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

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.

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.

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.