Inorganic Aluminum Concentrations Research Articles

Four decades of changing dissolved organic matter quality and stoichiometry in a Swedish forest stream

Dissolved organic matter (DOM) concentrations have risen by a factor of two or more across much of Europe and North America during recent decades. These increases have affected the carbon cycle, light regime, drinking water treatability, and the energy and nutrient budgets of lakes and streams. However, while trends in DOM quantity are well characterised, information on how/whether qualitative properties of DOM have changed are scarce. Here, we describe over 40 years of monitoring data from a forested headwater stream in the Gårdsjön experimental catchment, southwest Sweden, which provides a unique record of biogeochemical change, including optical and stoichiometric DOM quality metrics, spanning the entire period of recovery from acidification. For the period 1980–2020 we find a 71% reduction in decadal mean sulphate concentrations, and a similar reduction in inorganic aluminium concentrations, alongside a 64% increase in dissolved organic carbon (DOC) concentrations. Over the same period, colour (absorbance at 420 nm) increased almost twice as much as DOC, whereas dissolved organic nitrogen (DON) increased by only one third as much. These results demonstrate a shift in stream water composition, with DOM becoming dominated by highly coloured, complex, nitrogen-poor compounds. This material is likely more resistant to biological degradation, but more susceptible to photochemical degradation. Changes in DOM stoichiometry could lead to intensified nitrogen and/or phosphorus limitation in surface waters, while increased colour/DOC ratios could intensify light-limitation of primary production beyond that expected from DOC increases alone. We observed increases in organic matter associated metals (iron 117%, organically complexed aluminium 85%) that exceeded the increase in DOC, consistent with their increased mobilisation by more aromatic organic matter. All observed changes are consistent with recovery from acidification being the primary driver of change, implying that past acidification, and ongoing recovery, have profoundly affected terrestrial and aquatic biogeochemistry, ecology and the carbon cycle.

Evaluating acid-aluminum stress in streams of the Northeastern U.S. at watershed, fish community and physiological scales

In spite of overall improvements in air and water quality, biological stress from low pH and high concentrations of inorganic aluminum continue to impact fish and fish habitat in northeastern North America, with independent and interactive effects on individuals, populations and communities. Integrative indicators can therefore be useful in monitoring both impact and recovery across multiple scales. Using coupled water chemistry (pH, conductivity, and base cation and inorganic aluminum concentration), geographic (site elevation and watershed area) and biological (fish diversity, fish abundance, gill aluminum concentration and gill physiology) data, we developed an integrated indicator of acid-aluminum stress across the White and Green mountains in central New England, USA. As has been established in a number of previous studies, preliminary analysis clearly indicated that across all sites, inorganic aluminum concentration was consistently greatest during the spring season. Structural Equation modeling (SEM) revealed that toxic conditions (concurrent low pH and high concentrations of inorganic aluminum) were well summarized with an integrated toxicity score, related to both base cation concentrations and elevation, with sites at higher elevations more likely to experience toxic conditions as well as low base cation concentrations. Fish diversity and abundance generally trended negatively with toxicity score, with fewer cyprinids and sculpins at high toxicity score sites. In spite of considerable variation among individuals, gill aluminum was positively related to toxicity score for both Atlantic salmon and brook trout. Observed elevated gill aluminum levels associated with reduced gill metabolic activity in Atlantic salmon smolts from impacted systems likely result in impaired osmoregulatory function and seawater tolerance. Overall, our results suggest that the integrated toxicity score metric is associated with a syndrome of acute physiological stress, reduced abundance, and low species diversity for sensitive stream fishes in New England, and can likely serve as a reliable indicator of continued impairment or recovery of acid-aluminum vulnerable systems in this ecoregion.

