In 1854, Dr. John Snow stopped a cholera outbreak in London by mapping the sick residents' homes and the locations of the city water pumps. Most people who had fallen ill, it turned out, lived near the Broad Street pump, which Snow would later discover delivered pathogen-tainted water. Snow removed the pump's handle, and the outbreak abated.
Health researchers are now using geographic data to understand a different waterborne threat: lead poisoning. Researchers in North Carolina have used a mapping program called a geographic information system (GIS) to investigate the link between lead-contaminated water and an increasingly common water disinfectant.
Municipal water systems add disinfectant chemicals such as chlorine to the water. However, by-products of chlorine disinfection can be carcinogenic, so nearly a third of the nation's waterworks have switched to a disinfectant called chloramine.