Controls on the discharge of Chalk streams of the Berkshire Downs,UK

The water quality and biological functioning of major UK rivers draining permeable basins cannot be fully understood without knowledge of the spatial and temporal controls on the aquifer-river system. In this paper, hydrogeological features of Chalk streams feeding the Thames, the focus of this special issue, are described. Large-scale features, such as line sinks along the Thames and major tributaries such as the Kennet, tend to determine the regional groundwater flow patterns in the Chalk aquifer, whereas the complex characteristics of the Chalk dominate the local spatial and temporal patterns of groundwater discharge to the tributaries draining the dip slopes. These different controls are described and illustrated by particular reference to the Lambourn (a sub-catchment of the Kennet) and Pang catchments. Various issues requiring further investigation are also highlighted.     

The water quality of the River Thame in the Thames Basin of south/south-eastern England

The water quality of the River Thame, a tributary of the River Thames in the Thames basin, is described in relation to point and diffuse contaminant inputs and runoff from permeable and impermeable bedrock geology with their own characteristic water quality. The data is examined to see if the market town of Aylesbury in the upper part of the catchment influences water quality. Previous studies highlighted the influence of Aylesbury sewage treatment works (STW) on soluble reactive phosphorus (SRP) concentrations in the river before and after phosphorus (P) stripping at the STW. Variations in water quality along the river are described and the study indicates that, apart from SRP, water quality determinants seem to be relatively unaffected by Aylesbury. The Thame water quality is compared with other catchment typologies and it is very similar to that of the main stem of the Thames even though the Thames is mainly Chalk groundwater fed. Differences in water quality largely link to the amount of STW effluent within the rivers and to the endmember compositions of the groundwater and near surface water sources.     

Phosphorus concentrations in the River Dun, the Kennet and Avon Canal and the River Kennet, southern England

Variations in phosphorus (P) concentrations in an agriculturally impacted river draining a Chalk aquifer and an associated canal in the west of the Thames Basin, southern England are examined and linked to agricultural and sewage sources and within river/canal process controls. The study area comprises the River Dun, the adjacent River Kennet and the Kennet and Avon (K&A) Canal. Large seasonal variations are observed for soluble reactive phosphorus (SRP) and dissolved silicon (Si) with low concentrations in the spring and summer times when biological activity is high. The K&A Canal shows the largest SRP and Si concentration declines. This reflects high biological activity coupled with higher temperatures and higher water residence times. The extent of SRP removal is examined in relation to organic (uptake/release with phytoplankton growth/decay) and, to a lesser extent, inorganic (SRP coprecipitation with calcite) mechanisms. Boron (B) is used as a tracer of sewage sources. Agricultural inputs of both dissolved and particulate P (PP) can be important particularly under conditions where the catchment is wet and near surface/overland flow is important: sewage treatment works effluent and septic tank discharges to groundwater also probably provide a major component of the SRP occurring within the water column. The canal, and to a lesser extent the river, acts as sink for P in sewage effluent sources due to the high biological activity especially during the spring and summer. The aquifer probably acts as a major sink for agricultural and septic tank inputs of P.     

Predicting phosphorus concentrations in British rivers resulting from the introduction of improved phosphorus removal from sewage effluent

Phosphorus (P) concentration and flow data gathered during the 1990s for a range of British rivers were used to determine the relative contributions of point and diffuse inputs to the total P load, using the Load Apportionment Model (LAM). Heavily urbanised catchments were dominated by sewage inputs, but the majority of the study catchments received most of their annual phosphorus load from diffuse sources. Despite this, almost 80% of the study sites were dominated by point source inputs for the majority of the year, particularly during summer periods when eutrophication risk is greatest. This highlights the need to reduce sewage P inputs to improve the ecological status of British rivers. These modelled source apportionment estimates were validated against land-use data and boron load (a chemical marker for sewage).The LAM was applied to river flow data in subsequent years, to give predicted P concentrations (assuming no change in P source inputs), and these estimates were compared with observed concentration data. This showed that there had been significant reductions in P concentration in the River Thames, Aire and Ouse in the period 1999 to 2002, which were attributable to the introduction of P stripping at sewage treatment works (STW). The model was then used to forecast P concentrations resulting from the introduction of P removal at STW to a 2 or 1 mg l^− 1 consent limit. For the urbanised rivers in this study, the introduction of phosphorus stripping to a 1 mg l^− 1 consent level at all STW in the catchment would not reduce P concentrations in the rivers to potentially limiting concentrations. Therefore, further sewage P stripping will be required to comply with the Water Framework Directive. Diffuse P inputs may also need to be reduced before some of the highly nutrient-enriched rivers achieve good ecological status.     

