquifer sited in the Shabestar plain, Iran. First, the backpropagation neural network (ANN-BP) with seven neurons in its hidden layer is utilized to reproduce groundwater-level variations using the external input variables including the following: rainfall, average discharge, temperature, evaporation, and annual time series. In the next application, ant colony optimization is used to optimize and find initial connection weights and biases of a BP algorithm during the training phase (ACOR-BP). The results were found to be acceptable in terms of accuracy and demonstrated that a hybrid ACOR-BP model is a much more rigorous fitting prediction tool for groundwater-level forecasting. This study has shown that such a hybrid network can be used as viable alternative to physical-based models for simulating the reactions of the aquifer under conceivable future scenarios. In addition, it may be useful for reconstructing long periods of missing historical observations of the influencing variables.

Utilizing Successive Linearization Optimization to Control the Saltwater Intrusion Phenomenon in Unconfined Coastal Aquifers in Crete, Greece

In the present work, a simulation-optimization method is employed in order to manage saltwater intrusion in two unconfined coastal aquifers in Crete, Greece. The optimization formulation seeks to maximize groundwater withdrawal rates while maintaining the saltwater intrusion front at the current location or inhibiting it at locations closer to the coast. A combination of a groundwater flow model (MODFLOW) with the Ghyben-Herzberg saltwater front approximation and a sequential implementation of the Simplex algorithm (GWM) are employed. The results show that under the current pumping strategies, the saltwater intrusion front will continue to move inland, posing a serious threat to the groundwater quality of these regions. Optimal groundwater withdrawal scenarios that take into consideration the water needs of the local communities and environmental concerns are presented and discussed. In both case studies, significant reductions in pumping are required in order for the saltwater intrusion front to retract closer to the shoreline.

Groundwater sustainability assessment in coastal aquifers

The present work investigates the response of shallow, coastal unconfined aquifers to anticipated overdraft conditions and climate change effect using numerical simulation. The groundwater flow model MODFLOW and variable density groundwater model SEAWAT are used for this investigation. The transmissivity and specific yield estimated from the existing database range from 10 to 810 m 2/day and 0.08% to 10.92% respectively. After successful calibration with Nash–Sutcliffe efficiency greater than 0.80, the values of horizontal hydraulic conductivity and specific yield of the unconfined aquifer were set in the range 1.85–61.90 m/day and 0.006–0.24 respectively. After validating the model, it is applied for forecasting the aquifer’s response to anticipated future scenarios of groundwater draft, recharge rate and sea level rise. The findings of the study illustrate that saltwater intrusion is intensified in the area adjoining the tidal rivers, rather than that due to the sea alone. Of all the scenarios simulated, the immense negative impact on groundwater quality emerges due to overdraft conditions and reduced recharge with the areal extent of seawater intrusion exceeding about 67% (TDS >1 kg/m 3). The study also arrives at the conclusion that, regional sea level rise of 1 mm/year has no impact on the groundwater dynamics of the aquifer.

Estimation of the inland extending length of the freshwater–saltwater interface in coastal unconfined aquifers

ABSTRACTThe inland extending length of the freshwater–saltwater interface toe is useful in studies of seawater intrusion in coastal areas. The submarine fresh groundwater discharge in coastal zones is affected not only by hydraulic conductivity and hydraulic gradient of the aquifer, but also by the position of the interface. Two observation wells at different distances from the coast are required to calculate the fresh groundwater flow rate in coastal unconfined aquifers. By considering that the submarine groundwater discharge is equal to the groundwater flow rate, the length of the interface toe extending inland can be estimated when the groundwater flow is at a steady-flow state. Aquifers with horizontal and sloping confined beds and without/with unique surface vertical infiltration are considered. Examples used to illustrate the application of these methods indicate that the inland extending lengths of the interface toe in aquifers with vertical surface infiltration are much shorter than those in aquifers without vertical surface infiltration, and the length of the interface in aquifers with a horizontal confining lower bed are smaller than those in aquifers with a confining lower bed sloping towards the sea. The extent of the interface on the northwestern coast near the city of Beihai in southern Guangxi, China, on 18 January 2013 was estimated as 471–478 m.Editor M.C. Acreman Associate editor not assigned

