Estimating pathway-specific contributions to biodegradation in aquifers based on dual isotope analysis: Theoretical analysis and reactive transport simulations

At field sites with varying redox conditions, different redox-specific microbial degradation pathways contribute to total contaminant degradation. The identification of pathway-specific contributions to total contaminant removal is of high practical relevance, yet difficult to achieve with current methods. Current stable-isotope-fractionation-based techniques focus on the identification of dominant biodegradation pathways under constant environmental conditions. We present an approach based on dual stable isotope data to estimate the individual contributions of two redox-specific pathways. We apply this approach to carbon and hydrogen isotope data obtained from reactive transport simulations of an organic contaminant plume in a two-dimensional aquifer cross section to test the applicability of the method. To take aspects typically encountered at field sites into account, additional simulations addressed the effects of transverse mixing, diffusion-induced stable-isotope fractionation, heterogeneities in the flow field, and mixing in sampling wells on isotope-based estimates for aerobic and anaerobic pathway contributions to total contaminant biodegradation. Results confirm the general applicability of the presented estimation method which is most accurate along the plume core and less accurate towards the fringe where flow paths receive contaminant mass and associated isotope signatures from the core by transverse dispersion. The presented method complements the stable-isotope-fractionation-based analysis toolbox. At field sites with varying redox conditions, it provides a means to identify the relative importance of individual, redox-specific degradation pathways

Modeling population patterns of chemotactic bacteria in homogeneous porous media

The spatio-temporal distribution of subsurface microorganisms determines their efficiency in providing essential ecosystem services such as the degradation of organic matter, the remineralization of carbon and nitrogen, or the remediation of anthropogenic contaminants. Populations of motile, chemotactic bacteria have been shown to be capable of pattern formation even in the absence of environmental heterogeneities. Focusing on the water saturated domain of the subsurface (e.g., aquatic sediments, porous aquifers), we analyze this innate capability of bacterial populations in an idealized model of a homogeneous, saturated porous medium. Considering a linear array of connected, identical microhabitats populated by motile, chemotactic bacterial cells, we identify prerequisites for pattern formation, analyze types of patterns, and assess their impact on substrate utilization. In our model, substrate supplied to the microhabitats facilitates bacterial growth, and microbial cells can migrate between neighboring microhabitats due to (i) random motility, (ii) chemotaxis towards substrate, and (iii) self-attraction. A precondition for inhomogeneous population patterns is analytically derived, stating that patterns are possible if the self-attraction exceeds a threshold defined by the random motility and the steady state population density in the microhabitats. An individual-based implementation of the model shows that static and dynamic population patterns can unfold. Degradation efficiency is highest for homogeneous bacterial distributions and decreases as pattern formation commences. If during biostimulation efforts the carrying capacity of the microhabitats is successively increased, simulation results show that degradation efficiency can unexpectedly decrease when the pattern formation threshold is crossed

How the chemotactic characteristics of bacteria can determine their population patterns

Spatial distribution of soil microorganisms is relevant for the functioning and performance of many ecosystem processes such as nutrient cycling or biodegradation of organic matters and contaminants. Beside the multitude of abiotic environmental factors controlling the distribution of microorganisms in soil systems, many microbial species exhibit chemotactic behavior by directing their movement along concentration gradients of nutrients or of chemoattractants produced by cells of their own kind. This chemotactic ability has been shown to promote the formation of complex distribution patterns even in the absence of environmental heterogeneities. Microbial population patterns in heterogeneous soil systems might be, hence, the result of the interplay between the heterogeneous environmental conditions and the microorganisms' intrinsic pattern formation capabilities.In this modeling study, we combined an individual-based modeling approach with a reactive pore-network model to investigate the formation of bacterial patterns in homogeneous and heterogeneous porous media. We investigated the influence of different bacterial chemotactic sensitivities (toward both substrate and bacteria) on bacterial distribution patterns. The emerging population patterns were classified with the support of a geostatistical approach, and the required conditions for the formation of any specific pattern were analyzed.Results showed that the chemotactic behavior of the bacteria leads to non-trivial population patterns even in the absence of environmental heterogeneities. The presence of structural pore scale heterogeneities had also an impact on bacterial distributions. For a range of chemotactic sensitivities, microorganisms tend to migrate preferably from larger pores toward smaller pores and the resulting distribution patterns thus resembled the heterogeneity of the pore space. The results clearly indicated that in a porous medium like soil the distribution of bacteria may not only be related to the external constraints but also to the chemotactic behavior of the bacterial cells

