Understanding the cycling of critical oxyanion-forming trace metals is essential, considering their growing use in green technologies and their potential environmental risks. We investigated the source, transport, and fate of vanadium (V), molybdenum (Mo), and tungsten (W) across hemiboreal and boreal landscapes in Sweden, spanning 97 hemiboreal headwater streams, one boreal headwater stream, and two unregulated boreal rivers. Spatial and temporal variability was pronounced. V occurred at the highest concentrations, with hemiboreal headwaters averaging ∼78 μg/L compared to ∼8 μg/L for Mo and ∼0.2 μg/L for W. These concentrations exceeded those in boreal headwaters and rivers by factors of up to 300, reflecting enhanced weathering under milder hemiboreal conditions. Partitioning differed strongly among elements: Mo was predominantly dissolved, W occurred in both dissolved and particulate fractions, and V shifted from dominantly dissolved in hemiboreal streams to nearly equal distribution in boreal waters. Seasonal dynamics were most pronounced during spring snowmelt, when hydrological flushing and hyporheic exchange mobilized solutes from riparian soils and porewaters. Element-carrier relationships highlighted strong associations of V and W with Fe- and Al-rich phases, and Mo with organic matter, Si, and U, particularly at river mouths. Redox transitions in wetlands and hyporheic zones further mediated mobility, promoting V release and W transport while retaining Mo under reducing conditions.Our findings demonstrate that hydrology, redox dynamics, and landscape type jointly regulate the mobility and fate of V, Mo, and W from source to sea. The distinct behaviours of these elements highlight headwater streams as critical biogeochemical control points and underscore the need to integrate colloidal, redox, and hydrological processes into monitoring and modelling frameworks. These insights have implications for evaluating ecological risks and for critical raw material (CRM) exploration in northern landscapes under a changing climate.

During recent decades, much focus has been put on the iron (Fe) isotope ratios in soils, rivers, and oceans, while studies on the variation in headwater streams are scarce. Here we assess seasonal water chemical data from 104 hemiboreal headwater streams. Between summer and late autumn, decreasing Fe concentrations and simultaneously increasing sulfate and nitrate concentrations suggest a shift from reduced to oxidized conditions in the soils along the main groundwater flow paths. Fe isotope data, obtained from a subpopulation of 16 streams, show low δ56Fe ratios during summer drought, indicating an important influx of reduced groundwater to the streams with primarily Fe(II) as an important Fe source. In total, the δ56Fe data ranged between −0.8 ± 0.1 and 1.8 ± 0.1‰ with the lowest values in summer and maximum δ56Fe ratios in late autumn or spring, indicating an influx of more oxidized, less Fe(II) rich groundwater during those seasons. Local differences in δ56Fe ratios between the headwater streams, seemed to be driven by the different soil redox status of the catchments. The streams with the lowest δ56Fe ratios during summer are characterized by a small share (4.4 ± 6.6%) of wetlands, indicating discharge of reduced groundwater from mainly anoxic, moist, organic-rich mineral soils during drought. Relatively high total organic carbon (TOC) concentrations (2.4 ± 1.1 mM) and low pH (5.2 ± 0.8) may have restricted efficient Fe(II) oxidation in streamwater especially during the late autumn survey. Our results from hemiboreal headwater streams reveal the importance of climatic, pedogenic, and land cover-derived controls on the provenance of stream Fe loads that is likely broadly applicable to similar streams elsewhere.

The geochemistry of tungsten (W) in the environment is poorly studied. Tungsten usually occurs in low concentrations in natural waters and is not very mobile. For this study, we analyzed W together with molybdenum (Mo) in the dissolved and particulate fractions of two boreal estuaries during different seasons. Additionally, we sampled first-order streams that drain different landscape types and the receiving northern Baltic Sea. Furthermore, surface sediment from the estuaries was analyzed to obtain a comprehensive overview of the distribution of W and Mo in a boreal environment.

