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  • 1
    Language: English
    In: Water Research, 01 July 2016, Vol.98, pp.363-375
    Description: Groundwater under industrial sites is characterised by heterogeneous chemical mixtures, making it difficult to assess the fate and transport of individual contaminants. Quantifying the biological removal (attenuation) of nitrogen (N) is particularly difficult due to its reactivity and ubiquity. Here a multi-isotope approach is developed to distinguish N sources and sinks within groundwater affected by complex industrial pollution. Samples were collected from 70 wells across the two aquifers underlying a historic industrial area in Belgium. Below the industrial site the groundwater contained up to 1000 mg N l ammonium (NH ) and 300 mg N l nitrate (NO ), while downgradient concentrations decreased to ∼1 mg l DIN ([DIN] = [NH N] + [NO N] + [NO N]). Mean δ N-DIN increased from ∼2‰ to +20‰ over this flow path, broadly confirming that biological N attenuation drove the measured concentration decrease. Multi-variate analysis of water chemistry identified two distinct NH sources (δ N NH from −14‰ and +5‰) within the contaminated zone of both aquifers. Nitrate dual isotopes co-varied (δ N: −3‰ – +60‰; δ O: 0‰ – +50‰) within the range expected for coupled nitrification and denitrification of the identified sources. The fact that δ N NO values were 50‰–20‰ less than δ N NH values in the majority of wells confirmed that nitrification controlled N turnover across the site. However, the fact that δ N NO was greater than δ N NH in wells with the highest [NH ] shows that an autotrophic NO reduction pathway (anaerobic NH oxidation or nitrifier-denitrification) drove N attenuation closest to the contaminant plume. This direct empirical evidence that both autotrophic and heterotrophic biogeochemical processes drive N attenuation in contaminated aquifers demonstrates the power of multiple N isotopes to untangle N cycling in highly complex systems.
    Keywords: Ammonium Attenuation ; Groundwater ; Industrial Pollution ; Nitrate Reduction ; Nitrite Reduction ; Stable Isotopes ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 2
    Language: English
    In: Water Research, 01 October 2018, Vol.142, pp.373-382
    Description: Nitrogen fate and transport through contaminated groundwater systems, where N is both ubiquitous and commonly limits pollutant attenuation, must be re-evaluated given evidence for new potential microbial N pathways. We addressed this by measuring the isotopic composition of dissolved inorganic N (DIN = NH , NO , and NO ) and N functional gene abundances ( , , , ) from 20 to 38 wells across an NH , hydrocarbon, and SO contaminated aquifer. In-situ N attenuation was confirmed on three sampling dates (0, +6, +12 months) by the decreased [DIN] (4300 - 40 μM) and increased δ N-DIN (5‰–33‰) over the flow path. However, the assumption of negligible N attenuation within the plume was complicated by the presence of alternative electron acceptors (SO , Fe ), both oxidizing and reducing functional genes, and N oxides within this anoxic zone. Active plume N cycling was corroborated using an NO dual isotope based model, which found the fastest (∼10 day) NO turnover within the N and electron donor rich central plume. Findings suggest that N cycling is not always O limited within chemically complex contaminated aquifers, though this cycling may recycle the N species rather than attenuate N.
