Let's go with 20x40 as an example because you have 37k gallons. 1 inch of rain which would be a decent whopper of a storm would add 500 gallons and dilute you 1.3%.

At 8 FC if you left the pump on overnight, you'd end up with 7.9, unnoticeable, and pass/fail the OCLT as you would have with no storm.

If it's not well mixed then the rain water up top will have a more drastic affect.
 
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