Variations In Stream Water Chemistry Research Articles

Long‐term changes in stream water chemistry in small forested watersheds in the northern Kanto region

AbstractConcerns have been raised regarding the degradation of stream water quality due to the excessive influx of atmospheric deposition. This study aimed to reveal the long‐term variation in stream water chemistry in approximately 40 forested small watersheds in the northern Kanto region, based on 3 surveys conducted in 1991–1992, 2006–2007, and 2022. The factors influencing each dissolved element were investigated. Regarding long‐term variations in stream water chemistry, relatively stable concentrations of cations (Ca2+, Mg2+, K+, and Na+) and SiO2 were observed. However, the concentrations of anions (Cl−, , and ) decreased due to the improvement in the atmospheric environment and the diminishing effect of fertilization. Furthermore, the recent increase in stream water pH was attributed to an increase in bicarbonate (), compensating for the decrease in anion concentrations relative to cations. Geology was the most significant factor for inter‐watershed variations in cations and SiO2, and this influence remained relatively constant over 30 years. Forest practices, especially fertilization, had a significant effect on Cl− and , and contributed to higher concentrations in the fertilized watersheds than in the non‐fertilized watersheds over the 15 years following fertilization. Sulfate deposition was a significant influencing factor for , and the concentration fluctuated under long‐term variations in deposition over the past 30 years. Despite substantial acid deposition in the study area, the absence of stream water acidification could be attributed to the abundant cation supply from bedrock and volcanic ash, which underwent weathering processes.

Direct and Indirect Drivers of Energy and Nutrient Availability in Freshwater Ecosystems Across Spatial Scales

AbstractFreshwater ecosystems reflect the landscapes in which they are embedded. The biogeochemistry of these systems is fundamentally linked to climate and watershed processes that control fluxes of water and the mobilization of energy and nutrients imprinting as variation in stream water chemistry. Disentangling these processes is difficult as they operate at multiple scales varying across space. We examined the relative importance of climate, soil, and watershed characteristics in mediating direct and indirect pathways that influence carbon and nitrogen availability in streams and rivers across spatial scales. Our data set comprised landscape and climatic variables and 37,995 chemistry measurements of carbon and nitrogen across 459 streams and rivers spanning the continental United States. Models explained a small fraction of carbon and nitrogen concentrations at the continental scale (25% and 6%, respectively) but 61% and 40%, respectively, at smaller spatial scales. Hydrometeorological processes were always important in mediating the availability of solutes but the mechanistic implications were variable across spatial scales. The influence of hydrometeorology on concentrations was often not direct, rather it was mediated by soil characteristics for carbon and watershed characteristics for nitrogen. For example, the seasonality of precipitation was often important in determining carbon concentrations through its influence on soil moisture at biogeoclimatic spatial scales, whereas it had a direct influence on concentrations at the continental scale. Our results suggest that hydrometeorological forcing remains the consistent driver of energy and nutrient concentrations but the mechanism influencing patterns varies across broad spatial scales.

Stream health assessment using chemical and biological multi-metric models and their relationships with fish trophic and tolerance indicators

Abstract Nonsan Stream is a tributary of the Geum River that is strongly affected by urbanization and agricultural activities in the watershed. The present study was performed to diagnose the chemical and biological health of the stream from 2010 to 2016 using the multi-metric water pollution index (WPI) and index of biotic integrity (IBI) models. Monthly and annual variations in stream water chemistry were directly influenced by the monsoon season. Inputs of total phosphorus, suspended solids, and ionic substances, as well as dilution of total nitrogen, occurred during July and August. Chlorophyll increase was largely influenced by total phosphorus (R2 = 0.43, p

Stream water quality affected by interacting hydrological and biogeochemical processes in a riparian wetland

