Natural Radioactivity in Thermal Water in North-West Croatia

As part of the national survey of natural radioactivity levels in the environment the Radiation Protection Unit of the Institute for Medical Research and Occupational Health in Zagreb has been regularly monitoring the presence of natural radioactivity in Croatian thermal spring waters. Thermal waters are known as valuable natural resources of a country. They contain certain degree of natural radioactivity attributable to the elements of the uranium and thorium natural decay series. Among these elements, the most radiotoxic and the most important is radium which exists in several isotopic forms (226Ra and 228Ra). The focus of attention was the content of radium in the samples of thermal spring water from a spa in the northwestern part of Croatia. This water is mainly used for medical, bathing and recreational purposes. Measured activity concentrations of 226Ra were up to 6200 mBqL-1. Water samples were collected at different time periods over a couple of years and differences in activity were observed. 226Ra/228Ra ratio was up to two orders of magnitude higher as opposed to other thermal water samples from Croatia. Differences were probably caused by hydrological behavior of water in this part of Croatia with specific geological characteristics.