Factors Controlling Total Concentration and Aqueous Speciation of Aluminium in an Acidic Headwater Stream of the Bavarian Forest National Park: a Modelling Approach

The purpose of the present paper is to analyse factors controlling total concentration and aqueous speciation of aluminium in the Große Ohe River, using a thermodynamic equilibrium model and a mixing approach. A model compound for humic substances is derived on the basis of the relation between anion deficit and the organic carbon content in the river as well as literature data. An equilibrium speciation model for aluminium is set up, considering this model compound and relevant inorganic solutes. Although the model cannot be verified directly, its results may be viewed as qualitatively correct. Applying the model to measured stream water samples highlights that aqueous speciation of aluminium is mainly controlled by the pH value and discharge and that free aluminium concentrations reach clearly toxic levels during acidic episodes. Comparing measured concentrations of sulfate and H⁺ and calculated concentrations of Al³⁺ with solubility curves of gibbsite like minerals and jurbanite clearly shows that total aluminium concentrations are not controlled by equilibria with these mineral phases alone. The observed relationship can be better explained from a mixture of two distinct waters, representing lowflow and highflow chemistry, and the resulting equilibrium concentrations. This indicates that total aluminium concentration, in particular during high discharge events, is mainly controlled by the mixture of waters with differing chemistry and flowpaths.     

Influence of basin characteristics on baseflow and stormflow chemistry in the Great Smoky Mountains National Park,USA

Relationships between stream chemistry and elevation, area, Anakeesta geology, soil properties, and dominant vegetation were evaluated to identify the influence of basin characteristics on baseflow and stormflow chemistry in eight streams of the Great Smoky Mountains National Park. Statistical analyses were employed to determine differences between baseflow and stormflow chemistry, and relate basin‐scale factors governing local chemical processes to stream chemistry. Following precipitation events, stream pH was reduced and aluminium concentrations increased, while the response of acid neutralizing capacity (ANC), nitrate, sulfate, and base cations varied. Several basin characteristics were highly correlated with each other, demonstrating the interrelatedness of topographical, geological, soil, and vegetative parameters. These interrelated basin factors uniquely influenced acidification response in these streams. Streams in higher‐elevation basins (>975 m) had significantly lower pH, ANC, sodium, and silicon and higher nitrate concentrations (p < 0.05). Streams in smaller basins (<10 km²) had significantly lower nitrate, sodium, magnesium, silicon, and base cation concentrations. In stormflow, streams in basins with Anakeesta geology (>10%) had significantly lower pH and sodium concentrations, and higher aluminium concentrations. Chemical and physical soil characteristics and dominant overstory vegetation in basins were more strongly correlated with baseflow and stormflow chemical constituents than topographical and geological basin factors. Saturated hydraulic conductivity, of all the soil parameters, was most related to concentrations of stormflow constituents. Basins with higher average hydraulic conductivities were associated with lower stream pH, ANC, and base cation concentrations, and higher nitrate and sulfate concentrations. These results emphasize the importance of soil and geological properties influencing stream chemistry and promote the prioritization of management strategies for aquatic resources. Copyright © 2012 John Wiley & Sons, Ltd.     

Transformations of runoff chemistry in the Arctic tundra,Northwest Territories,Canada