Fluctuations of fresh-saline water interface and of water table induced by sea tides in unconfined aquifers

This study examines effects of tides on fluctuations of the fresh-saline water interface and the groundwater level in unconfined coastal aquifers using a two-dimensional numerical model. The time-lags of the simulated hydraulic heads and salinities fluctuations compared to sea level fluctuations are analyzed using cross-correlation analysis. The results show that both the fresh-saline water interface and the groundwater level are affected harmonically by sea tide fluctuations. However, significantly different time-lags are obtained between the hydraulic head in the deeper and upper parts of the aquifer, and between head and salinity in the fresh-saline water interface. The hydraulic head in the deeper part of the aquifer responses much faster to sea level fluctuations than in the upper part. Surprisingly, a similar difference is detected between the time-lag of the hydraulic head in the fresh-saline water interface and the time-lag of the salinity at the same location. Furthermore, the time-lag of the salinity in the fresh-saline water interface is similar to the time-lag of the water table. We suggest a comprehensive mechanism for tidal influence on the coastal groundwater system, in which two main processes act simultaneously. First, sea tide causes a pressure head wave which propagates into the saturated zone of the aquifer, governed by the diffusivity of the aquifer (Ks/Ss). Second, this pressure head wave is attenuated at the water table due to the unsaturated flow within the capillary fringe which occurs during groundwater level oscillations. Because the tidal forcing acts on the sea-floor boundary and the attenuation of the groundwater level due to capillary effect acts on the groundwater table, two dimensional distributions of time-lag and hydraulic head amplitude are created. The capillary effect in the unsaturated zone plays a key role not only in the water table fluctuations as shown previously, but also on the salinity fluctuations in the fresh-saline water interface. Unsaturated flow dynamics controls the actual movement of the entire fresh water body, which results in simultaneous fluctuations of the groundwater level and the fresh-saline water interface.

Geochemical evolution and salinization of a coastal aquifer via seepage through peaty lenses

The aim of this study was to determine the role of buried peaty lenses in the salinization process of an unconfined coastal aquifer. To unravel the complex biogeochemical processes that occur between the peat matrix and the flowing through groundwater, two monitoring wells were sampled using high-resolution multilevel straddle packers. Moreover, to quantify the salt release in the aquifer from the peat lenses, a 1-m length column experiment was run for 62 days including a long equilibration step of 37 days, an elution step of 5 days, a stop flow of 15 days and a second elution step of 5 days. The column was monitored using 20 cm spaced ports to allow a precise reconstruction of the biogeochemical processes occurring within the matrix. A numerical model with SEAWAT-4.0 accounting for variable density flow and transport was used to simulate the observed salt concentration eluted from the column. To correctly reproduce the observed concentrations, a dual-domain approach coupled with a mass loading rate from the peat layer had to be accounted for. The results of this study highlight the capability of peat lenses to salinize the aquifer, to release large amount of sulfate and phosphate and to increase the chloride–bromide ratio.

Effects of hydrogeological properties on sea-derived benzene transport in unconfined coastal aquifers.

