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How to calculate the 90th percentile of lead and copper contaminant levels in drinking water, using examples of 10 and 5 samples respectively. The document also covers different methods for calculating percentiles based on the number of samples collected.
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Site 1: 0.005 mg/L Site 2: 0.015 mg/L Site 3: 0.005 mg/L Site 4: 0.014 mg/L Site 5: 0.017 mg/L What is the 90 th percentile value? Site 6: 0.005 mg/L Site 7: 0.011 mg/L Site 8: 0.002 mg/L Site 9: 0.018 mg/L Site 10: 0.004 mg/L
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Calculate the Lead 90 th Percentile 5 samples collected Order and Rank lead results from lowest to highest: Rank 1 – 0.001 mg/L. Rank 2 – 0.003 mg/L Rank 3 – 0.005 mg/L Rank 4 – 0.012 mg/L Rank 5 – 0.026 mg/L Multiply the number of samples by 0.9 : 0.9 X 5 samples = 4.5. Therefore, calculate the average of the 4 th (0.012) and 5 th (0.026) highest ranked result to get the 90 th percentile result to compare to the Action Level. 0.012 mg/L + 0.026 mg/L = 0.019 mg/L 2 Lead 90th percentile = 0.019 mg/l (Action Level is 0.015 mg/l)
Calculate the Copper 90 th percentile 7 samples collected Interpolate: 0.9 X 7 samples = 6.3. Therefore, interpolate to get the 90 th percentile result to compare to the Action Level.
Calculate the Copper 90 th percentile 7 samples ( th %) = 6.3 ranked sample First, subtract the difference between the 6 th (1.73) & 7th (1.95) ranks. 1.95 – 1.73 = 0. Second, multiply the answer by 0.3 since the 90 th percentile is 0.3 higher than the 6 th ranked result in this case. 0.22 x 0.3 = 0. Next, add 0.066 to the 6 th ranked result (1.73). 1.73 + 0.066 = 1.8 mg/l Copper 90 th percentile = 1.8 mg/l (Action Level is 1.3 mg/l)