Riparian wetlands as both hydrological and biogeochemical hot spots often have a major impact on the release of solutes from headwater catchments. Numerous studies give some evidence of a rather complex interplay of hydrological and biogeochemical processes that is still poorly understood. This study seeks to address this challenge using a multivariate solute concentration data set from a small riparian headwater wetland. First, a non-linear variant of the Principal Component Analysis (Isomap) was performed in a preceding study to identify prevailing biogeochemical processes controlling water chemistry. Second, the scores of the components of the stream draining the wetland were subjected to a cluster analysis to identify typical biogeochemical patterns for different biogeochemical and hydrological boundary conditions.Four different clusters could be identified, which roughly followed a seasonal pattern, although modified by hydrological boundary conditions in the short-term. During the first three months of the year, Cluster 3 prevailed, indicating a discharge of rather unaltered shallow to mean depth groundwater. Cluster 3 was increasingly replaced by Cluster 2 and then subsequently by Cluster 1, indicating increasingly anoxic conditions, increasing denitrification and desulphurization, and increasing decomposition of organic carbon reflecting increasing biological activity and increasing water residence time within the wetland. However, stream water during stormflow after extended periods of low groundwater level in the second half of the growth season exhibited a very distinct pattern, represented by the fourth cluster. It indicated strong oxic conditions causing enhanced oxidation of sulphides, a corresponding decrease in pH values, and a substantial increase in the concentration of alkaline earth ions, manganese and in electric conductivity during the dry period.It is concluded that temporal variations in stream water chemistry clearly reflected the intensity of biological activity in the wetland, interacting with water table dynamics. Our results provide strong evidence for major effects of single extreme events like drought periods which are expected to become more frequent because of climate change.

Identifying the influence factors at multiple scales on river water chemistry in the Tiaoxi Basin, China

The catchment environment and landscape is widely used as a predictor of stream-ecosystem condition, and the extent of its influence is closely linked to spatial scale. The aim of this study was to identify the influence factors on river water chemistry at multiple scales in a basin, namely the catchment, riparian corridor, and river reach scales. Information about the catchment and riparian corridor landscapes, reach-scale river properties, and catchment environments and river water chemistry data were collected monthly from 31 streams across the Tiaoxi Basin from July 2011 to June 2012. We used redundancy analysis to identify the relative influences of multi-scale variables on nine water quality indexes over both the whole study period and three sub-periods (before, during, and after rainy seasons). Results showed that all the selected factors helped to explain variations in water chemistry, although the relative effects of these factors changed considerably with variation in the spatial and temporal scales. Stream water chemistry across the entire study period was more sensitive to physiography and landscape variable at the catchment scale than at the reach and riparian corridor scales. From dry seasons to the rainy season, the influence of physiography and landscape variable at the catchment scale decreased slightly, while the effects of variables at the reach and riparian corridor scales increased noticeably. Besides, the influence of variables at the catchment scale was relatively strong and stable while the impacts of variables at the local scale were relatively weak and fluctuated widely with seasons. The findings from this study may improve our understanding of the main drivers of variations in stream water chemistry in different spatial and temporal scales, and will help managers protect and restore stream water environments using a basin-scale perspective.

Redistribution of soil metals and organic carbon via lateral flowpaths at the catchment scale in a glaciated upland setting

Emerging evidence shows that interactions between soils and subsurface flow paths contribute to spatial variations in stream water chemistry in headwater catchments. However, few have yet attempted to quantify chemical variations in soils at catchment and hillslope scales. Watershed 3 (WS3) at Hubbard Brook Experimental Forest, New Hampshire, USA, was studied in order to better understand pedogenesis and its relationship to subsurface water dynamics. In WS3, 99 soil profiles were described, sampled by horizon, and assigned to a hydropedologic unit (HPU), a functional classification previously developed using landscape and morphological metrics which corresponded with distinct water table regimes. Soil samples were extracted with 1) citrate-dithionite (d) and analyzed for Fed and Mnd; and 2) acid ammonium oxalate (o) and analyzed for Alo, Feo and the rare earth elements Lao, Ceo, and Pro. Total organic C was also measured. These elements were redistributed via vertical and lateral podzolization. Typical (horizontally layered) podzols developed in the majority of the catchment due to predominantly vertical, unsaturated flow. However, lateral flow produced four other podzol types with distinct chemistry; thicker spodic horizons of laterally accumulating soils generally reflected larger pools of trace metals and subsoil organic C. The spatial distribution of positive cerium-anomalies (Ce/Ce*) in soil profiles proved to be a consistent hydropedologic indicator of lateral flow and seasonally high water table in three hillslopes. Despite occasional high water table in some of the HPUs, they were not hydric soils and were distinct from wetter podzols of coastal plains due to their high Fe content. This study suggests that vertical and lateral spatial variation in soil chemical composition, including the complexity of Ce distribution, as it relates to subsurface water dynamics should be considered when studying or predicting catchment scale functions such as stream solute export and biogeochemical processes.

