Application of multi-isotope data (O, D, C and S) to quantify redox processes in urban groundwater

•Urban groundwater (UGW) deterioration due to anthropogenic pollutants.•It is necessary to understand the hydrochemical processes that affect pollutants.•The quantification of UGW sources and the hydrochemical processes are required.•Multi-isotope ratios (δ18OH2O, δDH2O, δ34SSO4, δ18OSO4 and δ13CDIC) are used.•Mixing ratios and isotopes are successfully applied to quantify redox processes.

The different recharge sources and their mixing ratios were investigated in water samples from an urban aquifer. Two distinct zones from Barcelona city were compared: (1) Poble Sec and (2) Besòs River Delta, which are located in a densely populated area with a strong industrial impact. In this study 106 water samples were collected from July 2007 to May 2010 for hydrochemical and isotopic characterisation. The application of environmental isotopes coupled with hydrochemistry provided the necessary information to isotopically quantify groundwater recharge sources and evaluate the occurrence of redox processes. In the Besòs River Delta, a decrease in dissolved SO42- concentration and an increase in δ34SSO4 and δ18OSO4 values were observed in groundwater samples, both indicating SO42- reduction. Moreover, other chemical indicators supported a reducing environment, such as low or null levels of dissolved O2 and NO3-, the presence of NH4+ and an increase in dissolved Fe and As. The reducing conditions were probably induced by the organic C dissolved in water infiltrating from the River Besòs. In Poble Sec, the relationship between δ34SSO4 and δ18OSO4 showed a strong influence of sewage water infiltration into the aquifer. However, the aquifer is oxic and there is no influence of SO42- reduction.