The United States Environmental Protection Agency (EPA) has a radiation monitoring website called RadNet. From the homepage:
"The nationwide RadNet system monitors the nation’s air, drinking water, precipitation, and pasteurized milk to determine levels of radiation in the environment. RadNet sample analyses and monitoring results provide baseline data on background levels of radiation in the environment and can detect increased radiation from radiological incidents."
From the RadNet FAQ:
"RadNet is the only nationwide system that continuously monitors of ambient environmental radiation levels and those resulting from major nuclear accidents, such as the Fukushima nuclear reactor incident in Japan. RadNet data are used to inform the public, providing assurance if contamination levels are very low or helping to make science-based decisions about taking protective actions if contamination levels are high enough to warrant them."
The EPA has thirty locations where they take samples of rain, snow or sleet monthly and forward it to their lab to test it for gamma radiation. The sampling locations have been selected to give comprehensive and representative coverage of precipitation in the U.S. There are also 30 stations where milk samples are being taken quarterly and sent to the lab to be tested for gamma radiation as well as fission products such as iodine-131, barium-140 and cesium-137. Finally, the EPS takes drinking water samples from fifty sites across the U.S. quarterly and tests the samples for fission products.
The results from these analyses are provided in a database called Envirofacts. The first search parameter is the location.
The location input field will allow selection by state, city & state or EPA region.
Second, you need to select what you want to see results for which they call Media.
The Media field will allow selection of rain, milk, drinking water or air-filter.
The next field lets you select a particular radioisotope.
A selection of radioactive isotopes is presented.
And finally, you can choose which years you want information for.
Here is a set of search parameters.
You are given a choice of output units, either traditional units called picocuries or international system units called Becquerels.
Here is the results of the above search on Envirofacts.
Here are the results for Denver, Colorado in a search for Cesium-137 in precipitation.
The main purpose of the RadNet monitoring is to establish a base line for normal background radiation across the United States in monitor the air, rain, drinking water and milk for radiation above the safe levels due to nuclear explosions or radiation accidents at reactors.