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90th Percentile is a term use in conjunction with "water sampling standards as required under the Safe Drinking Water Act (SDWA) and amendments thereto."
To calculate the 90th percentile value, all sample results are placed in order from the lowest concentration (of specific contaminants) to the highest concentration. Each sample result gets a number, starting with the number 1 for the lowest. Next, you multiply the total number of samples collected by 0.9. The sample result with the number corresponding to this calculated value is the 90th percentile. In other words, if 90% of the samples are below the action levels (9 out of 10 samples), then the system is in compliance.
What type of contaminants are we talking about? The Environmental Protectio Agency provides a list: E. Coli, cyanide, arsenic, uranium, and more. To learn more about your city’s water quality, check out National Tapwater Database.
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