Frequent Questions

How are significant figures expressed when determining compliance with the arsenic maximum contaminant level (MCL)?

All MCLs are expressed in the number of significant digits permitted by the precision and accuracy of the specified analytical procedures. Data reported to the state or EPA should be reported to the nearest 0.001 mg/l. In calculating data for compliance purposes, it is necessary to round- off by dropping the last digit.  The last digit should be increased by one unit if the digit dropped is 5, 6, 7, 8, or 9. If the digit is 0, 1, 2, 3, or 4, do not alter the preceding number.

For example, analytical results for arsenic of 0.0105 mg/L would round off to 0.011 mg/L, while a result of 0.0104 mg/L would round off to 0.010 mg/L.
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