Both elements showed different distribution patterns during different seasons. While they decreased in the dissolved fraction during spring discharge, in winter, their concentrations were elevated. Molybdenum exhibited non-conservative behavior along the salinity gradient in winter, which was probably caused by its release from underlying sediments. In the particulate fraction, we found opposite behaviors for Mo and W, with higher particulate W and lower particulate Mo during spring discharge.

Molybdenum and W underwent fractionation from land to sea, indicating the different mobilities of these oxyanions. The Mo/W ratio in the dissolved fraction was mainly determined by the Mo concentration, as the W concentration varied only in a narrow range from first-order streams to the Bothnian Bay. In the particulate fraction, the Mo/W ratio appeared to be affected by scavenging processes and showed only small variations.

In natural waters, dissolved oxyanions often dominate over the particle-bound element fraction. Still, the scavenging of oxyanions by suspended particles might contribute significantly to their dynamic cycling and distribution. To investigate how oxyanions are affected by manganese (Mn) redox cycling, detailed depth profiles across the pelagic redox zone at the Landsort Deep, Baltic Sea, were collected for molybdenum (Mo), vanadium (V), and tungsten (W), for both dissolved (<0.22 µm) and suspended particulate (>0.22 µm) fractions.

All three oxyanions show a non-conservative behavior in the stratified Landsort Deep. Strong linear correlations with Mn in the particulate fraction in the redox zone of the Landsort Deep suggest that Mn redox cycling influences their distribution. In the dissolved fraction, Mo, V, and W exhibited rather different behavior. Molybdenum was depleted below the redox zone, while V was depleted only within the redox zone. Tungsten concentrations increased within the redox zone, being three times higher in the sulfidic zone than in the surface water. Unlike Mo, W shows no tendency for adsorption or co-precipitation under the prevailing weak sulfidic conditions in the deep water of the Landsort Deep and is, therefore, not exported to the underlying sediment.

The Landsort Deep data were compared with data from the northern Baltic Sea (Bothnian Bay, Kalix River and Råne River estuaries), where particulate iron (Fe) occurs in high abundance. The particulate fractions of Mo, V, and W decreased during mixing in these estuaries. Vanadium showed the most drastic reduction, with a decrease in dissolved and particulate fractions, indicating that different processes influence the distribution of these oxyanions.

The Landsort Deep in the Baltic Sea is a stratified basin with a pelagic redox zone separating the oxic water from the sulphidic water below. In this zone redox cycling of Mn plays an important role for the distribution of trace elements. Freshly formed Mn oxide acts as carrier for trace elements across the redox zone. In the underlying sulphidic zone the adsorbed trace elements are released when Mn is reduced and the Mn particles dissolve. Mn oxide has a negative surface charge at seawater pH conditions. Oxyanions would most likely adsorb on Fe oxides instead of Mn oxide due to the surface charge. The particulate Fe concentration in the redox zone of the Landsort Deep is 10-40 times lower than the concentration of particulate Mn. Therefore the Landsort Deep is an excellent place to study the role of Mn for cycling of oxyanions. This study shows uptake of Mo, V and W on Mn particles in the redox zone. Insights from this study may also be applicable for other oxyanion forming elements as P. The Landsort Deep in the Baltic Sea was sampled over a two-year period. Detailed profiles for the dissolved (<0.22 m) and particulate (>0.22 m) fraction were taken across the pelagic redox zone. While the pattern of dissolved Mn is constant during the sampling period maxima for particulate Mn vary in concentration and depth. Throughout the sampling period Mo, V and W are following Mn in the particulate fraction within the redox zone.