The transformation of snowmelt water chemical composition during melt, elution and runoff in an Arctic tundra basin is investigated. The chemistry of the water flowing along pathways from the surface of melting snow to the 95·5 ha basin outlet is related to relevant hydrological processes. In so doing, this paper offers physically based explanations for the transformation of major ion concentrations and loads of runoff water associated with snowmelt and rainfall along hydrological pathways to the stream outlet. Late‐lying snowdrifts were found to influence the ion chemistry in adjacent reaches of the stream channel greatly. As the initial pulse of ion‐rich melt water drained from the snowdrift and was conveyed through hillslope flowpaths, the concentrations of most ions increased, and the duration of the peak ionic pulse lengthened. Over the first 3 m of overland flow, the concentrations of all ions except for NO increased by one to two orders of magnitude, with the largest increase for K⁺, Ca²⁺ and Mg²⁺. This was roughly equivalent to the concentration increase that resulted from percolation of relatively dilute water through 0·25 m of unsaturated soil. The Na⁺ and Cl^? were the dominant ions in snowmelt water, whereas Ca²⁺ and Mg²⁺ dominated the hillslope runoff. On slopes below a large melting snowdrift, ion concentrations of melt water flowing in the saturated layer of the soil were very similar to the relatively dilute concentrations found in surface runoff. However, once the snowdrift ablated, ion concentrations of subsurface flow increased above parent melt‐water concentrations. Three seasonally characteristic hydrochemical regimes were identified in a stream reach adjacent to late‐lying snowdrifts. In the first two stages, the water chemistry in the stream channel strongly resembled the hillslope drainage water. In the third stage, in‐stream geochemical processes, including the weathering/ion exchange of Ca²⁺ and Mg²⁺, were the main control of streamwater chemistry. Copyright © 2006 John Wiley & Sons, Ltd.     

Impacts of acid emissions from Nevado del Ruiz volcano, Colombia, on selected terrestrial and aquatic ecosystems

Emissions of acidic gases and thermal waters from Nevado del Ruiz volcano have recently increased in concert with the November 13, 1985 eruption. This study examines the downwind and downstream effects of these emissions on alpine ecosystems high on the slopes of the volcano (4100 m) and on coffee plantations at lower elevations (< 2000 m) and greater distances from the active vent (> 30 km). Samples of bulk deposition, rain, soils, soil solutions, and streams were collected over a six-month period (January–July, 1987) to examine the impacts of this volcanogenic acidity.Bulk deposition falling on the higher slopes of the volcano is usually acidified; however, deposition reaching the distal coffee plantations seldom is acidic. The sources of the acids are hydrogen chloride and sulfur dioxide in the plume of the volcano. Although sulfur dioxide is by far the more abundant gas, hydrogen chloride is most responsible for acidification of rain falling on the slopes of the volcano. With distance from the vent, the chloride/sulfate ratio drops exponentially. The only major influence on regional precipitation chemistry in addition to the volcano appears to be land-use-related activities around the coffee plantations. Deposition on these areas is enriched by an order of magnitude in nitrate and base cations, compared to all other stations.Throughfall chemistry in the coffee plantations shows a dramatic response to occasional acid-rain events. A base-leaching process on coffee plant leaves is triggered by acid rain. For each equivalent of hydrogen ion in rain on the leaf surface, over 23 equivalents of potassium ion are leached from the leaf.In spite of this dramatic response by the vegetation, the plantation soils appear relatively unaffected by acidic deposition. In contrast, the alpine soils on the volcano exhibit low pHs, high sulfate and chloride concentrations in soil solutions, and high extractable sulfate concentrations. All of these factors indicate that these soils have undergone significant acid loading.While the deposition of the region is acidified by hydrogen chloride, the streams flowing off the volcano are apparently acidified by sulfuric acid in thermal waters discharging into the streams. The acidity of these streams decreases downstream, while the silica concentrations increase downstream. The composition of stream water is most influenced by thermal-water discharges as well as equilibrium dissolution of amorphous silica glass and non-equilibrium leaching of unweathered ash.The impacts of acid gases and thermal water released from the volcano appear to be restricted to ecosystems on the slope of the volcano. The only impact of Nevado del Ruiz on surrounding coffee plantations appears to be potassium leaching of coffee leaves from occasional acid-deposition events.     

Shenandoah Watershed Study-Virginia Trout Stream Sensitivity Study (SWAS-VTSSS): Stream water quality and hydrologic monitoring data for mid-Appalachian headwater streams

