Silicon concentrations in UK surface waters

Journal of Hydrology 304 (2005) 75–93 www.elsevier.com/locate/jhydrol

Silicon concentrations in UK surface waters Colin Neala,*, Margaret Neal, Brian Reynoldsb, Stephen C. Maberlyc, Linda Mayd, Robert C. Ferriere, Jennifer Smitha, Julie E. Parkerc a

Centre for Ecology and Hydrology Wallingford, Maclean Building, Crowmarsh Gifford, Wallingford, OXON, OX10 8BB, UK b Centre for Ecology and Hydrology Bangor, University of Wales Bangor, Deiniol Road, Bangor, Gwynedd, LL57 2UP, UK. Centre for Ecology and Hydrology Edinburgh, Bush Estate, Penicuik, Midlothian, EH26 0QB, UK c Centre for Ecology and Hydrology Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster, LA1 4AP, UK d Centre for Ecology and Hydrology Edinburgh, Bush Estate, Penicuik, Midlothian, EH26 0QB, UK e Macaulay Institute, Aberdeen, Scotland, AB15 8QH, UK Received 30 November 2003; revised 1 May 2004; accepted 1 July 2004

Abstract This paper describes the variations in silicon concentrations in UK waters for a wide range of catchment systems (near pristine, rural, and agricultural and urban impacted systems). The paper largely concerns silicon levels in streams, rivers and lakes based on extensive data collected as part of several research and monitoring initiatives of national and international standing. For a detailed study of an upland catchment in mid-Wales, information on atmospheric inputs and groundwater chemistries is provided to supply background information to cross link to the surface water chemistry. Several hundred streams/rivers and lakes are dealt with within the study, dealing with the main types of freshwater riverine and lacustrine environments. The streams/rivers vary from small ephemeral runoff to the major rivers of the UK. The geographical location of sites vary from local sites in mid-Wales, to regional studies across Scotland, to the major eastern UK rivers entering the North Sea and to acid sensitive upland sites across Wales, the English Lake District, Scotland and Northern Ireland. The surface waters range in silicon concentration from 0 to 19 mg-Si lK1 (average for individual sites vary between 0.7 and 7.6 mg-Si lK1) and there are some clear variations which link to two primary processes (1) the relative inputs of groundwaters enriched in silicon and near surface waters more depleted in silicon and (2) plankton uptake of silicon during the summer months under baseflow conditions. Thermodynamic analysis reveals that the waters are approximately saturated with respect to either quartz or chalcedony except for two circumstances when undersaturation occurs. Firstly, undersaturation occurs at pH less than 5.5 in the upland areas and this is because the waters are mainly sourced from the acidic organic soils which are depleted in inorganic minerals. Secondly, undersaturation occurs in the lowland rivers when biological activity is at its highest and this leads to silicon removal from the water column. Quartz equilibrium can be approached (at pHO5.5) mainly within the upland systems which are not aquifer recharge dominated. However, for the lowland systems that are groundwater recharge dominated, it is chalcedony saturation which is approached, and such saturation is often observed within groundwaters. Similar patterns of undersaturation in response to biological uptake are seen in lakes and the extent of silicon depletion increases with

* Corresponding author. Tel.: C44 1491 838800; fax: C44 1491 692424. E-mail address: [email protected] (C. Neal). 0022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2004.07.023

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biological productivity. Chalcedony oversaturation can occur for some UK rivers under baseflow conditions and this probably links to a higher rate of weathering. q 2004 Elsevier B.V. All rights reserved. Keywords: Silicon; Rivers; Groundwaters; Quartz; Chalcedony

