Hence, ground water generally has higher concentrations of 222 Rn than surface water, because the radon is continuously replenished by radioactive decay of 226 Ra present in rocks. Likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone because of diffusional losses to the atmosphere.[13][14] As a below-ground source of water, some springs—including hot springs—contain significant amounts of radon.[15] The towns of Boulder, Montana; Misasa; Bad Kreuznach, and the country of Japan have radium-rich springs which emit radon. To be classified as a radon mineral water, radon concentration must be above a minimum of 2 nCi/L (7 Bq/L).[16] The activity of radon mineral water reaches 2,000 Bq/L in Merano and 4,000 Bq/L in the village of Lurisia (Ligurian Alps, Italy).[5]