Hyporheic water exchange in a large hydropower-regulated boreal river – directions and rates

Widespread river regulation is known to modify river-aquifer interactions, influencing entire watersheds, but knowledge of the hyporheic flowpath along regulated rivers is limited. This study measured the hydraulic conductivity of the river bed and the aquifer, water levels and seepage fluxes in the heavily regulated Lule River in Northern Sweden, with the aim of characterising water exchange across the river-aquifer interface. While pristine rivers in the area are gaining, the Lule River was recharging the aquifer during 10% of the time. Daily river level fluctuations (typically ±0.25 m) directed ~3% of the total orthogonal flux across the river bed towards the aquifer, while during ~2% of the time the orthogonal fluxes were negligible (≤10–4 m d–1). A clogging layer on the river bed, most likely formed due to the modified river discharge, restricted river-aquifer exchange. The hyporheic zone had higher electrical conductivity than the river and the aquifer and electrical conductivity occasionally decreased following rising river water levels, with 3–5 hours' delay. Overall, hydropower regulation has severely altered the hydrological regime of the hyporheic zone in the Lule River.</jats:p

Potential biogeochemical and ecological development of a flooded tailings impoundment at the Kristineberg Zn-Cu mine, northern Sweden

The potential short-term (=102 years) and long-term (>102 years) biogeochemical and ecological effects of diverting stream water (pH 4.9-6.7) into a limed, flooded tailings impoundment (pH 8-12) were studied by combining geochemical and biological data. In the long-term perspective, the successional development of lakes was used as a natural analogue. Based on the vertical distribution of temperature and total dissolved solids (TDS<0.22 µm), the impoundment can be characterised as a continuous/discontinuous cold polymictic lake, with holomictic summer circulation. A re-inoculation study indicated that the growth of autotrophic, aerobic bacteria (presumably Acidithiobacillus ferrooxidans) presently is inhibited by the high pH in the impoundment. In a short-term perspective, termination of liming and diversion of stream water into the impoundment will result in a complex interplay between physical, biogeochemical and ecological effects. A reduced vertical mixing of the ~2-m-deep water column, dissolution of calcite and gypsum (compounds of a sludge formed in the impoundment) and an enhanced microbiological activity are major expected effects. The dissolution of calcite may act as a pH buffer and result in metal remobilisation from the sludge. Excluding autochthonous organic matter produced in the impoundment, streamwater input of suspended matter and formation of settling flocculants are expected to result in a sediment accumulation rate of ~1.5 mg cm-2 year-1 (1.6-3.3 cm/102 years). Settling allochthonous organic C (0.15-0.30 mg C cm-2 year -1) may serve as an oxygen barrier and as a reservoir of organic compounds capable of driving redox reactions. In a long-term perspective, a hydroseral development into a wetland/peatland can be expected, with a bog lake, poor fen or flat bog as final stage. This development presupposes a decreasing pH when liming is terminated and stream water is diverted into the impoundment. It also assumes that the impoundment will be similar to an acidified lake, and that the succession is driven by Sphagnum colonizing the impoundment. If the hydrological conditions/water level is affected (e.g., by climatic changes or a dam failure), a terrestrialization culminating in coniferous forest on peat soil may occur. © 2004 Elsevier B.V. All rights reserved.</p

Flow regulation effects on the hydrogeochemistry of the hyporheic zone in boreal rivers

