Not all months have 30 days and not all quarters have 90 days. How will this affect compliance tracking?
The term ?every 90 days? was included to eliminate the possibility that a system would take samples at the end of one quarter and then immediately again at the beginning of next quarter. Samples are not temporally distributed as intended when collected in this manner. The intent of the rule is to have a minimum period of time (e.g., two months) or interval that separates each monitoring event. EPA finds sampling every third month to be as protective of public health as sampling every 90 days because sampling at this frequency eliminates the possibility of sampling at the end of one quarter and then again immediately at the beginning of the next quarter. It also ensures that systems continue to monitor in the same four months (including the peak historical month) year after year without needing to account for quarters with differing numbers of days.
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