Jan Georg Wiederhold

River systems often retain the imprint of historical mercury (Hg) contamination over long time scales since decreasing trends after reduced Hg inputs are impeded by legacy storage in sediments. Here, we report temporal and spatial variations of total mercury (THg) concentrations in sediments of the Elbe River, Germany. Temporal THg trends (1996–2019) were investigated at five long-term monitoring sites in harbors and groyne fields. In contrast to well-documented declines of THg in suspended particulate matter (SPM) in recent decades, significant trends of decreasing THg in sediments were only found for two harbors, while the other sites exhibited no decreasing trends. Spatial distribution patterns of THg in sediments (0.11–10.6 mg kg⁻¹) showed increases in areas of the Middle Elbe downstream of the confluences of the tributary rivers Mulde and Saale. Similar spatial patterns of increasing concentrations were observed in floodplain soils (0.04–4.20 mg kg⁻¹) and SPM (0.37–1.84 mg kg⁻¹). Regardless of the spatial origin or sample type (sediments, floodplain soils, SPM), the binding forms of Hg studied by sequential extraction showed a clear dominance of strongly bound Hg extractable with 12 M HNO₃. While this seems to indicate a low mobility of Hg, uptake into aquatic food webs may occur following methylation. Our results demonstrate that the reduction of Hg releases into the Elbe River basin has so far only been able to improve sediment quality in some parts. Deposits of Hg-containing sediments will therefore remain a future problem with implications for the continued exceedance of environmental quality standards and the management of waterways.

Recently developed multi-element methods for filtered and unfiltered river water enable the analysis of 67 elements in a single analytical run, facilitating the assessment of multi-element fingerprints in monitoring programs. To elucidate the occurrence and possible pathways of emerging contaminants, it is essential to have baseline data for as many elements as possible to detect anomalies at specific environmental events. However, the variability of element concentrations in river water on the temporal and spatial scale is only poorly understood, causing considerable uncertainty for river water monitoring. Therefore, we conducted comprehensive sampling campaigns to assess the spatiotemporal variabilities of element concentrations in the river Rhine (Germany) at different discharge levels. Both, the long-term temporal (one-year data) and the spatial variability data revealed a distinct behavior of two element groups in relation to the discharge: elements showing a dilution effect (e.g.: B, Mg, S, K, Ca, Br, Sr, Mo, and U) or a co-rising effect (e.g.: Al, Si, P, Ti, V, Mn, Fe, Ni, Cu, Ga, As, Rb, Y, Cs, Ba, La, Ce, Pr, Nd, Sm, Gd, Pb, and Th) with higher discharge in the unfiltered river water. However, certain elements such as K, Ba or Gd displayed a variable behavior throughout the dataset, underlining the importance of collecting enough baseline data from various locations and conditions to detect anomalies. The analysis of unfiltered water samples in comparison with the filtered fraction allowed us to detect opposite behavior between the dissolved and particulate fractions for Co, Ni, As, Rb, Cs, and Gd. Cross-profile measurements were conducted to investigate spatial variabilities, revealing that lateral spatial gradients in element concentrations (up to a factor of 4) are more pronounced than depth gradients, likely caused by insufficient mixing of river influents or point-sources. Thus, fixed monitoring stations or single-point sampling for long-term data acquisition might not be able to capture the whole picture regarding element behavior in rivers.

