90th percentile lead sampling results
This data includes the 90th percentile values for the most recent round of water sampling, including results that require Public Water System (PWS) administrators to take action under the Lead and Copper Rule. See below for a brief summary of why this data is collected and what the Lead and Copper Rule is. To learn more about the PWS in the dataset, refer to the Public Water Supplier contact file.
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The Lead and Copper Rule and Public Water System (PWS) sampling
The Lead and Copper Rule requires that the PWS take action when the amount of lead or copper in a drinking water system is too high. The Rule applies to Community (COM) public water systems and Non-Transient Non-Community (NTNC) public water systems, such as those of schools and office buildings. As of January 2022, there were approximately 774 such water systems in Massachusetts.
The Action Level for lead is 0.015 milligrams per liter (mg/L) or 15 parts per billion (ppb). When the 90th percentile value of all sampling results collected during each monitoring period is greater than the Action Level, a PWS must act to reduce lead levels. In other words, no more than 10% of samples collected should be over the action level.
The number of samples that must be collected depends on the size of the population that PWS serves. For example, a PWS that serves fewer than 100 people must collect 5 samples, and a PWS that serves over 100,000 people must collect 100 samples. However, all collected samples that meet site selection, sample collection, EPA, and state requirements should be used in the 90th percentile calculation, even if the PWS collects more samples than required.
For more information on how to calculate the 90th percentile, see the EPA’s guidance on the Lead and Copper Rule (pages 34-36).