Forest damage and subsequent recovery alter the water composition in mountain lake catchments

Forest damage by insect infestation directly affects the trees themselves, but also indirectly affects water quality via soil processes. The changes in water composition may undergo different pathways depending on site-specific characteristics and forest components, especially the proportion of coniferous and deciduous trees. Here, we test whether changes in forest components and the intensity of disturbance can predict the chemical properties of water outflow from affected lake catchments. Information about forest regeneration (a phase dominated by deciduous trees) and the proportions of damaged and healthy coniferous trees and treeless areas were obtained from satellite data. The four study catchments of Prášilské, Laka, Plešné, and Čertovo lakes are geographically close and located in the same mountain range (Šumava Mts., Czech Republic) at similar altitude, but they differ in extents of forest disturbances and recoveries. The water quality measured at the lake catchment outflows differed, and better reflected the development of forest components and health than did meteorological (temperature and precipitation) or hydrological (discharge) variables. Several of the outflow properties (concentrations of inorganic aluminium, protons, potassium, calcium, magnesium, alkalinity, dissolved organic carbon (DOC), nitrate, and total phosphorus), responded catchment-specifically and with different delays to forest disturbance. The most pronounced differences occurred in DOC concentrations, which started to increase in the most disturbed Plešné and Laka catchments 7 and 6 years, respectively, after the peak in tree dieback, but did not increase significantly in the Prášilské catchment, which was disturbed several times during the last 3–4 decades. This study demonstrates an importance of extents of forest disturbances, the following changes in forest composition, and catchment-specific characteristics on water composition.

Acidification Impacts and Goals for Gauging Recovery of Brook Trout Populations and Fish Communities in Streams of the Western Adirondack Mountains, New York, USA

AbstractResults from several long‐term monitoring programs in the western Adirondack Mountains, New York, indicate that acid–base chemistry of headwater streams has remained unchanged or improved only marginally since the 1990s. A paucity of quantitative fishery data, however, limits our understanding of the pre‐acidified communities as well as present‐day impacts of acidification on fish assemblages, which impedes efforts to evaluate temporal trends and biological recovery in streams of the region. Fish communities were characterized at 48 streams (chemistry was assessed at 47 streams) in the western Adirondacks at least once during summer 2014–2016 to assess present‐day effects of acidification on fish assemblages, refine important relations, and identify biological targets and chemical effect thresholds that could help gauge biological recovery across the region. Concentrations of inorganic aluminum (Ali) exceeded chronic and acute toxicity thresholds (1.0 and 2.0 μmol/L) in 21.3% and 8.5%, respectively, of 47 study streams sampled during summer 2014–2016 and in 64.0% and 44.0% of 25 streams sampled during spring 2014–2015. In streams with summer Ali concentrations less than 1.0 μmol/L, community richness, density, and biomass averaged 2.0 species, 444.2 fish/0.1 ha, and 1,924.4 g/0.1 ha, respectively, whereas density and biomass of Brook Trout Salvelinus fontinalis populations averaged 280.8 fish/0.1 ha and 1,384.0 g/0.1 ha, respectively. These findings identify defensible targets for biological recovery and show that Ali toxicity is not a major concern for fish assemblages in most streams during summer base flow periods but is potentially a serious issue for fish in as many as two‐thirds of streams during spring high flows. Though additional data are needed to address several limitations and information gaps, results from this study provide a sound foundation to gauge biological recovery, detect future effects of climatic stressors, and help ensure that functional stream ecosystems can be sustained or restored in parts of the Adirondacks.

Fixed dose aluminum sulfate treatment to eradicate the salmon parasite Gyrodactylus salaris: effects on benthic macroinvertebrates