Sorption of mercury (Hg) in soils is suggested to be predominantly associated with organic matter (OM). However, there is a growing collection of research that suggests clay minerals and Fe/Mn-oxides are also important solid-phases for the sorption of soluble Hg in soil-groundwater systems. We use a series of (60 mL syringe based) column experiments to examine sorption and subsequent desorption of HgCl₂ solutions (Experiment 1 [EXP1]: 46.1 ± 1.1 mg L^-1; and Experiment 2 [EXP2]: 144 ± 6 mg L^-1) in low OM (0.16 ± 0.02 %) solid-phase aquifer materials. Analyses of total Hg concentrations, Hg speciation (i.e., pyrolytic thermal desorption (PTD)), and Hg stable isotopes are performed on both solid- and liquid-phase samples across sorption and desorption phases. Sorption breakthrough curve best fitted a Freundlich model. Despite the very low OM content, the Hg equilibrium sorptive capacity in these columns is very high: 1510 ± 100 and 2320 ± 60 mg kg^-1 for the EXP1 and EXP2, respectively, and is similar to those determined for high OM soils. Desorption fits exponential decay models and 46 ± 6 % and 58 ± 10 % of the sorbed Hg is removed from the solid-phase materials at the termination of desorption in EXP1 and EXP2, respectively. This desorption profile is linked to the initial release of easily exchangeable Hg(II) species physically sorbed to Fe/Mn-oxides and clay mineral surfaces and then slower release of Hg(II) species that have undergone secondary reaction to more stable/less soluble Hg(II) species and/or diffusion/transport into the mineral matrices. Hg stable isotope data support preferential sorption of lighter isotopes to the solid-phase materials with results indicating isotopically heavy liquid-phase and isotopically light solid-phase. The divergence of δ²⁰²Hg (describing mass dependent fractionation (MDF)) between liquid- and solid-phase continues into desorption and we attribute this to lighter isotopes being favoured in secondary processes occurring after initial sorption to the solid-phase materials (i.e., matrix diffusion, change in Hg(II) speciation, elemental Hg (Hg(0)) production) that lead to less exchangeable forms of Hg. Consequently, heavy isotopes are preferentially released during desorption. These observations agree with data from HgCl₂ contaminated sites. The secondary production of Hg(0) within the columns is confirmed by PTD analyses that indicate distinct Hg(0) release peaks in solid-phase samples at D) and distribution (K_D) coefficients are 77.9 ± 5.5 and 26.1 ± 3.0 mL g^-1 in EXP1, respectively, and 38.4 ± 2.7 and 12.4 ± 0.6 mL g^-1 in EXP2, respectively. These values are similar to values derived from column experiments on high OM soil and provide the basis for future Hg fate and transport modelling in soil-groundwater systems.

Sorption of mercury (Hg) in soils is suggested to be predominantly associated with organic matter (OM). However, there is a growing collection of research that suggests clay minerals and Fe/Mn-oxides are also important solid-phases for the sorption of soluble Hg in soil-groundwater systems. We use a series of (60 mL syringe based) column experiments to examine sorption and subsequent desorption of HgCl₂ solutions (Experiment 1 [EXP1]: 46.1 ± 1.1 mg L^-1; and Experiment 2 [EXP2]: 144 ± 6 mg L^-1) in low OM (0.16 ± 0.02 %) solid-phase aquifer materials. Analyses of total Hg concentrations, Hg speciation (i.e., pyrolytic thermal desorption (PTD)), and Hg stable isotopes are performed on both solid- and liquid-phase samples across sorption and desorption phases. Sorption breakthrough curve best fitted a Freundlich model. Despite the very low OM content, the Hg equilibrium sorptive capacity in these columns is very high: 1510 ± 100 and 2320 ± 60 mg kg^-1 for the EXP1 and EXP2, respectively, and is similar to those determined for high OM soils. Desorption fits exponential decay models and 46 ± 6 % and 58 ± 10 % of the sorbed Hg is removed from the solid-phase materials at the termination of desorption in EXP1 and EXP2, respectively. This desorption profile is linked to the initial release of easily exchangeable Hg(II) species physically sorbed to Fe/Mn-oxides and clay mineral surfaces and then slower release of Hg(II) species that have undergone secondary reaction to more stable/less soluble Hg(II) species and/or diffusion/transport into the mineral matrices. Hg stable isotope data support preferential sorption of lighter isotopes to the solid-phase materials with results indicating isotopically heavy liquid-phase and isotopically light solid-phase. The divergence of δ²⁰²Hg (describing mass dependent fractionation (MDF)) between liquid- and solid-phase continues into desorption and we attribute this to lighter isotopes being favoured in secondary processes occurring after initial sorption to the solid-phase materials (i.e., matrix diffusion, change in Hg(II) speciation, elemental Hg (Hg(0)) production) that lead to less exchangeable forms of Hg. Consequently, heavy isotopes are preferentially released during desorption. These observations agree with data from HgCl₂ contaminated sites. The secondary production of Hg(0) within the columns is confirmed by PTD analyses that indicate distinct Hg(0) release peaks in solid-phase samples at D) and distribution (K_D) coefficients are 77.9 ± 5.5 and 26.1 ± 3.0 mL g^-1 in EXP1, respectively, and 38.4 ± 2.7 and 12.4 ± 0.6 mL g^-1 in EXP2, respectively. These values are similar to values derived from column experiments on high OM soil and provide the basis for future Hg fate and transport modelling in soil-groundwater systems.