Holocene freshening has turned the Bothnian Bay, northern Baltic Sea into an oligotrophic basin. Sequestering of trace elements has changed significantly during the oligotrophication process. In principle, trace metals have been transferred from permanently buried sulfides to Fe–Mn-oxyhydroxides in the top layers of the sediment. The oxyhydroxide layers restrict the flux of trace metals from the sediment to the oxic bottom water. Hence, Fe–Mn cycling in the suboxic sediment enriches a number of trace metals in the surface sediment. Arsenic, Sn, Ge and Bi show enrichment in the Fe-oxyhydroxide layer, whereas Mo, Cd, Ni, Co, Cu, and Sb are enriched in the uppermost Mn-oxyhydroxide layer. This natural redox cycling in the sediment obscures pollution effects.The oligotrophication process started approximately 3500 years ago, reflected in decreasing deposition of Zn, a proxy for phytoplankton production, and formation of Mn oxyhydroxide layers. Similarly, Ba/Al data indicate a decrease in the pelagic input of plankton. Barium data also suggest that dissolved sulfide in the sediment never reached high concentrations. Germanium is closely related to Ba, suggesting that Ge can be used as a proxy for phytoplankton production. Vanadium, U, Re, and Mo all indicate that the bottom water never has been significantly sulfidic during the last 5500 years. Rhenium data indicate that the organic carbon oxidation rate has decreased during the last 5500 years. Cadmium follows the organic matter distribution, but started to increase 1000 YBP (years before present). The reason for this enhanced input of Cd is unclear.

Redox zones are defined by steep gradients of changing concentrations and changing redox potential. They form a transition zone between oxic and anoxic/sulfidic conditions, where nitrate, manganese and iron reduction occurs. These redox zones can be situated in the sediment as well as in the water column. In the Baltic Sea both types are found. In the Bothnian Bay, the northernmost part of the Baltic Sea, the water column is well oxygenated and the redox zone lies within the uppermost sediment (approximately 2 cm in extend). In the Baltic Proper several of the deeper basins are stratified with the redox zone hanging between 75 and 100 m in the water column reaching up to 20 m.The redox-sensitive trace metal manganese can be both electron donor and acceptor in redox zones depending on its oxidation state. Manganese is transformed between the dissolved Mn(II),(III) and the particulate Mn(III/IV). Hence, Mn plays an important role in trace metal cycling across redox zones in natural environments. Manganese particles serve as a carrier for adsorbed trace metals towards the anoxic/sulfidic zone in the water column and as a barrier in the sediment, which restricts dissolved trace elements from diffusing to the oxic zone.In this study water samples (dissolved fraction, <0.22 μm, and particulate fraction, >0.22 μm) from the pelagic redox zone in Landsort Deep, Baltic proper were analyzed. Furthermore, a sedimentary redox zone from the Bothnian Bay has been investigated. A Mn particle peak is detected within the pelagic redox zone at Landsort Deep. A strong correlation between these Mn particles and several oxyanions as Mo, V and W is observed. The oxyanions are adsorbed onto the freshly formed Mn particles in the redox zone, settle with the particles and are released when Mn particles are reduced to Mn2+ and dissolve. This mechanism can act as a pump for trace metals to the sulfidic zone, where the trace metals either can be enriched in the dissolved fraction or form sulfid particles.In the sediment Mn redox cycling leads to enrichment of trace metals in the top layer. The formation of a barrier of Mn-Fe hydroxides restricts trace metal exchange between bottom water and sediment. Freshening of the Bothnian Bay basin has led to an increased sequestering of trace metals in the uppermost sediment. Trace metal proxies show that primary production in the Bothnian Bay has decreased starting approximately 2500 years BP. That led to a shift in the deposition of sulfide forming elements mainly due to the lower input of reactive organic carbon from plankton and to the recent enrichment of elements together with Mn-Fe hydroxides.