River-aquifer interfaces are essential for ecosystem functioning in terms of nutrient exchange and biological habitat, but are greatly threatened world-wide. This study examined geochemical aspects of river-aquifer interaction in one regulated and one unregulated boreal river in Northern Sweden to determine whether the geochemical functioning of the hyporheic zone is affected by hydrological alterations, e.g. regulated river discharge and river-aquifer connectivity. In the unregulated Kalix River, the hyporheic pore water was well-oxygenated with orthogonal fluxes (≈0.6-0.7 m d-1) and acted as a sink for Fe, Mn, Al, NH4, and Ca, with fractional losses of 95%, 92%, 45%, 31%, and 15%, respectively. A corresponding elevation in the concentrations of these elements in the hyporheic sediment was observed, with higher saturation indices of Fe-, Mn-, and Al-bearing secondary minerals in hyporheic waters. In the regulated Lule River, hydraulic connectivity at the river-aquifer interface was altered by the presence of a clogging layer (0.04 m d–1). In addition, the river discharge oscillated daily, severely reducing exchange flows across the riverbed (<0.01 m d-1). As a result, the hyporheic pore water was suboxic, with elevated concentrations of filtered Fe and Mn (fractional increase of ≈3700% and ≈2500%, respectively) and other solutes (NH4, Si, S, Ca). A conceptual model revealed functional differences between geochemical features of the hyporheic zone of regulated and unregulated rivers. Overall, the results showed that hyporheic processes are altered along regulated rivers, with resulting impacts on the geochemistry of riverine, riparian and related marine ecosystems.</p

Nitrogen effluents from mine sites in northern Sweden - Environmental effects and removal of nitrogen in recipients

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High-resolution measurements of sulphur isotope variations in sedimentpore-waters by laser ablation multicollector inductively coupled plasma mass spectrometry

A novel combination of the technique of diffusive gradients in thin films (DGT) and laser ablation high-resolution multicollector inductively coupled plasma mass spectrometry was developed to study sulphur isotope variations of dissolved pore-water sulphide in freshwater and marine sediments. The technique enables two-dimensional mapping of isotopic variations (δ34S) in dissolved sulphide captured as solid Ag2S in DGT polyacrylamide gels. Measurements can be performed at a spatial resolution (~100 μm) relevant to microbiological processes and formation of individual iron sulphide grains in surface sediments. Values of δ34S measured in BaSO4–DGT gel isotope standards (δ34S=9.28±0.36‰ to 9.33±0.57‰) are within 1‰ of the accepted value determined with conventional analytical techniques (δ34S=10.13±0.29‰). Sulphur isotope measurements were performed in sediments from a eutrophic lake (Esthwaite Water, UK) contained in laboratory mesocosms. Bacterial sulphate reduction and sulphide formation in this sediment are predominantly localized to discrete, mm-sized microniches, where oxidation of labile organic matter such as fresh algae and faecal pellets drives the reduction of sulphate. The results emphasize the importance of microniches as localized, highly dynamic reaction sites in sediments, where significant shifts in δ34S of up to +20‰ relative to the local background were measured across microniches. The improved spatial resolution for pore-water sulphur isotope measurements, compared to that of conventional sampling and analytical techniques, is essential for improving our understanding of the global biogeochemical cycling of sulphur as well as trace metal–sulphide interactions in modern sediments

External carbon addition for enhancing denitrification modifies bacterial community composition and affects CH₄ and N₂O production in sub-arctic mining pond sediments.

Explosives used in mining operations release reactive nitrogen (N) that discharge into surrounding waters. Existing pond systems at mine sites could be used for N removal through denitrification and we investigated capacity in tailings and clarification pond sediments at an iron-ore mine site. Despite differences in microbial community structure in the two ponds, the potential denitrification rates were similar, although carbon limited. Therefore, a microcosm experiment in which we amended sediment from the clarification pond with acetate, cellulose or green algae as possible carbon sources was conducted during 10 weeks under denitrifying conditions. Algae and acetate treatments showed efficient nitrate removal and increased potential denitrification rates, whereas cellulose was not different from the control. Denitrifiers were overall more abundant than bacteria performing dissimilatory nitrate reduction to ammonium (DNRA) or anaerobic ammonium oxidation, although DNRA bacteria increased in the algae treatment and this coincided with accumulation of ammonium. The algae addition also caused higher emissions of methane (CH &lt;sub&gt;4&lt;/sub&gt; ) and nitrous oxide (N &lt;sub&gt;2&lt;/sub&gt; O). The bacterial community in this treatment had a large proportion of Bacteroidia, sulfate reducing taxa and bacteria known as fermenters. Functional gene abundances indicated an imbalance between organisms that produce N &lt;sub&gt;2&lt;/sub&gt; O in relation to those that can reduce it, with the algae treatment showing the lowest relative capacity for N &lt;sub&gt;2&lt;/sub&gt; O reduction. These findings show that pond sediments have the potential to contribute to mitigating nitrate levels in water from mining industry, but it is important to consider the type of carbon supply as it affects the community composition, which in turn can lead to unwanted processes and increased greenhouse gas emissions