The Shenandoah Watershed Study (established in 1979) and the Virginia Trout Stream Sensitivity Study (established in 1987) serve to increase understanding of hydrological and biogeochemical changes in western Virginia mountain streams that occur in response to acidic deposition and other ecosystem stressors. The SWAS-VTSSS program has evolved over its 40+ year history to consist of a temporally robust and spatially stratified monitoring framework. Currently stream water is sampled for water quality bi-hourly during high-flow events at three sites and weekly at four sites within Shenandoah National Park (SHEN), and quarterly at 72 sites and on an approximately decadal frequency at ~450 sites within the wider western Virginia Appalachian region. Stream water is evaluated for pH, acid neutralizing capacity (ANC), base cations (calcium, magnesium, sodium and potassium ion), acid anions (sulphate, nitrate and chloride), silica, ammonium, and conductivity with a subset of samples evaluated for monomeric aluminium and dissolved organic carbon. Hourly stream discharge (four sites) and in-situ measurements of conductivity, water and air temperature (three sites) are also measured within SHEN. Here we provide an overview and timeline of the SWAS-VTSSS stream water monitoring program, summarize the field and laboratory methods, describe the water chemistry and hydrologic data sets, and document major watershed disturbances that have occurred during the program history. Website links and instructions are provided to access the stream chemistry and time-series monitoring data in open-access federal databases. The purpose of this publication is to promote awareness of these unique, long-term data sets for wider use in catchment studies. The water chemistry and hydrologic data can be used to investigate a wide range of biogeochemical research questions and provide key inputs for models of these headwater stream ecosystems. SWAS-VTSSS is an ongoing program and quality assured data sets are uploaded to the databases annually.     

Influence of sea salt on stream water chemistry in an upland afforested catchment

The chemistry of bulk precipitation and stream water was monitored in an acidic afforested catchment at Llyn Brianne in upland Wales between 1985 and 1990. Throughfall, stemflow and soil water chemistry were also monitored between 1988 and 1989. Marine-derived solutes dominated the ionic composition of precipitation and stream water, which had mean Cl concentrations of 113 μequiv. 1^?1 and 245 μequiv. 1^?1, respectively. The higher concentrations in stream water reflect occult and dry deposition on the forest canopy and the effect of interception and transpiration losses. Chloride variations in stream water (112-454μequiv. 1^?1) were damped compared with bulk precipitation (28-762μequiv. 1^?1) due to the mixing of event (‘new’) water with pre-event (‘old’) water in the catchment soils. A storm episode monitored in the catchment in April 1989 was associated with high sea salt inputs and Cl concentrations in throughfall (1466μequiv. 1^?1) and storm runoff were exceptionally high (392μequiv. 1^?1). The Cl signal in stream water during the episode was consistent with an event (‘new’) water contribution to the storm response. However, a short-term hydrochemical budget estimated that although Cl outputs from the catchment during the event (1.17 kg ha^?1) were equivalent to 8% of inputs in throughfall and stemflow, the storm runoff was equivalent to 32% of effective precipitation. This indicates that pre-event (‘old’) water was the dominant source (> 75%) of storm runoff. Although sea salt inputs during the event had a marked impact on stream water chemistry, the anomalously high levels of acidity sometimes associated with sea salt events were not observed in this particular study.     

Experimental stream acidification — the influence of sediment and streambed vegetation

An acidification experiment was conducted on a small stream in the Loch Ard area of central Scotland. The stream was chosen because of its large, flow related, variation in pH (5.9-4.0). Two acid additions were made to approximately pH 3.5–3.7. The results indicated a strong correlation between labile aluminium and hydrogen, and a noticeable hysteresis in the response of calcium and hydrogen. It is hypothesised that divalent cation response is a result of ion-exchange mechanisms involving the streambed vegetation, with aluminium release resulting primarily from exchange reactions with streambed sediment stores. Data from a program of stream spot sampling have been analysed in an attempt to elucidate the contribution of different sources of aluminium under different flow conditions. Streambed sources of aluminium contribute significantly under low flow conditions; however, at high flow, additional sources of aluminium must contribute to match observed streamwater chemistry.     