This paper presents numerical investigations on quantifying the hydrodynamic effects of coastal environment factors, including tidal fluctuations, beach slopes, hydraulic conductivity, and hydraulic gradients on sea-derived benzene transport in unconfined coastal aquifers. A hydrologic transport and mixed geochemical kinetic/equilibrium reactions in saturated-unsaturated media model was used to simulate the spatial and temporal behaviors of the density flow and benzene transport for various hydrogeological conditions. Simulation results indicated that the tidal fluctuations lead to upper saline plumes (USPs) near the groundwater and seawater interfaces. Such local circulation zones trapped the seaward benzene plumes and carried them down in aquifers to the depth depending on the tide amplitudes and beach slopes across the coastal lines. Comparisons based on different tidal fluctuations, beach slopes, hydraulic conductivity, and hydraulic gradient were systematically conducted and quantified. The results indicated that areas with USPs increased with the tidal amplitude and decreased with the increasing beach slope. However, the variation of hydraulic conductivity and hydraulic gradient has relatively small influence on the patterns of flow fields in the study. The increase of the USP depths was linearly correlated with the increase of the tidal amplitudes. The benzene reactive transport simulations revealed that the plume migrations are mainly controlled by the local flow dynamics and constrained in the USP circulation zones. The self-cleaning process of a coastal aquifer is time-consuming, typically requiring double the time of the contamination process that the benzene plume reach the bottom of a USP circulation zone. The presented systematic analysis can provide useful information for rapidly evaluating seaward contaminants along a coastal line with available hydrogeological properties.

해안지역 비피압 충적 대수층에서의 흐름 및 분산(수치모형의 개발 및 적용)

해안지역 비피압 충적 대수층에서의 흐름 및 분산(수치모형의 개발 및 적용)

Estimation of the position of the sharp interface in littoral aquifers with hydraulic head differences within the fresh-water zone

In littoral zones where fresh groundwater discharges into the sea, a vertical difference in hydraulic head always exists within the fresh-water zone, i.e., the Dupuit assumption is not satisfied. The hydraulic head difference may affect the sharp interface and needs to be taken into account when position of the interface is determined. Examination of the equilibrium status of a point on the interface reveals that the location of the interface in a homogeneous and isotropic coastal unconfined aquifer can be determined based on the fresh-water head and the salt-water head at one point on the sharp interface. Considering a linear increase or decrease in the hydraulic head within the fresh-water zone, a method for estimating the position of the sharp interface is described on the basis of the fresh hydraulic heads at two arbitrary points in the fresh-water zone and the salt hydraulic head at one arbitrary point in the salt-water zone in the same vertical straight line in the littoral zone. The method using corresponding pressures at the points is also given. The Ghyben-Herzberg model is a specific case of the method. This method requires three nearest observation wells, two revealing the fresh-water zone and one tapping the deeper salt-water zone, or three transducers installed in one well, two in the fresh-water zone and one in the deeper salt-water zone. The Observations of piezometric heads in the northwest Beihai coast in China are used to illustrate the application of the method to determine the position of the sharp interface, which ranges from 32.56 to 49.15 m under the mean sea level during the specific observation period.

Coastal Aquifer Management Based on the Joint use of Density-Dependent and Sharp Interface Models

In pumping optimization of coastal aquifers, the evaluation of the objective function and constraints using density-dependent models is overwhelmed by complex and time-consuming numerical simulations. To address those cases where the available density-dependent model runs are very limited, due to excessive computational burden, an efficient optimization strategy is developed. The proposed methodology uses an efficient sharp interface model jointly with a complex density-dependent model in an evolutionary optimization algorithm. While most evaluations are based on the sharp interface model, the density-dependent model is selectively called to evaluate promising solutions and to improve the predictions of the sharp interface model through the adaptive modification of the saltwater-freshwater density ratio. The method is tested for pumping optimization problems in confined and unconfined coastal aquifers with multiple pumping wells. The optimal solutions are compared to those obtained by density-dependent as well as by sharp interface optimization alone. Under a very restrictive computational budget, the best feasible solution is attained in less than 25 density-dependent model runs for two optimization problems of 10 and 20 decision variables. The results indicate that this optimization method leads to good feasible solutions and that an improved estimation of optimal pumping rates can be achieved within a limited computational budget. The method could also stand as an efficient preliminary exploration of the optimal search space, to provide good feasible starting points for the implementation of more comprehensive methods of coastal aquifer management.

Effectiveness of a physical barrier for contaminant control in an unconfined coastal plain aquifer: the case study of the former industrial site of Bagnoli (Naples, southern Italy).