Declines in phosphorus concentration in the upper River Thames (UK): Links to sewage effluent cleanup and extended end-member mixing analysis

Phosphorus concentrations in the upper River Thames Basin (southeastern England) are described and linked to sewage effluent sources. Weekly surveys between 1997 and 2007 of the Thames and two of its major tributaries, the Thame and the Kennet indicated that phosphorus was mainly in soluble reactive (SRP) form. Baseflow concentrations in the Thames reduced from 1584 μg/l in 1998 to 376 μg/l in 2006 and from 2655 to 715 μg/l for the Thame. Flow response, flux and endmember mixing analysis indicated that these declines resulted from SRP reductions in sewage treatment works (STW) effluent following phosphorus stripping for the major STWs in the region. This was confirmed by comparing our analysis with direct measurements of SRP in the effluents based on Environment Agency data. A within-river loss under baseflow of ~ 64% (range 56-78%) of the SRP-effluent input was estimated for the Thames, with a near balance for the Thame. SRP concentrations in the Kennet were an order of magnitude lower than the Thames/Thame: non-point sources dominated and were important for all the rivers at high flows. It was concluded that removal of SRP from effluents would be insufficient SRP in the Thames and Thame to meet annual average environmental targets of 50 to 120 μg/l.The paper flags the value of combining hydrological/chemical tracing and concentration/flux approaches to data interrogation and the bonus of having actual measurements of the effluent. It highlights the need for fuller assessment of water storage/sediment/biota interactions for phosphorus and for caution in using boron as a long-term tracer for effluent inputs, its concentrations having declined markedly in response to reduced usage in washing powders: the value of using sodium as a tracer for examining SRP changes is shown.     

Phosphorus sources,speciation and dynamics in the lowland eutrophic River Kennet,UK

This paper examines the behaviour of phosphorus (P) in a lowland chalk (Cretaceous-age) stream, the upper River Kennet in southern England, which has been subject to P remediation by tertiary treatment at the major sewage treatment works in the area. The effects of treatment are examined in relation to boron, a conservative tracer of sewage effluent and in terms of the relative contributions of soluble reactive phosphorus (SRP) loads from point and diffuse sources, and in-stream SRP loads. These results indicate a baseline reduction in in-stream SRP concentrations immediately following P-treatment of approximately 72%. Subsequent high flows result in a greater contribution of diffuse inputs and increases in SRP levels relative to the initial post-treatment period. The dynamics of SRP and particulate phosphorus (PP) are examined under a wide range of river flow conditions. Given the flashy nature of near-surface runoff in the River Kennet, sub-weekly (daily automated) sampling was used to examine the dynamics in SRP and PP concentrations in response to storm events. Simple empirical models linking weekly SRP concentrations with flow were developed. The empirical models were successfully applied to the daily data, to partition TP measurements and provide an estimate of daily SRP and PP concentrations. Mass balance studies were used to examine net gains and losses along the experimental river reach and indicate large net losses (up to 60%) during the extreme low flows and high SRP concentrations prior to P-treatment, which may be linked to extensive epiphytic growth. Phosphorus dynamics and response to P-treatment are discussed in relation to hydrological controls in permeable chalk catchments and wider implications for eutrophication management are examined.     

