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And, do you know, I think that irrigation might have had something to do with how they died out.

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while Dr. Conway asked:

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Which reminds me, has anyone ever figured out how to deal with salt deposition in irrigated soils?

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This page, focusing on the southwestern United States, is a good introduction to the problems associated with irrigated agriculture. (Careful: it loads slowly and has been freezing up my browser).

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The main impacts of irrigated agriculture in the Southwest are due to the building of dams and the diversion of water, the waterlogging of soils, the overdraft of groundwater resources, and the salinization of soil and water. "Consumptive" use is a term used for water which is lost after its use, rather than immediately returned to its source for reuse. Consumptive use of water by irrigation can reach as high as 90% in the Southwest, and the table below lists the irrigated acreage for all the Southwestern states and the percentage of the total consumptive use of water that is accounted for by irrigation practices (El-Ashry and Gibbons 1986).

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Salinization is a problem that irrigated agriculture has had since the first large human civilizations in Mesopotamia. The fall of the ancient civilization in the fertile crescent is attributed to both waterlogging and soil salinization (Rhoades 1990).

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The author talks of desalination only in the context of removing excessive levels of salts from river water, or reducing or preventing further salt loading to soils:

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So far, it seems like the Bureau of Reclamation has relied on expensive, structurally complex projects such as the building of the huge desalinization plant at Yuma to deal with the problem of salinization in the southwest. The new emphasis for the Bureau should be on reducing deep percolation, minimizing salt leaching, switching to agricultural crops that need less water, and retiring highly saline and marginally productive lands (El-Ashry and Gibbons 1986). Other alternatives include installing special drains, ditch linings, sprinkler and drip irrigation systems, leveling farms to achieve more uniform water application, and decreasing the amount of land devoted to agriculture in the Southwestern United States.

As a final note, it is important to realize that salinity control projects can also be harmful to fish and wildlife. Many projects involve reducing seepage of irrigation water from canals and ditches. This practice does reduce salt loading, but it can also reduce or eliminate riparian habitat that relies on the seepage.

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I take this to mean that no practical large-scale method of recuperating saline soils has yet been developed or put in place, although I could be wrong and will keep researching.

Edited to add: as always on this subject, I recommend the book Cadillac Desert, by Mark Reiser. His focus, again, is the southwestern US, but he also goes into the history of civilizations based on irrigation agriculture.

[ April 28, 2002: Message edited by: 'lance ]

This is, of course, one hell of an oversimplification since positive and negative ions don't exist in isolation like that, but you can treat them that way if you want to fudge a little.

As you may gather, ion-exchange resins don't have an infinite capacity to "swap" ions. You may have noticed with your water softener that you're supposed to change the cartridges every so often.

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As you may imagine, the energy involved in such a process is quite prodigious.

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‘lance, I’m familiar with the CA salinization problems mentioned but I thought it was even somewhat worse in that this same problem served to compound certain pesticides as well, which in turn contaminated the ground water reserves. True?

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