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 SO42− concentration displayed in the middle towards the end of the wet season. The magnitude of peaks in SO42− concentration (13.5–60.6 μmolc/L) reflects deposition during the first 2 months of the wet season (March and April) in respective years (60.8–170 molc/ha). Release of SO42− with H+, which is retained in soil during the early wet season, may cause acidification later in the season. The deposition and concentration of SO42− declined over 6 years. However, the pH of stream water declined with increasing concentrations of SO42− and other major ions. The release of materials accumulated in the ecosystem was facilitated by the decrease in SO42− concentration/deposition and increased precipitation in the middle–late wet season. The retention-release cycle of SO42− largely contributed to both seasonal and interannual variations in stream water chemistry in the tropical savanna climate studied.

In-stream metabolism and atmospheric carbon sequestration in a groundwater-fed karst stream.

Atmospheric carbon sequestered in karst systems through dissolution of carbonate minerals is considered to have no net effect on long-term regional and global carbon budgets because precipitation of dissolved carbonate minerals emits CO2 back to the atmosphere. Even though recent studies have implied that rapid kinetics of carbonate dissolution coupled with the aquatic photosynthetic uptake of dissolve inorganic carbon (DIC) could facilitate a stable atmospheric C sink in karst rivers and streams, little is known about the magnitudes and long-term stability of this C sink. To assess in-stream biogeochemical processes and their role on stream C cycling, we measured diel cycles of water characteristics and chemical composition (temperature, pH, DO, SpC, DIC, Ca2+, δ13CDIC) in a groundwater-fed karst stream in southwest China. Our results show no diel variations at the groundwater discharge point (CK site) due to the absence of a sub-aquatic community (SAC). However, all hydrochemical parameters show significant diel cycle 1.3km downstream (LY site). Diel variations in pH, DO, and δ13CDIC were inversely related to diel changes in SpC, DIC, Ca2+ and pCO2. This result indicates that in-stream metabolism (photosynthesis and respiration) of SAC controls diel variations in stream water chemistry. Significant diel cycles of net ecosystem production (NEP) influences in-stream diel fluctuation of pH, DO, SIc, DIC, pCO2, Ca2+ and δ13CDIC, with gross primary production (GPP) dominating in day and ecosystem respiration (ER) dominating at the night. Absence of in-stream metabolism at CK enhances CO2 degassing from stream to the atmosphere, which is estimated to be 3-5 times higher than at LY. We estimate the carbon sink through in-stream metabolism of SAC to be 73tCkm-2a-1, which is around half the rate of the oceanic biological pump. These results imply in-stream photosynthesis sequesters DIC originating from karst weathering and controls CO2 evasion.