Quantification of organic pollutant degradation in contaminated aquifers using compound specific stable isotope analysis – Review of recent developments

Compound specific stable isotope analysis (CSIA) has been established as a viable tool for proving, characterizing and assessing degradation of organic pollutants within contaminated aquifers. The fractionation of stable isotopes during contaminant degradation leads to observable shifts in stable isotope ratios which can serve as an indicator for in situ pollutant degradation and allow for a quantitative assessment by means of the so-called Rayleigh (distillation) equation. This review highlights the recent developments of the Rayleigh equation approach for quantifying in situ degradation of organic pollutants in contaminated aquifers. The advantages and limitations of the Rayleigh equation approach are discussed and suggestions for improvements are given. Concepts are provided to estimate the uncertainty due to errors or variability of input parameters and how to deal with such uncertainty. Moreover, the applicability of the Rayleigh equation approach is evaluated regarding the heterogeneity and complexity of groundwater systems. For such systems, the review discusses the relevance of non-destructive processes, which affect the concentration (e.g., dispersive mixing) and potentially also the stable isotope ratio of contaminants (e.g., sorption, volatilization), and the resulting implications for the Rayleigh equation approach

How the chemotactic characteristics of bacteria can determine their population patterns

Spatial distribution of soil microorganisms is relevant for the functioning and performance of many ecosystem processes such as nutrient cycling or biodegradation of organic matters and contaminants. Beside the multitude of abiotic environmental factors controlling the distribution of microorganisms in soil systems, many microbial species exhibit chemotactic behavior by directing their movement along concentration gradients of nutrients or of chemoattractants produced by cells of their own kind. This chemotactic ability has been shown to promote the formation of complex distribution patterns even in the absence of environmental heterogeneities. Microbial population patterns in heterogeneous soil systems might be, hence, the result of the interplay between the heterogeneous environmental conditions and the microorganisms' intrinsic pattern formation capabilities.In this modeling study, we combined an individual-based modeling approach with a reactive pore-network model to investigate the formation of bacterial patterns in homogeneous and heterogeneous porous media. We investigated the influence of different bacterial chemotactic sensitivities (toward both substrate and bacteria) on bacterial distribution patterns. The emerging population patterns were classified with the support of a geostatistical approach, and the required conditions for the formation of any specific pattern were analyzed.Results showed that the chemotactic behavior of the bacteria leads to non-trivial population patterns even in the absence of environmental heterogeneities. The presence of structural pore scale heterogeneities had also an impact on bacterial distributions. For a range of chemotactic sensitivities, microorganisms tend to migrate preferably from larger pores toward smaller pores and the resulting distribution patterns thus resembled the heterogeneity of the pore space. The results clearly indicated that in a porous medium like soil the distribution of bacteria may not only be related to the external constraints but also to the chemotactic behavior of the bacterial cells

Estimating pathway-specific contributions to biodegradation in aquifers based on dual isotope analysis: Theoretical analysis and reactive transport simulations

At field sites with varying redox conditions, different redox-specific microbial degradation pathways contribute to total contaminant degradation. The identification of pathway-specific contributions to total contaminant removal is of high practical relevance, yet difficult to achieve with current methods. Current stable-isotope-fractionation-based techniques focus on the identification of dominant biodegradation pathways under constant environmental conditions. We present an approach based on dual stable isotope data to estimate the individual contributions of two redox-specific pathways. We apply this approach to carbon and hydrogen isotope data obtained from reactive transport simulations of an organic contaminant plume in a two-dimensional aquifer cross section to test the applicability of the method. To take aspects typically encountered at field sites into account, additional simulations addressed the effects of transverse mixing, diffusion-induced stable-isotope fractionation, heterogeneities in the flow field, and mixing in sampling wells on isotope-based estimates for aerobic and anaerobic pathway contributions to total contaminant biodegradation. Results confirm the general applicability of the presented estimation method which is most accurate along the plume core and less accurate towards the fringe where flow paths receive contaminant mass and associated isotope signatures from the core by transverse dispersion. The presented method complements the stable-isotope-fractionation-based analysis toolbox. At field sites with varying redox conditions, it provides a means to identify the relative importance of individual, redox-specific degradation pathways