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Compound-specific isotope analysis (CSIA) enables quantification of biodegradation by use of the Rayleigh equation. The Rayleigh equation fails, however, to describe the sequential degradation of chlorinated aliphatic hydrocarbons (CAHs) involving various intermediates that are controlled by simultaneous degradation and production. This paper shows how isotope fractionation during sequential degradation can be simulated in a 1D reactive transport code (PHREEQC-2). 12C and 13C isotopes of each CAH were simulated as separate species, and the ratio of the rate constants of the heavy to light isotope equaled the kinetic isotope fractionation factor for each degradation step. The developed multistep isotope fractionation reactive transport model (IF-RTM) adequately simulated reductive dechlorination of tetrachloroethene (PCE) to ethene in a microcosm experiment. Transport scenarios were performed to evaluate the effect of sorption and of different degradation rate constant ratios among CAH species on the downgradient isotope evolution. The power of the model to quantify degradation is illustrated for situations where mixed sources degrade and for situations where daughter products are removed by oxidative processes. Finally, the model was used to interpret the occurrence of reductive dechlorination at a field site. The developed methodology can easily be incorporated in 3D solute transport models to enable quantification of sequential CAH degradation in the field by CSIA.

There is increasing interest in combined carbon−chlorine compound-specific isotope analysis (CSIA) to differentiate between contaminant sources and to assess transformation processes. However, the significant abundance of polychlorinated molecules with several heavy chlorine isotopes complicates the evaluation of chlorine isotope trends. Therefore, the goal of this study was to develop a conceptual and mathematical framework that describes the expected chlorine isotope fractionation patterns during multistep transformation of chlorinated compounds. Reductive dechlorination of chlorinated ethenes served as an example. The study demonstrates that chlorine isotope trends can be simulated by reproducing the average behavior of light and heavy isotopes or by explicitly simulating molecules with different numbers of heavy isotopes (isotopologues). The calculations reveal that initial chlorine isotope ratios of products equal the isotope ratios of their parent compounds in the absence of secondary isotope effects, while steadily increasing during transformation. The slopes in dual isotope plots are linear for reactant and product during a one-step reaction. They become nonlinear for products that are degraded further but converge to characteristic slopes. Consideration of different scenarios reveals that combined carbon−chlorine isotope analysis bears high potential to differentiate between contaminant sources, to elucidate reaction mechanisms in laboratory studies, and to identify transformation processes in the field.

Enhanced In situ Biodenitrification (EIB) is a capable technology for nitrate removal in subsurface water resources. Optimizing the performance of EIB implies devising an appropriate feeding strategy involving two design parameters: carbon injection frequency and C:N ratio of the organic substrate nitrate mixture. Here we model data on the spatial and temporal evolution of nitrate (up to 1.2 mM), organic carbon (ethanol), and biomass measured during a 342 day-long laboratory column experiment (published in Vidal-Gavilan et al., 2014). Effective porosity was 3% lower and dispersivity had a sevenfold increase at the end of the experiment as compared to those at the beginning. These changes in transport parameters were attributed to the development of a biofilm. A reactive transport model explored the EIB performance in response to daily and weekly feeding strategies. The latter resulted in significant temporal variation in nitrate and ethanol concentrations at the outlet of the column. On the contrary, a daily feeding strategy resulted in quite stable and low concentrations at the outlet and complete denitrification. At intermediate times (six months of experiment), it was possible to reduce the carbon load and consequently the C:N ratio (from 2.5 to 1), partly because biomass decay acted as endogenous carbon to respiration, keeping the denitrification rates, and partly due to the induced dispersivity caused by the well developed biofilm, resulting in enhancement of mixing between the ethanol and nitrate and the corresponding improvement of denitrification rates. The inclusion of a dual-domain model improved the fit at the last days of the experiment as well as in the tracer test performed at day 342, demonstrating a potential transition to anomalous transport that may be caused by the development of biofilm. This modeling work is a step forward to devising optimal injection conditions and substrate rates to enhance EIB performance by minimizing the overall supply of, Peer Reviewed, Postprint (author's final draft)

