Migeru:

The EPA limit for I-131 is 3 pCi/L, or 0.111 Bq/L

Note the following: the EPA's acceptable level of Iodine in drinking water equals the Minimum Detectable Activity. That is, statistical noise is of the same order as the EPA acceptable limit.

How we calculate Minimum Detectable Activity (MDA) | The Nuclear Engineering Department At UC Berkeley

The Minimum Detectable Activity, or MDA, represents the smallest quantity of a radioisotope which can be detected with 99.7% confidence in one of our systems (rainwater, or the various air sampling systems). It is fundamentally based on the statistical variation of detector counts in the region where a peak from the isotope would appear. If the statistical variation is greater than the counts from an actual amount of radioactivity of that isotope, it is not statistically significant and it is not detected.

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As of 3/24/2011, the MDAs for our five measured isotopes in rainwater are:

Te-132	0.074 Bq/L
I-131	0.115 Bq/L
I-132	0.115 Bq/L
Cs-134	0.106 Bq/L
Cs-137	0.084 Bq/L

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