River Chemistry Research Articles

The immediate environmental effects of tephra emission

The Earth’s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere. These tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents. The interaction of tephra with or in receiving environments may induce an array of physical, chemical and biological effects. The consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent, small eruptions, while larger and rarer events may have acute, regional-scale impacts. It is, therefore, necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems. We collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post-eruptive effects of tephra emission into the atmosphere, onto vegetation, soil or ice/snow surfaces and in aquatic systems. We further consider the repercussions that may be induced in the weeks to years afterwards. In the atmosphere, tephra can influence cloud properties and air chemistry by acting as ice nuclei (IN) or by offering sites for heterogeneous reactions, respectively. Tephra on vegetation causes physical damage, and sustained coverage may elicit longer-term physiological responses. Tephra deposits on soils may alter their capacity to exchange gas, water and heat with the atmosphere or may have a specific chemical effect, such as nutrient input or acidification, on sensitive soils. Tephra deposition onto snow or ice may affect ablation rates. Rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged (months or years) erosion from the tephra-covered catchment. In the first weeks after deposition, tephra leaching may affect river chemistry. The abundance and speciation of phytoplankton populations in lakes may be altered by tephra-induced changes in water chemistry or sediment–water nutrient cycling. In the oceans, tephra deposition may fertilise Fe-limited waters, with potential impacts on the global carbon cycle. Embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies, combined with consideration of the interconnectivity of induced impacts within and between different receiving environments.

Estimating groundwater discharge to rivers from river chemistry surveys

AbstractEnvironmental tracer methods have been used to quantify groundwater discharge to rivers for the past few decades. A number of different tracers have been used in these studies, including individual ion concentrations, electrical conductivity, stable isotopes 2H and 18O, and the dissolved gases helium, chlorofluorocarbons and radon. This paper discusses the assumptions of the method, as well as its resolution and accuracy. The method will be most accurate when the tracer concentration in groundwater is very distinct from that in the river. On the basis of typical parameters, groundwater inflow rates as low as 5 mm/day can usually be estimated with electrical conductivity and ion tracers. A lower limit of resolution of approximately 2 mm/day is usually possible with radon, principally because the ratio of the river concentration to the groundwater concentration will be higher. However, hyporheic exchange can also contribute radon to the river. Where this process is significant, it is more difficult to estimate groundwater inflow from radon activities in the river, thus reducing the accuracy of the method. For CFCs, the lower limit of resolution is approximately 30 mm/day. Helium has not been widely used but can potentially be very accurate if the groundwater is old.The method assumes steady‐state conditions and so can only be applied when river flows are stable. Sampling resolution is also particularly important for dissolved gases, and uncertainty in where groundwater inflow occurs between sampling points can cause large uncertainty in inflow rates if the distance between sample locations is large. Poor mixing of solutes within the river can limit the method if the river is wide and shallow. When correctly applied, however, the environmental tracer method is able to provide robust estimates of groundwater discharge at a scale and accuracy that is not possible with most other methods. Copyright © 2012 John Wiley & Sons, Ltd.

Rock denudation rates and organic carbon exports along a latitudinal gradient in the Hudson, James, and Ungava bays watershed