    Keywords: Contaminated Groundwater ; Stable Isotopes ; Nitrite ; Chemolithotrophic N Cycling ; Biodegradation ; Nitrate Dual Isotopes ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 3
    Language: English
    In: Water Research, 2011, Vol.45(20), pp.6688-6698
    Description: Current understanding of the dynamics of sulfur compounds inside constructed wetlands is still insufficient to allow a full description of processes involved in sulfur cycling. Experiments in a pilot-scale horizontal subsurface flow constructed wetland treating high sulfate-containing contaminated groundwater were carried out. Application of stable isotope approach combined with hydro-chemical investigations was performed to evaluate the sulfur transformations. In general, under inflow concentration of about 283 mg/L sulfate sulfur, sulfate removal was found to be about 21% with a specific removal rate of 1.75 g/m ·d. The presence of sulfide and elemental sulfur in pore water about 17.3 mg/L and 8.5 mg/L, respectively, indicated simultaneously bacterial sulfate reduction and re-oxidation. 70% of the removed sulfate was calculated to be immobilized inside the wetland bed. The significant enrichment of S and O in dissolved sulfate (δ S up to 16‰, compared to average of 5.9‰ in the inflow, and δ O up to 13‰, compared to average of 6.9‰ in the inflow) was observed clearly correlated to the decrease of sulfate loads along the flow path through experimental wetland bed. This enrichment also demonstrated the occurrence of bacterial sulfate reduction as well as demonstrated by the presence of sulfide in the pore water. Moreover, the integral approach shows that bacterial sulfate reduction is not the sole process controlling the isotopic composition of dissolved sulfate in the pore water. The calculated apparent enrichment factor (  = −22‰) for sulfur isotopes from the δ S vs. sulfate mass loss was significantly smaller than required to produce the observed difference in δ S between sulfate and sulfide. It indicated some potential processes superimposing bacterial sulfate reduction, such as direct re-oxidation of sulfide to sulfate by oxygen released from plant roots and/or bacterial disproportionation of elemental sulfur. Furthermore, 41% of residual sulfate was calculated to be from sulfide re-oxidation, which demonstrated that the application of stable isotope approach combined with the common hydro-chemical investigations is not only necessary for a general qualitative evaluation of sulfur transformations in constructed wetlands, but also leads to a quantitative description of intermediate processes. ► Stable isotope assessment was performed to evaluate sulfur transformations. ► Significant enrichment of δ S and δ O in sulfate strongly demonstrated bacterial sulfate reduction. ► Re-oxidation and disproportionation of reduced sulfur compounds are potential processes. ► Stable isotope assessment leads to a quantitative description of intermediate processes.
    Keywords: Constructed Wetland ; Bacterial Sulfate Reduction ; Sulfide Re-Oxidation ; Stable Isotopes ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 4
    Language: English
    In: Water Research, 01 May 2015, Vol.74, pp.203-212
    Description: Constructed wetlands are important ecosystems with respect to nitrogen cycling. Here we studied the activity and abundance of nitrogen transforming bacteria as well as the spatial distribution of nitrification, anaerobic ammonium oxidation (anammox), and denitrification processes in a horizontal subsurface-flow constructed wetland. The functional genes of the nitrogen cycle were evenly distributed in a linear way along the flow path with prevalence at the superficial points. The same trend was observed for the nitrification and denitrification turnover rates using isotope labeling techniques. It was also shown that only short-term incubations should be used to measure denitrification turnover rates. Significant nitrate consumption under aerobic conditions diminishes nitrification rates and should therefore be taken into account when estimating nitrification turnover rates. This nitrate consumption was due to aerobic denitrification, the rate of which was comparable to that for anaerobic denitrification. Consequently, denitrification should not be considered as an exclusively anaerobic process. Phylogenetic analysis of hydrazine synthase ( ) gene clones indicated the presence of and anammox species in the constructed wetland. Although anammox bacteria were detected by molecular methods, anammox activity could not be measured and hence this process appears to be of low importance in nitrogen transformations in these freshwater ecosystems.
    Keywords: Nitrification ; Anammox ; Aerobic Denitrification ; Abundance ; Activity ; Constructed Wetland ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 5
    Language: English
    In: Water Research, 01 March 2018, Vol.130, pp.185-199
    Description: Nitrate contamination in ground- and surface water is a persistent problem in countries with intense agriculture. The transition zone between rivers and their riparian aquifers, where river water and groundwater interact, may play an important role in mediating nitrate exports, as it can facilitate intensive denitrification, which permanently removes nitrate from the aquatic system. However, the in-situ factors controlling riparian denitrification are not fully understood, as they are often strongly linked and their effects superimpose each other. In this study, we present the evaluation of hydrochemical and isotopic data from a 2-year sampling period of river water and groundwater in the riparian zone along a 3rd order river in Central Germany. Based on bi- and multivariate statistics (Spearman's rank correlation and partial least squares regression) we can show, that highest rates for oxygen consumption and denitrification in the riparian aquifer occur where the fraction of infiltrated river water and at the same time groundwater temperature, are high. River discharge and depth to groundwater are additional explanatory variables for those reaction rates, but of minor importance. Our data and analyses suggest that at locations in the riparian aquifer, which show significant river water infiltration, heterotrophic microbial reactions in the riparian zone may be fueled by bioavailable organic carbon derived from the river water. We conclude that interactions between rivers and riparian groundwater are likely to be a key control of nitrate removal and should be considered as a measure to mitigate high nitrate exports from agricultural catchments.
