Density-dependent Numerical Model Research Articles

Evaluation of Fresh Groundwater Lens Volume and Its Possible Use in Nauru Island

A proper management of fresh groundwater lenses in small islands is required in order to avoid or at least limit uncontrolled saltwater intrusion and guarantee the availability of the resource even during drought occurrences. An accurate estimation of the freshwater volume stored in the subsoil is a key step in the water management decision process. This study focused on understanding the hydrogeological system behaviour and on assessing the sustainable use of the groundwater resource in Nauru Atoll Island (Pacific Ocean). A first phase, concerning the hydrogeological characterization of the island, highlighted the occurrence of few drought-resilient freshwater lenses along the seashore. The second part of the study focused on the characterization of a freshwater lens found in the northern coastal area and identified such area as the most suitable for the development of groundwater infrastructures for water withdrawal. The characterization activities allowed quantifying the freshwater lens thickness and volume in order to assess the capability to satisfy the population water demand. A geo-electrical tomography survey was carried out, and a 3D density-dependent numerical model was implemented in SEAWAT. The model results demonstrated that in small islands freshwater can unexpectedly accumulate underground right along the seashore and not in the centre of the island as is commonly believed. Furthermore, the model can constitute a useful tool to manage the groundwater resources and would allow the design of sustainable groundwater exploitation systems, avoiding saltwater intrusion worsening.

Spatio-temporal mapping of salinity in the heterogeneous coastal aquifer

The interface between freshwater and saltwater in the coastal aquifer is a main factor that is affected by environmental threat and excessive pumping. In this study, a density-dependent numerical model was utilized to simulate the salt distribution of a coastal aquifer in the Talar region near the Caspian Sea, Mazandaran, Iran. To simulate the aquifer salinity and head contour over 2-year stress period, SEAWAT model in groundwater modeling system (GMS) package was used. Moreover, different variogram models based on 21 observation points were tested to find a best fit between observed and simulated total dissolved solids (TDS). A genetic algorithm was coupled with GMS model to identify the optimal sill ratio, range, and nugget value of each variogram. The interface between saltwater and freshwater estimated by the optimal variogram model and compared with the result of sharp interface and variable density flow assumption. The result indicates that the optimal Gaussian variogram as best estimation of spatial TDS can be used for estimation of temporal salinity with a coefficient determination about 0.91. Also, an integration of spherical and exponential variogram shows the high performance for mapping spatial TDS and neutralizes the underestimation behavior of single variogram. A comparison between variogram model and analytical solution indicates that the interface length in the eastern part of aquifer for Gaussian model is relatively small than sharp interface relation. The maximum saltwater penetration based on sharp interface and SEAWAT results is about 1600 m and 1750 m, respectively. Additionally, maximum saltwater intrusion is observed in the western part of aquifer which has a large number of active wells.

Numerical modeling of seawater intrusion in Zhoushuizi district of Dalian City in northern China

Since 1980s, seawater intrusion in coastal aquifers caused by groundwater over-abstraction has led to extensive deterioration in groundwater quality and quantity and has been fazing local residents in Zhoushuizi district of the metropolitan Dalian City in northern China. In this study, a three-dimensional (3D) density-dependent numerical model was constructed to simulate the seawater intrusion process in heterogeneous coastal aquifers in Zhoushuizi district of the metropolitan Dalian City. Considering that the groundwater flow in karst aquifers in northern China is relatively uniform, approximately following Darcy’s law, the fracture-karst aquifer in Zhoushuizi district of Dalian City was simplified as an equivalent porous medium. To further identify the hydrogeological parameters of the aquifers in the study area, the model was calibrated and validated using the observation heads and concentrations. Based on the current groundwater abstraction conditions of the study area, the calibrated and validated model was then applied to predict the dynamics and trend of seawater intrusion for the following 30 years from 2010 to 2040 under different rainfall scenarios. The overall extent of seawater intrusion in the future would be even more severe under different prediction scenarios. This 3D seawater intrusion model provides the theoretical basis for implementing a reasonable allocation of groundwater resource, which may significantly affect the sustainability of water resources.

Impact of Climate Change on Salinization of Coastal Water Resources

A density-dependent numerical model was set up to quantify the actual and future (2050) salinization of a coastal aquifer in the Po Delta (Italy). SEAWAT 4.0 was used for this purpose, while PEST was used for automatic inverse parameter calibration. The use of high-resolution multi-level sampling allowed obtaining insights in the salinity stratification within the aquifer and on the vertical fluxes induced by the reclamation drainage network. The calibrated model was employed to build up a scenario investigating the effects of the projected relative sea level rise on groundwater salinity by 2050. This scenario allowed to identify the zones that resulted more affected by the relative sea level rise and to quantify the increase in salinization of groundwater, the salt loads export towards surface waters and the changing volumes of freshwater. Results show that the Po Delta will experience a significant salinization by 2050 and that the major cause is autonomous salinization via seepage of saline groundwater rather than enhanced seawater intrusion due to sea level rise.