This study documents chemical denudation rates (CDR) in the Canadian Shield and Interior Platform. It focuses on the dissolved chemistry of rivers flowing into the Hudson, James, and Ungava bays (HJUB). Major ions, strontium, neodymium, and dissolved organic carbon (DOC) concentrations were monitored in four rivers (Koksoak, Great Whale, La Grande, and Nelson). Six other rivers flowing into the HJUB were sampled during baseflow and snowmelt conditions. The rivers of the Canadian Shield exhibit major cation concentrations ranging between 62 and 360 µmol/L, [Nd] of 0.57–4.72 nmol/L, and variable [DOC] of 241–1777 µmol/L. In comparison, the Nelson River (Interior Platform) shows higher major cation concentrations (1200–2276 µmol/L), lower [Nd] (0.14–0.45 nmol/L), and intermediate [DOC] (753–928 µmol/L). Altogether, the studied rivers export 8 × 106 t/year of dissolved major cations and 50 t/year of dissolved Nd towards the HJUB. Basin-scale rock denudation rates (RDR) range from 1.0 to 5.3 t·km–2·year–1 and are essentially controlled by lithology, as illustrated by the relationship established between RDR and the proportion of sedimentary and volcanic rocks (%S + %V) within the basins: RDR = 0.08(%S + %V) + 0.9. In contrast, dissolved Nd exports (and likely other insoluble elements) seem to be dependent upon organic matter leaching, as illustrated by the empirical coupling between Nd and DOC exports. These fluxes decrease northwards, likely in response to the hydroclimatic gradient. The CDR evaluated within the Canadian Shield are among the lowest on the planet, and the alkalinity generated by rock weathering remains small with respect to DOC exports.

Seasonal sensitivity of weathering processes: Hints from magnesium isotopes in a glacial stream

Seasonal changes in river chemistry offer the potential to assess how weathering processes respond to changing meteorological parameters and ultimately how chemical weathering might respond to climatic parameters. Systematic seasonal variations in magnesium isotope ratios (the 26Mg/24Mg ratio expressed as δ26Mg in per mil units) are reported in stream waters from a mono-lithological granitic, weathering-limited, first order catchment from the Swiss Alps (Damma glacier). Rain, ground, and pore-waters, in addition to plants, rocks, mineral separates and soil are also reported. The concentration response of the river waters is attenuated compared to the large changes in discharge. However, the systematic trends in the isotope data imply that either the source of the Mg changes in a systematic manner, or that the process by which Mg is released into solution changes as a function of discharge. The two first order observations in the data that need to be explained are 1) the systematic enrichment in 24Mg in the stream waters compared to the granitic rocks they drain and 2) a systematic increase in δ26Mg in the waters during the summer melt season. Both observations (which are similar to many other rivers draining silicate rock) can either be accounted for by 1) conservative mixing between at least two different sources of Mg (in addition to precipitation inputs), or 2) process related fractionation. If the stream water compositions can be rationalised by multi-component mixing, there is at least one unidentified component with a δ26Mg < − 1.2‰. This is considered unlikely. Multiple physicochemical processes could fractionate Mg isotope ratios such as 1) preferential leaching of 24Mg, 2) exchange of Mg onto (or from) mineral surfaces and into interlayer sites of clays, 3) uptake by plants, and 4) 26Mg could be preferentially retained during the formation of secondary phases, such as clays, amorphous phases or oxides. These processes are not mutually exclusive and distinguishing between them at a field scale is not trivial, but significant biological uptake is improbable at this site. Unless there is a non-identified external input of Mg, 26Mg must be accumulating in solid phase residues in the catchment because of at least one physicochemical process. Such processes are likely well described, at least in the first order by a Rayleigh distillation model. Simple calculations illustrate how much 26Mg would accumulate in the catchment per unit time. In the first order, the isotopic enrichments in the solids are so small that they would not be detectable for the time-scales that are relevant to this field site, in spite of the marked impact on the water chemistry. The seasonal signal detected by Mg isotope ratios is promising for using them (with a better understanding of fractionation mechanisms) to quantify how specific weathering processes impact upon both export fluxes, and retention of elements within catchments.