    Keywords: Riparian Zone ; Nitrate Contamination ; Nitrate Stable Isotopes ; River-Groundwater Interaction ; Denitrification ; Engineering
    ISSN: 0043-1354
    E-ISSN: 1879-2448
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  • 6
    Language: English
    In: Science of the Total Environment, 01 April 2018, Vol.619-620, pp.1579-1588
    Description: This work aims to (1) identify the most conductive conditions for the generation of greenhouses gases (GHGs) in groundwater (e.g., hydrogeological contexts and geochemical processes) and (2) evaluate the indirect emissions of GHGs from groundwater at a regional scale in Wallonia (Belgium). To this end, nitrous oxide (N O), methane (CH ) and carbon dioxide (CO ) concentrations and the stable isotopes of nitrate (NO ) and sulphate were monitored in 12 aquifers of the Walloon Region (Belgium). The concentrations of GHGs range from 0.05 μg/L to 1631.2 μg/L for N O, 0 μg/L to 17.1 μg/L for CH , and 1769 to 100,514 ppm for the partial pressure of CO (pCO ). The highest average concentrations of N O and pCO are found in a chalky aquifer. The coupled use of statistical techniques and stable isotopes is a useful approach to identify the geochemical conditions that control the occurrence of GHGs in the aquifers of the Walloon Region. The accumulation of N O is most likely due to nitrification (high concentrations of dissolved oxygen and NO and null concentrations of ammonium) and, to a lesser extent, initial denitrification in a few sampling locations (medium concentrations of dissolved oxygen and NO ). The oxic character found in groundwater is not prone to the accumulation of CH in Walloon aquifers. Nevertheless, groundwater is oversaturated with GHGs with respect to atmospheric equilibrium (especially for N O and pCO ); the fluxes of N O (0.32 kg N O-N Ha y ) and CO (27 kg CO Ha y ) from groundwater are much lower than the direct emissions of N O from agricultural soils and fossil-fuel-related CO emissions. Thus, indirect GHG emissions from the aquifers of the Walloon Region are likely to be a minor contributor to atmospheric GHG emissions, but their quantification would help to better constrain the nitrogen and carbon budgets.
    Keywords: Greenhouses Gases ; Groundwater ; Indirect Emissions ; Belgium ; Walloon Region ; Environmental Sciences ; Biology ; Public Health
    ISSN: 0048-9697
    E-ISSN: 1879-1026
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  • 7
    Language: English
    In: Science of the Total Environment, 15 February 2018, Vol.615, pp.773-783
    Description: Elevated nitrate concentrations are a thread for water supply and ecological integrity in surface water. Nitrate fluxes obtained by standard monitoring protocols at the catchment outlet strongly integrate spatially and temporally variable processes such as mobilization and turnover. Consequently, inference of dominant nitrate sources is often problematic and challenging in terms of effective river management and prioritization of measures. Here, we combine a spatially highly resolved assessment of nitrate concentration and fluxes along a mesoscale catchment with four years of monitoring data at two representative sites. The catchment is characterized by a strong land use gradient from pristine headwaters to lowland sub-catchments with intense agricultural land use and wastewater sources. We use nitrate concentrations in combination with hydrograph separation and isotopic fingerprinting methods to characterize and quantify nitrate source contribution. The hydrological analysis revealed a clear dominance of base flow during both campaigns. However, the absolute amounts of discharge differed considerably from one another (outlet: 1.42 m s in 2014, 0.43 m s in 2015). Nitrate concentrations are generally low in the pristine headwaters (〈 3 mg L ) and increase downstream (15 to 16 mg L ) due to the contribution of agricultural and wastewater sources. While the agricultural contribution did not vary in terms of nitrate concentration and isotopic signature between the years, the wastewater contribution strongly increased with decreasing discharge. Wastewater-borne nitrate load in the entire catchment ranged between 19% (2014) and 39% (2015). Long-term monitoring of nitrate concentration and isotopic composition in two sub-catchment exhibits a good agreement with findings from spatially monitoring. In both datasets, isotopic composition indicates that denitrification plays only a minor role. The spatially highly resolved monitoring approach helped to pinpoint hot spots of nitrate inputs into the stream while the long-term information allowed to place results into the context of intra-annual variability.