Data visar att det är svårt att karakterisera och ta fram en ”typ-älv” för spårmetalltransportentill Bottenviken. Det är stora skillnader i transporten av spårmetaller mellan kustnära vattendrag,som dränerar sediment som tidigare avsatts i Bottenviken, skogsälvar och fjällälvar. Dessutom ärskillnaderna stora mellan älvar av samma typ. Trots att Kalixälv och Torneälv dränerar samma typav berggrund och lösa avlagringar är kemin olika för flera spårmetaller. Skillnaderna är i grundenberoende på vittringsförhållanden i dräneringsområdet och vattenföringen, men späds på av analyssvårigheteroch avsaknaden av filtrerade prov.I stort sett visar järn, krom, aluminium och totalt organiskt kol liknade mönster, även om de iblandinte är korrelerade i varje älv. Zink, kadmium, kobolt och nickel är korrelerade till mangan, meni flera älvar är inte sambandet uppenbart. Arsenik är linjärt korrelerat till järn söder om Piteälven.Halterna av järn är låga i utbyggda älvar, vilket indikerar att järntransporten minskar när en älvbyggs ut. Zinkvärdena i monitoringdata verkar vara påverkade av kontamination. De flesta kadmiumvärdenligger nära detektionsgränsen och överlag är kadmiumhalterna inte tillförlitliga.Övervakningsprogrammet bygger på ofiltrerade syrasatta vattenprov. Problemet med sådana provär att analysvärdena ”hoppar”. Syrasättningen löser inte upp bergartsfragment men löser järnmanganoxidhydroxidpartiklar.Partiklar påverkar med andra ord analysen på ett oregelbundet sätteftersom mängden, och typen a partiklar, varierar kraftigt speciellt vid höga flöden.Proven bör filtreras i fält, enklast genom sprutfiltrering. Filtren kan sparas för analys eller baralagras. Oberoende om filtren analyseras eller inte blir den lösta fraktionen bättre definierad än utanfiltrering. Filtrering förändrar inte nämnvärt provtagningskostnaden och analyskostnaden blir densamma (utan filteranalys). Filtrering bör införas omgående. För att kunna jämföra med tidigare årbör både ofiltrerad och filtrerad fas analyseras under ett eller två år.I vissa system bör även den partikulära fasen bestämmas. Detta är speciellt viktigt för att förståförändringar i järn och mangantransporten, eftersom dessa grundämnen styr flera spårmetaller ochfosfor. Brunifieringen av svenska vattendrag är tydlig i flera större och mindre vattendrag. Brunifieringenberor på förhöjda halter av bla. järn och mangan. För att förstå orsakerna till brunifieringmåste både filtrerade prov och partiklar analyseras.Markanvändning och dess betydelse för transporten av järn och mangan är en viktig forskningsfråga.Dels för att det finns gränsvärden uppsatta för säker mänsklig konsumtion, dels för attdessa grundämnen styr biotillgängligheten av fosfor i Östersjön, via en serie av komplicerade biogeokemiskaprocesser. Detaljerade kunskaper, på molekylnivå, om järn-mangan oxidhydroxideroch deras biogeokemi är viktig.Eftersom det är järn-mangan oxidhydroxider som till stor del reglerar utbytet mellan vatten ochsediment blir tillgången av dessa grundämnen central. Källorna för både järn och mangan är påland. Hanteringen av markfrågor kan med andra ord direkt påverka tillflödet av järn och mangan.Kopplingen mellan marktyp-markanvändning och utflödet av järn och mangan är idag litestuderat. Fosfortransporten i sötvatteninflödet är också starkt korrelerad till järnoxidhydroxider.Biotillgängligheten av den tillförda fosforn är lite studerat. Nya data antyder att fosforn inte ärdirekt biotillgänglig. Detta kan bero på ett komplext samspel mellan fosfor-järn-fotoreduktion, enkoppling som inte studerats i Östersjön.2Höga halter fosfor sitter bundet i ytsediment i Bottniska viken. En sänkning av redoxnivån skullefrigöra fosforn och systemet skulle snabbt bli eutroft. Tröskelvärdet för hur mycket näringsämnensom kan tillföras utan att förändra syreförhållandena är okänt. Bottenhavet och Bottenvikenär idag viktiga sänkor för fosfor. Hur stabila dessa sänkor är för miljöförändringar blir därför enviktig fråga. Eftersom Bottenhavet i stort sett saknar Mn-barriären så är denna bassäng troligenkänsligare för förändringar jämfört med Bottenviken.Kadmium visar en stigande trend i sedimentdata från Bottenviken. Inga andra metaller, förutombly, visar en sådan klar förändring. Orsaken till förändringen kan bero på landhöjningen (exponeringav sulfidleror på land) och oligotrofieringen av Bottenviken, men mekanismen är inte klarlagdoch bör studeras i detalj.