A vertical engineered barrier (VEB) coupled with a water treatment plant was surveyed in the framework of a vast remedial action at the brownfield site of the former ILVA of Bagnoli steel making facility located in western Naples, Italy. The VEB was put in place to minimize contaminant migration from the brownfield site toward the sea at the shorelines sites of Bagnoli and Coroglio. The efficiency of the VEB was monitored through 12 piezometers, 8 at the Bagnoli shoreline and 4 at the Coroglio shoreline. Concentrations of inorganic and organic pollutants were examined in upstream and downstream groundwater relative to the VEB. The mean levels of Al, As, Fe, and Mn largely exceeded the legal limits, 10-15-fold, whereas that of Hg was up to 3-fold the rules. The VEB decreased the outlet concentrations only at certain specific location of the barrier, four times for Al, 6-fold for Hg, and by 20% for Mn with means largely exceeding the rules. At the other sites, the downstream water showed marked increases of the pollutants up to 3-fold. Outstanding levels of the hydrocarbons > 12 were detected in the inlet water with means of some hundred times the limits at both sites. Likewise most of screened inorganic pollutants, the downstream water showed marked increases of the hydrocarbons up to ~113%. The treatment plant was very effective, with removal efficiencies >80% for As, Al, Fe, and Mn. The study evidenced the need to put alternative groundwater remedial actions.

Water table salinization due to seawater intrusion

AbstractSeawater intrusion (SWI) is a significant threat to freshwater resources in coastal aquifers around the world. Previous studies have focused on SWI impacts involving salinization of the lower domain of coastal aquifers. However, under certain conditions, SWI may cause salinization of the entire saturated zone of the aquifer, leading to water table salinization (WTS) in unconfined aquifers by replacing freshwater within the upper region of the saturated zone with seawater, thereby posing a salinity threat to the overlying soil zone. There is presently limited guidance on the extent to which WTS may occur as a secondary impact of SWI. In this study, physical experiments and numerical modeling were used to explore WTS associated with SWI in various nontidal, unconfined coastal aquifer settings. Laboratory experiments and corresponding numerical simulations show that significant WTS can occur under active SWI (i.e., the freshwater hydraulic gradient slopes toward the land) because the cessation of freshwater discharge to the sea and the subsequent landward flow across the entire sea boundary eventually lead to water table salinities approaching seawater concentration. WTS during active SWI is larger under conditions of high hydraulic conductivity, rapid SWI, high dispersivity and for deeper aquifers. Numerical modeling of four published field cases demonstrates that rates of WTS of up to 60 m/yr are plausible. Under passive SWI (i.e., the hydraulic gradient slopes toward the sea), minor WTS may arise as a result of dispersive processes under certain conditions (i.e., high dispersivity and hydraulic conductivity, and low freshwater discharge). Our results show that WTS is probably widespread in coastal aquifers experiencing considerable groundwater decline sustained over several years, although further evidence is needed to identify WTS under field settings.

Cellular automata simulation of saltwater intrusion in coastal aquifer

The wasteful consumption of freshwater in heavily populated coastal areas usually consist the basic reason for the intrusion of saltwater into the coastal aquifers. In order to avoid such catastrophic scenarios, their prediction is of utter significance. Underground water systems are highly complex and the water flow is extremely dynamic, thus making the prediction of this phenomenon a difficult task. For this reason, a two dimensional Cellular Automaton (CA) was designed enabling both the qualitative and quantitative simulation and illustration of the saltwater intrusion into an unconfined coastal aquifer. The presented results ensure the robustness of the proposed CA model taking full advantage of its inherent parallelism and local connectivity.

