Complex sentence: They said that the the water is safe to drink and posed no immediate risk of harm, however, long term exposure to low level radiation is associated with stochastic health effects; the greater the exposure, the more likely the health effects are to occur.
Split into simpler sentences: They said that the the water is safe to drink and posed no immediate risk of harm. Long term exposure to low level radiation is associated with stochastic health effects; the greater the exposure, the more likely the health effects are to occur.