The role of fluid residence time and topographic scales in determining chemical fluxes from landscapes

The role of fluid residence time and catchment length scales in controlling the chemical composition of rivers is evaluated by comparing numerical simulations and scaling arguments to concentration–discharge data from small catchments. The analysis suggests that poorly-crystalline aluminosilicates are an important control on the composition of stream waters and therefore chemical equilibrium between the dissolving and precipitating phases determines the maximum concentration and the maximum silicate weathering flux. The modeling results suggest that the residence time of fluid relative to the residence time required to approach chemical equilibrium can be used to assess the controls on solute fluxes in small catchments, and possibly larger rivers. Catchments that show little variability in concentration with discharge (or “chemostatic behavior”) likely have average fluid residence times that exceed the time required to reach chemical equilibrium. Conversely, decreases in concentration with increasing discharge are explained by average residence times shorter than required to approach chemical equilibrium, resulting in dilution. Solute fluxes are also strongly impacted by the distribution of fluid residence times in a basin. The fluid residence time model provides an alternative framework for assessing both the relationship between discharge and concentration observed for individual catchments, and controls on the solute fluxes of rivers. If fluid residence times are a dominant control on weathering fluxes, the chemistry of different rivers could vary entirely as a function of the nature subsurface flow paths and the composition of the system at equilibrium, which is complex to predict and strongly coupled to biological processes, temperature and the composition of the system. As a result of thermodynamic and hydrologic restrictions on the amount of weathering, global solute fluxes may depend more strongly on the geometry, relief, runoff and permeability of basins then on temperature and rates of erosion.

Paleozoic multiple accretionary and collisional processes of the Beishan orogenic collage

The dissolved chemistry of rivers has been extensively studied to elucidate physical and climatic controls of chemical weathering at local to global spatial scales, as well as the impacts of chemical weathering on climate over short to geologic temporal scales. Within this effort, mixing models with Monte Carlo uncertainty propagation are a common tool for inverting measurements of dissolved river chemistry to distinguish among contributions from end-members with distinct elemental and/or isotopic compositions. However, the methods underlying prior river inversion models have typically been opaque. Here we present Mixing Elements ANd Dissolved Isotopes in Rivers (MEANDIR), a set of MATLAB scripts that enable highly customizable inversion of dissolved river chemistry with Monte Carlo propagation of uncertainty. First, we present an overview of the mathematics underlying MEANDIR. This overview includes, among other topics, derivation of equations for mass balance, implementation of chlorine critical values, construction of cost functions, normalization to the sum of dissolved variables, quantification of river sulfate sourced from pyrite oxidation, resolution of petrogenic organic carbon oxidation, representation of secondary phase formation with isotopic fractionation, and calculation of the impact of weathering on atmospheric carbon dioxide. Second, we apply MEANDIR to five previously published datasets to demonstrate the sensitivity of results to parameter choices. We invert data from two global compilations of river chemistry (Gaillardet and others, 1999; Burke and others, 2018), the major element chemistry and sulfate sulfur isotope ratios of rivers in the Peruvian Amazon (Torres and others, 2016), the major element chemistry of Icelandic rivers (Gíslason and others, 1996), and the major and trace element chemistry of water samples from the Mackenzie River (Horan and others, 2019). MEANDIR and its user guide are freely available online.

Movement of manganese contamination through the Critical Zone

Humans have transferred large quantities of metals from the lithosphere to the Earth’s surface, drastically altering the natural flow of these elements. The geographic dispersal of many metals and their impacts on the environment are unknown. Here, existing datasets are compiled to assess how anthropogenic inputs of Mn to the air have altered soil and water chemistry over time. Although levels of Mn in the air have declined in recent decades, soils throughout the USA and Europe are enriched in Mn, revealing past contamination near zones of industrial input. Examination of river chemistry indicates a similar decline in Mn and can be used to evaluate the removal of Mn from soils. We use a small watershed, the Susquehanna/Shale Hills Critical Zone Observatory, as a focus site to investigate geochemical mass balance models and find that rapid biocycling contributes to the retention of Mn in this affected ecosystem.