Closed-form analytical solutions for assessing the consequences of sea-level rise on groundwater resources in sloping coastal aquifers

The impacts of sea-level rise due to climate change on seawater intrusion in sloping-shore coastal unconfined aquifers are investigated. The study provides four closed-form analytical solutions for: (1) assessing the change in water-table elevation of coastal aquifers resulting from sea-level rise; (2) calculating the magnitude of the change of the saltwater inland toe migration within coastal aquifers in a context of sea-level rise; (3) measuring the change of groundwater travel times through coastal aquifers in a context of sea-level rise, and (4) calculating the change in the quantity (changes of volume) of coastal fresh groundwater resources in a context of sea-level rise. The solutions apply to Dupuit-flow type conditions considering one-dimensional horizontal flow for homogenous and isotropic unconfined aquifers recharged by constant surface infiltration and discharging to the ocean, under steady-state conditions and assuming a sharp interface of the saltwater/freshwater transition zone. Examples are provided to illustrate how to apply the new solutions and conduct sensitivity analyses of the critical parameters involved in the equations. Taking into account the angle of the shore slope, these solutions constitute new analytical tools aiming to better measure the effects of land-surface inundation and anticipate changes in groundwater resources in coastal aquifers in a context of climate change.

Groundwater freshening following coastal progradation and land reclamation of the Po Plain, Italy

Many coastal areas historically were inundated by seawater, but have since undergone land reclamation to enable settlements and farming. This study focuses on the coastal unconfined aquifer in the Po Plain near Ravenna, Italy. Freshwater is present as isolated thin (1–5 m) lenses on top of brackish to saline water. Historical maps show large areas of sea inundation until approximately 150–200 years ago when coastal progradation and construction of the drainage canals began. Since then, the aquifer has been freshening from recharge. A three-dimensional SEAWAT model is used to simulate a 200-year freshening history, starting with a model domain that is saturated with seawater, and applying recharge across the top model layer. Calibration to the observed concentrations for discrete depths within many monitoring wells is remarkably good. The current distribution of freshwater is largely controlled by the drainage network. Within and adjacent to the drains, the groundwater has high salinity due to up-coning of salt water. Between drains, the surface layers of the aquifer are fresh due to the flushing action of recharge. The modeling results are consistent with cation exchange processes revealed in the groundwater chemistry and with freshwater lenses identified in electrical resistivity soundings.

A correction for Dupuit–Forchheimer interface flow models of seawater intrusion in unconfined coastal aquifers

Interface flow models that use the Dupuit-Forchheimer (DF) approximation for assessing the freshwater lens and the seawater intrusion in coastal aquifers lack representation of the gap through whic ...

Submarine groundwater discharge and nutrient addition to the coastal zone of the Godavari estuary

Submarine groundwater discharge (SGD) represents a significant pathway of materials between land and sea, especially as it supplies nutrients, carbon and trace metals to coastal waters. To estimate SGD fluxes to the Godavari estuary, India, we used multiple tracers: salinity, Si, 223Ra, 224Ra, 228Ra and 226Ra. Tracer abundances were elevated in groundwater from the unconfined coastal aquifer and in surface water from the near shore zone; these enrichments decreased to low levels offshore, indicative of groundwater discharge. A model based on the decay of 224Ra relative to 228Ra was used to determine apparent water ages of various bays within the estuary. These ages ranged from 2.6 to 4.8d during November 2011. Knowing the water age, the distribution of radium in the estuary, and the radium isotopic composition of groundwater enabled us to calculate SGD fluxes to the estuary. These fluxes (in units of 106m3d−1) were on the order of 5 in the Gautami Godavari estuary, 20–43 in the Vasishta Godavari estuary, and about 300 in Kakinada bay, where enhanced ion exchange processes and redox-controlled cycling in the mangrove ecosystem may contribute to higher fluxes. These estimates of water fluxes allowed us to determine the magnitude and seasonal variability in the nutrient fluxes to the estuary associated with SGD. These nutrient fluxes (in units of mmolm−2d−1) ranged from 1–19 (N), 0.6–2.6 (P), and 5–40 (Si) in Gautami Godavari; 19–40 (N), 2.6–5.5 (P), and 200 (Si) in Vasishta Godavari; and 120–140 (N), 10 (P), and 220 (Si) in Kakinada bay. The high SGD fluxes to Kakinada bay contribute significant nutrients to this bay; considerably lower SGD fluxes to Vasishta Godavari still contribute significant nutrients to this estuary. Thus SGD represents a major source of new nutrients to these coastal ecosystems. For the entire Godavari estuarine system, SGD fluxes contribute (48–88)×109molDICy−1 and (51–94)×109molTAy−1. These fluxes represent ~54 and ~62% of the riverine DIC and TA fluxes to the Godavari estuarine system. This study provides baseline data against which future changes in nutrient and carbon fluxes due to urbanization and economic growth over this region can be compared.