Early diagenesis and isotopic composition of lead in Lake Laisan, northern Sweden

Water column (dissolved/suspended phase, sediment traps) and sediment data (pore-water, solid-phase sediment) were combined with stable Pb and Pb-210 isotope data to trace the early diagenetic behaviour and geochemical cycling of Pb in Lake Laisan, a lake which has received large quantities of anthropogenic Pb since the early 1940s. Early diagenetic remobilisation of Pb is indicated by a subsurface pore-water Pb maximum (120 mug 1(-1)) in the oxic surface layer of the sediment, where the solid-phase Pb concentration is 3400-4600 mug g(-1). The remobilisation of Pb appears to be caused by a pH-controlled desorption of Pb from solid-phase sediment, which is consistent with a model describing surface complexation of Pb(II) on hydrous goethite surfaces. The diffusive Pb flux from the subsurface pore-water maximum towards the sediment surface (36 mug cm(-2) year(-1)) exceeds the depositional Pb flux (8.6 mug cm(-2) year(-1)) by approximately a factor of four, indicating that Pb is highly mobile in the sediment. Stable Pb isotope data and a mass balance calculation suggest that Pb diffusing upwards is, to a large extent, trapped in the surface sediment. Lead that may diffuse into the slightly alkaline lake water appears to be efficiently sorbed to suspended particulate matter, resulting in low dissolved Pb concentrations in the water column (0.040-0.046 mug 1(-1)). Sorption of Pb to suspended particulate matter is consistent with the elevated suspended particulate Pb concentrations in the hypolimnion (3800-4000 mug g(-1)), and the fact that the stable Pb isotopic compositions of suspended matter and pore-water are similar

Tracing anthropogenic sources of Tantalum and Niobium in Bothnian Bay sediments, Sweden

Abstract Purpose This study aims to evaluate temporal trends of Tantalum (Ta) and Niobium (Nb) concentrations in northern Baltic Sea sediments with focus on the potential impact from the Rönnskär sulfide ore smelter, located 15 km east of Skellefteå in northern Sweden. The potential of Ta or Nb to serve as tracers for environmental pollution caused by the electronic waste is compared. Lastly, correlations between Ta and Nb concentrations and those of major redox elements are investigated. Methods A 35-cm-deep core was collected in the harbor bay (Kallholmsfjärden) outside of the smelter. A secondary 6-m-deep sediment core was collected in the deep Bothnian Bay for comparative purposes. Element screening analysis was conducted the using ICP-SFMS for the Kallhomsfjärden core and a combination of ICP-SFMS and ICP-OES for the Bothnian Bay core. In the final analysis, a 5-step sequential extraction technique was preformed to allow for better prediction of the fate and mobility of Ta and Nb. Results and discussion In the vicinity of the smelter, Ta concentration increases from 0.42 to 3.8 ppm from the time coinciding with the beginning of electronic waste processing. Conversely, Nb concentration remained stable at background levels throughout the core at 6.33 ± 0.78 ppm. The Nb/Ta ratio thus changed from 14.5 to 1.7, reflecting an increase of anthropogenic input of Ta into the bay sediments. In the pre-industrial part of the sediment, concentrations of both elements follow concentration of aluminosilicates in the core. In recent sediments, however, the anthropogenic Ta exhibits an association with hydrous ferric oxides (HFOs) and organic matter. Conclusions Notable increases in Ta concentrations began following the beginning of scrap metal and electronic waste processing at the Rönnskär smelter. Anthropogenic Ta introduced in the upper portion of the sediment and are more associated with HFOs and organic material than natural Ta seen in the deeper parts of the core where detrital Ta is most common. Niobium was not affected by the Rönnskär smelter and displayed no notable change to the preindustrial background. </jats:sec