Characteristics and driving factors of surface water chemistry of Wujiang watershed

The changes in the water chemistry of rivers can reflect influence of anthropogenic activities on the water environment to some extent. To understand the relationship between the spatial distribution of the eco-environment of the watershed and the characteristics of water chemistry and geochemistry of rivers, firstly, the digital Wujiang watershed was built, and then the sub-watersheds were delineated, taking the sample points as sub-watershed outlets based on GIS. Secondly, using the function of spatial analyst of GIS, the statistical features of eco-environment (such as lithology and land use/cover) of each sub-watershed were calculated according to their respective classification. Finally, the correlation between the spatial distribution of lithology of the sub-watersheds and their corresponding 87Sr/86Sr ratio of river water, the correlation between NO3−/HCO3−, Cl−/HCO3−, SO42−/HCO3− and anthropogenic activities, respectively, and the correlation between the fraction of green vegetation of the sub-watershed and their corresponding flux of TDS (total dissolved solids) were analyzed quantitatively. The results justify that the 87Sr/86Sr ratio of river water is highly dependent on the lithologic feature of the watershed and indicate that anthropogenic activities are one of the main sources of NO3− and SO42− of river waters. The output of TDS is highly dependent on the percentage of vegetation cover of the watershed.

Contribution of deep groundwater to the weathering budget in a rapidly eroding mountain belt, Taiwan

We analysed the water chemistry of a 6-yr time series of the Liwu River, Taiwan, as well as spot samples of tributaries and groundwater. In this steep and well-drained catchment impacted by typhoon events, the river carries a high dissolved load (average of 320 mg/L) dominated by Ca 2+ and Mg 2+ (73 Eq% and 21 Eq% of the cationic charge), and SO 4 2− and HCO 3 − (48 Eq% and 51 Eq% of the anionic charge). Hysteresis in Ca/Na-discharge data and absence of hysteresis in matching Ca/Sr-discharge data are interpreted as reflecting a mixing of three components. While river chemistry at low- to medium-flow reveals a binary mixing between a rapid surface runoff characterised by the chemistry of first-order tributaries and a component likely to have a deep groundwater chemistry, samples collected at high-flow show the contribution of a third water source not immediately mobile and therefore called slow surface runoff. The relative contribution of these end-members has been estimated through hydrograph separations, and then validated by modelling of the observed riverine 87Sr/ 86Sr variations (0.0013) during a typhoon. The 37-yr average estimates that 16 ± 3% of the river discharge comes from the deep groundwater reservoir while slow and rapid surface runoff provide 21 ± 5% and 63 ± 5%, respectively. The slow surface runoff is the least well characterised of the end-members but the modelled evolution of water chemistry along its flow-path suggests that cations (except Na) and sulphate are removed from the water in the slow surface runoff source region by the combination of biological uptake, bacterial reduction and pedogenesis (clays and carbonate precipitation) — processes that might be partly driven by the evapo-transpiration that removes 75% of the water within this reservoir. The silicate cation denudation rate for the Liwu is 18 t km − 2 yr − 1 , which is one of highest measured so far in the world for felsic lithologies. This high value has to be modulated by the facts that 1) deep chemical weathering of silicate accounts for more than 1/3 of this flux, and 2) sulphuric-acid weathering is significant in this catchment. In Taiwan, the input of chemicals in river flux from deep reservoirs is likely to be promoted by rapid uplift of fractured rocks, but the underlying processes (chemical reaction in a porous media) responsible for such significant groundwater contribution are also present in alluvial and sedimentary basins.