Biogeochemistry on the Range: The Established Researcher

Photo by David Hamilton, used with permission. Anthony is much too kind. The real genesis of my interest, research, and writings about biogeochemistry began at Dartmouth College, where I rubbed shoulders with Bill Reiners and his first graduate student, Peter Vitousek. Peter and Bill's paper, “Ecosystem succession and nutrient retention—hypothesis” (1975, BioScience 25:376–381) along with Noye Johnson and his colleagues’ paper, “A working model for the variation in stream water chemistry at the Hubbard Brook Experimental Forest, New Hampshire” (1969, Water Resources Research 5:1353–1363) documented how the movement of the important nutrient elements for plants is closely controlled by biology, whereas the non-essential elements move through ecosystems, controlled by the laws of solubility and other physical processes. I have carried that observation through my subsequent work, leading to the Chihuahuan Desert in southern New Mexico. Deserts have often been overlooked by biogeochemists, as places that are too dry for much to happen that is very interesting. I differ from that opinion. In deserts, you can see the stark contrast of what is controlled by biology and what is not. In a 1990 paper, “Biological feedbacks in global desertification” (Science 247:1043–1048), my Long-Term Ecological Research (LTER) colleagues at the Jornada Experimental Range formulated the Teeter-Totter model of desertification, in which landscapes tip from grassland to shrubland when perturbed by overgrazing and other external stressors. With shrub encroachment, the model predicted that the distribution of important soil nutrients would become heterogeneous across the landscape. My 1996 paper that Anthony referred to was a test of that hypothesis: comparing the spatial variation in important soil nutrients (N and P) to non-essential elements (e.g., Na and Li) across the landscape of desert grassland and shrubland habitats. We included sites in the Mojave Desert to document the nutrient distribution in long-term shrublands and sites in the grasslands of Colorado as an example of what to expect in a long-term grassland. A few years ago, I was gratified to see research by Jobbagy and Jackson (2001, Biogeochemistry 53:51–77) showing that the differing distributions we saw across the desert landscape were paralleled by differing distributions of soil nutrients with depth in ecosystems worldwide. All these are examples of biogeochemistry—the control of the chemical conditions at the surface of the Earth, on land and at sea, by the distribution and activities of organisms. The characteristics of our planet that we consider normal would not be as they are without life on Earth. G. E. Hutchinson saw it, Alfred Redfield saw it, and Herb Bormann saw it. And, to the distress of many, these characteristics are now being upset disproportionately by a single species, Homo sapiens, whose changes to the atmosphere, climate, and biodiversity threaten the persistence of the biosphere as we have come to know it.

Riparian and in-stream controls on nutrient concentrations and fluxes in a headwater forested stream

Abstract. Headwater streams are recipients of water sources draining through terrestrial ecosystems. At the same time, stream biota can transform and retain nutrients dissolved in stream water. Yet studies considering simultaneously these two sources of variation in stream nutrient chemistry are rare. To fill this gap of knowledge, we analyzed stream water and riparian groundwater concentrations and fluxes as well as in-stream net uptake rates for nitrate (NO3−), ammonium (NH4+), and soluble reactive phosphorus (SRP) along a 3.7 km reach on an annual basis. Chloride concentrations (used as conservative tracer) indicated a strong hydrological connection at the riparian–stream interface. However, stream and riparian groundwater nutrient concentrations showed a moderate to null correlation, suggesting high in-stream biogeochemical processing. In-stream net nutrient uptake (Fsw) was highly variable across contiguous segments and over time, but its temporal variation was not related to the vegetative period of the riparian forest. For NH4+, the occurrence of Fsw > 0 μg N m−1 s−1 (gross uptake > release) was high along the reach, while for NO3−, the occurrence of Fsw < 0 μg N m−1 s−1 (gross uptake < release) increased along the reach. Within segments and dates, Fsw, whether negative or positive, accounted for a median of 6, 18, and 20% of the inputs of NO3−, NH4+, and SRP, respectively. Whole-reach mass balance calculations indicated that in-stream net uptake reduced stream NH4+ flux up to 90%, while the stream acted mostly as a source of NO3− and SRP. During the dormant period, concentrations decreased along the reach for NO3−, but increased for NH4+ and SRP. During the vegetative period, NH4+ decreased, SRP increased, and NO3− showed a U-shaped pattern along the reach. These longitudinal trends resulted from the combination of hydrological mixing with terrestrial inputs and in-stream nutrient processing. Therefore, the assessment of these two sources of variation in stream water chemistry is crucial to understand the contribution of in-stream processes to stream nutrient dynamics at relevant ecological scales.