Chlorophyll-a in the rivers of eastern England

Chlorophyll-a concentration variations are described for two major river basins in England, the Humber and the Thames and related to catchment characteristics and nutrient concentrations across a range of rural, agricultural and urban/industrial settings. For all the rivers there are strong seasonal variations, with concentrations peaking in the spring and summer time when biological activity is at its highest. However, there are large variations in the magnitude of the seasonal effects across the rivers. For the spring-summer low-flow periods, average concentrations of chlorophyll-a correlate with soluble reactive phosphorus (SRP). Chlorophyll-a is also correlated with particulate nitrogen (PN), organic carbon (POC) and suspended sediments. However, the strongest relationships are with catchment area and flow, where two straight line relationships are observed. The results indicate the importance of residence times for determining planktonic growth within the rivers. This is also indicated by the lack of chlorophyll-a response to lowering of SRP concentrations in several of the rivers in the area due to phosphorus stripping of effluents at major sewage treatment works. A key control on chlorophyll-a concentration may be the input of canal and reservoir waters during the growing period: this too relates to issues of residence times. However, there may well be a complex series of factors influencing residence time across the catchments due to features such as inhomogeneous flow within the catchments, a fractal distribution of stream channels that leads to a distribution of residence times and differences in planktonic inoculation sources. Industrial pollution on the Aire and Calder seems to have affected the relationship of chlorophyll-a with PN and POC. The results are discussed in relation to the Water Framework Directive.     

Calcite saturation in eastern UK rivers

Calcite saturation in eastern UK rivers is assessed in relation to the potential kinetic inhibition of calcite precipitation. Two well established inhibitors are considered: soluble reactive phosphorus (SRP, i.e. inorganic monomeric phosphorus); and dissolved organic carbon (DOC). The rivers show a range of calcite saturation levels from approximately 1 hundredth to approximately 100-fold. The greatest range occurs for the northernmost river considered, the Tweed, where the waters range from highly unsaturated to highly oversaturated. The lowest range occurs for the most southerly rivers (the Great Ouse and the Thames) where the waters are consistently oversaturated with respect to calcite. The contrasting patterns relate to a greater diversity of water quality within the northern regions. Thus, during the winter, the main waters are derived from the upland areas with acidic soils and low weathering rates. During the summer baseflow periods, groundwater inputs are more important and high photosynthesis results in particularly high pHs and calcite oversaturation. In contrast, for the southern rivers, the main source of water during both the summer baseflow and the winter highflow periods comes from calcium carbonate rich aquifer sources. Statistical analysis of pH vs. the logarithm of the calcite saturation index for each river indicates strong linear features with individual gradients of approximately 1. This linearity results from an autocorrelation (the logarithm of the saturation index is calculated from the pH) and this indicates that calcite solubility controls are not operative in any of the rivers examined. A comparison of calcite saturation levels and SRP and DOC concentrations show a pattern inconsistent with kinetic hindrance, although some structure is observed, probably due to the mixing reactions between point and diffuse sources of water with contrasting chemistry.     

Sewage-effluent phosphorus: a greater risk to river eutrophication than agricultural phosphorus?

Phosphorus (P) concentrations from water quality monitoring at 54 UK river sites across seven major lowland catchment systems are examined in relation to eutrophication risk and to the relative importance of point and diffuse sources. The over-riding evidence indicates that point (effluent) rather than diffuse (agricultural) sources of phosphorus provide the most significant risk for river eutrophication, even in rural areas with high agricultural phosphorus losses. Traditionally, the relative importance of point and diffuse sources has been assessed from annual P flux budgets, which are often dominated by diffuse inputs in storm runoff from intensively managed agricultural land. However, the ecological risk associated with nuisance algal growth in rivers is largely linked to soluble reactive phosphorus (SRP) concentrations during times of ecological sensitivity (spring/summer low-flow periods), when biological activity is at its highest. The relationships between SRP and total phosphorus (TP; total dissolved P+suspended particulate P) concentrations within UK rivers are evaluated in relation to flow and boron (B; a tracer of sewage effluent). SRP is the dominant P fraction (average 67% of TP) in all of the rivers monitored, with higher percentages at low flows. In most of the rivers the highest SRP concentrations occur under low-flow conditions and SRP concentrations are diluted as flows increase, which is indicative of point, rather than diffuse, sources. Strong positive correlations between SRP and B (also TP and B) across all the 54 river monitoring sites also confirm the primary importance of point source controls of phosphorus concentrations in these rivers, particularly during spring and summer low flows, which are times of greatest eutrophication risk. Particulate phosphorus (PP) may form a significant proportion of the phosphorus load to rivers, particularly during winter storm events, but this is of questionable relevance for river eutrophication. Although some of the agriculturally derived PP is retained as sediment on the river bed, in most cases this bed sediment showed potential for removal of SRP from the overlying river water during spring and summer low flows. Thus, bed sediments may well be helping to reduce SRP concentrations within the river at times of eutrophication risk. These findings have important implications for targeting environmental management controls for phosphorus more efficiently, in relation to the European Union Water Framework Directive requirements to maintain/improve the ecological quality of impacted lowland rivers. For the UK rivers examined here, our results demonstrate that an important starting point for reducing phosphorus concentrations to the levels approaching those required for ecological improvement, is to obtain better control over point source inputs, particularly small point sources discharging to ecologically sensitive rural/agricultural tributaries.     

