Seawater intrusion is one of the major threats to freshwater resources in coastal areas, often exacerbated by groundwater overexploitation. Mitigation measures are needed to properly manage aquifers, and to restore groundwater quality. This study integrates three computational tools into a unified framework to investigate seawater intrusion in coastal areas and to assess strategies for managing groundwater resources under natural and human-induced stresses. The three components are a three-dimensional hydrogeological model for density-dependent variably saturated flow and miscible salt transport, an automatic calibration procedure that uses state variable outputs from the model to estimate selected model parameters, and an optimization module that couples a genetic algorithm with the simulation model. The computational system is used to rank alternative strategies for mitigation of seawater intrusion, taking into account conflicting objectives and problem constraints. It is applied to the Gaza Strip (Palestine) coastal aquifer to identify a feasible groundwater management strategy for the period 2011–2020. The optimized solution is able to: (1) keep overall future abstraction from municipal groundwater wells close to the user-defined maximum level, (2) increase the average groundwater heads, and (3) lower both the total mass of salt extracted and the extent of the areas affected by seawater intrusion.

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.

Offshore hydrogeology has been much less studied compared to onshore hydrogeology. The marine Quaternary system in Hong Kong (China) consists of interlayers of aquitards and aquifers and was part of the Pearl River Delta when the sea level was low before the Holocene. Core samples from six offshore boreholes were collected to measure the chloride concentration in the system by adding deionized water. A method was proposed to convert the sediment chloride into that of the original pore water. A one-dimensional sedimentation-transport model was developed to simulate the historical conservative transport of the reconstructed pore-water chloride. The model integrates present knowledge of stratigraphy and the historical evolution of the geological system. The chloride concentration profiles show that the chloride decreases from an average of 13,800 mg/L in the first marine unit to an average of 5,620 mg/L in the first aquifer. At the bottom of one borehole, the concentration is only 1,420 mg/L. The numerical model shows that the vertical chloride distribution is due to diffusion-controlled downward migration of seawater. The second marine unit obstructs the downward migration, indicating its low permeability and good aquitard integrity. The relatively fresh or brackish water in deep aquifers protected by the aquitard has the potential to be used as drinking water following some treatment, or at least as raw water with much cheaper desalinization compared with using seawater. The methodology and findings in this study are instructional for other coastal areas with similar geology and history in the South China Sea.

Results are presented for a quantitative and qualitative analysis of the relationship between hydrogeological and environmental elements characterizing the areas of groundwater-dependent ecosytems (GDEs) located in the Kampinos National Park in central Poland. Statistical analysis was used to assess the seasonal and long-term variability of groundwater conditions. A geographic information system (GIS)-based model enabled the visualization of the test results. Objectification of spatial relationships between hydrogeological and environmental elements was carried out using factor analysis. The statistical analysis of groundwater levels in the period 1999–2013 confirmed the sequence of wet and dry years. The calculation enabled the determination of the range of groundwater-level changes, but no specific trends were observed with respect to these changes. Moreover, the widespread belief that the lowering of the water table in presented GDEs is due to anthropogenic pressure and climate change was not confirmed. The factor analysis showed that GDE areas are characterized by a considerable homogeneity of abiotic elements and locally occurring heterogeneous regions, mainly related to anthropogenic pressure. Dependency between the type of plant community and depth to the water table in the typical GDEs was not defined by the delimiting factors.

Although impacts of land-use changes on groundwater recharge have been widely demonstrated across diverse environmental settings, most previous research has focused on the role of agriculture. This study investigates recharge impacts of tree plantations in a century-old experimental forest surrounded by mixed-grass prairie in the Northern High Plains (Nebraska National Forest), USA. Recharge was estimated using solute mass balance methods from unsaturated zone cores beneath 10 experimental plots with different vegetation and planting densities. Pine and cedar plantation plots had uniformly lower moisture contents and higher solute concentrations than grasslands. Cumulative solute concentrations were greatest beneath the plots with the highest planting densities (chloride concentrations 225–240 % and sulfate concentrations 175–230 % of the grassland plot). Estimated recharge rates beneath the dense plantations (4–10 mm yr⁻¹) represent reductions of 86–94 % relative to the surrounding native grassland. Relationships between sulfate, chloride, and moisture content in the area’s relatively homogenous sandy soils confirm that the unsaturated zone solute signals reflect partitioning between drainage and evapotranspiration in this setting. This study is among the first to explore afforestation impacts on recharge beneath sandy soils and sulfate as a tracer of deep drainage.

The beneficial use of large-scale geophysical surveys in combination with numerical modeling for assessing water resources problems in coastal areas is demonstrated. A 5,000-year long historical evolution of the regional distribution of salinity beneath a coastal lagoon in Denmark is simulated in a stage-wise approach using a two-dimensional variable-density flow and transport model and compared with an interpreted resistivity distribution from transient electromagnetic data. A sequence of multi-layer unconfined/confined aquifers with non-continuous aquitards is needed to match observations in terms of complexity in resistivity/salinity distribution, deep-seated low resistivity zones (trapped residual saltwater), and presence of groundwater discharge tubes with high resistivities indicating both near and off-shore discharge of fresh groundwater. Refreshening of the lagoon system is ongoing and simulations show that this process has been most rapid during the last ∼300 years, but will continue at a slower rate for the next many hundreds of years. The development of the lagoon over the last 5,000 years, the associated changes in salinity and the present-day control of lagoon salinity are responsible for these processes. Finally, simulation results show that the groundwater influx to the lagoon is significant. The estimated fluxes correspond to 168 % of net precipitation on the lagoon or 17 % of the discharge from the largest river into the lagoon.