Spatio-temporal variation in stream water chemistry in a tropical urban watershed

Urban activities and related infrastructure alter the natural patterns of stream physical and chemical conditions. According to the Urban Stream Syndrome, streams draining urban landscapes are characterized by high concentrations of nutrients and ions, and might have elevated water temperatures and variable oxygen concentrations. Here, we report temporal and spatial variability in stream physicochemistry in a highly urbanized watershed in Puerto Rico. The main objective of the study was to describe stream physicochemical characteristics and relate them to urban intensity, e.g., percent impervious surface cover, and watershed infrastructure, e.g., road and pipe densities. The Rio Piedras Watershed in the San Juan Metropolitan Area, Puerto Rico, is one of the most urbanized regions on the island. The Rio Piedras presented high solute concentrations that were related to watershed factors, such as percent impervious cover. Temporal variability in ion concentrations lacked seasonality, as did all other parameters measured except water temperature, which was lower during winter and highest during summer, as expected based on latitude. Spatially, stream physicochemistry was strongly related to watershed percent impervious cover and also to the density of urban infrastructure, e.g., roads, pipe, and building densities. Although the watershed is serviced by a sewage collection system, illegal discharges and leaky infrastructure are probably responsible for the elevated ion concentration found. Overall, the Rio Piedras is an example of the response of a tropical urban watershed after major sewage inputs are removed, thus highlighting the importance of proper infrastructure maintenance and management of runoff to control ion concentrations in tropical streams.

Initial Effects of Wood Ash Application on the Stream Water Chemistry in a Boreal Catchment in Central Sweden

Due to increased whole-tree harvesting in Swedish forestry, concern has been raised that a depletion of nutrients in forest soil will arise. The Swedish Forest Agency recommends compensation fertilization with wood ash to ensure that unwanted effects are avoided in the nutrient balance of the forest soil and in the quality of surface water. In this investigation, the chemistry of two first-order streams, of which one was subjected to a catchment scale treatment with 3 tonnes of self-hardened wood ash/ha in the fall of 2004, was monitored during 2003–2006. Large seasonal variations in stream water chemistry made changes due to ash application difficult to detect, but evaluating the ash treatment effects through comparison of the stream water of the treated catchment with the reference was possible via statistical tools such as randomized intervention analysis in combination with cumulative sum charts. The wood ash application did not yield any significant effect on the pH in the stream water and hence did not affect the bicarbonate system. However, dissolved organic carbon increased, a previously unreported effect of WAA, bringing about an increase of organic anions in the stream water. The wood ash application also induced significant increases for Ca, Mg, K, Si, Cl and malonate, of which K was most prominent. Although significant, the changes induced by the wood ash application were all small compared to the seasonal variations. As a tool to counteract acidification of surface waters, WAA seems to have limited initial effects.

Linking soil- and stream-water chemistry based on a Riparian Flow-Concentration Integration Model

Abstract. The riparian zone, the last few metres of soil through which water flows before entering a gaining stream, has been identified as a first order control on key aspects of stream water chemistry dynamics. We propose that the distribution of lateral flow of water across the vertical profile of soil water chemistry in the riparian zone provides a conceptual explanation of how this control functions in catchments where matrix flow predominates. This paper presents a mathematical implementation of this concept as well as the model assumptions. We also present an analytical solution, which provides a physical basis for the commonly used power-law flow-load equation. This approach quantifies the concept of riparian control on stream-water chemistry providing a basis for testing the concept of riparian control. By backward calculation of soil-water-chemistry profiles, and comparing those with observed profiles we demonstrate that the simple juxtaposition of the vertical profiles of water flux and soil water chemistry provides a plausible explanation for observed variations in stream water chemistry of several major stream components such as Total Organic Carbon (TOC), magnesium, calcium and chloride. The "static" implementation of the model structure presented here provides a basis for further development to account for seasonal influences and hydrological hysteresis in the representation of hyporheic, riparian, and hillslope processes.