1. Introduction Silicon in natural waters is primarily derived from the weathering of silicates and aluminosilicates in the bedrock and soils of an area (Berner and Berner, 1996; Drever, 1997) but there is considerable complexity to the terrestrial cycle for silicon and there can be intense internal cycling within the biomass (Conley, 2002) with both inorganic and biogenic mineral breakdown and storage (Jongmans et al., 1997; Van Breemen et al., 2000a,b; Conley et al., 2000; Conley, 2002; Hoffland et al., 2003; Berner and Berner, 1996) cross linked to issues of climatic type, climatic variability and hydrology (Meybeck, 1980; White and Blum, 1995). Silicon is an important nutrient in surface waters. For example, it is an essential nutrient for one group of microalgae, the diatoms (Reynolds, 1984). Also, there can be important relationships between silicon and the major nutrients, phosphorus in particular (Conley et al., 2000). While silicon is important to aquatic biology in relation to its nutrient role, it is also of significance to aquatic biology indirectly for acidic systems by reducing the environmental harm from aluminium release due to acidic deposition (Birchall et al., 1989; Exley et al., 1991). In this paper, the concentration and distribution of silicon in surface and groundwaters in the UK is assessed. For this, there are several main components. A report is given on the distribution of silicon in rainfall, cloud water, stream water and groundwater for the Plynlimon area in mid Wales to show the variations from atmospheric inputs to ground and stream water outputs. This work provides the output from almost 20 years of monitoring for one of the major UK catchment studies of acidic and acid sensitive upland systems (Neal, 1997a,b): it is augmented by an extension to a regional study for Wales (Neal et al., 1998). The Plynlimon findings link to issues of upland typologies and to reducing the environmental harm from aluminium: this latter aspect is not dealt with in this paper, as this is covered

in an earlier publication (Neal, 1995). The results are then set within a broader national framework based on a monitoring programme for upland sites across the UK (The UK Acid Waters Monitoring Network, UKAWMN; Monteith and Evans (2000), web site http://www.geog.ucl.ac.uk). Then, information collected as part of a community research initiative (Land Ocean Interaction Study (LOIS): Leeks and Jarvie, 1998; Huntley et al., 2001) dealing with chemical, sediment and water fluxes to and within the North Sea is presented for eastern UK rivers is linked to associated studies for other eastern UK rivers (Neal and Robson, 2000; Neal and Whitehead, 2002). Longterm data from Loch Leven, Loch Saugh and spatial data from twenty lakes in the English Lake District are presented for comparative purposes and there is also a summary of Harmonized Monitoring Scheme data for rivers and lakes across Scotland. Within the study, thermodynamic analysis of solubility controls adds weight to the issues of silicon regulation. Further, variations in silicon concentrations are linked to typologies and endmember/endmembermixing issues. Typologies refer to specific types of environment such as rural, agricultural and urban systems, while endmembers represent parts of the catchment system where specific hydrogeochemical provenances occur (e.g. acidic soil waters or groundwaters where weathering of the bedrock occurs). Thus, this paper brings together major sources of information to provide a national picture of silicon concentrations and solubility controls for the UK waters across a wide range of typological and hydrogeochemical provenance settings.

2. Resource 2.1. Study areas The data presented here covers a wide part of the UK and the locations are illustrated in Fig. 1. The details of the study areas are as follows.

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Fig. 1. Location map for the various surveys of silicon.

2.1.1. Plynlimon The headwaters of the River Severn (Afon Hafren, Afon Hore and Nant Tanllwyth) drain a hill top plateau dominated by acid moorland and the Hafren Forest in the lower half of the catchment. The soils in the area are a mixture of upland acid soil types dominated by peaty podzol with subsidiary peaty gley with deep peat in the moorland plateau area: the bedrock is fractured Lower Palaeozoic mudstones, shales and grits. The Hafren Forest, mainly Sitka

spruce with some Norway spruce, larch and lodgepole pine, planted in various phases from the mid 1940s to late 1960s. The water quality of the upper Severn is variable, with baseflow waters of good quality (calcium bicarbonate type) and more acidic and aluminium bearing water of much poorer quality occurring under stormflow conditions. For this study, information is drawn from monitoring programmes of rainfall, cloud water, throughfall, stemflow, stream water and groundwater covering time frames from

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one to twenty years (Neal and Kirchner, 2000, provide details of locations and monitoring periods): the general water quality functioning of the area is provided by Neal et al. (1997a,b,c) and broader aspects are covered in a special issue of Hydrology and Earth System Sciences (1997, vol. 1; Neal, 1997a). 2.1.1.1. Welsh regional survey. Within the Plynlimon study, the impacts of conifer harvesting were examined in detail However, to put the findings within a broader context, a regional survey was undertaken across Wales for small catchment areas (2–5 ha in area) for conifer forested and harvested systems. For this study campaigns were undertaken for baseflow and stormflow conditions with the help of Forest Enterprise staff with one to four surveys between autumn 1995 and spring 1998 (Neal et al., 1998). Within this paper, the silicon concentration results of this regional survey are presented.

