Reactive transport modelling to assess pesticide dissipation at the sediment-water interface

Rivers that are hydrologically connected to an agro-ecosystem act as a source or sink of pollutants transported by surface runoff and subsurface water. The Sediment-Water Interface (SWI) of rivers is a critical boundary for river dynamics where hydrological and biogeochemical processes tightly control pesticide dissipation. Transport processes govern pesticide transit time and distribution across the SWI depending on the water flow and hyporheic exchanges. Simultaneously, reactive processes such as sorption and biodegradation are responsible for retardation or actual degradation of pesticides within the porous sediment. However, knowledge on the interplay of these processes at the SWI remain sparse mostly because a physically-based generalized framework to model transport and reactivity at fluid-porous interfaces is still lacking. Here, we combine model development and laboratory experiments to investigate the effects of representative hydrological conditions on pesticide transport at the SWI.An innovative discrete flow-transport model accounting for sorption was developed to consider the pure fluid layer (via Navier-Stokes model) and the porous medium (via Darcy-Brinkmann model). Advanced and appropriate numerical techniques are implemented to solve the coupled models (Navier-Stokes and Darcy-Brinkmann) without any interface conditions or empirical transfer functions. Conservative (NaCl) and non-conservative (Foron Blue 291 &#8211; sorptive) tracer experiments were performed within a 15 cm long and 10 cm deep recirculated river model (3 < equivalent length < 30 km) to characterize pesticide transport at the SWI. Configurations with the contaminant in the sediment (sediment as contaminant source) or contaminations from the overlying water (sediment as a sink) were tested. Simulated tracer concentrations fitted well to measured concentrations over a range of laminar flows representative of low Strahler order rivers (Re < 700, bulk velocities 10 < &#160;U < 100 mm.s-1). In all flow conditions, the first few mm of sediment constituted the most dynamic layer, which was controlled by advective processes. In contrast, the tracer in deeper sediment layers undergone diffusive transport with lower exchange rates. Sorption was also observed to significantly increase residence time within the sediment and to slow down the progression of the tracer plume into the sediment. The times required to reach the bottom of the river model rose up to 12 times as compared with the non-sorptive tracer, indicating limited hyporheic exchanges with increasing sorption.To account for biodegradation at the SWI, the model is further extended to include degradation kinetics and stable isotope fractionation of organic micropollutants. Changes of stable isotope ratios of the remaining, non-degraded pool of pollutants over time or across the sediment layer is used as a proxy of in situ biodegradation. Biodegradation is interpreted as a function of oxygen zonation within the sediment. This model is eventually tested against tracer experiments with caffeine, which is used here as a fast degrading anthropogenic micropollutant. Patterns of micropollutant dissipation at the SWI arising from these developments will be further extrapolated at river reaches within an agricultural catchment (Souffel catchment, France). Altogether, this study will help understanding how rivers influence pesticide transport, storage and degradation at the catchment scale.

Download Full-text

Cadmium uptake and transport processes in rice revealed by stable isotope fractionation and Cd-related gene expression

The Science of The Total Environment ◽

10.1016/j.scitotenv.2021.150633 ◽

2022 ◽

Vol 806 ◽

pp. 150633

Author(s):

Songxiong Zhong ◽

Xiaomin Li ◽

Fangbai Li ◽

Yingmei Huang ◽

Tongxu Liu ◽

...

Keyword(s):

Gene Expression ◽

Stable Isotope ◽

Isotope Fractionation ◽

Transport Processes ◽

Cadmium Uptake ◽

Related Gene ◽

Stable Isotope Fractionation ◽

Uptake And Transport

Download Full-text

Continual in-situ monitoring of pore water stable isotopes in the subsurface

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-10-13293-2013 ◽

2013 ◽

Vol 10 (11) ◽

pp. 13293-13331 ◽

Cited By ~ 3

Author(s):

T. H. M. Volkmann ◽

M. Weiler

Keyword(s):