Spatial distribution of dissolved constituents in Icelandic river waters

Summary In this study we map the spatial distribution of selected dissolved constituents in Icelandic river waters using GIS methods to study and interpret the connection between river chemistry, bedrock, hydrology, vegetation and aquatic ecology. Five parameters were selected: alkalinity, SiO2, Mo, F and the dissolved inorganic nitrogen and dissolved inorganic phosphorus mole ratio (DIN/DIP). The highest concentrations were found in rivers draining young rocks within the volcanic rift zone and especially those draining active central volcanoes. However, several catchments on the margins of the rift zone also had high values for these parameters, due to geothermal influence or wetlands within their catchment area. The DIN/DIP mole ratio was higher than 16 in rivers draining old rocks, but lowest in rivers within the volcanic rift zone. Thus primary production in the rivers is limited by fixed dissolved nitrogen within the rift zone, but dissolved phosphorus in the old Tertiary catchments. Nitrogen fixation within the rift zone can be enhanced by high dissolved molybdenum concentrations in the vicinity of volcanoes. The river catchments in this study were subdivided into several hydrological categories. Importantly, the variation in the hydrology of the catchments cannot alone explain the variation in dissolved constituents. The presence or absence of central volcanoes, young reactive rocks, geothermal systems and wetlands is important for the chemistry of the river waters. We used too many categories within several of the river catchments to be able to determine a statistically significant connection between the chemistry of the river waters and the hydrological categories. More data are needed from rivers draining one single hydrological category. The spatial dissolved constituent distribution clearly revealed the difference between the two extremes, the young rocks of the volcanic rift zone and the old Tertiary terrain.

River solute fluxes reflecting active hydrothermal chemical weathering of the Yellowstone Plateau Volcanic Field, USA

In the past few decades numerous studies have quantified the load of dissolved solids in large rivers to determine chemical weathering rates in orogenic belts and volcanic areas, mainly motivated by the notion that over timescales greater than ~ 100 kyr, silicate hydrolysis may be the dominant sink for atmospheric CO 2, thus creating a feedback between climate and weathering. Here, we report the results of a detailed study during water year 2007 (October 1, 2006 to September 30, 2007) in the major rivers of the Yellowstone Plateau Volcanic Field (YPVF) which hosts Earth's largest “restless” caldera and over 10,000 thermal features. The chemical compositions of rivers that drain thermal areas in the YPVF differ significantly from the compositions of rivers that drain non-thermal areas. There are large seasonal variations in river chemistry and solute flux, which increases with increasing water discharge. The river chemistry and discharge data collected periodically over an entire year allow us to constrain the annual solute fluxes and to distinguish between low-temperature weathering and hydrothermal flux components. The TDS flux from Yellowstone Caldera in water year 2007 was 93 t/km 2/year. Extensive magma degassing and hydrothermal interaction with rocks accounts for at least 82% of this TDS flux, 83% of the cation flux and 72% of the HCO 3 − flux. The low-temperature chemical weathering rate (17 t/km 2/year), calculated on the assumption that all the Cl − is of thermal origin, could include a component from low-temperature hydrolysis reactions induced by CO 2 ascending from depth rather than by atmospheric CO 2. Although this uncertainty remains, the calculated low-temperature weathering rate of the young rhyolitic rocks in the Yellowstone Caldera is comparable to the world average of large watersheds that drain also more soluble carbonates and evaporates but is slightly lower than calculated rates in other, less-silicic volcanic regions. Long-term average fluxes at Yellowstone are likely ~ 20% higher than those in the abnormally dry water year 2007, but the protocol used in this study can be easily adaptable to track future changes in low-temperature weathering and hydrothermal flux components, which could provide better monitoring of magmatic unrest.

Water chemistry and the distribution of Podostemaceae in Ghana

The physico-chemical parameters of several rivers in southern Ghana were determined during the dry season. Other environmental features, e.g. canopy cover, rock substrate type, etc. of each site sampled were also recorded. Four Podostemaceae species were recorded in 10 of the rivers sampled. The distribution of the Podostemaceae appeared to relate to availability of rock substrate, open canopy and some level of plant nutrients, e.g. phosphates and nitrates in the water. Further data must be collected at the sites where Podostemaceae grow in the tropics, particularly in Africa, to enable a better understanding of the contribution of river chemistry and other environmental parameters to the distribution of the Podostemaceae.