Sorption of mercury (Hg) in soils is suggested to be predominantly associated with organic matter (OM). However, there is a growing collection of research that suggests clay minerals and Fe/Mn-oxides are also important solid-phases for the sorption of soluble Hg in soil-groundwater systems. We use a series of (60 mL syringe based) column experiments to examine sorption and subsequent desorption of HgCl₂ solutions (Experiment 1 [EXP1]: 46.1 ± 1.1 mg L^-1; and Experiment 2 [EXP2]: 144 ± 6 mg L^-1) in low OM (0.16 ± 0.02 %) solid-phase aquifer materials. Analyses of total Hg concentrations, Hg speciation (i.e., pyrolytic thermal desorption (PTD)), and Hg stable isotopes are performed on both solid- and liquid-phase samples across sorption and desorption phases. Sorption breakthrough curve best fitted a Freundlich model. Despite the very low OM content, the Hg equilibrium sorptive capacity in these columns is very high: 1510 ± 100 and 2320 ± 60 mg kg^-1 for the EXP1 and EXP2, respectively, and is similar to those determined for high OM soils. Desorption fits exponential decay models and 46 ± 6 % and 58 ± 10 % of the sorbed Hg is removed from the solid-phase materials at the termination of desorption in EXP1 and EXP2, respectively. This desorption profile is linked to the initial release of easily exchangeable Hg(II) species physically sorbed to Fe/Mn-oxides and clay mineral surfaces and then slower release of Hg(II) species that have undergone secondary reaction to more stable/less soluble Hg(II) species and/or diffusion/transport into the mineral matrices. Hg stable isotope data support preferential sorption of lighter isotopes to the solid-phase materials with results indicating isotopically heavy liquid-phase and isotopically light solid-phase. The divergence of δ²⁰²Hg (describing mass dependent fractionation (MDF)) between liquid- and solid-phase continues into desorption and we attribute this to lighter isotopes being favoured in secondary processes occurring after initial sorption to the solid-phase materials (i.e., matrix diffusion, change in Hg(II) speciation, elemental Hg (Hg(0)) production) that lead to less exchangeable forms of Hg. Consequently, heavy isotopes are preferentially released during desorption. These observations agree with data from HgCl₂ contaminated sites. The secondary production of Hg(0) within the columns is confirmed by PTD analyses that indicate distinct Hg(0) release peaks in solid-phase samples at D) and distribution (K_D) coefficients are 77.9 ± 5.5 and 26.1 ± 3.0 mL g^-1 in EXP1, respectively, and 38.4 ± 2.7 and 12.4 ± 0.6 mL g^-1 in EXP2, respectively. These values are similar to values derived from column experiments on high OM soil and provide the basis for future Hg fate and transport modelling in soil-groundwater systems.