    Keywords: Nitrogen Isotope of Nitrate ; Meso-Scale Isotope Pattern ; Nitrate Pollution ; Surface Water ; Water Quality ; Environmental Sciences ; Biology ; Public Health
    ISSN: 0048-9697
    E-ISSN: 1879-1026
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  • 8
    Language: English
    In: Journal of Hydrology, 2011, Vol.406(1), pp.66-72
    Description: ► Application of δ H/δ O for the description of the hydrological situation in a coastal barrier system. ► Barrier groundwater is mainly recharged by precipitation. ► Hydrological scenario is influenced by different barrier vegetation types. This paper presents isotopic composition (δ O, δ H) of groundwater, lagoon water, and seawater collected in a coastal lagoon system in Southern Brazil as a tool to define groundwater sources within a permeable barrier that separates the lagoon from the ocean. The isotopic signature of the barrier groundwater differs both from the isotopic composition of lagoon water and seawater. The groundwater is significantly lighter, indicating that the major recharge source to the barrier is precipitation, rather than lagoon water. In the southern and central part of the barrier no interaction of groundwater with lagoon water was observed. In the northern part, however, lagoon water intrusion into the barrier was identified. This effect is apparently caused by different vegetation types on the barrier influencing subsurface hydrological processes. The central and the southern areas are characterised by natural dune vegetation (high infiltration, low evapotranspiration rates) whereas the northern part is covered by extensive pine tree plantations (low infiltration, high evapotranspiration rates). The presence of pine trees may enhance lagoon and seawater intrusion into the shallow aquifer and decrease fresh submarine groundwater discharge.
    Keywords: Stable Isotopes ; Geochemical Tracers ; Permeable Sediments ; Hydrologic Cycle ; Submarine Groundwater Discharge ; Subterranean Estuaries ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 9
    Language: English
    In: Environmental Monitoring and Assessment, 2015, Vol.187(3), pp.1-15
    Description: Freshwater discharge into the coastal sea is of general interest for two reasons: (i) It acts as vehicle for the transport of contaminants or nutrients into the ocean, and (ii) it indicates the loss of significant volumes of freshwater that might be needed for irrigation or drinking water supply. Due to the large-scale and long-term nature of the related hydrological processes, locating and quantitatively assessing freshwater discharge into the sea require naturally occurring tracers that allow fast, inexpensive and straightforward detection. In several studies, the standard water parameters electrical conductivity (EC) and pH have proven their suitability in this regard. However, while distribution patterns of EC and pH in the coastal sea indicate freshwater discharge in general, a separation between discharging surface water and submarine groundwater discharge (SGD) is not possible with these alone. The naturally occurring radionuclide radon-222 has been shown to be useful in the quantification of SGD and its distinction from surface runoff. This study aimed to evaluate and compare the informative value of the three parameters—EC, pH and radon concentration—in detecting and quantifying SGD by carrying out a case study in a bay located in western Ireland. The results reveal that radon activity is the most sensitive parameter for detecting SGD. However, only the combined evaluation of radon, EC and pH allows a quantitative allocation of groundwater and surface water contributions to the overall freshwater discharge into the sea. This conclusion is independently supported by stable isotope data measured on selected samples.
    Keywords: Coastal freshwater discharge ; Submarine groundwater discharge ; Radon ; Stable isotopes of water ; Environmental tracers
    ISSN: 0167-6369
    E-ISSN: 1573-2959
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  • 10
    In: Water Resources Research, March 2011, Vol.47(3), pp.n/a-n/a
    Description: Naturally occurring radon‐222 was evaluated for its use in estimating groundwater flow velocities using single‐well tests. Investigations were carried out for four different well scenarios, which revealed the advantages and limitations of the approach. On one hand, it was shown that radon is useful as an environmental tracer because of (1) the low costs of the method, (2) the avoidance of any artificial tracer injection into the aquifer, (3) the immediate availability of results, and (4) the need for only a single monitoring well. On the other hand, several potential sources of error were identified, including poor sampling, inadequate hydraulic connection of the well because of a clogged screen, and an unsuitable well diameter resulting in excessively long or short well water residence times. The practical approach is supported by in‐depth theoretical considerations. General recommendations are presented concerning the use of radon as an environmental tracer for groundwater flow assessment.
    Keywords: Radon ; Groundwater Flow Velocity ; Single Well Test ; Tracer
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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