Melt water chemistry and its impact on stream water quality

Samples of snowpack leachate were collected over a 60 day period of the spring melt season in 1988 and 1989 at a 10 km² upland catchment in the Cairngorm mountains of Scotland. These were analysed for major ions to assess snowpack chemistry dynamics through the spring and to assess the melt water influence on stream water chemistry. The data clearly show preferential elution of sulphate and nitrate over chloride and hydrogen over the other cations during the early melt of 1988. Following the addition of ions to the snow surface, either as snow or later in the season as rain, the elution sequence is reproduced. Comparison of leachate chemistry with stream chemistry samples taken at the basin outlet indicate that snow pack melt water contributes directly to stream water. The stream water chemistry signal is, however, noisy and the stream concentrations are considerably damped relative to the snowpack leachate. This is thought to be a consequence of differential melting within the catchment as the snowpack at lower altitudes is at a more advanced stage of melt and so holds fewer solutes and mixing with groundwater contributions. Temperature observations at different altitudes within the catchment support this interpretation.     

Sources of variability in springwater chemistry in Fool Creek,a high-elevation catchment of the Rocky Mountains,Colorado, USA

Springs are the point of origin for most headwater streams and are important regulators of their chemical composition. We analysed solute concentrations of water emerging from 57 springs within the 3 km² Fool Creek catchment at the Fraser Experimental Forest and considered sources of spatial variation among them and their influence on the chemical composition of downstream water. On average, calcium and acid neutralizing capacity (bicarbonate-ANC) comprised 50 and 90% of the cation and anion charge respectively, in both spring and stream water. Variation in inorganic chemical composition among springs reflected distinct groundwater sources and catchment geology. Springs emerging through glacial deposits in the upper portion of the catchment were the most dilute and similar to snowmelt, whereas lower elevation springs were more concentrated in cations and ANC. Water emerging from a handful of springs in a geologically faulted portion of the catchment were more concentrated than all others and had a predominant effect on downstream ion concentrations. Chemical similarity indicated that these springs were linked along surface and subsurface flowpaths. This survey shows that springwater chemistry is influenced at nested spatial scales including broad geologic conditions, elevational and spatial attributes and isolated local features. Our results highlight the role of overlapping factors on solute export from headwater catchments.     

POTASSIUM CHEMISTRY OF A SMALL UPLAND STREAM FOLLOWING A MAJOR DROUGHT

Previously unpublished water quality data are used to explore the potassium chemistry of a small upland stream following the 1976 drought in England. The behaviour of potassium is a complex response to several factors: hydrological pathways operating during periods of storm runoff; sediment inputs; and the chemical properties of the transporting water. Analyses of ‘hysteresis loops’ for a series of storms show that the relationship between suspended sediment and potassium concentrations is not simple; spatial and temporal variations in surface and subsurface stormflow add complexity. In addition to the specific discussion of potassium, data are presented to show the recovery of stream discharge, and of sediment and solute concentrations during the immediate post-drought period. © 1997 by John Wiley & Sons Ltd.     

Modelling stream acidification in afforested catchments: Long-term reconstructions at two sites in central Scotland

A conceptual model of the combined effects of afforestation and acidic deposition is applied to two forested sites in central Scotland. Refinements are made to the model inputs specifically to include: increased dry deposition to the forests (in excess of the dry deposition expected for moorland sites) as the forest canopy develops; uptake of ions by the growing forests; and increased evapotranspiration (and thus decreased water yield) as the forests mature. The model is calibrated using a fuzzy optimisation technique which incorporates uncertainty in target variables (stream base cation concentrations and soil exchangeable bases) and uncertainty in selecting values for fixed and adjustable parameters which describe the physico-chemical characteristics of the catchments. Simulated present-day stream and soil chemistry closely match observed values at both sites. The calibrated models indicate that while the patterns of acidification in the two catchments are broadly similar, some differences do exist between the sites in the responses of the soils to acidic deposition and afforestation. It is concluded that the calibrated models provide a tool for: (a) comparison of the relative effects of deposition and afforestation on soil and surface water acidification; (b) assessment of the likely effects of reductions in future deposition combined with future forestry management practices.     