Gyrodactylus salaris is an ectoparasite invasive to Norway that is highly virulent to pre-smolt stages of Atlantic salmon (Salmo salar). To eliminate the parasite without killing the salmon host, aqueous aluminum sulfate and sulfuric acid (AlS treatment) have been applied to infested rivers for short durations. When the River Lærdalselva and its tributaries were treated in the summer of 2012, we studied communities of macroinvertebrates in a small stream that received AlS for 18 days. We monitored pH, inorganic aluminum (Ali) and collected quantitative samples of macroinvertebrates. Concentrations of inorganic aluminum (Ali) were generally in the range of 20–50 µg/L, but episodes up to 95 µg/L were recorded. Six of the fourteen recorded macroinvertebrate taxa were, to some degree, negatively affected by the treatment. The acid sensitive ephemeropterans Baetis subalpinus and Baetis rhodani were almost eliminated from the treated site (p < 0.001 for both compared to the untreated site). Halfway into the treatment, abundances of Baetis were approximately 15 individuals per m2 at the treated site. Densities of Baetis at the untreated site were above 250 individuals per m2 throughout the treatment. The plecopteran Isoperla grammatica, the trichopteran Rhyacophila nubila, and the dipterans Chironomidae and Simuliidae also responded to the treatment, but the effects were not as prominent as for Baetis. For R. nubila and I. grammatica, the decline in population sizes could be a behavioral response related to lower prey densities (Baetis, Simuliidae, and Chironomidae). The reduction of Baetis led to a prominent reduction in the number of grazers and scrapers and gatherers/collectors (p = 0.004 for both groups). No such responses were found for the other feeding groups. Significant reductions in populations of Baetis can affect ecosystem functioning and, thereby, have implications for pre-smolt survival following AlS treatments.

Re-examining the effects of episodic acidity on macroinvertebrates in small conifer-forested streams in Ireland and empirical evidence for biological recovery

Research throughout Europe has indicated the continuing reductions in acidic aerial deposition and associated improvements in freshwater pH. However, information on macroinvertebrate responses to these on-going changes is rare. During 2009, 2010 and 2011, 34 peat-dominated catchments in three geologically distinct acid-sensitive regions of Ireland were sampled for macroinvertebrates (6 occasions) and chemistry (3 seasons) to assess the effects of reduced sulphate and nitrate deposition in an ecoregion with historically low acidic deposition. Streams drained moorland catchments without forest, catchments with 20%–50 % (low) and catchments with forest cover and >50% (high) forest cover. Results indicate ecological improvements have occurred in many forested catchments when compared with findings in the early 1990s, especially on metamorphic and igneous geologies. However, episodically low pH (<5.7) and high inorganic aluminium concentrations (>150μg l−1), associated with storm-flow events, resulted in periods of ecological degradation. Nevertheless, the reductions in ecological health appeared seasonal, associated with antecedent hydrochemistry and climate. Interestingly, acid-sensitive Baetidae were present in the majority of forested streams in Ireland, owing to the recent improvements in water quality. However, caution must be advised as there are still many hydrochemical and ecological uncertainties in peat-soil forested catchments, owing to the high concentrations of dissolved organic carbon, climate change and changes to catchment management.

Re-examining the effects of episodic acidity on macroinvertebrates in small conifer-forested streams in Ireland and empirical evidence for biological recovery

:ABSTRACTResearch throughout Europe has indicated the continuing reductions in acidic aerial deposition and associated improvements in freshwater pH. However, information on macroinvertebrate responses to these on-going changes is rare. During 2009, 2010 and 2011, 34 peat-dominated catchments in three geologically distinct acid-sensitive regions of Ireland were sampled for macroinvertebrates (6 occasions) and chemistry (3 seasons) to assess the effects of reduced sulphate and nitrate deposition in an ecoregion with historically low acidic deposition. Streams drained moorland catchments without forest, catchments with 20%–50 % (low) and catchments with forest cover and >50% (high) forest cover. Results indicate ecological improvements have occurred in many forested catchments when compared with findings in the early 1990s, especially on metamorphic and igneous geologies. However, episodically low pH (<5.7) and high inorganic aluminium concentrations (>150μg l−1), associated with storm-flow events, resulted in periods of ecological degradation. Nevertheless, the reductions in ecological health appeared seasonal, associated with antecedent hydrochemistry and climate. Interestingly, acid-sensitive Baetidae were present in the majority of forested streams in Ireland, owing to the recent improvements in water quality. However, caution must be advised as there are still many hydrochemical and ecological uncertainties in peat-soil forested catchments, owing to the high concentrations of dissolved organic carbon, climate change and changes to catchment management.

