In every transaction in this country be it a sale or rental, the seller or owner of the item is required to disclose all known about the item that would materially affect its desirability to a buyer or what a willing buyer would pay for it.From what you have written, supposedly there was an event causing concern for you more than fifty (50) years ago. Look in the neighborhood where you are renting. I presume that there are people living nearby. Ask your neighbors if they have any concerns over what supposedly happened fifty (50) years ago.If you have concerns about the water used for your unit and the water is provided by the local municipality, contact it about questions as to its quality.Your landlord has no responsibility to have the water and land tested for you at this time. If the soil and water were contaminated with unacceptable levels of radiation, it would be evident from simply looking at the surrounding vegetation.