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

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

Water quality, nutrients and the European union's Water Framework Directive in a lowland agricultural region: Suffolk, south-east England

The water quality of 13 rivers in the lowland, agricultural county of Suffolk is investigated using routine monitoring data for the period 1981 to 2006 collected by the Environment Agency of England and Wales (EA), and its predecessors, with particular emphasis on phosphorus (as total reactive phosphorus, TRP) and total (dissolved and particulate) oxidised nitrogen (TOxN — predominantly nitrate NO₃). Major ion and flow data are used to outline fundamental hydrochemical characteristics related to the groundwater provenance of base-flow waters. Relative load contributions from point and diffuse sources are approximated using Load Apportionment Modelling for both TRP and TOxN where concurrent flow and concentration data are available. Analyses indicate a mixture of point and diffuse sources of TRP, with the former being dominant during low flow periods, while for TOxN diffuse sources dominate.Out of 59 sites considered, 53 (90%) were found to have annual average TRP concentrations greater than 0.05 mg P l^− 1, and 36 (61%) had average concentrations over 0.120 mg P l^− 1, the upper thresholds for ‘High’ and ‘Good’ ecological status, respectively. Correspondingly, for TOxN, most of the rivers are already within 70% of the 11.3 mg N l^− 1 threshold, with two rivers (Wang and Ore) being consistently greater than this.It is suggested that the major challenge is to characterise and control point-source TRP inputs which, being predominant during the late spring and summer low-flow period, coincide with the peak of primary biological production, thus presenting the major challenge to achieving ‘good’ ecological status under the Water Framework Directive. Results show that considerable effort is still required to ensure appropriate management and develop tools for decision-support.     

Nutrient hydrochemistry for a groundwater-dominated catchment: the Hampshire Avon, UK

The patterns in nitrate and phosphorus sources, loads and concentrations in a groundwater-dominated lowland catchment, the Hampshire Avon, are examined and water quality signatures are used to identify a typology of headwater stream types. The major separations in water quality are linked to geology and groundwater chemistry as modified by the impacts of point source sewage effluents. The water quality of the major tributaries and the main stem of the River Avon are linked to the relative contributions of these source types, the impact of further direct effluent inputs to the main channel and in-stream processing. The tributaries and main stem of the Avon act as net sinks for total reactive phosphorus (TRP). Low concentrations of TRP were found in the Chalk groundwater and the groundwater system acts as an efficient buffer, removing and retaining TRP from water draining from the catchment surface into the aquifer. Thermodynamic analysis of calcium carbonate (CaCO3) solubility controls indicates that this natural 'self-cleansing mechanism' system within the groundwater may be directly linked to CaCO3-P co-precipitation within the aquifer matrix.     