Solute dynamics during bank storage flows and implications for chemical base flow separation

Chemical base flow separation is a widely applied technique in which contributions of groundwater and surface runoff to streamflow are estimated based on the chemical composition of stream water and the two end‐members. This method relies on the assumption that the groundwater end‐member can be accurately defined and remains constant. We simulate solute transport within the aquifer during and after single and multiple river flow events, to show that (1) water adjacent to the river will have a concentration intermediate between that of the river and that of regional groundwater and (2) the concentration of groundwater discharge will approach that of regional groundwater after a flow event but may take many months or years before it reaches it. In applying chemical base flow separation, if the concentration in the river prior to a flow event is used to represent the pre‐event or groundwater end‐member, then the groundwater contribution to streamflow will be overestimated. Alternatively, if the concentration of regional groundwater a sufficient distance from the river is used, then the pre‐event contribution to streamflow will be underestimated. Changes in concentration of groundwater discharge following changes in river stage predicted by a simple model of stream‐aquifer flows show remarkable similarity to changes in river chemistry measured over a 9 month period in the Cockburn River, southeast Australia. If the regional groundwater value was used as the groundwater end‐member, chemical base flow separation techniques would attribute 8% of streamflow to groundwater, as opposed to 25% if the maximum stream flow value was used.

Impact of solid waste disposal on nutrient dynamics in a sandy catchment

Groundwaters impacted by mature landfill leachate are generally enriched in ammonium. In order to assess the dynamics of ammonium exchanges between leachates and the water system inside a sandy permeable catchment we measured ammonium, nitrate and chloride concentrations in the stream and in sediment pore waters of the streambed of a landfill impacted aquifer. Geophysical investigation methods complemented the biogeochemical survey. The studied zone is a 23 km² catchment located in a coastal lagoon area sensitive to eutrophication risk. Ammonium concentrations in the river were up to 800 µmol l − 1 during low water period in summer. Three surveys of the river chemistry showed a regular increase in ammonium, nitrate and chloride concentrations along a 1 km section of the watercourse, downstream the landfill, implying that the leachate plume exfiltrates along this section. Sediment cores collected within this section showed all an increase in ammonium concentrations with depth in pore waters as a consequence of the landfill leachate dispersion, as attested by a simultaneous increase in chloride concentrations. Nitrate enrichment in the river water was due to nitrification of ammonium at the interface between groundwater and streamwater. The apparent nitrification rate obtained was within values reported for turbid estuaries, although the river contained very little suspended particulate matter. Actually, pore water chemistry suggests that nitrification occurred for the most part in subsurface permeable sediments, rather than in stream water. The overall topographic, hydrological, geochemical, and geoelectrical data set permit to estimate the extension of the chloride and ammonium plume. The estimation of the apparent ammonium plume velocity is 23 m year − 1 whereas the chloride plume velocity should be 50 m year − 1. The river is the outlet of the impacted groundwaters. Considering that the input of ammonium from the landfill is balanced by the present day output via the river, the residence time of ammonium in the aquifer is between 7 and 18 years.

Ferrolysis induced soil transformation by natural drainage in Vertisols of sub-humid South India

In sub-humid South India, recent studies have shown that black soil areas (Vertisols and vertic Intergrades), located on flat valley bottoms, have been rejuvenated through the incision of streambeds, inducing changes in the pedoclimate and soil transformation. Joint pedological, geochemical and geophysical investigations were performed in order to better understand the ongoing processes and their contribution to the chemistry of local rivers. The seasonal rainfall causes cycles of oxidation and reduction in a perched watertable at the base of the black soil, while the reduced solutions are exported through a loamy sand network. This framework favours a ferrolysis process, which causes low base saturation and protonation of clay, leading to the weathering of 2:1 then 1:1 clay minerals. Maximum weathering conditions occur at the very end of the wet season, just before disappearance of the perched watertable. Therefore, the by-products of soil transformation are partially drained off and calcareous nodules, then further downslope, amorphous silica precipitate upon soil dehydration. The ferrolysed area is fringing the drainage system indicating that its development has been induced by the streambed incision. The distribution of 14C ages of CaCO 3 nodules suggests that the ferrolysis process started during the late Holocene, only about 2 kyr B.P. at the studied site and about 5 kyr B.P. at the watershed outlet. The results of this study are applied to an assessment of the physical erosion rate (4.8 × 10 − 3 m/kyr) since the recent reactivation of the erosion process.