Stable Isotopes ◽

Water Vapor ◽

Stable Isotope ◽

Monitoring System ◽

Pore Water ◽

Sampling Method ◽

Transport Processes ◽

In Situ Monitoring ◽

Subsurface Water

Abstract. The stable isotope signature of pore water provides an integral fingerprint of water origin, flow path, transport processes, and residence times and can thus serve as a powerful tracer of hydrological processes in the unsaturated and saturated zone. However, the full potential of stable isotopes to quantitatively characterize subsurface water dynamics is yet unfolded due to the difficulty in obtaining extensive detailed and continual measurements of spatiotemporally variable pore water signatures. With the development of field-deployable laser-based isotope analyzers, such measurements are now becoming feasible. This study presents the development and application of a functional, automatable, and cost-efficient system for non-destructive continual in-situ monitoring of pore water stable isotope signatures with high resolution. The monitoring system uses automatic-controllable valve arrays to continuously extract diluted soil air water vapor via a branching network of multiple small microporous probes into a commercial isotope analyzer. Soil temperature observations are used to convert obtained vapor phase into liquid phase water isotope signatures, but these can also be obtained based on vapor concentration measurements. In-situ sampling was conducted at six depths for each of three plots planted with varying vegetation on an experimental site in SW Germany. Two different methods based on advective and diffusive soil water vapor probing were employed suitable under unsaturated and all (including saturated) moisture conditions, respectively. The advective sampling method was applied using multiple permanently installed probes (continual mode) and using a single probe subsequently inserted to sample the various locations (push-in mode), while the diffusive sampling method was applied in push-in mode only. Using a specific identical treatment onsite calibration approach along with basic corrections for instrument bias and temperature dependent free water-vapor isotopic equilibrium fractionation, the monitoring system facilitated inference of normalized liquid pore water isotopic composition with sufficiently high accuracy and precision at sampling intervals of less than four minutes and resolved the isotopic variability along natural depth profiles. Comparison indicated that the presented in-situ approaches may be used interchangeably with each other and with concurrent laboratory-based direct equilibration measurements of destructively collected samples, such that the choice of method will depend upon the task and anticipated conditions of sampling. The introduced sampling techniques provide powerful tools towards a detailed quantitative understanding of dynamic and heterogeneous shallow subsurface and vadose zone processes.

Download Full-text

Technical note: Unresolved aspects of the direct vapor equilibration method for stable isotope analysis (δ18O, δ2H) of matrix-bound water: unifying protocols through empirical and mathematical scrutiny

Hydrology and Earth System Sciences ◽

10.5194/hess-25-5219-2021 ◽

2021 ◽

Vol 25 (9) ◽

pp. 5219-5235

Author(s):

Benjamin Gralher ◽

Barbara Herbstritt ◽

Markus Weiler

Keyword(s):

Stable Isotope ◽

Data Quality ◽

Stable Isotope Analysis ◽

Isotope Analysis ◽

Bound Water ◽

Transport Processes ◽

Subsurface Water ◽

Water Volume ◽

Equilibration Time ◽

Matrix Bound

Abstract. The direct vapor equilibration laser spectrometry (DVE-LS) method has been developed for obtaining matrix-bound water stable isotope data in soils, the critical zone, and bedrock, deriving therefrom subsurface water flow and transport processes and, ultimately, characterizing, for example, groundwater recharge and vulnerability. Recently, DVE-LS has been increasingly adopted due to its possible high sample throughput, relative simplicity, and cost-efficiency. However, this has come at the cost of a non-unified standard operation protocol (SOP), and several contradictory suggestions regarding protocol details do exist which have not been resolved to date. Particularly, sample container material and equilibration times have not yet been agreed upon. Beside practical constraints, this often limits DVE-LS applicability to interpreting relative isotope dynamics instead of absolute values. It also prevents data comparability among studies or laboratories, and several previous comparisons of DVE-LS with other, more traditional approaches of water extraction and subsequent stable isotope analysis yielded significant discrepancies for various sample matrices and physical states. In a series of empirical tests, we scrutinized the controversial DVE-LS protocol details. Specifically, we tested 10 different easily available and cost-efficient inflatable bags previously employed or potentially suitable for DVE-LS sample collection and equilibration. In storage tests similar to the DVE-LS equilibration process but lasting several weeks, we quickly found heat-sealed bags made of laminated aluminum (Al) sheets to be superior by several orders of magnitude over more frequently used freezer bags in terms of evaporation safety and accompanying adverse isotope effects. For the first time, Al-laminated bags allow the applied equilibration time to be adapted exclusively to sample requirements instead of accepting reduced data quality in a trade-off with material shortcomings. Based on detailed physical considerations, we further describe how to calculate the minimum available container headspace and sample-contained liquid water volume and how their ratio affects analytical precision and accuracy. We are confident that these guidelines will expand DVE-LS applicability and improve data quality and comparability among studies and laboratories by contributing to a more unified, physically well-founded SOP based on more appropriate components.