Sorption of mercury (Hg) in soils is suggested to be predominantly associated with organic matter (OM). However, there is a growing collection of research that suggests clay minerals and Fe/Mn-oxides are also important solid-phases for the sorption of soluble Hg in soil-groundwater systems. We use a series of (60 mL syringe based) column experiments to examine sorption and subsequent desorption of HgCl₂ solutions (Experiment 1 [EXP1]: 46.1 ± 1.1 mg L^-1; and Experiment 2 [EXP2]: 144 ± 6 mg L^-1) in low OM (0.16 ± 0.02 %) solid-phase aquifer materials. Analyses of total Hg concentrations, Hg speciation (i.e., pyrolytic thermal desorption (PTD)), and Hg stable isotopes are performed on both solid- and liquid-phase samples across sorption and desorption phases. Sorption breakthrough curve best fitted a Freundlich model. Despite the very low OM content, the Hg equilibrium sorptive capacity in these columns is very high: 1510 ± 100 and 2320 ± 60 mg kg^-1 for the EXP1 and EXP2, respectively, and is similar to those determined for high OM soils. Desorption fits exponential decay models and 46 ± 6 % and 58 ± 10 % of the sorbed Hg is removed from the solid-phase materials at the termination of desorption in EXP1 and EXP2, respectively. This desorption profile is linked to the initial release of easily exchangeable Hg(II) species physically sorbed to Fe/Mn-oxides and clay mineral surfaces and then slower release of Hg(II) species that have undergone secondary reaction to more stable/less soluble Hg(II) species and/or diffusion/transport into the mineral matrices. Hg stable isotope data support preferential sorption of lighter isotopes to the solid-phase materials with results indicating isotopically heavy liquid-phase and isotopically light solid-phase. The divergence of δ²⁰²Hg (describing mass dependent fractionation (MDF)) between liquid- and solid-phase continues into desorption and we attribute this to lighter isotopes being favoured in secondary processes occurring after initial sorption to the solid-phase materials (i.e., matrix diffusion, change in Hg(II) speciation, elemental Hg (Hg(0)) production) that lead to less exchangeable forms of Hg. Consequently, heavy isotopes are preferentially released during desorption. These observations agree with data from HgCl₂ contaminated sites. The secondary production of Hg(0) within the columns is confirmed by PTD analyses that indicate distinct Hg(0) release peaks in solid-phase samples at D) and distribution (K_D) coefficients are 77.9 ± 5.5 and 26.1 ± 3.0 mL g^-1 in EXP1, respectively, and 38.4 ± 2.7 and 12.4 ± 0.6 mL g^-1 in EXP2, respectively. These values are similar to values derived from column experiments on high OM soil and provide the basis for future Hg fate and transport modelling in soil-groundwater systems.

Sorption of mercury (Hg) in soils is suggested to be predominantly associated with organic matter (OM). However, there is a growing collection of research that suggests clay minerals and Fe/Mn-oxides are also important solid-phases for the sorption of soluble Hg in soil-groundwater systems. We use a series of (60 mL syringe based) column experiments to examine sorption and subsequent desorption of HgCl₂ solutions (Experiment 1 [EXP1]: 46.1 ± 1.1 mg L^-1; and Experiment 2 [EXP2]: 144 ± 6 mg L^-1) in low OM (0.16 ± 0.02 %) solid-phase aquifer materials. Analyses of total Hg concentrations, Hg speciation (i.e., pyrolytic thermal desorption (PTD)), and Hg stable isotopes are performed on both solid- and liquid-phase samples across sorption and desorption phases. Sorption breakthrough curve best fitted a Freundlich model. Despite the very low OM content, the Hg equilibrium sorptive capacity in these columns is very high: 1510 ± 100 and 2320 ± 60 mg kg^-1 for the EXP1 and EXP2, respectively, and is similar to those determined for high OM soils. Desorption fits exponential decay models and 46 ± 6 % and 58 ± 10 % of the sorbed Hg is removed from the solid-phase materials at the termination of desorption in EXP1 and EXP2, respectively. This desorption profile is linked to the initial release of easily exchangeable Hg(II) species physically sorbed to Fe/Mn-oxides and clay mineral surfaces and then slower release of Hg(II) species that have undergone secondary reaction to more stable/less soluble Hg(II) species and/or diffusion/transport into the mineral matrices. Hg stable isotope data support preferential sorption of lighter isotopes to the solid-phase materials with results indicating isotopically heavy liquid-phase and isotopically light solid-phase. The divergence of δ²⁰²Hg (describing mass dependent fractionation (MDF)) between liquid- and solid-phase continues into desorption and we attribute this to lighter isotopes being favoured in secondary processes occurring after initial sorption to the solid-phase materials (i.e., matrix diffusion, change in Hg(II) speciation, elemental Hg (Hg(0)) production) that lead to less exchangeable forms of Hg. Consequently, heavy isotopes are preferentially released during desorption. These observations agree with data from HgCl₂ contaminated sites. The secondary production of Hg(0) within the columns is confirmed by PTD analyses that indicate distinct Hg(0) release peaks in solid-phase samples at D) and distribution (K_D) coefficients are 77.9 ± 5.5 and 26.1 ± 3.0 mL g^-1 in EXP1, respectively, and 38.4 ± 2.7 and 12.4 ± 0.6 mL g^-1 in EXP2, respectively. These values are similar to values derived from column experiments on high OM soil and provide the basis for future Hg fate and transport modelling in soil-groundwater systems.