Salinity can also impact the suitability of water for irrigation and livestock watering. Salinity hazard is assessed by measuring the electrical conductivity of an aqueous extract of the overburden. It can be present in the form of salts in solution, or adsorbed onto clays. Sodium is highly mobile and can readily leach from exposed spoil. Once mobilized it can migrate upward from the spoil into the root zone.
Sodium can also cause dispersion and swelling of clays, especially montmorillonite.
High levels of sodium in the root zone causes deterioration of soil structure that restricts water movement, aeration and plant development. The suitability of water supplies for irrigation and livestock watering can also be affected by high levels of sodium. The SAR is a ratio used to express the relative activity of sodium ions in exchange reactions with soil where ionic concentrations are expressed in milliequivalents per liter. Although selenium is an essential nutrient, it can also be extremely toxic. Care must be taken not to place selenium overburden material in the recharge zone where it could contaminate underlying aquifers.