FORT DODGE, Iowa — Allison Roderick has a warning and a pledge for rural residents of her county: The water from their wells could be contaminated, but the government can help make it safe.

Roderick is the environmental health officer for Webster County in north-central Iowa, where a few thousand rural residents live among sprawling corn and soybean fields. Many draw their water from private wells, which are exempt from most federal testing and purity regulations. Roderick spreads the word that they aren’t exempt from danger.