Isotope characterization of an in situ biodenitrification pilot-test in a fractured aquifer

Laboratory and field-scale experiments were developed to test the viability of in situ heterotrophic denitrification in a fractured aquifer and were characterized using chemical, microbial and isotopic tools. Native microbiota were stimulated and NO 3 - reduction was effectively achieved by addition of a C source (ethanol or glucose) as well as a P source (disodium hydrogen phosphate). Transient NO 2 - accumulation was observed at all scales, as was some evidence for incomplete denitrification and coexistence of other respiration processes (such as Fe or SO4 reduction) and autotrophic denitrification. The N and O isotope fractionation obtained at the laboratory scale (−13.0‰ and −17.1‰ for εN and −8.9‰ and −15.1‰ for εO in ethanol and glucose-amended experiments, respectively), resulting in εN/εO values of 1.46 (ethanol-amended experiment), and 1.13 (glucose-amended), allowed the calculation of the field-scale denitrification percentage. Sulfate isotopic characterization, on the other hand, proved that autotrophic denitrification linked to sulfide oxidation could be occurring along with heterotrophic denitrification. Overall, results suggest that stimulated heterotrophic denitrification could be applied as a remedial alternative in fractured media. However, efforts must be addressed to control microbial population and stability as a key issue to avoid the decrease of groundwater quality due to incomplete denitrification or secondary respiratory processes. As demonstrated in this work, insight into such issues can be efficiently studied by means of isotopic techniques.