If you are required to collect more than five samples:
- Place lead results in ascending order (from lowest to highest value).
- Assign each sample a number, 1 for lowest value.
- Multiply the total number of samples by 0.9.
- Compare the 90th percentile level to the action level of 0.015 mg/L (can also be expressed as 15 parts per billion (ppb)). If your 90th percentile value is higher than 0.015 mg/L, you have an exceedance.
Repeat this procedure for copper sample results, except compare the 90th percentile copper level against its action level of 1.3 mg/L. If your 90th percentile value is greater than 1.3 mg/L, you have an exceedance.
If you are required to collect five samples:
- Place lead or copper results in ascending order.
- Take the average of the 4th and 5th highest sample. This is your 90th percentile level.
- Compare the 90th percentile level against the lead action level of 0.015 mg/L or copper action level of 1.3 mg/L.
You can also use this Excel spreadsheet (XLSX), which will automatically calculate your 90th percentile once you enter your Lead and Copper results in the respective cells.