The friendly waitress at the Pretty Prairie Steak House delivers tumblers of tap water as soon as diners take their seats. Across Main Street, the Wagon Wheel Café offers the same courtesy. Customers may also order coffee or iced tea, but it all starts at the same tap, and everyone is fine with that. This blasé attitude about drinking water surprised me: everyone in this little farm town in Reno County, Kansas, knew beyond the shadow of a doubt that the liquid flowing from the municipal water tower was highly contaminated with nitrate, a chemical compound derived from fertilizer…