River water quality of the River Cherwell: an agricultural clay-dominated catchment in the upper Thames Basin, southeastern England

The water quality of the River Cherwell and a tributary of it, the Ray, are described in terms of point and diffuse sources of pollution, for this rural area of the upper Thames Basin. Point sources of pollution dominate at the critical ecological low flow periods of high biological activity. Although the surface geology is predominantly clay, base flow is partly supplied from springs in underlying carbonate-bearing strata, which influences the water quality particularly with regards to calcium and alkalinity. The hydrogeochemistry of the river is outlined and the overall importance of urban point sources even in what would normally be considered to be rural catchments is stressed in relation to the European Unions Water Framework Directive. Issues of phosphorus stripping at sewage treatment works are also considered: such stripping on the Cherwell has reduced phosphorus concentrations by about a factor of two, but this is insufficient for the needs of the Water Framework Directive.     

Water quality of Scottish rivers: spatial and temporal trends

The Harmonised Monitoring Scheme (HMS) was initiated in 1974 and represents the best-organised source of temporal and spatial data describing water quality for the major mainland rivers of Scotland, England and Wales. This paper presents the first detailed analysis of HMS data for Scotland, and identifies temporal changes in water quality from 1974 to 1995. From a concomitant analysis of catchment land cover characteristics, water quality indices have been linked with potential 'drivers' of change, influencing both point and diffuse sources. Nitrate concentrations between rivers are highly correlated with the amount of arable land, and relationships exist between grassland cover, orthphosphate-P and suspended solids concentrations. Similarly, urban catchments are highly correlated with ammonium-N, orthophosphate-P and suspended solids. Spatial and temporal trends in water quality for the rivers of Scotland are discussed.     

The role of climate on inter-annual variation in stream nitrate fluxes and concentrations

In recent decades, temporal variations in nitrate fluxes and concentrations in temperate rivers have resulted from the interaction of anthropogenic and climatic factors. The effect of climatic drivers remains unclear, while the relative importance of the drivers seems to be highly site dependent. This paper focuses on 2-6 year variations called meso-scale variations, and analyses the climatic drivers of these variations in a study site characterized by high N inputs from intensive animal farming systems and shallow aquifers with impervious bedrock in a temperate climate. Three approaches are developed: 1) an analysis of long-term records of nitrate fluxes and nitrate concentrations in 30 coastal rivers of Western France, which were well-marked by meso-scale cycles in the fluxes and concentration with a slight hysteresis; 2) a test of the climatic control using a lumped two-box model, which demonstrates that hydrological assumptions are sufficient to explain these meso-scale cycles; and 3) a model of nitrate fluxes and concentrations in two contrasted catchments subjected to recent mitigation measures, which analyses nitrate fluxes and concentrations in relation to N stored in groundwater. In coastal rivers, hydrological drivers (i.e., effective rainfall), and particularly the dynamics of the water table and rather stable nitrate concentration, explain the meso-scale cyclic patterns. In the headwater catchment, agricultural and hydrological drivers can interact according to their settings. The requirements to better distinguish the effect of climate and human changes in integrated water management are addressed: long-term monitoring, coupling the analysis and the modelling of large sets of catchments incorporating different sizes, land uses and environmental factors.     

Faecal-indicator concentrations in waters draining lowland pastoral catchments in the UK: relationships with land use and farming practices

Faecal-indicator budget studies have shown marine bathing water quality at two small UK coastal resorts, Staithes and Newport, to be adversely affected by riverine inputs from lowland pastoral catchments (J. Chartered Inst. Water Environ. Mangt. 12 (1998) 414). The present paper reports on presumptive coliform (PC), presumptive Escherichia coli (PE) and presumptive streptococci (PS) concentrations at 43 sampling points on watercourses within these catchments, and on their relationship with land use and livestock-related management practices, such as grazing and slurry/manure applications. The results show > 10-fold elevations in geometric mean faecal-indicator concentrations under high-flow conditions, compared with low flow, with maximum high-flow geometric mean PC, PE and PS concentrations of 2.6 x 10(6), 1.8 x 10(6) and 4.4 x 10(5) cfu/100 ml, respectively. High-flow geometric mean concentrations exhibit highly significant positive correlations with land use/management variables associated with intensive livestock farming, both within the individual catchments and in the two combined. Additional factors, such as antecedent weather conditions and topography, contribute to inter-catchment variability in water quality. Although inputs from diffuse and point sources of pollution were not quantified, point sources (e.g. runoff from farm yards) seem likely to be significant. The findings suggest that it may be possible to develop generic statistical models to predict microbial water quality from land use and farm management data. They also provide indirect evidence that channel bed sediment 'stores' closely reflect land use within their catchments and that there is little die-off of organisms along watercourses.     

