Global overview of saltwater intrusion problem

Sustainable management of natural water resources and food security in the face of changing climate conditions is critical to the livelihood of coastal communities. Worldwide, groundwater resources are the primary source of freshwater in coastal regions and represent about 31% of the earth’s freshwater. Population growth increases water demands and leads to over-pumping from the aquifers. Climate change and over-pumping, combined with the sea water level rise (SLR), adversely impact the water shortage problem. For instance, about 85% of total groundwater abstractions in Egypt are exploited from Egypt’s Nile Delta aquifer. Saltwater intrusion (SWI) is considered one of the primary sources of fresh groundwater pollution, impacting drinking water and irrigation standards and endangering future exploitation of coastal aquifers. The leading causes of SWI include groundwater flow changes, aquifers over-abstraction, barometric pressure, seismic waves, dispersion, climate change by global warming, and sea-level fluctuation, among others. The intensively exploited regions around the world are generally located within the world’s coastal aquifers, with around 70%.

Global spatial distribution of groundwater resources and coastal cities impacted by SWI (Source: www.whymap.org).

In Mediterranean regions, the coastal aquifers and low land areas, e.g., along Egypt’s Mediterranean coast, are vulnerable to climate change due to SLR, saltwater intrusion on freshwater resources, agricultural resources, tourism, and human settlement. An SLR between 18 and 58 cm is expected by 2100; about 95% of coastal areas worldwide will be affected by SLR. Over-pumping and groundwater contamination by natural and artificial activities are depleting fresh groundwater bodies. Groundwater contamination is damaged by four sources: environmental, domestic, industrial, and agricultural activities.  On the other side, agricultural water management in coastal agriculture plays a vital role in the agricultural production system. Utilizing salty water to irrigate crops will lead to an accumulation of salts in the soil and increase soil salinity. In some cases, it will result in soil degradation and loss of arable land. Increasing inundation and saltwater intrusion (SWI) will likely adversely affect agricultural production and the associated beach access for tourism.

Schematic representation of SWI and coastal earth fill.

Also, increased human impact; make clear that there is an immediate necessity to manage aquifer use and predict future conditions under various management strategies. The aquifers’ groundwater pollution by SWI has a major impact on groundwater; also, the management of SWI interface in the coastal regions is an environmental issue and sustains the fresh groundwater resources. The salinity differences in the coastal line are the main cause of the different environmental parameters. It has been seen that the soil parameter changes as salinity decreases. Clay, loam, and organic matter will increase with distance from the sea. Floristic zonal sequences express these changes. It has been suggested that the top 60 cm of fill material to retard SWI should be loamy texture soil characterized by an equal portion of sand, silt, and clay, considered the best soil texture for the agriculture production layer. Therefore, it is highly important that groundwater resource management and protection practices consider controlling SWI to maintain an ‘equilibrium’ between pumped water and the available aquifer recharge.

Methods to mitigate saltwater intrusion

Different methods were developed to control SWI in coastal aquifers. Such measures include the relocation of pumping wells, reduction of pumping rates, natural and artificial recharge, abstraction of saline water, combined recharge, and abstraction techniques that include installing subsurface physical barriers (e.g., cut-off walls or dams), and using earthen fill materials among others.

Optimization of pumping rates by reducing abstraction and changing the location of wells is applied for managing the coastal aquifer SWI.

The subsurface physical barriers method reduces the aquifer permeability and controls the basin’s saline water inflow. The barriers are impervious structures constructed to manage the seawater inflow and increase the groundwater storage or potentiality in a coastal aquifer. These physical barriers can be divided into two types based on the position of groundwater flow opening towards the coast. The first is the cut-off wall installed perpendicular to the aquifer flow direction; the second is subsurface dams embedded on the aquifer bedrock, as are storage dams. The construction materials of barriers use sheet piles and cement or chemical grout.

Saltwater intrusion control using subsurface barriers, cut-off wall (left side), and subsurface dam (right side).

Abstraction-recharge process is a hydraulic technique used to mitigate the coastal aquifer SWI and retard the variable density interface towards the shore. Artificial recharge could be applied using a storage dam, pondage, and/or injection wells where the waterflood is infiltrated into the aquifer by increasing freshwater storage volume.