Temporal and spatial variability of cation and silica export in an alpine watershed,Emerald Lake,California

A reaction set of possible mineral weathering reactions is proposed to explain observed cation and silica export for the Emerald Lake watershed, a small Sierra Nevada, California catchment. The reaction set was calculated through a stoichiometric mole‐balance method, using a multiyear record of stream flow and snowpack chemical analyses and site‐specific mineral compositions. Reaction‐set calculations were intended to explore how the processes controlling stream cation and silica export depend on differing bedrock mineralogy across the catchment as snowmelt and runoff patterns change over the year. Different regions within the watershed can be differentiated by lake inflow subdrainages, each exhibiting different stream‐flow chemistry and calculated weathering stoichiometry, indicating that different silica and cation generation processes are dominant in wet steep portions of the catchment. Short‐term differences in stream concentrations were assumed to reflect ion exchange equilibria and rapid biological processes, whereas long‐term persistent stream concentration differences in different areas of the catchment were assumed to reflect spatial variability in mineral weathering stoichiometry. Mineralogical analyses of rock samples from the watershed provided site‐specific chemical compositions of major mineral species for reaction calculations. Reaction sets were evaluated by linear regression of calculated versus observed differences between snowmelt and stream‐flow chemistry and by a combined measure. Initially, single weathering reactions were balanced and evaluated to determine the reactions that best explained observed stream chemical export. Next, reactions were combined, using mineral compositions from different rock types to estimate the dependence of ion fluxes on lithology. The seasonal variability of major solute calculated fluxes is low, approximately one order of magnitude, relative to the observed three orders of magnitude variability in basin discharge. Reaction sets using basin‐averaged lithology and Aplite lithologies gave superior explanations of stream chemical composition. Copyright © 2004 John Wiley & Sons, Ltd.     

Alkalinization and acidification of stream water with changes in atmospheric deposition in a tropical dry evergreen forest of northeastern Thailand

Field surveys on atmospheric deposition and stream water chemistry were conducted in an evergreen forest in northeastern Thailand characterized by a tropical savanna climate with distinct dry and wet seasons. Atmospheric deposition of ion constituents by throughfall and stemflow was shown to increase in the beginning and end of the wet season, reflecting the precipitation pattern. The pH and electrical conductivity of stream water increased with alkalinity and base cation concentrations due to mineralization of organic matter by the first rain and retention of anions in soil during the start of the wet season. After initial alkalinization, the pH and alkalinity declined rapidly with the highest SO₄^2? concentration displayed in the middle towards the end of the wet season. The magnitude of peaks in SO₄^2? concentration (13.5–60.6 μmol_c/L) reflects deposition during the first 2 months of the wet season (March and April) in respective years (60.8–170 mol_c/ha). Release of SO₄^2? with H⁺, which is retained in soil during the early wet season, may cause acidification later in the season. The deposition and concentration of SO₄^2? declined over 6 years. However, the pH of stream water declined with increasing concentrations of SO₄^2? and other major ions. The release of materials accumulated in the ecosystem was facilitated by the decrease in SO₄^2? concentration/deposition and increased precipitation in the middle–late wet season. The retention‐release cycle of SO₄^2? largely contributed to both seasonal and interannual variations in stream water chemistry in the tropical savanna climate studied.     

The seasonal variation of streamwater chemistry in three forested Mediterranean catchments

Streamwater chemistry is described for three streams draining undisturbed, evergreen broad-leaved forested catchments on phyllites in NE Spain: two streams with no or negligible flow in summer are located in the Prades massif, and one perennial stream is in the wetter Montseny mountains. Weekly data for a study period of 2–4 years are provided to (1) describe the seasonal variations in streamwater chemistry, (2) analyse the relationship between stream discharge and solute concentrations using a two-component mixing model and (3) search for patterns of temporal variation in stream solute concentrations after discounting the effects of discharge. At Prades, concentrations of all analysed ions, except NO⁻₃, showed marked seasonal variations in stream water, whereas at Montseny only ions related to mineral weathering (HCO⁻₃, Na⁺, Ca²⁺ and Mg²⁺) showed strong seasonality. Ion concentrations were more closely dependent on instantaneous discharge at Montseny than at Prades. The residuals of the relationship between solute concentrations and discharge retained a strong seasonality at Prades, but not at Montseny. These differences are related to the major hydrochemical processes that determine the streamwater chemistry at each site. The same processes are probably operative in the three catchments, but are of varying relative importance. At Montseny, the mixing of waters of different chemical composition seems to be the major process controlling streamwater chemistry, although the soilwater end-member composition predicted by the mixing model applied did not match the measured soilwater chemistry. In the drier Prades catchments, the two major hydrochemical processes determining the seasonal variation of streamwater chemistry are (1) the restart of flow after the summer drought, which flushes out the solutes accumulated during the dry period, and (2) the seasonal changes in groundwater chemistry that result from the interplay of water residence time, temperature and CO₂ partial pressure. In Mediterranean catchments with relatively high precipitation, such as Montseny, the seasonal variation in the streamwater chemistry is largely determined by the same processes as at humid-temperate sites, whereas in drier Mediterranean catchments, such as Prades, the major hydrochemical processes are clearly distinct.     

