International audienceThe geochemistry of groundwaters from the Pannonian Basin (Hungary and Romania) were analysed to elucidate the general geochemistry and processes controlling elevated levels of arsenic. Using cluster analysis 4 main groups were identified. Group 4, influenced by geothermal and saline inputs (Na-HCO3 to Na-Cl type, high Li, Si, NH4 and B, low δ7Li values) with low As levels (mean 33.1 μg/L, range <0.5 to 240 μg/L). Group 3 waters, sampling only dug wells with geochemistry influenced by rain water and surface contamination (Ca-Mg-HCO3 type, elevated K, NO3, S(tot) and Cl) and also with low As (mean 1 μg/L, range <0.5 to 2.1 μg/L). Groups 1 and 2 represented general reducing groundwaters of the area (Mg/Ca-HCO3 to Na-HCO3). Despite such similarities, both groups had different redox characteristics, (i) group 1 indicative of waters dominated by methanogenesis (high CH4, low to absent S(tot)), (ii) group 2 indicative of waters dominated by SO4-reduction (low CH4, moderate S(tot)), and (iii) both group 1 and 2 having similar ranges of Fe concentrations indicating that Fe-reduction is occurring equally in both groups. Arsenic levels in group 1 (methanogenesis) were high, ranging from 23.4 to 208 μg/L (mean 123 μg/L) and predominantly as As(III). Levels in group 2 (SO4-reducing) were much lower, ranging from <0.5 to 58 μg/L (mean 11.5 μg/L), again predominantly as As(III). Typically low levels of As in group 4 waters (geothermal), combined with positive correlations between δ7Li and As(tot) indicate that elevated As is not from an external input, but is released due to an in-aquifer process. Geochemical reasoning implies As mobilisation is controlled by redox-processes, most likely reductive dissolution of As bearing Fe-oxides, known to occur in sediments from the area. More important however is an overlying retention mechanism determined by the presence or absence of SO4.. Ongoing SO4-reduction will release S2-, removing As from solution either by the formation of Assulphides, or from sorption onto Fe-sulphide phases (group 2). In the group dominated by methanogenesis (group 1), any As released by reductive dissolution is not removed from solution and can rise to the high levels observed. Levels of organic carbon are thought to be the ultimate control on the redox conditions in these 2 groups. High levels of organic carbon (as found in group 1) would quickly exhaust any SO4 present in the waters, driving the system to methanogenesis and subsequent high levels of As. Group 2 has much lower concentrations and so SO4 is not exhausted. Therefore, As levels in waters of the Pannonian Basin are controlled not by release but by retention mechanisms, ultimately controlled by levels of TOC and SO4 in the waters