Connectivity of the Tisza River System: trace element and isotopic constraints

At the watershed scale, a number of complex biogeochemical processes govern riverine geochemistry, and the use of multiple isotopic and trace element analyses has the potential to elucidate these dynamic processes. Such a study was undertaken within the Tisza River Basin (TRB) which is the largest river basin within the Carpathian Basin of Central Europe. The TRB stretches 157,186 km2 and encompasses Romania, Hungary, Serbia, Slovakia, and Ukraine. Geologically, the TRB covers the Pannonian Basin, the Eastern, Western and Southern Carpathians and the Apuseni Mountains. These units have a very complex geology and lithology. The formations range from igneous and metamorphic rocks to sediments, covering a time span from Paleozoic to Quaternary. Our study aimed to determine the influence of various bedrock lithologies on the water chemistry of tributaries and the impact of the biogeochemical and anthropogenic factors on the downstream chemical evolution of the rivers. Twenty-three sampling locations were chosen within the TRB based on the dominant bedrock lithology of the drainage area. In June of 2013, water and rock samples were collected from the Apuseni Mountains and the Southern Carpathians, sampling two main tributary systems - the Cris (Körös) and Mure (Maros) Rivers - and the Lower Tisa (Tisza) River. At each sampling location, field parameters (temperature, pH, dissolved oxygen, and specific conductivity) were measured and water samples were retrieved. Water samples were analyzed for alkalinity, major and minor cations (Fe, Al, Si, Mn, Cu, Sr, Cd, Ni, Zn, Ca, Mg, Na, Se, Pb, As, K) and anions (SO42-, NO3-, Cl-, F-, PO43-), as well as 87Sr/86Sr ratios, and δ2D and δ18O values. Tributaries drained primarily by carbonates tended to exhibit more of an effect on the downstream water chemistry compared to areas drained by silicate rocks, which had little effect on mainstem rivers. Additionally, δ2D and δ18O values (ranging from -80.53 to -41.97‰ for δ2D and -12.44 to 9.99‰ for δ18O) indicate large differences in chemical processes within each of the studied river systems. By better understanding these processes and the overall connectivity of such dynamic river systems, more focused efforts can be made in the conservation and management of the water resources.