What toxins appear as green in soil?

Question

If you dug a ditch and found the soil to be green and it stayed green after the dirt dried out, what would you think was there?

If you took a sample of it to a lab, what would you have it tested for?

Answer 1

Copper.

Copper combines with carbon dioxide and water in the atmosphere to form chalky, green copper carbonate:

Cu + CO2 + H2O -> CuCO3 + H2

This is what is known as a "patina" on copper used ornamentally in construction. Copper in the soil might be due to old construction material getting buried or perhaps copper wire was disposed of. Of course, copper ocurrs naturally as well and the green patch might be a vein of copper ore. Another possibility is a leaking sewage line. Copper sulfate is used as a root remover in speptic lines. The sulfate is soluable but turns into the insoluable carbonate upon contact with organic material.

Answer 2

Depends on the shade of green.

In most cases, the soil below the level of oxygenation is green because of iron.  Iron forms greenish colored minerals when it is in the 2+ state, and reddish-orange minerals in the 3+ state (a generality).  The green isn't a bright green, though, it is a dull to darker green in most cases (I have seen a lot of soils and never really seen anything that I would call a bright green).

A common soil profile goes from brownish to orangish material near the surface (except when there is a lot of vegetable matter, then black is common) through a transition zone where the soil is mottled brown (orange) and green, to essentially all green.  This is particularly true in my experience when the soil has a lot of clay.

Now, if you are seeing some really bright green streaks, or light bluish-green coloration, I would start to think heavy metals like copper, cobalt or chromium maybe, or others.

If I were taking any soil to a lab to be tested because I suspected contamination, I would test for metals, total petroleum hydrocarbons, monocyclic aromatic hydrocarbons (MAHs) and polycyclic aromatic hydrocarbons (PAHs), unless there was some historical reason to believe that there were other contaminants there.