3.

4.

2.1.2. Regional UK rivers and lakes This dataset comprises a range of acidic, acid sensitive and neutral to moderately basic sites with a range of geological settings across England, Scotland, Wales and Northern Ireland. The sites comprise: 1. Acidic and acid sensitive sites were studied as part of the UK Acid Waters Monitoring Network (UKAWMN) information on which is available via the UKAWMN web site at www.geog.ucl.ac. uk/ukawmn/. There are 22 sites in total, with an equal split of lake and stream sites: the regional breakdown is 9 sites (6 lakes and 3 streams) in Scotland, five sites in England (two lakes in the Lake District and three rivers elsewhere), 4 sites (2 lakes and 2 streams) in Wales and 4 sites (3 streams and 1 lake) in Northern Ireland. 2. Loch Leven is situated in the Perth and Kinross area of south-eastern Scotland. The data provided here was collected by CEH Edinburgh: this data provides more detailed temporal resolution through the year for the lakes which were otherwise usually less frequently monitored. The catchment is surrounded by the Ochil Hills (mostly andesite and basaltic lavas) to the north-west, the Lomond Hills to the north-east and the Cleish Hills (sandstone and mudstone) in the south. Much of

5.

6.

7.

the lower part of the catchment basin comprises Upper Devonian sandstone overlain by glacial sand and gravel deposits. Roughly 80% of the catchment is under agricultural use, about 11% is accounted for by woodland (predominantly coniferous) and some 2% by the major settlements. Rural houses, roads, water, playing fields, etc. make up the remainder. Loch Saugh and its catchment on the eastern edge of the Grampian Mountains is studied by the Macaulay Institute (MI) as one of its field research centres. The catchment is mainly short-term and permanent grassland cover and the underlying geology is schist and sandstone (www.macaulay. ac.uk/glensgh.htm). Data from twenty lakes in the Lake District (northwest England) are provided from a survey by CEH Lancaster which illustrates the effects of phosphorus loadings on biological influences on silicon levels. These Cumbrian lakes were created during the last glaciation and their catchments lie in upland areas of high rainfall on relatively hard slates or volcanic rock of Ordovician or Silurian age (Talling, 1999). The main land use is unimproved rough pasture grazed by sheep, but there are also areas of improved grassland and bracken. Regional river water data for Scottish rivers (56 monitoring points) as collected as part of the Harmonized Monitoring Programme. These cover a wide range of land use, soils and geological settings. Five eastern UK river basins, the Tweed, Wear, Humber, Great Ouse and Thames, as collected as part of the Land Ocean Interaction Study (LOIS, Neal and Robson, 2000). Four streams in Devon within the upper Taw catchment in Devon. These streams are part of the Institute of Grassland and Environmental Research (IGER) North Wyke experimental catchments. Two correspond to the upper Taw, one, site (the upper site) drains Dartmoor (granite) while the other further downstream drains both the Granite and Carboniferous sediments. The Two other sites drain Carboniferous sediments (Den Brook and Drewston) and for Den Brook there is highly gleyed soils with low permeability and high