Download Full-text

Technical note: Controversial aspects of the direct vapor equilibration method for stable isotope analysis (δ18O, δ2H) of matrix-bound water: Unifying protocols through empirical and mathematical scrutiny

10.5194/hess-2021-255 ◽

2021 ◽

Author(s):

Benjamin Gralher ◽

Barbara Herbstritt ◽

Markus Weiler

Keyword(s):

Stable Isotope ◽

Data Quality ◽

Stable Isotope Analysis ◽

Isotope Analysis ◽

Bound Water ◽

Transport Processes ◽

Subsurface Water ◽

Water Volume ◽

Equilibration Time ◽

Matrix Bound

Abstract. The direct vapor equilibration laser spectrometry (DVE-LS) method has been developed for obtaining matrix-bound water stable isotope data in soils, the critical zone and bedrock, deriving therefrom subsurface water flow and transport processes and, ultimately, characterising e.g. groundwater recharge and vulnerability. Recently, DVE-LS has been increasingly adopted due to its possible high sample throughput, relative simplicity and cost-efficiency. However, this has come at the cost of a non-unified standard operation protocol (SOP) and several contradictory suggestions regarding protocol details do exist which have not been resolved to date. Particularly, sample container material and equilibration times have not yet been agreed upon. Beside practical constraints, this often limits DVE-LS applicability to interpreting relative isotope dynamics instead of absolute values. It also prevents data comparability among studies or laboratories and several previous comparisons of DVE-LS with other, more traditional approaches of water extraction and subsequent stable isotope analysis yielded significant discrepancies for various sample matrices and physical states. In a series of empirical tests, we scrutinized the controversial DVE-LS protocol details. Specifically, we tested ten different easily available and cost-efficient inflatable bags previously employed or potentially suitable for DVE-LS sample collection and equilibration. In storage tests similar to the DVE-LS equilibration process but lasting several weeks, we quickly found heat-sealed bags made of laminated Aluminum (Al) sheets to be superior by several orders of magnitude over more frequently used freezer bags in terms of evaporation-safety and accompanying adverse isotope effects. For the first time, Al-laminated bags allow the applied equilibration time to be adapted exclusively to sample requirements instead of accepting reduced data quality in a trade-off with material shortcomings. Based on detailed physical considerations, we further describe how to calculate the minimum available container headspace and sample-contained liquid water volume and how their ratio affects analytical precision and accuracy. We are confident, that these guidelines will expand DVE-LS applicability and improve data quality and comparability among studies and laboratories by contributing to a more unified, physically well-founded SOP based on more appropriate components.