Forecasting monthly groundwater level fluctuations in coastal aquifers using hybrid Wavelet packet–Support vector regression

AbstractThis research demonstrates the state-of-the-art capability of Wavelet packet analysis in improving the forecasting efficiency of Support vector regression (SVR) through the development of a novel hybrid Wavelet packet–Support vector regression (WP–SVR) model for forecasting monthly groundwater level fluctuations observed in three shallow unconfined coastal aquifers. The Sequential Minimal Optimization Algorithm-based SVR model is also employed for comparative study with WP–SVR model. The input variables used for modeling were monthly time series of total rainfall, average temperature, mean tide level, and past groundwater level observations recorded during the period 1996–2006 at three observation wells located near Mangalore, India. The Radial Basis function is employed as a kernel function during SVR modeling. Model parameters are calibrated using the first seven years of data, and the remaining three years data are used for model validation using various input combinations. The performance of b...

Assessment of sharp-interface approach for saltwater intrusion prediction in an unconfined coastal aquifer exposed to pumping

Water over-exploitation becomes a common problem in coastal aquifers as it disarranges the dynamic equilibrium of saltwater and freshwater and causes saltwater intrusion (SWI). Mathematical simulations become a necessary tool nowadays to predict SWI under future pumping scenarios. This study aims to assess the validity of sharp-interface approach for an unconfined coastal aquifer subjected to pumping by comparison with sand-tank observation and dispersive approach results. The comparison was in terms of (1) transient movement of saltwater toward the well screen and (2) prediction of well salinity in times. The sharp-interface approach produced acceptable results, although it over-predicted the toe position of saltwater wedge. Salinity of extracted water was less predicted by sharp-interface modeling. The sharp-interface approach was then applied for a synthetic field-scale unconfined aquifer in steady condition with different pumping rates and well placements to explore the sensitivity of the modeling. The results were compared with salinity contours of dispersive modeling. The sharp-interface approach produced better result for higher pumping rates where the saltwater was reached to the well screen. Additionally, the results of fully penetrating wells (compare to partially penetrating one) and also closer location of well to shoreline matched better with the dispersive modeling outputs. In real cases, where the saltwater may wend a long distance toward the well screen, the sharp-interface modeling weakly matched with the dispersive modeling specially in terms of well salinities that is attributed to wider mixing zone.

Investigation of sea level rise effect on saltwater intrusion in an unconfined coastal aquifer using sharp-interface approach

A numerical sharp-interface model is developed to predict saltwater intrusion (SWI) into an unconfined coastal aquifer due to sea level rise (SLR). The model is applied to a conceptual field-scale aquifer to explore the sensitivity of the intrusion mechanism to various involved parameters. In all of the simulations, appropriate SLR rate has been applied to the sharp-interface model. The 90 years unsteady results showed that changes in hydraulic conductivity, rainfall recharge and freshwater inflow have significant effect on intrusion but the model is not very sensitive to different values of porous media porosity and specific yield. Afterward, the seaside saltwater head is raised and the toe of new time dependent salt wedge is determined. The results demonstrated that the SLR leads to more SWI. This excess intrusion is not very clear when SLR rate is low (e.g., 0.2 mm/year) but saltwater intrudes significantly further when the rate gets the higher values (e.g., 8 mm/year).