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surface/near-surface runoff. For the Taw, there are also two rainfall sites. Samples from all sites were collected on a weekly basis. The details of the LOIS basins are as follows. The Tweed, a Site of Special Scientific Interest (SSSI), spans the eastern border between Scotland and England. It has a basin with low-intensity agriculture with uplands of moorland and rough pasture for hill sheep farming, to the west, and arable tilled regions in the lowlands to the east. The geology is largely Ordovician and Silurian age greywacke, shale and mudstone, Old Red Sandstone and Carboniferous age shale, greywacke and limestone, with some Old Red Sandstone Age intrusive granites and extrusive basic lavas to the south of the basin. The basins’ typology is ‘rural to pristine’ and the water is of good quality. The Wear, located in northeast England, has been influenced by derelict lead–zinc and coal mining activity. The underlying geology is Carboniferous age Limestone, Millstone Grit, Coal Measure shales and mudstones while the upper half of the catchment has moorland with extensive heavy metal vein mineralization, historic lead–zinc mining activity and spoil heaps. The lower reach of the Wear has extensive arable farmland areas as well as urban centres such as Durham and, near its estuary, Sunderland. The basin has a long history of coal mining, leading to problems over the discharge of mine-water to the river, and sand/aggregate and shale extraction near to the river. The water quality is fair to good except where sewage and mine drainage sources reduce its quality. The Humber Rivers drain into the North Sea at the Humber estuary in northern England, providing a major drainage area for the eastern UK. The bedrock varies from sedimentary rocks of Carboniferous and New Red Sandstone age sandstone, grit and limestone in the uplands (to the west) and Jurassic and Cretaceous age sandstone and limestone in the lowlands (to the east). The socio-economic and water quality functioning of the Humber basin largely divides into northern and southern areas. Firstly, the northern Humber Rivers comprise the Yorkshire Ouse and its two tributaries, the Wharfe and the Yorkshire Derwent. These rivers drain moorland in the eastern uplands and agricultural land in the lowlands to the west of the basin and the water quality is typically ‘good to fair’. However, there is some deterioration due to metal

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release from contaminated flood plain materials derived from historic mining activity and sewage inputs of nutrients from the conurbations. Secondly, the southern Humber Rivers (the Aire, Calder, Don and Trent) drain from moorland and rural source areas in the upper parts of the catchment through to the industrial heartland of north-eastern and north-central England. For these ‘urban/industrial rivers’, water quality is generally ‘fair to good’ with some stretches ‘poor to very poor’. The poorer water quality is a consequence of both diffuse and point sources of pollutants of nutrients, micro-organics and metals. The Great Ouse provides the main river basin of the southern part of the English midlands and East Anglia. It drains Upper Jurassic clays and Cretaceous Chalk into the Wash and it flows through several market towns and major lowland areas of intensive agriculture. The water chemistry is of moderate to good quality. The Thames, the major river of southeastern England, drains rural and agricultural areas with several urban conurbations including several market towns and the city of London at its estuary. The bedrock is of mixed sedimentary geology of Oolitic limestone in the headwater areas and Oxford clays and minor Cretaceous age sedimentary rocks in the lower parts of the basin. Within this study, both the main stem of the river and two of its tributaries, the Pang and Kennet are examined. Detailed information on the locations of the LOIS monitoring sites, the social, economic and geological settings as well as the water quality functioning are provided in four special issues of Science of the Total Environment (1997, vol. 194/195; 1998, vol. 210/211; 2000, vol. 251/252; 2002, vol. 282/283). 2.2. Chemical and thermodynamic analysis 2.2.1. Chemical analysis Silicon concentrations in the water have been determined using automated colourimetric analysis involving the formation of molybdosilicic acid as either a blue or yellow complex (e g Truesdale and Smith, 1976). The silicon measured is one of ‘total dissolved silicon’ as it comprises several species (both unionised and ionised forms). For the present study, the term ‘silicon concentration’ is used to denote ‘total’ to save on space and improve clarity.

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2.2.2. Thermodynamic analysis Within this study, thermodynamic evaluation has been made in terms of saturation indices for SiO2 minerals (quartz, chalcedony and amorphous SiO2) as expressed in logarithmic form For this, allowance is made for the species H4SiO04, H3SiOK1 and H2SiOK2 4 4 together with temperature compensation. The thermodynamic data for silicon speciation and amorphous silica, chalcedony and quartz solubility are provided from the PHREEQC database (Parkhurst, 1995). The saturation index (SI) for a water sample was determined by straightforward calculation within a spreadsheet and no allowance was made for ionic strength: this is only of second order importance. The relevant equations are:SiO2 ðsÞ C 2H2 O Z H4 SiO04 KSiO2 solid Z ½H4 SiO04  C H4 SiO04 Z H3 SiOK1 4 CH C 0 k1 Z ½H3 SiOK1 4 ½H =½H4 SiO4  C H4 SiO04 Z H2 SiOK2 4 C 2H C 2 0 k2 Z ½H2 SiOK2 4 ½H  =½H4 SiO4 

