In parallel testing, the French Institute for Radiological Protection and Nuclear Safety (IRSN), the national public institution monitoring nuclear and radiological risks, found iodine 131 in milk.
In normal times, no trace of iodine-131 should be detectable in rainwater or milk.
The Euratom Directive of 13 May 1996 establishes the general principles and safety standards on radiation protection in Europe.
In response to thousands of inquiries from citizens concerned about fallout from the Fukushima nuclear disaster in Europe, CRIIRAD has compiled an information package on the risks of radioactive iodine-131 contamination in Europe.
The document, published on 7 April, advises against consuming rainwater and says vulnerable groups such as children and pregnant or breastfeeding women should avoid consuming vegetables with large leaves, fresh milk and creamy cheese.
The risks related to prolonged contamination among vulnerable groups of the population can no longer be considered "negligible" and it is now necessary to avoid "risky behaviour," CRIIRAD claimed.
However, the institute underlines that there is absolutely no need to lock oneself indoors or take iodine tablets.
CRIIRAD says its information note is not limited to the situation in France and is applicable to other European countries, as the level of air contamination is currently the same in Belgium, Germany, Italy and Switzerland, for instance.
Data for the west coast of the United States, which received the Fukushima radioactive fallout 6-10 days before France, reveals that levels of radioactive iodine-131 concentration are 8-10 times higher there, the institute says.