Identification of sources and transformations of nitrate in Cr(VI)-impacted alluvial aquifers by a hydrogeochemical and δ15N-NO3- and δ18O-NO3- isotopes approach

A coupled methodology of nitrogen isotopes, hydrogeochemical characterization, multivariate statistical analysis, and SIAR Bayesian modeling has been employed to identify the sources of NO3- and N transformation processes in three alluvial aquifers (Schinos, Thiva, and Central Evia) located in central Greece where geogenic Cr(VI) co-occurs with agricultural activity and rural development. Hexavalent chromium concentrations exceed 50 mu g/L in many sampling stations of the studied groundwater bodies, while nitrate contamination is evident in all three study areas with concentrations well over 50 mg/L. The mean delta N-15-NO3- and delta(18)omicron-NO3- values are 6.67 +/- 1.77 parts per thousand and 2.68 +/- 1.77 parts per thousand in C. Evia, 8.72 +/- 4.74 parts per thousand and 3.96 +/- 4.57 parts per thousand in Schinos and 4.44 +/- 1.71 parts per thousand and 2.91 +/- 1.02 parts per thousand in Thiva, respectively. Domestic sewage and N-bearing fertilizers are contributing in various degrees to the observed nitrification which is the dominant transformation process of N in the studied aquifers. Multivariate statistics indicated that the main processes identified in the study areas are salinization, silicate dissolution, and groundwater contamination due to fertilizer use. It is suggested that ultramafic rock-related alluvial aquifers must be closely monitored in terms of nutrient inputs as an effective measure for controlling Cr(VI) release in groundwater.

Automated summary

A coupled methodology of nitrogen isotopes, hydrogeochemical characterization, multivariate statistical analysis, and SIAR Bayesian modeling was used to identify the sources of NO3- and N transformation processes in three alluvial aquifers in central Greece. The study found that the groundwater in these aquifers is contaminated with hexavalent chromium and nitrate. The dominant nitrogen transformation process was nitrification, which was influenced by domestic sewage and N-bearing fertilizers. Multivariate statistical analysis revealed that salinization, silicate dissolution, and groundwater contamination due to fertilizer use were the main processes in the study areas.