Why can pyrite-rich ground exposed by thaw keep generating acid after the first flush of water has passed?

Answer: Iron cycling re-attacks pyrite

Iron cycling re-attacks pyrite ✓ — Correct. In acid drainage chemistry, pyrite first releases ferrous iron, sulfate, and acidity; then ferric iron can become a strong oxidizer that attacks more pyrite. Microbes can speed parts of that iron cycle. The system is not just a first rinse of dirty water - it can become a reaction network as long as water, oxygen, and fresh sulfide surfaces remain available.

Dilution makes more acid — Dilution usually weakens acidity at a particular spot; it does not create new acid by itself. The source is the reaction between exposed sulfide minerals, oxygen, water, and iron chemistry. A larger river may buffer the acid while still carrying the products downstream, which is why dilution can hide part of the signal rather than stop the source.

Rust seals the rock shut — Rust can clog some pores locally, but it does not reliably seal the whole source shut. Rust-colored iron often appears after dissolved iron leaves the most acidic zone and oxidizes in better-buffered water. Meanwhile, thawed flow paths can keep feeding water and oxygen to fresh rock surfaces, so the acid source can persist.