Ann: Lake Way Project and Financing Update, page-124

Rainfall is not the problem. It only dilutes what's already in the pond. From my understanding SO4 assumed enough water would evaporate to allow unwanted salts to drop out of solution before running continually into the harvest ponds, it didn't, so those ponds are now contaminated with unwanted salts still in suspension.
Different salts would drop out of solution at different times, look at it like distilling.
That's just based on my understanding and a foggy memory of high school chemistry.