½SiTotal dissolved  K2 Z ½H4 SiO04  C ½H3 SiOK1 4  C ½H2 SiO4 

½H4 SiO04  Z ½SiTotal dissolved =ð1 C k1 ½HCK1 C k2 ½HCK2 Þ SI Z Log10 ½H4 SiO04  K Log10 ½KSiO2 sol  Where the square brackets denote molar concentration (and as an approximation, activity), ‘k’ with a suffix represents the various equilibrium constants and K SiO2 solid is the equilibrium constant for SiO2/chalcedony/quartz. In terms of saturation level as linked to the silicon concentrations found in the water, Fig. 2 is provided to show what the equilibrium concentrations are in the case of chalcedony and quartz in relation to temperature: amorphous silica concentrations are not

Fig. 2. The dissolved silicon concentrations for waters in equilibrium with quartz and chalcedony in relation to variations in temperature and pH.

shown as, its solubility is much higher and all the waters examined in this study are undersaturated with respect to this phase. In Fig. 2, two important features are shown. Firstly, data are plotted across the normal temperature range of UK surface waters to show the effect of temperature on solubility. The salient features are as follows. † As temperature increases, so does the silica solubility and between 5 and 25 8C, concentrations almost double. † Silicon concentrations are about double for chalcedony compared to quartz at a given temperature and pH: this is linked to the lower crystallinity of chalcedony. † Below pH 9, silicon concentrations are almost constant. This is due to the small contribution of K2 that increase in importance H3SiOK 4 and H2SiO4 as pH increases. † Above pH 9 silicon concentrations increase almost exponentially with pH as a result of the growing K2 importance of H3SiOK 4 and H2SiO4 . Here, the levels of saturation are largely presented in graphical form as plots of saturation index against pH, rather than as silicon concentration against pH as part of the scatter in the latter case is linked to temperature variation: the saturation index is temperature compensated. However, it must be remembered that in plotting the saturation index against pH, the two

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Table 1 Silicon concentration (mg-Si lK1) statistics for the Plynlimon (up to 20 years of data: Neal and Kirchner (2000)), Welsh regional (up to 2 years of data: Neal et al. (1998)) and UK acid waters monitoring programme. catchments (up to 14 years of data: www.geog.ucl.ac.uk/ukawmn/) Average

Rainfall Cloud water Throughfall Stemflow Streams Groundwater Streams Streams Lakes

Average Flow-weighted

Plynlimon 0.06 0.06 0.07 0.05 0.20 0.16 0.22 0.16 1.45 1.13 2.58 NA Welsh regional survey 1.35 NA UK Acid waters monitoring programme 1.70 1.35 0.60 NA

Minimum

Maximum

Average low-flow/ low-vol. of catch

Average high-flow/ high-vol. catch

0.00 0.00 0.00 0.00 0.00 0.70

4.00 1.50 0.55 1.25 5.60 7.20

0.09 0.07 0.33 0.34 1.78 2.65

0.04 0.05 0.1 0.06 0.97 2.19

0.20

4.50

1.32

1.37

0.40 0.09

3.43 2.15

2.04 NA

1.15 NA

axes are not completely independent as the saturation index is calculated from the dissolved silicon concentration and the pH. However, this interdependence is not important, because (a) for most waters the pH is not K2 to be sufficiently high for H3SiOK 4 and H2SiO4 important and (b) at high pH the waters are actually undersaturated with respect to quartz and chalcedony and there is no clear trend at higher pHs which would correspond to an auto-correlation.