A regional examination of episodic acidification response to reduced acidic deposition and the influence of plantation forests in Irish headwater streams

Episodic surface water acidification is common in many regions worldwide; the driving processes are dependent on a variety of physicochemical and climatic characteristics, and acid deposition pressures, which have changed significantly over the last two decades. This study provided a unique opportunity to re-examine the drivers of acidity in an environment of low anthropogenic input. In three geologically distinct acid-sensitive regions of Ireland during 2009–2010, 34 headwater streams were evaluated in peat-dominated catchments draining moorlands without forest, 20–50% (low) forest cover and >50% (high) forest cover. Results indicated episodic acidity/alkalinity loss in headwater streams, despite significant reductions in acid deposition. Both the differences in pH between base and storm-flow (∆pH) and the number of pH events≤5.5 were higher in forested streams. Dissolved organic carbon and inorganic aluminium concentrations were also higher in forested catchments. The primary driver of acidity was strong organic anions, which generally increased with increasing forest cover. Base-cation dilution was also prominent in west and southern regions, while surprisingly chlorine anion acidity from sea-salts had little or no influence on stream acidity. The contributions of excess non-marine sulphate (xSO4) and nitrate (NO3) to storm-water were low, with no observed increases in xSO4 with increasing forest cover, although contributions of NO3 were higher in forested catchments in the east. The results suggest that episodic acidification in Ireland is primarily driven by organic acids. However in peat dominant catchments, plantation forest, climate change and/or reductions in xSO4 appear to also be having an effect on stream pH from increased DOC, with some forested streams previously unaffected by deposition now showing low pH (<5.5) during storm-flow. As quantified from this study, observed changes in stream acidification in Ireland may provide a better understanding of future chemical responses to declining acid deposition and climate change elsewhere.

Trends in the hydrochemistry of acid-sensitive surface waters in the UK 1988–2008

We conducted complementary linear and non-linear statistical modelling of the first 20 years of hydrochemical data from the 22 lakes and streams of the UK Acid Waters Monitoring Network (AWMN) in order to assess temporal patterns and rates of change in indicators of the key drivers of surface water acidification and acidity status. Over the period 1988–2008, concentrations of non-marine sulphate (i.e. of anthropogenic origin) fell in line with reductions in non-marine sulphur deposition, the principal driver of acidification. Most of the decline was confined to the latter half of the 1990s. Whilst these reductions were substantial, concentrations in recent samples from the most contaminated sites remained several times higher than those in the most remote, low-deposition regions. Nitrate (NO3−) concentrations also declined slightly at several sites in northern England and Wales, possibly reflecting a recent regional reduction in N deposition. Further north, NO3− concentrations increased slightly in some Scottish sites, reflecting a continued dominance of climatic variation on nitrogen cycling in this region. A combination of unusually high rainfall and seasalt inputs in the early years, gradual long-term reductions in hydrochloric acid deposition, and later and more substantial reductions in sulphur deposition, appeared to account for relatively linear increases in an alkalinity-based estimate of Acid Neutralising Capacity ([AB-ANC]) with time. However, variation between sites in the size of the long-term trend in [AB-ANC] was most clearly linked to the size of the long-term reduction in sulphate concentration.The chemical characteristics of changes in [AB-ANC] across the network were dependent on rates of change in acid deposition, the extent to which sites have previously acidified, and average dissolved organic carbon concentration ([DOC]). In the most acidified waters, the response in acidity to reductions in acid deposition was dominated initially by large reductions in inorganic aluminium concentrations ([Alinorg]. Responses in pH have become clearer at these sites as [Alinorg] has begun to stabilise. Over the wider network, pH has been rising more rapidly in sites with lower concentrations of organic acids. A substantial proportion of the deposition-driven increase in [AB-ANC] at several sites is accounted for by increases in [DOC]. For the non-acidified but acid-sensitive waters in the far north and west, changes in [DOC] represent the only clear response to the small changes in sulphur deposition.In the more acidified sites, a reduction in the size of occasional extreme concentrations of Alinorg, that occur after periods of high rainfall or seasalt deposition events, has been more rapid than the declines in average levels, implying that at most sites the persistence of these events should not be seen as an additional barrier to ecological recovery. In a comparison of geographically paired sites with forested and moorland catchments, consistently higher levels of inorganic aluminium concentration and lower AB-ANC provided clear evidence that the former group had acidified to a greater extent. There were few significant differences in the rate at which non-marine sulphate concentration declined or AB-ANC increased and the forested sites therefore remain in a more acidified condition.

