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Wouldn’t this type of concentration happen directly as a result of any irrigation system? If you keep transferring water to the same land, anything that doesn’t get absorbed by plant matter and picked or evaporate would stay in the soil. So without some other process to leech the metals, it’d keep building up. Main determinant would be the baseline rate of the source water.

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Why wouldn't they accumulate by natural processes exactly the same?

When you look at the ice cores, the "emissions" were higher just before the last glacial maximum.

The only reason why the metals weren't there 5000 years ago was that the soils had gotten recently stripped. The whole concept was invented by insane people.

When there's a natural process that consistently brings groundwater into a place where it evaporates you do see exactly this. Flood plains, salt flats, vernal pools, dry lake beds etc.

Irrigation water (especially if pumped from aquifers) may have more metals in it than rainwater.

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