3. Results A statistical summary of the variations in concentrations of dissolved silicon for the Plynlimon, Welsh regional survey and the UK acid waters monitoring network is presented in Table 1 in terms of average, flow weighted average and minimum and maximum flow, together with low-flow/low-volume of catch and high-flow/high-volume of catch. Table 2 provides

Table 2 Silicon concentration statistics for eastern UK Rivers and tributaries (up to 5 years of data: Neal and Robson, 2000) Site

Average

Average flow-weighted

Minimum

Maximum

Average low-flow

Average high-flow

Tweed: Ormiston Mill Tweed: Boleside Tweed: Norham Wear: Sunderland Br. Swale: Catterick Br. Swale: Thornton Man. Ure: Boroughbridge Nidd: Skip Bridge Ouse: Clifton Bridge Ouse: Acaster Malbis Derwent: Bubwith Wharfe: Tadcaster Aire: Beal Bridge Calder: Methley Bridge Don: Sprotborough Br. Trent: Cromwell Lock Great Ouse: Gt Paxton Thames: Howberry Pk Kennet: upper reaches Pang: Tidmarsh

1.28 1.72 1.53 2.19 1.39 1.74 1.40 2.32 1.71 1.82 2.95 1.21 3.72 3.77 3.73 3.26 3.08 4.25 7.48 6.90

1.72 2.08 1.98 2.27 1.36 1.86 1.37 2.33 1.82 1.92 3.20 1.33 3.48 2.33 3.79 3.53 3.30 4.36 7.56 6.61

0.09 0.28 0.00 0.45 0.11 0.15 0.01 0.00 0.00 0.00 0.39 0.07 0.06 1.59 0.63 0.01 0.00 0.35 4.70 4.60

2.95 3.62 7.02 3.00 3.92 4.63 3.62 5.10 6.23 5.53 7.79 2.70 16.20 10.65 11.72 13.04 5.51 6.70 10.00 8.70

1.75 1.43 2.05 1.84 0.73 1.52 1.58 1.90 1.76 1.85 2.04 0.84 3.65 4.06 3.14 2.56 3.34 5.18 7.58 7.42

1.83 2.09 2.11 2.23 2.02 2.15 1.39 2.35 1.92 2.39 3.40 1.33 3.08 3.60 3.96 3.60 3.42 4.52 7.77 5.98

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comparable data for the eastern UK Rivers, but in this case, statistical summaries for each river is provided as there is greater variability in silicon concentrations. For the streams, low- and high-flow silicon concentration estimation, the data for the bottom and top 10% of flows, respectively, were estimated except for the acid waters survey and the acid waters monitoring network data for lakes where no flow values were available. However, for the Welsh regional survey, low- and high-flows were monitored using specific campaigns for dry (baseflow) and wet (high surface runoff) conditions. For rainfall, the low- and highvolumes of catch were used based on the weekly sampling. The salient features of silicon concentration levels are described below and a summary of the thermodynamic information is subsequently given. 3.1. Plynlimon Silicon concentrations in rainfall and cloud water are similar and relatively low averaging around 0.07 mg-Si lK1: range 0–4.0 and 0–1.5 mg-Si lK1, respectively. Throughfall and stemflow also have low silicon concentrations, both averaging around 0.21 mg-Si l K1 with a range of 0–0.55 and 0–1.25 mg-Si lK1, respectively. For the inputs (rainfall and cloud water) and the canopy cycling (throughfall and stemflow), silicon concentrations are higher for small volumes of catch. This feature represents the varying dilution of a limited supply of particulate and aerosol materials containing soluble silicon in the atmosphere and on the vegetation surfaces. In relation to the levels of silicon in stemflow and throughfall relative to the atmospheric inputs, some scavenging of aerosol and particulate materials might add to inputs via cycling through the vegetation. However, there is also a concentration mechanism linked to the rainfall associated with evaporation from the vegetation surfaces. In order to estimate if the increase is associated with cycling, the ratio of silicon to chloride in rainfall, cloud water and throughfall and stemflow has been estimated using flow weighted values that link more directly to flux than straight averages: chloride effectively comes in as an atmospheric source and is of low hydrobiogeochemical reactivity. The respective values are 0.01, 0.001, 0.005 and 0.005. On this basis, there is no clear