Aluminium concentrations in Swedish forest streams and co-variations with catchment characteristics

The negative effects of elevated concentrations of inorganic aluminium on aquatic organisms are well documented. Acid deposition is often cited as a main driver behind the mobilisation and speciation of aluminium in soils and surface waters. In the study, we tested the hypothesis that sulphur deposition is the main driver for elevated concentrations of inorganic aluminium in 114 base poor, boreal Swedish streams. However, the deposition of anthropogenic sulphate has decreased substantially since it peaked in the 1970s, and at the current deposition levels, we hypothesise that local site parameters play an important role in determining vulnerability to elevated concentrations of inorganic aluminium in boreal stream waters. Presented here are the results of a principal components analysis of stream water chemistry, acid deposition data and local site variables, including forest composition and stem volume. It is shown that the concentrations of both organic and inorganic aluminium are not explained by either historical or current acid deposition, but are instead explained by a combination of local site characteristics. Sites with elevated concentrations of inorganic aluminium were characterised by small catchments (<500 ha) dominated by mature stands of Norway spruce with high stem volume. Using data from the Swedish National Forest Inventory the area of productive forest land in Sweden with a higher vulnerability for elevated inorganic aluminium concentrations in forests streams is approximately 1.5 million hectares or 7% of the total productive forest area; this is higher in the south of Sweden (10%) and lower in the north (2%). A better understanding of the effects of natural processes and forest management in controlling aquatic inorganic aluminium concentrations is therefore important in future discussions about measures against surface water acidification.

Thresholds for Survival of Brown Trout during the Spring Flood Acid Pulse in Streams High in Dissolved Organic Carbon

AbstractThe survival of brown trout Salmo trutta embryos and first‐year juveniles was studied using in situ bioassays during the snowmelt‐driven spring flood in 12 streams in northern Sweden. Unlike in most previous studies on the impact of acidity on brown trout, the streams in this study were high in dissolved organic carbon (DOC), and as a result organic acids play a primary role in controlling pH. During the spring flood period DOC concentrations increased strongly in most streams and, in combination with dilution of acid‐neutralizing capacity, resulted in a decrease in pH and an increase in total and inorganic monomeric aluminum. High mortality of brown trout juveniles occurred during the spring flood and was best correlated to the high H+ concentration. No toxic effect could be directly attributed to measured inorganic aluminum concentrations. An empirical model to predict juvenile brown trout mortality in DOC‐rich streams was developed, and based on these predictions a critical chemical threshold of pH 4.8‐5.4 is proposed for first‐year juveniles. There was high embryo and yolk sac fry survival during the spring flood, even at sites with pH as low as 4.0, suggesting that the pH threshold in DOC‐rich waters is lower than the previously established threshold for low‐DOC systems. We discuss the complex role that DOC has in humic‐rich surface waters, where it both causes a pH decrease and protects against aluminum toxicity in fish. The results suggest that first‐year juveniles are likely to be the stage most vulnerable to the effects of episodic pH depression associated with the snowmelt period in northern boreal systems. This results from asynchrony in the seasonality of the spring flood acid pulse and the seasonality of trout embryo development, which is slow in cold northern waters.

Controlled exposure of brown trout to humic water limed to different pH and inorganic aluminium concentrations

Fish that accumulated more than 800 μg Al/g dw gill and 900 μg Fe/g dw gill died at pH 4.6 but survived at higher pH. Fish experienced severe physiological responses when gill-Al exceeded 600 μg Al ...

