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The data used in this analysis is from the U.S. Environmental Protection Agency’s (EPA)  data. In order to determine the states with the worst water quality for swimming, researchers calculated the percentage of total acreage or mileage of assessed, untreated waters that were impaired for primary water contact. Impaired waters are bodies of water polluted by various sources such as industrial waste, sewage, or agricultural runoff, which makes them unsuitable for their designated uses. Under the mandate of the Clean Water Act, states are required to identify these polluted waters every two years and to take actions towards reducing this pollution. In some cases, broader recreational-use categories were used in states where primary water contact is not distinguished, and only waters designated with sufficient information were considered assessed. Untreated recreational-use waters, unlike chlorinated swimming pools, contain water that has not undergone a disinfection or treatment process to maintain good microbiological quality for recreation.
For clarity, acreage-based water bodies were grouped together, including: lakes, ponds, reservoirs, and wetlands, while mileage-based water bodies such as creeks, rivers, and streams were analyzed collectively. Further, the latest organizational submission data for each state was used, and other water bodies—such as oceans, Great Lakes, beaches, and bays—were excluded due to state-to-state data inconsistencies in categorization and reporting. For additional context, the percentage of primary-contact-use waters assessed, total assessed primary-contact-use waters impaired, total primary-contact-use waters assessed, and total primary-contact-use waters not assessed were calculated for both acreage- and mileage-based water bodies.
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