Chemical erosion rates of river basins of the Ganga system in the Himalaya: Reanalysis based on inversion of dissolved major ions, Sr, and 87Sr/86Sr

Available data on chemical and Sr isotopic composition of the headwaters of the Ganga and its tributaries in the Himalaya are analyzed using inverse and the forward models to quantify the silicate and carbonate erosion rates in their basins and to parameterize the controlling factors of contemporary chemical erosion. The estimates of silicate‐derived cations based on the inverse model are lower on average by ∼20% than that derived from the forward model, a result attributable to differences in the release ratios of major ions between the two models. The inverse model yields the best fit values for the release ratio of various major ions based on measured river chemistry and therefore is more likely to be representative of the basins, unlike the forward model, in which the release ratios are assumed. Among the various tributaries, the Kosi and the Bhagirathi have relatively larger contributions from silicate weathering to the cation budget with high 87Sr/86Sr, consistent with the lithologies of their basins, which are dominated by silicates. The Ganga river system in the Himalaya is known to be supersaturated with respect to calcite during lean flow periods. The inverse model calculates the extent of Ca loss to be between ∼10%–40% with the highest for the Gandak. The silicate and carbonate erosion rates for these Himalayan rivers are relatively higher, in the ranges of 9.9–17.6 and 12.4–92.3 tons/km2/yr, respectively, compared to other global rivers. The higher chemical erosion rates in these river basins could be due to intense physical erosion caused by their higher relief and runoff.

Factors influencing the acid–base (pH) balance in the Baltic Sea: a sensitivity analysis

Using calculations based on the marine carbon system and on modelling, the sensitivity of Baltic Sea surface pH was examined. Transient long-term calculations demonstrated that the marine carbon system adjusts to lateral boundary conditions within some decades, as does salinity. Climate changes in temperature or salinity will only marginally affect the acid–base (pH) balance. Wetter or dryer climate will also play a minor role in the pH balance. The direct effect on seawater pH of acid precipitation over the Baltic Sea surface was demonstrated to be small. Acidification due to river transport of dissolved organic carbon (DOC) into the marine system seems marginal although mineralization of terrestrial DOC may cause extra marine acidification, but the effect has yet to be quantified. Increased nutrient load may increase the amplitude in the pH seasonal cycle and increase the acidification during winter time. Fossil fuel burning is likely to have both a direct and indirect effect through increased CO2 levels, altering seawater pH as well as changing the river chemistry. This may severely threaten some species in the Baltic Sea, particularly in the Northern Baltic.

Decreasing boron concentrations in UK rivers: Insights into reductions in detergent formulations since the 1990s and within-catchment storage issues

The changing patterns of riverine boron concentration are examined for the Thames catchment in southern/southeastern England using data from 1997 to 2007. Boron concentrations are related to an independent marker for sewage effluent, sodium. The results show that boron concentrations in the main river channels have declined with time especially under baseflow conditions when sewage effluent dilution potential is at its lowest. While boron concentrations have reduced, especially under low-flow conditions, this does not fully translate to a corresponding reduction in boron flux and it seems that the “within-catchment” supplies of boron to the river are contaminated by urban sources. The estimated boron reduction in the effluent input to the river based on the changes in river chemistry is typically around 60% and this figure matches with an initial survey of more limited data for the industrial north of England. Data for effluent concentrations at eight sewage treatment works within the Kennet also indicate substantial reductions in boron concentrations: 80% reduction occurred between 2001 and 2008. For the more contaminated rivers there are issues of localised rather than catchment-wide sources and uncertainties over the extent and nature of water/boron stores. Atmospheric sources average around 32 to 61% for the cleaner and 4 to 14% for the more polluted parts. The substantial decreases in the boron concentrations correspond extremely well with the timing and extent of European wide trends for reductions in the industrial and domestic usage of boron-bearing compounds. It clearly indicates that such reductions have translated into lower average and peak concentrations of boron in the river although the full extent of these reductions has probably not yet occurred due to localised stores that are still to deplete.

