Metal bioaccumulation in the freshwater snail Lymnaea stagnalis under the influence of thermal waters in the Mura River side channel

The discharge of spent geothermal waters may pose a potential environmental risk and lead to significant ecological disturbances associated with increased concentrations of metals and their potential accumulation in the freshwater organisms [1,2]. The aim of this study was to investigate spatial differences in metal concentrations (Ba, Cs, Li, Mn, Na, Rb, Sr, and Zn) in surface water and whole-body tissues of L. stagnalis collected from two branches of the Mura River. The first site was located near Sveti Martin na Muri (SM), potentially influenced by thermal water discharge from thermal resort Sveti Martin, while the second site was near Podturen (PT), approximately 20 km downstream. Almost all measured elements were present at higher concentrations in the surface water at SM site compared to PT site. The most pronounced difference was observed for Li, which concentrations were approximately 24-fold higher at the SM site. Similarly, Cs concentrations were nearly 13 times higher, while Rb, Sr, Ba, and Na concentrations were up to six times higher at SM site compared to PT. Such elevated concentrations of alkali and alkaline earth metals are consistent with the characteristic chemical composition of geothermal waters [3]. In contrast, Mn and Zn concentrations were 55–75% higher at the PT site, possibly reflecting additional downstream influences such as agricultural runoff. A similar spatial pattern was observed in the whole-body tissues of L. stagnalis. Concentrations of all analyzed elements, except Mn and Zn, were up to 3 times higher in snails collected at the SM site compared to those from PT. The obtained results indicate that thermal water discharge near SM alters the local water chemistry and enhances the bioavailability and accumulation of thermal-water associated elements in L. stagnalis, highlighting the need for further monitoring to better understand the long-term ecological consequences of this type of water pollution.