Seasonal cycles of dissolved constituents in streamwater in two forested catchments in the mid-Atlantic region of the eastern USA

Streamwater discharge and chemistry of two small catchments on Catoctin Mountain in north-central Maryland have been monitored since 1982. Repetitive seasonal cycles in stream-water chemistry have been observed each year, along with seasonal cycles in the volume of stream discharge and in groundwater levels. The hypothesis that the observed streamwater chemical cycles are related to seasonal changes in the hydrological flow paths that contribute to streamflow is examined using a combination of data on groundwater levels, shallow and deep groundwater chemistry, streamwater discharge, streamwater chemistry, soil-water chemistry, and estimates of water residence times. The concentrations of constituents derived from rock weathering, particularly bicarbonate and silica, increase in streamwater during the summer when the water table is below the regolith-bedrock interface and stream discharge consists primarily of deep groundwater from the fractured-bedrock aquifer. Conversely, the concentrations in streamwater of atmospherically derived components, particularly sulfate, increase in winter when the water table is above the regolith-bedrock interface and stream discharge consists primarily of shallow groundwater from the regolith. Tritium and chlorofluorocarbon (CFC) measurements suggest that the groundwater in these systems is young, with a residence time of less than several years. The results of this study have implications for the design of large-scale water-quality monitoring programs.     

The role of chemical weathering in the neutralization of acidic deposition

Chemical weathering of rocks and minerals is a key factor which mitigates acidic deposition and affects water chemistry. It supplies cations and alkalinity to the surface water, groundwater, ion-exchange complex, and vegetation in the watershed. The kinetics of chemical weathering have not been determined in the field, but based on laboratory experiments, the rate of weathering has a fractional order dependency on hydrogen ion and organic ligand concentration in bulk solution. Watersheds with the greatest degree of hydrologic and geologic sensitivity can produce only 200–500 eq/ha·yr of cations or alkalinity for export. This is equivalent to 100 cm/yr of precipitation with a pH of 4.3–4.6 or an annual sulfur deposition of 1.0–2.5 g S/m²·yr. When acid and sulfur deposition are greater than these levels, extremely sensitive lakes may become acidified. To illustrate this point, a simple steady-state model is applied to lakes in regions where acidification of lakes has been reported.     

Use of soil‐streamwater relationships to assess regional patterns of acidic deposition effects in the northeastern USA

Declines of acidic deposition levels by as much as 50% since 1990 have led to partial recovery of surface waters in the northeastern USA but continued depletion of soil calcium through this same period suggests a disconnection between soil and surface water chemistry. To investigate the role of soil‐surface water interactions in recovery from acidification, the first regional survey to directly relate soil chemistry to stream chemistry during high flow was implemented in a 4144‐km² area of the Catskill region of New York, where acidic deposition levels are among the highest in the East. More than 40% of 95 streams sampled in the southern Catskill Mountains were determined to be acidified and had inorganic monomeric aluminum concentrations that exceeded a threshold that is toxic to aquatic biota. More than 80% likely exceeded this threshold during the highest flows, but less than 10% of more than 100 streams sampled were acidified in the northwestern portion of the region. Median Oa horizon soil base saturation ranged from 50% to 80% at 200 sites across the region, but median base saturation in the upper 10 cm of the B horizon was less than 20% across the region and was only 2% in the southern area. Aluminum is likely to be interfering with root uptake of calcium in the mineral horizon in approximately half the sampled watersheds. Stream chemistry was highly variable over the Catskill region and, therefore, did not always reflect the calcium depletion of the B horizon that our sampling suggested was nearly ubiquitous throughout the region. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Fluxes of inorganic carbon from two forested catchments in the Appalachian mountains