Variation in Streamwater Chemistry Throughout the Hubbard Brook Valley

Streamwater chemistry was measured at 100-m intervals in all streams of the Hubbard Brook Valley, NH during ‘spring’ (May–July) and during ‘fall’ (October–December) 2001. Overall, streamwater chemistry was very similar during these two periods, but fall median concentrations were consistently higher than spring values, except for ANC, pH, NO3 − and PO4 3−, which had lower values in fall. Median concentrations for NH4 + were approximately the same in spring and fall. Stream chemistry varied throughout the Hubbard Brook Valley by elevation, channel length, drainage area and type of drainage, but most of the variability in stream chemistry was subtle and relatively small. Overall, there were relatively large (two- to 10-fold) changes in chemistry with longitudinal distance of wetted channel, elevation and/or size of drainage area in some streams and for some elements (e.g., H+, Aln+, DOC), but other chemical concentrations changed relatively little (e.g., Cl−, dissolved Si). The main Hubbard Brook, a fifth-order stream at the mouth of the Valley, was remarkably constant in chemistry throughout its length, except where human disturbance near the mouth changed the chemistry. Differences in vegetation, geologic substrates and wetland areas were related to changes in pattern of streamwater chemistry throughout the Valley.

Modelling the effects of climate change on an acidic upland stream

Most modelling studies of soil and surface water recovery from acidification assume a constant influence of climate over the simulation period. Given the likelihood of future climate change, and recent identification of links between climatic fluctuations and surface water trends on decadal time scales, an attempt is made to simulate a number of climate-related effects on the recovery of an acidified stream in mid-Wales. Empirical relationships were derived to model past and future variations in (i) runoff, as a function of rainfall and temperature; (ii) stream chloride concentration and sea-salt deposition, as a function of the North Atlantic Oscillation Index (NAOI); and (iii) stream dissolved organic carbon (DOC) concentration, as a function of summer temperatures and sulphur deposition. Incorporating these relationships in the MAGIC dynamic model reproduced a significant part of the observed variation in streamwater chemistry. Current scenarios of rising UK temperature and NAOI, and falling rainfall and acid deposition, were used to forecast effects of climate change on soil and water recovery from acidification. Results suggest that increasing DOC would have a significant negative impact on soil recovery, with elevated organic acidity lowering soil water pH and severely depleting soil base saturation. The resulting increase in soil base cation export would, however, have a positive impact on streamwater recovery. For increased sea-salt deposition, effects are essentially the opposite; increased base cation inputs enhance the recovery of soil base saturation, but displace acidity to the stream. However, simulated effects of raising DOC considerably outweighed those of increased sea-salt deposition. It is argued that greater consideration of these, and other, climatic effects on biogeochemical cycles needs to be taken when predicting the recovery of soils and surface waters from acidification.

Variability of Lotic Macroinvertebrate Assemblages and Stream Habitat Characteristics Across Hierarchical Landscape Classifications

Streams are naturally hierarchical systems, and their biota are affected by factors effective at regional to local scales. However, there have been only a few attempts to quantify variation in ecological attributes across multiple spatial scales. We examined the variation in several macroinvertebrate metrics and environmental variables at three hierarchical scales (ecoregions, drainage systems, streams) in boreal headwater streams. In nested analyses of variance, significant spatial variability was observed for most of the macroinvertebrate metrics and environmental variables examined. For most metrics, ecoregions explained more variation than did drainage systems. There was, however, much variation attributable to residuals, suggesting high among-stream variation in macroinvertebrate assemblage characteristics. Nonmetric multidimensional scaling (NMDS) and multiresponse permutation procedure (MRPP) showed that assemblage composition differed significantly among both drainage systems and ecoregions. The associated R-statistics were, however, very low, indicating wide variation among sites within the defined landscape classifications. Regional delineations explained most of the variation in stream water chemistry, ecoregions being clearly more influential than drainage systems. For physical habitat characteristics, by contrast, the among-stream component was the major source of variation. Distinct differences attributable to stream size were observed for several metrics, especially total number of taxa and abundance of algae-scraping invertebrates. Although ecoregions clearly account for a considerable amount of variation in macroinvertebrate assemblage characteristics, we suggest that a three-tiered classification system (stratification through ecoregion and habitat type, followed by assemblage prediction within these ecologically meaningful units) will be needed for effective bioassessment of boreal running waters.

