Use of nitrogen and oxygen isotopes of dissolved nitrate to trace field-scale induced denitrification efficiency throughout an in-situ groundwater remediation strategy

摘要

In the framework of the Life+ InSiTrate project, a pilot-plant was established to demonstrate the viability of inducing in-situ heterotrophic denitrification to remediate nitrate (NO3-)-polluted groundwater. Two injection wells supplied acetic add by pulses to an alluvial aquifer for 22 months. The monitoring was performed by regular sampling at three piezometers and two wells located downstream. In the present work, the pilot-plant monitoring samples were used to test the usefulness of the isotopic tools to evaluate the efficiency of the treatment. The laboratory microcosm experiments determined an isotopic fractionation (epsilon) for N-NO3- of -12.6 parts per thousand and for O-NO3- of -13.3 parts per thousand. These epsilon N-15(NO3)/(N2) and epsilon O-18(NO3)/N2 values were modelled by using a Rayleigh distillation equation to estimate the percentage of the induced denitrification at the pilot-plant while avoiding a possible interference from dilution due to non-polluted water inputs. In some of the field samples, the induced NO3- reduction was higher than 50% with respect to the background concentration. The held samples showed a reduced slope between delta O-18-NO3 and delta N-15-NO3 (0.7) compared to the laboratory experiments (1.1). This finding was attributed to the reoxidation of NO2- to NO3- during the treatment The NO3- isotopic characterization also permitted the recognition of a mixture between the denitrified and partially or non-denitrified groundwater in one of the sampling points. Therefore, the isotopic tools demonstrated usefulness in assessing the implementation of the field-scale induced denitrification strategy. (C) 2019 Elsevier B.V. All rights reserved.