The following is taken from the EPA web site:
How can strontium-90 affect people's health?
Strontium-90 is chemically similar to calcium, and tends to deposit in bone and blood-forming tissue (bone marrow). Thus, strontium-90 is referred to as a "bone seeker." Internal exposure to Sr-90 is linked to bone cancer, cancer of the soft tissue near the bone, and leukemia.
Risk of cancer increases with increased exposure to Sr-90. The risk depends on the concentration of Sr-90 in the environment, and on the exposure conditions.
What is EPA doing about strontium-90?
EPA protects people and the environment from Sr-90 by establishing standards for the clean-up of contaminated sites, by setting limits on the amount of Sr-90 (and other radionuclides) that may be released to the air, and by setting limits on the amount of strontium-90 (and other radionuclides) that may be present in public drinking water.
EPA uses its authority under the Comprehensive Environmental Response, Compensation, and Liability Act (commonly known as "Superfund"

to set standards for the clean-up of existing contaminated sites. Cleanups must meet all environmental requirements that are relevant or applicable, including state regulations and regulations issued in connection with other federal environmental laws.
When these types of regulations are unavailable, or not protective enough, EPA sets site-specific cleanup levels. Site-specific standards limit the chance of developing cancer because of exposure to a site-related carcinogen (such as strontium-90) to between one in 10,000 and one in 1,000,000.
EPA's Superfund Radiation Webpage
EPA's Superfund Hotline: 1-800-424-9346 or 1-800-535-0202