Factors regulating the spatial and temporal distribution of solute concentrations in a major river system in NE Scotland

The River Dee in NE Scotland, an oligotrophic soft water system, has a catchment area of approx. 2100 km 2, its source in the Cairngorm mountains being approx. 140 km from its outlet to the North Sea at Aberdeen. A comprehensive sampling strategy and analytical programme, commensurate with the size and nature of the Dee system, have been established for major water quality determinands to identify the controls on, and origins of, dissolved species throughout the system at a range of catchment scales and over a range of flow regimes. Fifty-nine sites covering a range of catchment types and scales were therefore sampled bi-weekly for 1 year. At the basin scale, there is a general downstream increase in determinand concentrations. This produces strong linear relationships between many determinands which are unrelated in terms of a common terrestrial process or origin. At the sub-catchment scale, however, specific hydrochemical processes control streamwater chemistry. The Dee basin divides into two distinct geographic regions in terms of land use (upland and lowland) which produce clear differences in water chemistry. Individual sub-catchments can also be grouped in terms of temporal variations in streamwater chemistry. The strength of the relationship between weathering-derived ionic concentrations and flow in the upland sub-catchments has lead to the identification of specific concentration limits in sub-catchments which can be used as characteristics of soil water and groundwater end-members. This provides a basis for the prediction of upland weathering-derived component concentrations for each sub-catchment at a range of flows.

Variation in stream water chemistry and hydrograph separation in a small drainage basin

In the present study, the change in chemical composition of stream water was investigated for a small Atlantic salmon stream (Catamaran Brook) of the Miramichi River system in New Brunswick, Canada. Chemical composition of runoff and groundwater flow was established, as were relations between concentration of dissolved materials and discharge. Specific storm events were analysed to determine changes in chemistry and to carry out a hydrograph separation using specific chemical parameters. The hydrograph separation was used to identify the relative contribution of groundwater flow to total streamflow. By selective sampling of stream water during high flow (runoff) and low flow (groundwater) periods it was possible to observe the range in chemical composition of many parameters in Catamaran Brook. Most relationships between concentration of chemical parameters and discharge were significant at P < 0.0001, with sodium having the highest coefficient of determination (r2 = 0.849). Concentration returned to pre-storm levels in approximately 10 days following an event. As observed in previous studies, the peak groundwater flow plays an important role during the storm hydrograph and can account for as much as 91 % of the total peak flow for small events. For higher flow events in Catamaran Brook, the groundwater flow contribution was markedly lower (55% of total streamflow). The composite hydrograph separation revealed that conductivity, as a single parameter, provided the best results in representing the composite separation.

Aluminium speciation variations in an acidic upland stream draining the Hafren spruce forest, Plynlimon, Mid-Wales

The speciation of aluminium in an acidic stream, draining a spruce plantation-forest in Mid-Wales, is examined using an established model based on an equilibrium thermodynamic approach. Trivalent aluminium and aluminium complexes with fluoride, with dissolved organic matter and with silica were abundant: aluminium hydroxy-fluorides and sulphates were much less so. There is a large scatter in the results, primarily due to variations in stream water chemistry at a given pH, rather than to an effect of temperature. It is concluded that aluminium silica complexes can be an important part of the total dissolved aluminium concentration: at pHs > 5, they comprise typically 20% of the total dissolved aluminium in the stream water.

Mixing and uptake of solutes in catchments: model development

Simple mixing models are widely used to explain temporal variations in streamwater chemistry and to infer the relative contributions of old and new water during runoff events. These models assume conservative mixing of fixed end members and ignore solute uptake while new water is resident in the ground. A simple modelling approach is proposed which combines two-component mixing with solute uptake using a first-order kinetic rate equation. In this approach new and old water have the same origin in precipitation but old water, by virtue of its longer residence time, is more enriched. Equations are presented and illustrated for predicting the chemical evolution of old, new and mixed water and for inferring new water contributions. One consequence of the model is that solute rating plots do not exhibit a unique and reversible concentration-discharge relationship but a set of shifted counter-clockwise loops. Methods of parameter estimation are discussed. If intensive time-series data are not available, estimation of the chemical uptake parameter is not possible for a single solute but is possible for two solutes given information on several runoff events.