Groundwater salinity drivers

Natural drivers

• Deposition of marine sediments: During this process, seawater fills the voids of the sediments (connate water). Often it remains trapped inside the sediment matrix for a very long period, even long after the marine environment has stopped. 
• Sea level variations: Rising sea levels may cause flooding of coastal land by seawater, either for a long period (marine transgressions) or a short one (storm flood events, tsunamis). In addition, they boost lateral seawater intrusion into coastal aquifers that are hydraulically connected to the sea. Reduced sea levels, like those prevailing on a global scale during the Pleistocene, create conditions for intensified flushing of coastal saline groundwater domains.
• Meteorological processes and the hydrological cycle: On a worldwide scale, evaporation contributes steadily to the formation saline groundwater, in particular by producing evaporates. Rainfall, on the other hand, tends to have an opposite effect and activates those parts of the hydrological cycle that may flush and refresh bodies of saline groundwater.
• Climate change: The climate change anticipated for the current century (admittedly, not an entirely ‘natural' phenomenon) is assumed to cause a global rise in atmospheric temperatures and rising sea levels. This will intensify the risks of sea water intrusion. Especially in areas where in addition the rainfall becomes less, salinity levels of groundwater may increase by intensified evaporation and by stronger anthropogenic pressures.

Saltwater intrusion wedge

Anthropogenic drivers 

• Coastal protection, land reclamation and drainage: These human activities have a strong impact on local and regional hydrological conditions. On the one hand, they may reduce the encroachment of sea water into the aquifers. However, if drainage results in declined groundwater levels, then this may change the groundwater regimes in the sense that connate water  migrates into formerly fresh aquifers and lateral intrusion of seawater becomes more intensive.
• Groundwater abstraction modifies the local subsurface hydrodynamic pressure field causing flow towards the well. If saline groundwater is part of the subsurface system, then it tends to become mobilized.
• Irrigation tends to increase gradually the salinity levels in soil water, surface water systems and/or aquifers. This is because the crop evapotranspiration leaves a residue of dissolved substances in the soil. These effects are most pronounced under arid conditions.
• Disposal of waste & pollution: Examples of pollution leading to groundwater salinisation are the subsurface injection of saline water as applied in the oil industry, the operation of waste disposal sites, use of road salt in winter (for de-icing purposes).

Center pivot irrigation