Reconstruction of Long-Term Changes in Lake water Chemistry, Zooplankton and Benthos of a Small, Acidified High-Mountain Lake: Magic Modelling and Palaeolimnogical Analysis

Starolesnianske pleso is a small and shallow acid lake in the High Tatra Mountains, situated at 2000 m above sea level, on granitic bedrock, with sparse and thin soil cover. When detailed measurements began in the 1980s Starolesnianske pleso had pH below 5 and only one species of cladoceran zooplankton, the ubiquitous Chydorus sphaericusPalaeolimnological investigations show changes in cladoceran zooplankton and chironomid zoobenthic assemblages since about 1920 and a major change in 1970–1980. The dynamic acidification model MAGIC was used to reconstruct changes in water chemistry over the past 150 years. The results from MAGIC agree well with the sediment record; pH levels gradually decreased from 6.5 in the mid-1800s to about 5.6 in 1920 (first response in biota) to below 5.0 by 1970, and concentrations of inorganic aluminium rose dramatically beginning about 1960. In the 1990s the lake water chemistry showed clear signs of reversal in acidification brought about by a major decline in S deposition.

Deposition Inputs, Water Quality and Liming Effectiveness

Abstract Rainfall quantity and quality, and stream and loch water quality have been monitored throughout the Loch Fleet Project. This has allowed the effects of liming of parts of the catchment to be monitored, and assessments made of the effectiveness and duration of these treatments. Rainfall over 1989–1993 fluctuated around the long-term average of 2100 mm. Over this time, year by year variations in acid or sulphate loading from deposition was evident and quite substantial. There was no trend, however, reflecting reductions in national sulphur emissions. Liming parts of the catchment in 1986 increased stream and loch water pH and calcium concentrations and reduced inorganic aluminium concentrations. Improved stream and loch water quality has been maintained for more than eight years since liming.

Survival of crucian carp,Carassius carassius, exposed to a high low-molecular weight inorganic aluminium challenge

Crucian carp were exposed to three combinations of pH and inorganic aluminium concentrations for 25 days. Mortality, plasma chloride and haematocrit, and gill aluminium content were investigated. No mortality was observed in any of the three exposures. Crucian carp survived acidic Al-rich water (pH 5.17) with a high concentration of low-molecular weight inorganic Al species (242 µgl−1) for 25 days. These fish had lowered plasma chloride levels and high amounts of aluminium on their gills compared to fish exposed to acidic Al-poor water (pH 5.16 and 12 µgl−1 Ali) and control water (pH 6.29 and 4 µgl−1 Ali). Haematocrit was the same in all three exposures. Because aluminium was not acutely toxic to the anoxia tolerant crucian carp, the present results give support to the hypothesis that Al polymerization with subsequent hypoxia is of high importance for the mechanism of acute Al toxicity to fish.

Efficacy of Shoal Liming for Rehabilitation of Lake Trout Populations in Acid-Stressed Lakes

We tested the efficacy of adding crushed limestone of various grades (particle sizes) to spawning shoals to increase survival of eggs and embryos of lake trout Salvelinus namaycush in acidic lakes. Spawning lake trout did not avoid shoals where limestone was applied, and egg density from natural spawning was the same in both treated and untreated areas. Experimental limestone treatment increased pH and calcium while decreasing concentrations of inorganic aluminum in the interstitial water of the spawning shoals. Under ambient conditions in which control embryo mortality was high, incubation within limestone substrate significantly increased the survival of embryos. We find that shoal liming is an effective management technique to improve lake trout embryo survival in acidic lakes where whole-lake water quality can still support juvenile and adult lake trout.

Fish population losses from Adirondack Lakes: The role of surface water acidity and acidification

