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The Aral Sea region has a problem with salinization of soils and drinking water, e.g. water coming from wells. As I understand it, it's caused by unsustainable cotton farming, but what is the exact mechanism? This publication says something about over-irrigation of soils and liberation of salts locked beneath soil surface, but because of the way the sentence is phrased ("The result is..."), I'm not sure it's what triggered it initially.

The result is that over-irrigated soils have accumulated excessive amounts of water capable of liberating salt locked deep beneath the soil surface.

Sergey Zolotarev
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  • https://en.wikipedia.org/wiki/Soil_salinity specifically the part titled "Salinity due to irrigation" – John Jul 19 '21 at 02:02
  • One thing to consider is how was ground water replenished in the region prior to the use of prolonged massive irrigation schemes & how is it replenished now. It is well known all water contains some salt. With decades of extensive use of irrigation water salt has accumulated in soils. Depending on the level of the water tables in the region (proximity to the land surface), the salt accumulated in the soil can contaminate underground water & thus contaminate once potable water. – Fred Jul 19 '21 at 05:27
  • @John It's not detailed and clear enough. If all water contains salt, even rainfall, then what difference does it make if it's irrigation and not precipitation? – Sergey Zolotarev Jul 19 '21 at 06:43
  • The amount of salt in rain water is much less than water from rivers which flow over minerals (all forms of rocks) accumulating dissolved salts as they flow. Salt in rain water is due to salt based aerosols acting a nucleus for water droplets for form on. – Fred Jul 19 '21 at 09:03
  • @SergeyZolotarev look at the image, read the whole article, you can also follow links, dry land salinity and environmental impact of irrigation each has it's own page for instance. precipitation adds little water at a time, irrigation adds huge amounts which can bring deeper minerals to the surface. – John Jul 19 '21 at 13:57

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the soil already contains salts, dry soils are especially prone to this as there is not enough water movement to move salts away. there are several ways the salt can get there, from slowly accumulating due to precipitation or from direct weathering of the minerals in or around the soil. leeching just drives it deep. Irrigation then remobilizes this salt bringing it to the surface, instead of staying in the deeper parts of soil. remember irrigation adds orders of magnitude more water to dry soils then they had before, completely changing how the soil behaves.

enter image description here

https://www.researchgate.net/figure/Hydrological-processes-and-salinity-development-commonly-found-in-cropping-systems-in_fig3_228776735

John
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  • Re "from slowly accumulating due to precipitation". Could you expand/clarify this? Most precipitation should be free of dissolved minerals, as the water originates from evaporation. There is probably some aerosolized salt spray in the direct vicinity of large bodies of saltwater. – njuffa Jul 19 '21 at 20:29
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    rain varies quite a lot in its composition, most rain water contains microscopic amounts of salt which over millions of years can accumulate, this can come from sea water and aerosolized dust pulled out of the atmosphere by rain. remember rain can be transported thousands of miles before it falls. – John Jul 19 '21 at 20:50
  • Thanks, but I'm still not sure how exactly irrigation brings the salt up – Sergey Zolotarev Oct 17 '22 at 22:20
  • It adds more water, which dissolves some of the salt then through capillary action brings it towards the surface. – John Oct 17 '22 at 22:54