Lead and Copper 90th Percentile Examples
Systems Collecting More Than Five Samples
Step 1: Place lead/copper results in ascending order (from lowest to highest value).
Step 2: Assign each sample a number, 1 for lowest value.
Step 3: Multiply the total number of samples by 0.9.
Step 4: Compare the 90th percentile level to the action level. If the 90th percentile value
is higher than the action level, there is an exceedance.
Example
Sample Results: 0.020, 0.010, 0.006, 0.005, 0.000, 0.015, 0.006, 0.005, 0.002, 0.000
Step 1: 0.000, 0.000, 0.002, 0.005, 0.005, 0.006, 0.006, 0.010, 0.015, 0.020
Step 2:
1
2
3
4
5
6
7
8
9
10
0.000 0.000 0.002 0.005 0.005 0.006 0.006 0.010 0.012 0.020
Step 3: 10 x 0.9 = 9* (corresponds to sample #9)
If this is not a whole number, round to the nearest whole number. Round down to the
nearest whole number if the decimal is 0.4 or less and round up to the nearest whole
number if the decimal is 0.5 or higher.
Step 4: Sample #9 is 0.012 < 0.015
Systems Collecting Less Than Five Samples
Step 1: Place lead/copper results in ascending order (from lowest to highest value).
Step 2: Take the average of the 4th and 5th highest sample. This is the 90th percentile
level.
Step 3: Compare the 90th percentile level to the action level. If the 90th percentile value
is higher than the action level, there is an exceedance.
Example
Sample Results: 0.008, 0.006, 0.020, 0.000, 0.001
Step 1: 0.000, 0.001, 0.006, 0.008, 0.020
Step 2: (0.008 + 0.020)/2 = 0.014
Step 3: 0.014 <0.015