evidence of significant cycling: indeed, there may be some uptake of silicon by the vegetation from the rainfall. Silicon concentrations are generally higher within the streams compared to the rainfall, cloud water and within-canopy on average by a factor of about 10 fold. The average across the streams is 1.35 mg-Si lK1 and a range of average values across the different streams of 0.69–2.26 mg-Si lK1: the total range in concentration is 0–4.5 mg-Si lK1. The lowest silicon average concentrations are for the small streams of the Tanllwyth catchments. These streams (Tan North and Tan South) drain gley soils with low permeability. They effectively represent soil water outputs with very little shallow groundwater inputs. With a high organic carbon content and low silicon contents to weather and release silicon, low concentrations are perhaps to be expected. The highest stream silicon concentrations occur for two small streams draining podzols (average silicon concentrations 1.98 and 2.26 mg-Si lK1) and these soils have a higher permeability and contain weatherable silicon. Further, being perennial in nature, the two streams contain groundwater components that maintain flow even under drought conditions: groundwater, as shown below, is on average enriched in silicon relative to the streams. For some of the streams, the catchments have either been felled or partially felled: visual inspection of the data shows no clear change associated with felling and this aspect is not analysed in further detail here. The streams show systematic variations in silicon concentration as a function of flow with higher concentrations under baseflow conditions and the lowest concentrations at high flows (Fig. 3). The relationship is curvilinear and there is a strong linear relationship between silicon concentration and the logarithm of flow (r2Z0.647 with NZ857). This feature relates to the relative inputs of groundwater and soil water as a function of flow. Under baseflow conditions the streams are mainly supplied from the groundwater areas where inorganic components that can weather and partially neutralize acidic waters (e.g. aluminosilicates) from the soil zone reside (these waters are enriched in calcium, magnesium and bicarbonate). Correspondingly, under stormflow conditions, the waters are mainly derived from the acidic and organic rich soil zone where acidic conditions prevail and there is very limited

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Fig. 3. Silicon concentration relationships with flow, the logarithm of flow and Gran alkalinity for the Afon Hore. For the plot concentrations above 3.5 mg-Si lK1 are excluded (values go up to 6 mg-Si lK1) to show most clearly the main trends: the high values correspond to very low flows and to occasionally anomalously high values.

inorganic material for weathering: these waters are acidic, aluminium bearing and base cation depleted. In Fig. 3, a plot is provided of silicon concentration against Gran alkalinity to illustrate one of the main features of the soil-groundwater mixing relationships in the stream. Gran alkalinity provides a measure of the extent of the weathering for positive values where essentially it represents bicarbonate concentrations. For positive Gran alkalinities, there is a linear relationship between silicon and Gran alkalinity concentrations: this corresponds to simple twocomponent mixing—the groundwaters have positive

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Gran alkalinities and are bicarbonate bearing while the soils are more silicon and bicarbonate depleted and have negative Gran alkalinities. For negative Gran alkalinities, there seems to be a curvilinear relationship between silicon concentration and Gran alkalinity. For negative Gran alkalinities the Gran alkalinity is negatively and linearly related to the hydrogen ion and the trivalent aluminium concentration. Thus, the lowest silicon levels occur under the most acidic and the most aluminium bearing cases. This linear feature may link to the inputs of (1) near surface highly acidic but silicon and aluminium depleted soil waters from the organic horizons and (b) acidic and aluminium bearing waters from the lower soils—a region where secondary aluminosilicates minerals (the clays) might precipitate. For comparative purposes, Table 3, provides a summary of silicon concentrations in forest, moorland and peat soils at Plynlimon based on a compilation by Robson (1993) of data from Reynolds et al. (1986, 1989). It shows that the soil waters have silicon concentrations in the range 0–3.5 mg-Si lK1 with an average of about 1.5, 0.8 and 0.3 mg-Si lK1 in the forest, moorland and peat soils, respectively. The streams can have very low concentrations of silicon (less than 0.3 mg-Si lK1), but these occur on a very infrequent basis (less than 2% of the time). Further, the low concentrations cannot be related to particular months when biological activity may come into play. It is not clear why particularly low values occur in the streams. The groundwaters typically exhibit the highest silicon concentrations with an average of 2.58 mgSi lK1 and a range in average of 1.18–4.47 mg-Si lK1 across the sites (total range 0.7–7.2 mg-Si lK1. Under dry periods when groundwater levels are at their deepest, silicon concentrations are at their highest while under wet conditions when groundwater levels are nearest the surface, silicon concentrations are at their lowest. However, the difference in silicon concentration between dry and wet conditions is much smaller than for that observed in the inputs, canopy cycling and stream water counterparts. There is no clear indication of why there are differences in silicon concentration across the sites. However, the highest silicon concentrations occur for two boreholes which have particularly high Gran alkalinities (typically greater than 1000 mEq lK1 as compared to