Inferring the interconnections between surface water bodies, tile-drains and an unconfined aquifer–aquitard system: A case study

Summary Shallow lenses in reclaimed coastal areas are precious sources of freshwater for crop development, but their seasonal behaviour is seldom known in tile-drained fields. In this study, field monitoring and numerical modelling provide a robust conceptual model of these complex environments. Crop and meteorological data are used to implement an unsaturated flow model to reconstruct daily recharge. Groundwater fluxes and salinity, water table elevation, tile-drains’ discharge and salinity are used to calibrate a 2D density-dependent numerical model to quantify non-reactive solute transport within the aquifer–aquitard system. Results suggest that lateral fluxes in low hydraulic conductivity sediments are limited, while water table fluctuation is significant. The use of depth-integrated monitoring to calibrate the model results in poor efficiency, while multi-level soil profiles are crucial to define the mixing zone between fresh and brackish groundwater. Measured fluxes and chloride concentrations from tile-drains not fully compare with calculated ones due to preferential flow through cracks.

A 3D density-dependent model for assessment and optimization of water management policy in a coastal carbonate aquifer exploited for water supply and fish farming

Summary The Ansedonia promontory (southern Tuscany, Italy) is characterized by the presence of fish farms that pump thermal saline groundwater. The water is extracted from a carbonate aquifer with high permeability due to fracturing and karstification that is also exploited for irrigation purposes and domestic use. Such exploitation has led to the degradation of groundwater quality, producing conflict among the different users. The conceptualization of the aquifer allowed the development of a 3D finite element density-dependent numerical model using the FEFLOW code. The slightly negative freshwater budget in the very humid hydrologic year of 2004-2005 revealed that the aquifer was overexploited, especially due to the extraction of freshwater (along with seawater) from fish farm wells and pumping from public supply wells. The model was also used to forecast the quantitative and qualitative evolution of resources over time, thus testing the effects of different management hypotheses. Results demonstrate that the sustainable management of the aquifer mostly depends on withdrawals from public supply wells; the quantity extracted by fish farms only significantly affects the freshwater/saltwater interface and, locally, the salinity of groundwater. Actions to counteract seawater intrusion are proposed.

Numerical modelling of seawater intrusion in Shenzhen (China) using a 3D density-dependent model including tidal effects

During the 1990s, groundwater overexploitation has resulted in seawater intrusion in the coastal aquifer of the Shenzhen city, China. Although water supply facilities have been improved and alleviated seawater intrusion in recent years, groundwater overexploitation is still of great concern in some local areas. In this work we present a three-dimensional density-dependent numerical model developed with the FEFLOW code, which is aimed at simulating the extent of seawater intrusion while including tidal effects and different groundwater pumping scenarios. Model calibration, using waterheads and reported chloride concentration, has been performed based on the data from 14 boreholes, which were monitored from May 2008 to December 2009. A fairly good fitness between the observed and computed values was obtained by a manual trial-and-error method. Model prediction has been carried out forward 3 years with the calibrated model taking into account high, medium and low tide levels and different groundwater exploitation schemes. The model results show that tide-induced seawater intrusion significantly affects the groundwater levels and concentrations near the estuarine of the Dasha river, which implies that an important hydraulic connection exists between this river and groundwater, even considering that some anti-seepage measures were taken in the river bed. Two pumping scenarios were considered in the calibrated model in order to predict the future changes in the water levels and chloride concentration. The numerical results reveal a decreased tendency of seawater intrusion if groundwater exploitation does not reach an upper bound of about 1.32 × 104 m3/d. The model results provide also insights for controlling seawater intrusion in such coastal aquifer systems.

Evaluation of climate change effects in a coastal aquifer in Morocco using a density-dependent numerical model

A density-dependent numerical groundwater model was applied to study the climate change impact in a shallow aquifer in the Mediterranean coast of Morocco, the Saïdia aquifer. The stresses imposed to the model were derived from the IPCC emission scenarios and included recharge variation and sea level rise. The main effect of the climate change in the Saïdia aquifer will be a decrease in renewable resources, which in the worst-case scenario may decrease to 50–60% of present-day values, due to the decline in recharge and to a reduced inflow from the adjacent Triffa aquifer. The water quality will be affected mostly in the area immediately adjacent to the seashore, where salinity may increase up to 30 g/l. Localised areas may see a decrease in salinity due to the induced freshwater recharge from Oued Moulouya River and diminished inflow from high-salinity springs.

Occurrence of salt water above fresh water in dynamic equilibrium in a coastal groundwater flow system near De Panne, Belgium

A salt water lens is found above fresh water under the shore between Dunkerque (France) and Nieuwpoort (Belgium). This inverse density distribution is in a dynamic equilibrium. It develops due to the infiltration of salt water on the back shore during high tide. Under this salt water lens, water infiltrated in the adjacent dune area flows towards the sea and discharges at the seabed. This water quality distribution differs from the classic salt water wedge under fresh water described in the literature. Here, the evolution to this water quality distribution is simulated with a density dependent numerical model. A large tidal range, shore morphology and a permeable groundwater reservoir are the main conditions for the observed water quality distribution. By altering these conditions, intermediate water quality distributions between the classic salt water wedge and the one discussed here develop. Based on these simulations, it is expected that similar kinds of inverse density distribution could be present in a number of coastal areas, which have tides, a gently sloping shore and a permeable substratum.