This study uses long‐term records of stream chemistry, discharge and air temperature from two neighbouring forested catchments in the southern Appalachians in order to calculate production of dissolved CO₂ and dissolved inorganic carbon (DIC). One of the pair of catchments was clear‐felled during the period of the study. The study shows that: (1) areal production rates of both dissolved CO₂ and DIC are similar between the two catchments even during and immediately after the period of clear‐felling; (2) flux of total inorganic carbon (dissolved CO₂+ DIC) rises dramatically in response to a catchment‐wide acidification event; (3) DIC and dissolved CO₂ are dominantly released on the old water portion of the discharge and concentrations peak in the early autumn when flows in the study catchments are at their lowest; (4) total fluvial carbon flux from the clear‐felled catchment is 11·6 t km⁻² year⁻¹ and for the control catchment is 11·4 t km⁻² year⁻¹. The total inorganic carbon flux represents 69% of the total fluvial carbon flux. The method presented in the study provides a useful way of estimating inorganic carbon flux from a catchment without detailed gas monitoring. The time series of dissolved CO₂ at emergence to the stream can also be a proxy for the soil flux of CO₂. Copyright © 2005 John Wiley & Sons, Ltd.     

Comparison of methods for calculating annual solute exports from six forested appalachian watersheds

Six methods were compared for calculating annual stream exports of sulfate, nitrate, calcium, magnesium and aluminum from six small Appalachian watersheds. Approximately 250–400 stream samples and concurrent stream flow measurements were collected during baseflows and storm flows for the 1989 water year at five Pennsylvania watersheds and during the 1989–1992 water years at a West Virginia watershed. Continuous stream flow records were also collected at each watershed. Solute exports were calculated from the complete data set using six different scenarios ranging from instantaneous monthly measurements of stream chemistry and stream flow, to intensive monitoring of storm flow events and multiple regression equations. The results for five of the methods were compared with the regression method because statistically significant models were developed and the regression equations allowed for prediction of solute concentrations during unsampled storm flows. Results indicated that continuous stream flow measurement was critical to producing exports within 10% of regression estimates. For solutes whose concentrations were not correlated strongly with stream flow, weekly grab samples combined with continuous records of stream flow were sufficient to produce export estimates within 10% of the regression method. For solutes whose concentrations were correlated strongly with stream flow, more intensive sampling during storm flows or the use of multiple regression equations were the most appropriate methods, especially for watersheds where stream flows changed most quickly. Concentration–stream flow relationships, stream hydrological response, available resources and required level of accuracy of chemical budgets should be considered when choosing a method for calculating solute exports. © 1997 John Wiley & Sons, Ltd.     

Hydrological flow paths controlling stream chemistry in Japanese forested watersheds

Water sources and flow paths contributing to stream chemistry were evaluated in four Japanese forested watersheds with steep topography (slopes ≥30°). Stream chemistry during periods without rainfall and during events with less than 100 mm of precipitation was similar to seepage water chemistry, but markedly different from that of soil water which had higher concentrations of NO⁻₃ and Ca²⁺ and lower concentrations of Na⁺ and HCO⁻₃. Also, stream Cl⁻ concentrations in a Cl⁻‐treated watershed did not increase either during events with less than 100 mm of total rainfall or at baseflow conditions, even three years after the Cl⁻ treatment. These results suggest that groundwater within bedrock fissures of Paleozoic strata had a long residence time and was a major contributor to steam water under baseflow conditions and even during small precipitation events (≤100 mm). In contrast, for large precipitation events (≥100 mm), stream chemistry became more similar to soil water chemistry, especially within the steepest watershed. Also, for large precipitation events, stream Cl⁻ concentrations in the Cl⁻‐treated watershed increased markedly. These results suggest that soil water was a major contributor to stream waters only during these large events. Copyright © 1999 John Wiley & Sons, Ltd.