Paper No. 1
Presentation Time: 8:10 AM
TESTING CONCEPTUAL MODELS FOR PROCESSES CONTROLLING NITRATE REDUCTION COUPLED TO IRON(II) OXIDATION DURING A FIELD-SCALE TRANSPORT EXPERIMENT
Natural gradient transport experiments were conducted by injecting native groundwater amended with ~ 1100 uM NaBr (Br- as a conservative tracer) and 1000 uM NaNO3 into the anoxic zone of a wastewater plume. The anoxic zone had elevated dissolved and sorbed Fe(II) concentrations. Nitrate reduction was observed within 1 m of the injection, initially producing stoichiometric quantities of N2O (up to 100 uM N2O). Nitrate breakthrough was accompanied by perturbations in aqueous concentrations of Fe(II). In a subsequent injection at the same site, NO3- was reduced more rapidly, N2O concentrations > 150 uM were observed, and aqueous Fe(II) concentrations decreased to < 5 uM. Filterable hydrous Fe(III) oxide (HFO), the expected product of Fe(II) oxidation, was observed in groundwater during NO3- breakthrough. Wastewater-derived phosphate concentrations decreased from ~ 65 to < 1.5 uM and naturally occurring As(V) and As(III) concentrations decreased from ~ 100 nM each to 7 and 30 nM, respectively, during the experiments. Three conceptual models were tested in reactive transport model simulations. All three models used the same set of parameters for equilibrium cation and anion sorption models on the sediments and on freshly precipitated HFO obtained from other experiments (sediments) or taken from the literature (HFO). Model 1, which assumed rate-limited reduction of NO3- to N2O and N2O to N2 coupled to Fe(II) oxidation, captured the principal trends in the experimental results. Model 2, which assumed rate-limited reduction of NO3- to N2O and N2O to NH4+ coupled to Fe(II) oxidation, over-predicted the observed decreases in Fe(II) and sulfate concentrations (owing to sorption on the excessive HFO produced) and the observed increases in NH4+ concentrations. Model 3, which assumed rate-limited NO3- reduction to NO2- and N2O reduction to N2 coupled to organic matter oxidation but equilibrium NO2- reduction to N2O coupled to Fe(II) oxidation, under-predicted observed decreases in Fe(II) concentrations and over-predicted observed total dissolved CO2 concentrations. Our findings suggest that nitrate can effectively attenuate concentrations of Fe(II), phosphate, and As(III/V), common contaminants in anaerobic plumes, while producing N2 and HFO with co-precipitated phosphate and As as the major products.