A reliable prediction of hydrograph responses in mountainous headwater catchments requires a mechanistic understanding of the coupled hydro-climatic processes in these regions. This study shows that only a small fraction of the total area in a pre-Alpine headwater catchment actively regulates streamflow responses to hydro-climatic forcing, which facilitates the application of a parsimonious framework for hydrograph time-series prediction. Based on landscape analysis and hydrometric data from the Upper Rietholzbach catchment (URHB, 0.94 km², northeast Switzerland), a conceptual model was established. Here, the rainfall-event-driven contribution of surface runoff and subsurface flow (event flow) accounts for around 50 % of total river discharge. The event-flow hydrograph is generated from approximately 25 % of the entire area consisting of riparian zones (8 %) and adjacent hillslopes (17 %), each with characteristic streamflow-generating mechanisms. Baseflow generation is attributed to deep groundwater discharge from a fractured-rock aquifer covering ∼75 % of the catchment area. A minimalistic model, that represents event flow as depletion of two parallel linear reservoirs, verified the conceptual model of the URHB with adequate hydrograph simulations (R ² = 0.67, Nash-Sutcliffe efficiency (NSE) = 0.64). Hereby, the expansion of the event-flow contributing areas was found to be particularly significant during long and high-intensity rainfall events. These findings provide a generalized approach for the large-scale characterization of groundwater recharge and hydrological behavior of mountainous catchments with similar landscape properties.

Closed basins are catchments whose drainage networks converge to lakes, salt flats or alluvial plains. Salt flats in the closed basins in arid northern Chile are extremely important ecological niches. The Salar del Huasco, one of these salt flats located in the high plateau (Altiplano), is a Ramsar site located in a national park and is composed of a wetland ecosystem rich in biodiversity. The proper management of the groundwater, which is essential for the wetland function, requires accurate estimates of recharge in the Salar del Huasco basin. This study quantifies the spatio-temporal distribution of the recharge, through combined use of isotopic characterization of the different components of the water cycle and a rainfall-runoff model. The use of both methodologies aids the understanding of hydrological behavior of the basin and enabled estimation of a long-term average recharge of 22 mm/yr (i.e., 15 % of the annual rainfall). Recharge has a high spatial variability, controlled by the geological and hydrometeorological characteristics of the basin, and a high interannual variability, with values ranging from 18 to 26 mm/yr. The isotopic approach allowed not only the definition of the conceptual model used in the hydrological model, but also eliminated the possibility of a hydrogeological connection between the aquifer of the Salar del Huasco basin and the aquifer that feeds the springs of the nearby town of Pica. This potential connection has been an issue of great interest to agriculture and tourism activities in the region.

Increased irrigation in the Neishaboor watershed, Iran, during the last few decades has caused serious groundwater depletion, making the development of comprehensive mitigation strategies and tools increasingly important. In this study, SWAT and MODFLOW were employed to integratively simulate surface-water and groundwater flows. SWAT and MODFLOW were iteratively executed to compute spatial and temporal distributions of hydrologic components. The combined SWAT-MODFLOW model was calibrated (2000–2010) and validated (2010–2012) based on streamflow, wheat yield, groundwater extraction, and groundwater-level data. This multi-criteria calibration procedure provided greater confidence for the partitioning of water between soil storage, actual evapotranspiration, and aquifer recharge. The SWAT model provided satisfactory predictions of the hydrologic budget for the watershed outlet. It also provided good predictions of irrigated wheat yield and groundwater extraction. The 10-year mean annual recharge rate estimated using the combined model varied greatly, ranging from 0 to 960 mm, with an average of 176 mm. This result showed good agreement with the independently estimated annual recharge rate from an earlier study. The combined model provides a robust tool for the sustainable planning and management of water resources for areas with stressed aquifers where interaction between groundwater and surface water cannot be easily assessed.

Groundwater has provided 50–90 % of the total water supply in Antelope Valley, California (USA). The associated groundwater-level declines have led the Los Angeles County Superior Court of California to recently rule that the Antelope Valley groundwater basin is in overdraft, i.e., annual pumpage exceeds annual recharge. Natural recharge consists primarily of mountain-front recharge and is an important component of the total groundwater budget in Antelope Valley. Therefore, natural recharge plays a major role in the Court’s decision. The exact quantity and distribution of natural recharge is uncertain, with total estimates from previous studies ranging from 37 to 200 gigaliters per year (GL/year). In order to better understand the uncertainty associated with natural recharge and to provide a tool for groundwater management, a numerical model of groundwater flow and land subsidence was developed. The transient model was calibrated using PEST with water-level and subsidence data; prior information was incorporated through the use of Tikhonov regularization. The calibrated estimate of natural recharge was 36 GL/year, which is appreciably less than the value used by the court (74 GL/year). The effect of parameter uncertainty on the estimation of natural recharge was addressed using the Null-Space Monte Carlo method. A Pareto trade-off method was also used to portray the reasonableness of larger natural recharge rates. The reasonableness of the 74 GL/year value and the effect of uncertain pumpage rates were also evaluated. The uncertainty analyses indicate that the total natural recharge likely ranges between 34.5 and 54.3 GL/year.