Biodegradation is one of the most favored and sustainable means of removing organic pollutants from contaminated aquifers but the major steering factors are still surprisingly poorly understood. Growing evidence questions some of the established concepts for control of biodegradation. Here, we critically discuss classical concepts such as the thermodynamic redox zonation, or the use of steady state transport scenarios for assessing biodegradation rates. Furthermore, we discuss if the absence of specific degrader populations can explain poor biodegradation. We propose updated perspectives on the controls of biodegradation in contaminant plumes. These include the plume fringe concept, transport limitations, and transient conditions as currently underestimated processes affecting biodegradation.

Enhanced in-situ biodenitrification (EIB) is a potential technology for remediating nitrate-polluted groundwater. EIB aims to create optimal biodenitrification conditions through the addition of carbon sources, enabling the autochthonous microbial community to degrade nitrate via different redox pathways. Biogeochemical numerical models are useful tools for predicting and designing such biodenitrification applications. Compound-specific stable isotope analysis (CSIA) is another valuable method for determining the degree of nitrate transformation. Therefore, incorporating isotope fractionation in biogeochemical models combines the two tools and is a key step in the development of reactive transport models of EIB under field conditions. In this work, we developed such an integrated model using the Phreeqc code and calibrated the model with batch scale experimental data using either ethanol or glucose as external carbon sources. The model included the following: microbiological processes —exogenous and endogenous nitrate respiration coupled to microbial growth and decay; geochemical processes —precipitation or dissolution of calcite; and isotopic fractionation —δ15N-NO3−, δ18O-NO3−, and δ13C-DIC, incorporating the full δ13C isotope geochemistry involved in EIB. The modeled results fit well with the hydrochemical and isotopic experimental data. The model also incorporated nitrite accumulation observed during the glucose experiment. The biogeochemical model indicates that, depending on the added carbon source, calcite precipitates (using ethanol) or dissolves (using glucose). In both cases, changes in hydraulic conductivity can be induced for actual and long-term EIB applications. The incorporation of isotope fractionation in the model better enables to account for other natural attenuation processes, such as dilution and dispersion, in EIB applications at field scale. Both calibrated enrichment factors (+ 8‰ for ethanol and + 17‰ for glucose) suggest that an inverse fra, Peer Reviewed, Postprint (published version)

Compound-specific stable isotope analysis (CSIA) has, in combination with model-assisted interpretation, proven to be a valuable approach to quantify the extent of organic contaminant degradation in groundwater systems. CSIA data may also provide insights into the origin and transformation of diffuse pollutants, such as pesticides and nitrate, at the catchment scale. While CSIA methods for pesticides have increasingly become available, they have not yet been deployed to interpret isotope data of pesticides in surface water. We applied a coupled subsurface-surface reactive transport model (HydroGeoSphere) at the hillslope scale to investigate the usefulness of CSIA in the assessment of pesticide degradation. We simulated the transport and transformation of a pesticide in a hypothetical but realistic two-dimensional hillslope transect. The steady-state model results illustrate a strong increase of isotope ratios at the hillslope outlet, which resulted from degradation and long travel times through the hillslope during average hydrological conditions. In contrast, following an extreme rainfall event that induced overland flow, the simulated isotope ratios dropped to the values of soil water in the pesticide application area. These results suggest that CSIA can help to identify rainfall-runoff events that entail significant pesticide transport to the stream via surface runoff. Simulations with daily rainfall and evapotranspiration data and one pesticide application per year resulted in small seasonal variations of concentrations and isotope ratios at the hillslope outlet, which fell within the uncertainty range of current CSIA methods. This implies a good reliability of in-stream isotope data in the absence of transport via surface runoff or other fast transport routes, since the time of measurement appears to be of minor importance for the assessment of pesticide degradation. The analysis of simulated isotope ratios also allowed quantification of the contribution of tw