EPA Historical I-131 Levels In Calif. Milk

Ok. So, we have an I-131 reading of 1.14 Bq/L on the milk with a best
by date of 4/21. Why was there I-131 in Calif. milk even before Chernobyl?

The EPA historical I-131 data for milk in Calif. shows a high of 1.39 Bq/L
(Los Angeles) back in Jul of 1986 (Chernobyl was April 1986). But, the EPA
data also shows a level of 0.814 Bq/L (Sacramento) back in April of
***1982***. Any idea why there was this level of I-131 4 years before
Chenobyl?

http://oaspub.epa.gov/enviro/erams_query.simple_output?Llocation=State&s...

Top 14 highest levels listed below. Not sure about "Combined Standard
Uncertainty (CSU)", which could dramtically raise or lower the level:

Location Medium Sample Date Procedure Name Nuclides/Radiation Result Combined Standard Uncertainty MDC Unit
LOS ANGELES, CA PASTEURIZED MILK 15-Jul-86 Gamma spectroscopy Iodine-131 1.39 0.233 --- Bq/L
SACRAMENTO, CA PASTEURIZED MILK 6-Oct-88 Gamma spectroscopy Iodine-131 1.07 0.444 --- Bq/L
SACRAMENTO, CA PASTEURIZED MILK 3-Jun-86 Gamma spectroscopy Iodine-131 0.892 0.178 --- Bq/L
SAN FRANCISCO, CA PASTEURIZED MILK 4-May-89 Gamma spectroscopy Iodine-131 0.888 0.407 --- Bq/L
SACRAMENTO, CA PASTEURIZED MILK 2-Apr-82 Gamma spectroscopy Iodine-131 0.814 0.666 --- Bq/L
SAN FRANCISCO, CA PASTEURIZED MILK 6-Nov-81 Gamma spectroscopy Iodine-131 0.777 0.407 --- Bq/L
LOS ANGELES, CA PASTEURIZED MILK 11-Feb-85 Gamma spectroscopy Iodine-131 0.766 0.263 --- Bq/L
SAN FRANCISCO, CA PASTEURIZED MILK 8-Aug-86 Gamma spectroscopy Iodine-131 0.762 0.318 --- Bq/L
LOS ANGELES, CA PASTEURIZED MILK 14-Nov-86 Gamma spectroscopy Iodine-131 0.699 0.315 --- Bq/L
SAN FRANCISCO, CA PASTEURIZED MILK 4-Mar-88 Gamma spectroscopy Iodine-131 0.681 0.322 --- Bq/L
SAN FRANCISCO, CA PASTEURIZED MILK 19-Feb-86 Gamma spectroscopy Iodine-131 0.67 0.222 --- Bq/L
SACRAMENTO, CA PASTEURIZED MILK 28-Feb-89 Gamma spectroscopy Iodine-131 0.666 0.407 --- Bq/L
SACRAMENTO, CA PASTEURIZED MILK 29-Jul-87 Gamma spectroscopy Iodine-131 0.629 0.37 --- Bq/L
LOS ANGELES, CA PASTEURIZED MILK 8-Sep-80 Gamma spectroscopy Iodine-131 0.618 0.266 --- Bq/L