Assessment of natural and anthropogenic contamination sources in a Mediterranean aquifer by combining hydrochemical and stable isotope techniques.

The Atalanti basin is an intensively cultivated area in central Greece, facing groundwater quality deterioration threats due to natural and anthropogenic-related contamination sources. A combination of statistical and hydrogeochemical techniques, and stable isotope compositions (δ2 H-H2 O and δ18 Ο-Η2 Ο, δ15 Ν-ΝΟ3 - and δ18 Ο-ΝΟ3 - , δ34 S-SO4 2- and δ18 O-SO4 2- ) were applied to elucidate the origin of salinity and nitrate contamination, and shed light on the potential associations between geogenic Cr(VI) and NO3 - sources and transformations. Nitrate and Cr(VI) concentrations reached up to 337 mg L-1 and 76.1 μg L-1 , respectively, exceeding WHO threshold values in places. The cluster of samples with the high salinity was mostly influenced by irrigation return flow and marine aerosols, and less by seawater intrusion, as evidenced by the ionic ratios (e.g., Na+ /Cl- ) and the stable isotopes of oxygen and hydrogen in water, and sulphur and oxygen in sulphates. The δ15 Ν-ΝΟ3 - and δ18 O-NO3 - values ranged from +2.0 ‰ to +14.5 ‰ and + 0.3 ‰ to +11.0 ‰, respectively. We found that the dominant sources of NO3 - in groundwater were fertilizers in the central part of the area and sewage waste in the northern part around the residential area of Livanates. The occurrence of denitrification was evident in the northern part of the basin, where the DO levels were lowest (≤ 2.2 mg L-1 ), whereas nitrification of NH4 + -fertilizers prevailed in the central part. High Cr(VI) values (≥ 20 μg/l) were associated with the lowest deviation of the measured from the theoretical nitrification δ18 Ο-NO3 - values, whereas the lowest Cr(VI) values were observed in the denitrified water samples. Our isotope findings revealed the strong influence of redox conditions on the biogeochemical transformations of N species and the mobilization of Cr(VI) that will help improve the understanding of the fate of these contaminants from the unsaturated zone to the groundwater in areas of agricultural and urban land use.