Unit 8: Water Resources // Section 7: Water Salinization
When freshwater resources become saline, they can no longer be used for irrigation or drinking. Saline water is toxic to plants, and high sodium levels cause dry soils to become hard and compact and reduce their ability to absorb water. Irrigation water becomes toxic to most plants at concentrations above 1,300 milligrams/liter; for comparison, the salinity of seawater is about 35,000 mg/l (footnote 12). Salinity is not dangerous to humans, but water becomes nonpotable for human consumption at about 250 mg/l.
Groundwater extraction and irrigation can increase salt concentrations in water and soils in several ways. First, irrigation increases the salinity of soil water when evaporation removes water but leaves salt behind. This occurs when irrigation water contains some salt and irrigation rates are not high enough to flush the salt away. Saline water in the vadose zone can then contaminate surface water and soils. Irrigation has caused high salinity levels in areas including the cotton growing region near the Aral Sea in Central Asia, the lower reaches of the Colorado River, and California's Central Valley (Fig. 12).
Figure 12. Fields in central California suffering from severe salinization
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Source: © United States Department of Agriculture, Agricultural Research Service.
Irrigation can also cause salinization by raising the water table and lifting saline groundwater near the surface into the root zone. This occurs when irrigation efficiency is poor, so a large fraction of irrigation water infiltrates into the soil, and groundwater flow is slow. A similar problem occurs in some regions when trees are cut down, reducing transpiration and increasing the rate at which water flushes through the vadose zone. The increased infiltration flushes high concentrations of salt to the water table and lifts the water table toward the surface. This process has severely affected the Murray-Darling Basin in Australia.
A third type of salinization occurs in coastal areas, where excessive groundwater pumping draws seawater into aquifers and contaminates wells. In coastal aquifers freshwater floats on top of denser seawater. When this lens of freshwater is diminished by withdrawals, seawater rises up from below. Because world populations are increasing particularly rapidly in coastal regions, seawater intrusion is a threat in many coastal aquifers.
A recent analysis by scientists at the Institute of Ecosystem Studies found that salinity levels have also increased significantly in urban and suburban areas in the northeastern United States. The authors attributed this rise to two main factors: use of salts for de-icing roads in winter and increased levels of street paving. These trends deliver concentrated bursts of saline runoff to local water bodies after storms and floods. "As coverage by impervious surfaces increases, aquatic systems can receive increased and pulsed applications of salt, which can accumulate to unsafe levels in ground and surface waters over time," the authors observe (footnote 13).