Download Full-text

Continual in situ monitoring of pore water stable isotopes in the subsurface

Hydrology and Earth System Sciences ◽

10.5194/hess-18-1819-2014 ◽

2014 ◽

Vol 18 (5) ◽

pp. 1819-1833 ◽

Cited By ~ 60

Author(s):

T. H. M. Volkmann ◽

M. Weiler

Keyword(s):

Stable Isotopes ◽

Stable Isotope ◽

Pore Water ◽

Transport Processes ◽

Water Dynamics ◽

In Situ Monitoring ◽

Subsurface Water ◽

Efficient System ◽

Stable Isotope Signatures

Abstract. Stable isotope signatures provide an integral fingerprint of origin, flow paths, transport processes, and residence times of water in the environment. However, the full potential of stable isotopes to quantitatively characterize subsurface water dynamics is yet unfolded due to the difficulty in obtaining extensive, detailed, and repeated measurements of pore water in the unsaturated and saturated zone. This paper presents a functional and cost-efficient system for non-destructive continual in situ monitoring of pore water stable isotope signatures with high resolution. Automatic controllable valve arrays are used to continuously extract diluted water vapor in soil air via a branching network of small microporous probes into a commercial laser-based isotope analyzer. Normalized liquid-phase isotope signatures are then obtained based on a specific on-site calibration approach along with basic corrections for instrument bias and temperature dependent isotopic fractionation. The system was applied to sample depth profiles on three experimental plots with varied vegetation cover in southwest Germany. Two methods (i.e., based on advective versus diffusive vapor extraction) and two modes of sampling (i.e., using multiple permanently installed probes versus a single repeatedly inserted probe) were tested and compared. The results show that the isotope distribution along natural profiles could be resolved with sufficiently high accuracy and precision at sampling intervals of less than four minutes. The presented in situ approaches may thereby be used interchangeably with each other and with concurrent laboratory-based direct equilibration measurements of destructively collected samples. It is thus found that the introduced sampling techniques provide powerful tools towards a detailed quantitative understanding of dynamic and heterogeneous shallow subsurface and vadose zone processes.

Download Full-text

Unravelling the process of petroleum hydrocarbon biodegradation in different filter materials of constructed wetlands by stable isotope fractionation and labelling studies

Biodegradation ◽

10.1007/s10532-021-09942-1 ◽

2021 ◽

Author(s):

Andrea Watzinger ◽

Melanie Hager ◽

Thomas Reichenauer ◽

Gerhard Soja ◽

Paul Kinner

Keyword(s):

Stable Isotope ◽

Constructed Wetlands ◽

Hydrogen Isotope ◽

Petroleum Hydrocarbon ◽

Isotope Fractionation ◽

Isotope Effects ◽

Filter Material ◽

Hydrocarbon Biodegradation ◽

Stable Isotope Fractionation ◽

Filter Materials

AbstractMaintaining and supporting complete biodegradation during remediation of petroleum hydrocarbon contaminated groundwater in constructed wetlands is vital for the final destruction and removal of contaminants. We aimed to compare and gain insight into biodegradation and explore possible limitations in different filter materials (sand, sand amended with biochar, expanded clay). These filters were collected from constructed wetlands after two years of operation and batch experiments were conducted using two stable isotope techniques; (i) carbon isotope labelling of hexadecane and (ii) hydrogen isotope fractionation of decane. Both hydrocarbon compounds hexadecane and decane were biodegraded. The mineralization rate of hexadecane was higher in the sandy filter material (3.6 µg CO2 g−1 day−1) than in the expanded clay (1.0 µg CO2 g−1 day−1). The microbial community of the constructed wetland microcosms was dominated by Gram negative bacteria and fungi and was specific for the different filter materials while hexadecane was primarily anabolized by bacteria. Adsorption / desorption of petroleum hydrocarbons in expanded clay was observed, which might not hinder but delay biodegradation. Very few cases of hydrogen isotope fractionation were recorded in expanded clay and sand & biochar filters during decane biodegradation. In sand filters, decane was biodegraded more slowly and hydrogen isotope fractionation was visible. Still, the range of observed apparent kinetic hydrogen isotope effects (AKIEH = 1.072–1.500) and apparent decane biodegradation rates (k = − 0.017 to − 0.067 day−1) of the sand filter were low. To conclude, low biodegradation rates, small hydrogen isotope fractionation, zero order mineralization kinetics and lack of microbial biomass growth indicated that mass transfer controlled biodegradation.

Download Full-text