Mixing effects on apparent reaction rates and isotope fractionation during denitrification in a heterogeneous aquifer

Abstract

[1] Gradients in contaminant concentrations and isotopic compositions commonly are used to derive reaction parameters for natural attenuation in aquifers. Differences between field-scale (apparent) estimated reaction rates and isotopic fractionations and local-scale (intrinsic) effects are poorly understood for complex natural systems. For a heterogeneous alluvial fan aquifer, numerical models and field observations were used to study the effects of physical heterogeneity on reaction parameter estimates. Field measurements included major ions, age tracers, stable isotopes, and dissolved gases. Parameters were estimated for the O 2 reduction rate, denitrification rate, O 2 threshold for denitrification, and stable N isotope fractionation during denitrification. For multiple geostatistical realizations of the aquifer, inverse modeling was used to establish reactive transport simulations that were consistent with field observations and served as a basis for numerical experiments to compare sample-based estimates of "apparent" parameters with "true" (intrinsic) values. For this aquifer, non-Gaussian dispersion reduced the magnitudes of apparent reaction rates and isotope fractionations to a greater extent than Gaussian mixing alone. Apparent and true rate constants and fractionation parameters can differ by an order of magnitude or more, especially for samples subject to slow transport, long travel times, or rapid reactions. The effect of mixing on apparent N isotope fractionation potentially explains differences between previous laboratory and field estimates. Similarly, predicted effects on apparent O 2 threshold values for denitrification are consistent with previous reports of higher values in aquifers than in the laboratory. These results show that hydrogeological complexity substantially influences the interpretation and prediction of reactive transport.Citation: Green, C. T., J. K. Böhlke, B. A. Bekins, and S. P. Phillips (2010), Mixing effects on apparent reaction rates and isotope fractionation during denitrification in a heterogeneous aquifer, Water Resour. Res., 46, W08525,

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

“…Fractionation coefficients ( 15 ε) derived from both the cross-spring analysis and (5.27-7.49) the Ichetucknee River springs time series (7.37) are within the lower range of values reported for other aquifers (Mariotti, 1986;Bohlke et al, 2002;Green et al, 2008), and other marine and freshwater environments (Sigman et al, 2005;Granger et al, 2008), but are lower than some recent experimental values (Knoller et al, 2011). These relatively low values may indicate diffusion-constrained NO in the UFA (Sebilo et al, 2003), but could also be an artefact of mixing between distinct water sources (Green et al, 2010).…”

“…For the subset of springs for which 3 H/ 3 He ages were available, we estimated zero-order denitrification rates as the concentration of excess N 2 divided by water age (Green et al, 2008). Because we lack data for discrete geochemical end-members within each spring, we are unable to quantitatively address the effects of mixing (Green et al, 2010) on our estimates of denitrification rates or O 2 thresholds.…”

“…This calculation assumes that fractionation is constant in space and time within the UFA, and ignores potential mixing effects (Green et al, 2010). To assess the effects of denitrification at the regional scale, we compared the distributions of observed δ 15 N and estimated δ 15 N source , using the reference values reported by Heaton (1986), whose distinguish synthetic fertilizer, organic waste, and their mixtures as <6 ‰ for synthetic sources, >9 ‰ for animal-derived sources, and intermediate values for mixtures.…”

A model-based assessment of the potential use of compound-specific stable isotope analysis in river monitoring of diffuse pesticide pollution

“…In particular, assumptions concerning reaction kinetics can be validated by comparing model results to measured concentration and CSIA data (D'Affonseca et al, 2011;Pooley et al, 2009;Prommer et al, 2009;Atteia et al, 2008). IF-RTMs have also proven useful to study the transformation of the agrochemical pollutant nitrate (Chen and MacQuarrie, 2004;Green et al, 2010). As opposed to the simulation of concentrations of a degrading contaminant, which solely yields a certain concentration reduction between the emission source and a downgradient point, these studies have demonstrated that IFRTMs allow for the quantification of in situ degradation and the distinction between destructive and non-destructive processes.…”

Sources and fate of nitrate in groundwater at agricultural operations overlying glacial sediments

“…earthen manure storage, temporary manure piles, or fertilizer) and the 10 complexity of hydrogeologic systems (Spalding and Exner, 1993;Rodvang et al, 2004;Showers et al, 2008;Kohn et al, 2016). These spatial and temporal variations can result in complex subsurface solute distributions that are difficult to interpret using classical transect studies or numerical groundwater models (Green et al, 2010;Baily et al, 2011).…”