A model for nitrate-nitrogen transport and denitrification in the river Thames

A mathematical model describing the mass flow, including denitrification, of nitrate in the main river Thames has been developed. A three year period, including severe drought and flooding, was used to test the model. Good agreement was found between the nitrate concentrations observed at several sites on the river Thames and those calculated by the model. The model be used to establish the sensitivity of nitrate levels in the Thames above intakes to changes in tributary inputs which could be affected by various management strategies.     

The strategic significance of wastewater sources to pollutant phosphorus levels in English rivers and to environmental management for rural, agricultural and urban catchments

The relationship between soluble and particulate phosphorus was examined for 9 major UK rivers including 26 major tributaries and 68 monitoring points, covering wide-ranging rural and agricultural/urban impacted systems with catchment areas varying from 1 to 6000 km² scales. Phosphorus concentrations in Soluble Reactive (SRP), Total Dissolved (TDP), Total (TP), Dissolved Hydrolysable (DHP) and Particulate (PP) forms correlated with effluent markers (sodium and boron) and SRP was generally dominant signifying the importance of sewage sources. Low flows were particularly enriched in SRP, TDP and TP for average SRP > 100 μg/l indicating low effluent dilution. At particularly low average concentrations, SRP increased with flow but effluent sources were still implicated as the effluent markers (boron in particular) increased likewise. For rural areas, DHP had proportionately high concentrations and SRP + DHP concentrations could exceed environmental thresholds currently set for SRP. Given DHP has a high bioavailability the environmental implications need further consideration. PP concentrations were generally highest at high flows but PP in the suspended solids was generally at its lowest and in general PP correlated with particulate organic carbon and more so than the suspended sediment in total.Separation of pollutant inputs solely between effluent and diffuse (agriculture) components is misleading, as part of the “diffuse” term comprises effluents flushed from the catchments during high flow. Effluent sources of phosphorus supplied directly or indirectly to the river coupled with within-river interactions between water/sediment/biota largely determine pollutant levels.The study flags the fundamental need of placing direct and indirect effluent sources and contaminated storage with interchange to/from the river at the focus for remediation strategies for UK rivers in relation to eutrophication and the WFD.     

Seasonal nutrient dynamics in a chalk stream: the River Frome, Dorset, UK

Chalk streams provide unique, environmentally important habitats, but are particularly susceptible to human activities, such as water abstraction, fish farming and intensive agricultural activity on their fertile flood-meadows, resulting in increased nutrient concentrations. Weekly phosphorus, nitrate, dissolved silicon, chloride and flow measurements were made at nine sites along a 32 km stretch of the River Frome and its tributaries, over a 15 month period. The stretch was divided into two sections (termed the middle and lower reach) and mass balances were calculated for each determinand by totalling the inputs from upstream, tributaries, sewage treatment works and an estimate of groundwater input, and subtracting this from the load exported from each reach. Phosphorus and nitrate were retained within the river channel during the summer months, due to bioaccumulation into river biota and adsorption of phosphorus to bed sediments. During the autumn to spring periods, there was a net export, attributed to increased diffuse inputs from the catchment during storms, decomposition of channel biomass and remobilisation of phosphorus from the bed sediment. This seasonality of retention and remobilisation was higher in the lower reach than the middle reach, which was attributed to downstream changes in land use and fine sediment availability. Silicon showed much less seasonality, but did have periods of rapid retention in spring, due to diatom uptake within the river channel, and a subsequent release from the bed sediments during storm events. Chloride did not produce a seasonal pattern, indicating that the observed phosphorus and nitrate seasonality was a product of annual variation in diffuse inputs and internal riverine processes, rather than an artefact of sampling, flow gauging and analytical errors.