Changes over time in the species composition of fish communities in Adirondack lakes were assessed to determine (1) the approximate numbers offish populations that have been lost and (2) the degree to which fish population losses may have resulted from surface water acidification and acidic deposition. Information on the present‐day status offish communities was obtained by the Adirondack Lakes Survey Corporation, which surveyed 1469 Adirondack lakes in 1984–1987 (53% of the total ponded waters in the Adirondack ecological zone). Two hundred and ninety‐five of these lakes had been surveyed in 1929–1934 during the first statewide biological survey; 720 had been surveyed in one or more years prior to 1970. Sixteen to 19% of the lakes with adequate historical data appeared to have lost one or more fish populations as a result of acidification. Brook trout and acid‐sensitive minnow species had experienced the most widespread effects. Populations of brook trout and acid‐sensitive minnows had been lost apparently as a result of acidification from 11% and 19%, respectively, of the lakes with confirmed historical occurrence of these taxa. By contrast, fish species that tend to occur primarily in lower elevation and larger lakes, such as largemouth and smallmouth bass and brown trout, have experienced little to no documented adverse effects. Lakes that were judged to have lost fish populations as a result of acidification had significantly lower; pH and, in most cases, also had higher estimated concentrations of inorganic aluminum and occurred at higher elevations than did lakes with the fish species still present. No other lake characteristics were consistently associated with fish population losses attributed to acidification. The exact numbers and proportions of fish populations affected could not be determined because of limitations on the quantity and quality of historical data. Lakes for which we had adequate historical data to assess long‐term trends in fish communities were significantly larger and deeper and have higher pH than do Adirondack lakes in general; thus, fish communities adversely affected by acidification and acidic deposition may be underrepresented in this study.

Prediction of future short-term stream chemistry — a modelling approach

The problem of predicting future short-term chemical behaviour in acidic and acid-sensitive streams is addressed. A relatively simple method is presented which combines a chemical technique for splitting the hydrograph into ground water and soil water components, based on the conservative component acid neutralization capacity, with the long-term hydrochemical model (MAGIC) in a ‘two-box’ mode. This method, coupled with a chemical speciation program (ALCHEMI), is used to assess short-term variations in stream chemistry with change in atmospheric deposition chemistry. The method is applied for a semi-natural moorland catchment in mid-Wales (Afon Gwy). For both hydrogen ion and inorganic aluminium, the modelled stream mixing relationships are non-linear, particularly at low hydrogen ion concentrations. The results also show that the relationship between hydrogen ion and inorganic aluminium concentrations varies with time in the stream. This result has important implications as it implies that aluminium concentrations will not recover, with reduced anthropogenic sulphur deposition, as rapidly as has previously been thought. Two methods, for use with critical load evaluation of ecological stress, are presented for describing the changes occurring: the hydrogen ion and inorganic aluminium concentration duration curve; the hydrogen ion and inorganic aluminium incident frequenty diagram.

Stormflow hydrochemistry of a small Welsh upland catchment

The hydrochemistry of a small upland catchment during a summer storm, following six weeks of low flow, is described. The storm exhibited several unusual features which may be attributed to the dry antecedent conditions. Streamwater concentrations of all ions analysed increased during the event but the timing of the increases varied according to the initiation of flows controlling the supply of particular ions. The increase in the concentration of calcium in the stream coincided with onset of overland flow, and appears to result from flushing of accumulated weathering products from a narrow strip of mire which provides a major pathway for storm water in the catchment. Natural soil pipes appear to be the main source of the increase in streamwater concentrations of both organically and inorganically complexed aluminium. Concentrations of inorganic aluminium in pipe water were considerably greater than those observed during previous events, possibly another result of the long period without flow. During the stream recession, inorganic aluminium remained high, reflecting a contribution from aluminium-rich throughflow. It is concluded that the chemistry of the stream at any given time is controlled by the relative importance of contributing sources of water which is, in turn, controlled by hydrological factors such as antecedent conditions and rainfall intensity and duration.

Predicting the long-term variations in stream and lake inorganic aluminium concentrations for acidic and acid sensitive catchments

A modification to the one-box version of the MAGIC model of stream and lake acidification is given which presents a new hypothesis for describing inorganic aluminium controls in streams draining acidic catchments. This modification entails the removal of any assumption of Al(OH) 3 solubility control in the stream. Success of this modification is demonstrated for data collected from a regional survey of Welsh streams and lakes. The modified model is used with data from the acidic and acid impacted Dargall Lane catchment of the Galloway region of South West Scotland to provide an example of the long-term changes in stream acidity and inorganic aluminium concentration.