You statement about drinking rain water is misleading
Thanks for publishing these measurements. Your statement here is misleading:
At this level, you would need to drink 632 liters of this rain water to obtain the same radiation effects you obtain on a round-trip flight between San Francisco and Washington D.C
You have measured concentrations of I-131 up to 8 Bq/L (216pCi/L). EPA guidelines for drinking water have a Maximum Contamination Level of .11 (3pCi/L). This is also stated as 4 millirems per year.
EPA Facts About Iodine
Your measured concentrations exceed EPA guidelines by 72 times.
As others have stated the exposure on a flight is not ingesting radionuclide beta emitters.