A glass of water pulled from a mountain reservoir looks cleaner than water from a well tucked behind a gas station — but looks are deceiving. Surface water sits open to the sky, to wildlife, to runoff, and to every other source of contamination that groundwater is naturally shielded from, and that exposure carries a specific, well-documented risk: the parasites Giardia lamblia and Cryptosporidium, along with assorted viruses, survive quite comfortably in raw surface water, and Cryptosporidium in particular shrugs off ordinary chlorine disinfection almost entirely. Because of that risk, Congress and the U.S. Environmental Protection Agency (EPA) built an entire family of drinking water regulations — the Surface Water Treatment Rules (SWTR) — specifically so that a water system drawing from a river, lake, or reservoir never hands a customer a glass of water carrying these pathogens