Dissolved silica mobilization in the conterminous USA

Silicate weathering mobilizes “fresh” dissolved silica (DSi). The major factors governing DSi mobilization by chemical weathering on continental or global scales have not been satisfactorily quantified. Furthermore, influence of regional variations of proposed factors on large scale DSi mobilization is not properly assessed. A continental-scale, process-oriented, empirical model for DSi mobilization is developed to assess this research gap. The model is calibrated on river chemistry data from 142 monitoring stations from the conterminous USA, selected for minimal anthropogenic and water–body influence in their catchments. The average area of the catchments is 3890 km 2. The average observed DSi yield of the catchments is 2.68 t SiO 2 km − 2 a − 1 . The model calculates DSi yield as subject to catchment attributes, i.e. climate, lithology, land cover and morphology. As lithological source data for the model, a new lithological map of North America was developed. The high spatial resolution of the new map allows assessment of lithology classes with mapped extents as small as 0.5 km 2. The average lithology polygon size is 75 km 2. The developed multi-lithological, non-linear, lumped model for annual DSi mobilization describes 89% of the observed variance of DSi yield. It uses lithology proportion and runoff as predictors. With runoff, DSi yield increases differently for individual lithological classes. Basic igneous rocks show the highest DSi yields with respect to a given runoff. Consolidated sedimenary lithological classes yield DSi in the reversed order of their defined silicon contents. Of all lithological classes, the least DSi per runoff is mobilized from acid plutonic rocks. Apart from the two major predictors, analysis of the model also provides evidence for an influence of temperature and land cover on DSi mobilization. Comparison with existing studies shows that controlling factors on DSi mobilization vary regionally. Thus, studies calibrated in different regions result in significantly different DSi yields for comparable lithological classes. This emphasizes the need for global DSi mobilization models to be regionally calibrated.

Landscape elements and river chemistry as affected by river regulation – a 3-D perspective

Abstract. We tested the hypothesis whether individual land classes within a river catchment contribute equally to river loading with dissolved constituents or whether some land classes act as "hot spots" to river loading and if so, are these land classes especially affected by hydrological alterations. The amount of land covered by forests and wetlands and the average soil depth (throughout this paper soil refers to everything overlying bedrock i.e. regolith) of a river catchment explain 58–93% of the variability in total organic carbon (TOC) and dissolved silicate (DSi) concentrations for 22 river catchments in Northern Sweden. For the heavily regulated Luleälven, with 7 studied sub-catchments, only 3% of the headwater areas have been inundated by reservoirs, some 10% of the soils and aggregated forest and wetland areas have been lost due to damming and further hydrological alteration such as bypassing entire sub-catchments by headrace tunnels. However, looking at individual forest classes, our estimates indicate that some 37% of the deciduous forests have been inundated by the four major reservoirs built in the Luleälven headwaters. These deciduous forest and wetlands formerly growing on top of alluvial deposits along the river corridors forming the riparian zone play a vital role in loading river water with dissolved constituents, especially DSi. A digital elevation model draped with land classes and soil depths which highlights that topography of various land classes acting as hot spots is critical in determining water residence time in soils and biogeochemical fluxes. Thus, headwater areas of the Luleälven appear to be most sensitive to hydrological alterations due to the thin soil cover (on average 2.7–4.5 m) and only patchy appearance of forest and wetlands that were significantly perturbed. Hydrological alterations of these relatively small headwater areas significantly impacts downstream flux of dissolved constituents and their delivery to receiving water bodies.