Schematic presentation of a storage dam (upper) and a pond recharge (lower) applied mainly in wet regions.

In general, the artificial recharge lake capacity has demonstrated a great potential to minimize the SWI in humid and wet regions with a high level of flooding. Combining physical barriers during dry seasons and recharge wells for wet seasons may result in suitable measures to manage the SWI in semi-arid and semi-humid regions.

The TRAD methodology includes Treatment and Recharge of wastewater, Abstraction, and Desalination of brackish water. The TRAD method is an effective tool to mitigate SWI, is more economical, has a less environmental impact, and is more sustainable in coastal areas. The SWI could be decreased considerably by using the TRAD method in hyper-arid and arid regions.

The land reclamation technique uses the landfill, which impacts the freshwater resources in coastal areas. Land reclamation in coastal zones is an artificial mechanism to extend the coast to the sea using specified earthen fill placed at the desired geometry and slope, significantly impacting groundwater resources in the coastal region. These artificial fill areas provide increased land to meet growing urbanization needs and possibly reduce SWI. This technique changes and increases the groundwater flow and the discharges in the coastal direction and may develop a new zone for a freshwater body. Reducing groundwater pumping and land reclamation can help move the transition zone toward the shoreline.

Schematic representation of TRAD method.

The land reclamation technique uses the landfill, which impacts the freshwater resources in coastal areas. Land reclamation in coastal zones is an artificial mechanism to extend the coast to the sea using specified earthen fill placed at the desired geometry and slope, significantly impacting groundwater resources in the coastal region. These artificial fill areas provide increased land to meet growing urbanization needs and possibly reduce SWI. This technique changes and increases the groundwater flow and the discharges in the coastal direction and may develop a new zone for a freshwater body. Reducing groundwater pumping and land reclamation can help move the transition zone toward the shoreline.

Saline water and freshwater volume in Biscayne aquifer, Florida, USA, for different landfill scenarios.

The hydraulic benefits of the technique are penetrating the fresh groundwater into the reclamation zone, delaying the saline water inflow rates toward the aquifer, increasing the distance between production wells and the shoreline, providing a larger area to deal with natural rainfall, and improving life in coastal zones.

The reclamation areas worldwide support the agriculture demands and land use in the coastal regions. The increase in landfill width and decrease in the artificial aquifer permeability reduces the SWI and protects the freshwater aquifer. The fill area could be recharged by natural harvesting of precipitation and flash flooding and manage this water from discharge to the sea or artificially by surface water from agriculture or treatment water and desalination using the wave energy in the oceans and sea for production of freshwater. These factors will support surface water supplies that accommodate saline water intrusion and reflect the coupling between surface water and groundwater. Many studies showed that the groundwater heads and discharge were increased after reclamation. Also, land reclamation affects the aquifer system and is a long-lasting process.  Finally, the best strategy to mitigate the SWI and minimize desalination costs also depends on the precipitation rates.

Reference

El Shinawi, A., Kuriqi, A., Zelenakova, M., Vranayova, Z., & Abd-Elaty, I. (2022). Land subsidence and environmental threats in coastal aquifers under sea level rise and over-pumping stress. Journal of Hydrology, 608, 127607.
Abd-Elaty, I., Kushwaha, N. L., Grismer, M. E., Elbeltagi, A., & Kuriqi, A. (2022). Cost-effective management measures for coastal aquifers affected by saltwater intrusion and climate change. Science of The Total Environment, 836, 155656.
Abd-Elaty, I., Pugliese, L., & Straface, S. (2022). Inclined Physical Subsurface Barriers for Saltwater Intrusion Management in Coastal Aquifers. Water Resources Management, 1-15.
Abd-Elaty, I., & Zelenakova, M. (2022). Saltwater intrusion management in shallow and deep coastal aquifers for high aridity regions. Journal of Hydrology: Regional Studies, 40, 101026.
Abd-Elaty, I., Straface, S., & Kuriqi, A. (2021). Sustainable saltwater intrusion management in coastal aquifers under climatic changes for humid and hyper-arid regions. Ecological Engineering, 171, 106382.