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Table 3 A summary of averages and ranges in silicon concentrations and quartz saturations in Plynlimon forest, moorland and peat soils (up to 7 years of data) Si (mg-Si lK1) average Oh Eag Bs C Oh Eag Bs C Peat

Forest 0.81 1.54 1.37 1.99 Moorland 0.39 1.04 0.87 0.92 Peat 0.28

Si (mg-Si lK1) minimum

Si (mg-Si lK1) maximum

SIQtz average

SIQtz minimum

SIQtz maximum

0.08 0.62 0.76 1.51

2.69 3.50 2.10 2.91

K0.32 K0.04 K0.09 0.07

K1.31 K0.44 K0.35 K0.05

0.20 0.31 0.09 0.23

0.00 0.70 0.56 0.53

1.09 1.60 1.79 1.79

K0.64 K0.22 K0.29 K0.27

!K2 K0.39 K0.48 K0.51

K0.19 K0.03 0.02 0.02

0.00

1.09

K0.78

!K2

K0.19

For the saturation calculations, an average temperature of 10 8C has been used. The soils typically have a total depth of less than 1 m: details of the profiles are provided in Reynolds et al. (1988a).

the norm of less than 100 mEq lK1), which indicate the highest extent of weathering. However, there is only a very poor correlation between Gran alkalinity and silicon concentration in general and the variations in silicon concentrations are much lower than that for Gran alkalinity (i.e. the silicon concentrations are buffered much more than Gran alkalinity). 3.1.1. Welsh regional survey For the 67 catchments monitored in this survey, the data may be summarized as follows. † Across the sites, the range of concentrations is 0.2– 4.5 mg-Si lK1. † The range of average concentrations for the individual sites is 0.45–3.35 mg-Si lK1. † The range in baseflow and stormflow concentrations for the individual sites are 0.13–3.35 mg-Si lK1 (average 1.32 mg-Si lK1) and 0.18–3.25 mg-Si lK1 (average 1.35 mg-Si lK1), respectively. In some cases, the concentrations are relatively low compared to most of the streams (less than 1 mgSi lK1), and this fits with the dominance of soil water inputs, while the higher values are more reflective of high groundwater inputs as reflected in the streams under baseflow condition and the groundwaters

themselves (greater than 2 mg-Si lK1). In terms of the relationship between silicon and the components indicative of weathering, there is a strong positive linear correlation with magnesium (r2Z0.455, NZ 398), but much weaker links with components such as calcium and Gran alkalinity as well as pH. Thus, it seems that factors other than the relative inputs of soil and groundwaters come into play to determine silicon levels in the runoff from small streams draining low permeability waters in Wales. 3.2. Regional UK Rivers and lakes Silicon concentrations vary in the range 0.5– 5.02 mg-Si lK1 for the rivers monitored within the UKAWMN having on average higher concentrations than the lakes by about a factor of about two. For the UKAWMN rivers, the average concentration across the sites is 1.70 mg-Si lK1 with individual catchments having average values in the range 0.87–3.16 mgSi lK1. Silicon concentrations in these rivers decrease as flow increases: silicon concentrations at high flows average 1.15 mg-Si lK1 while low-flows average 2.04 mg-Si lK1 in line with the effect described earlier for the Plynlimon streams in terms of groundwatersoil water mixing. Correspondingly, the average for the lake sites as collected within the UKAWMN programme is 0.60 mg-Si lK1 with a range in average

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for the various sites of 0.32–1.33 mg-Si lK1: no data is available on flow related effects, but dampening of the input signal would of course be expected. For the CEH Lancaster study of 20 lakes in the Lake District, the average silicon concentration was 0.67 mg-Si lK1 with a range in average across the different lakes of 0.32–1.11 mg-Si lK1 and a total range of 0.07– 1.27 mg-Si lK1 for all the data points. The lower concentrations observed in the lakes implies silicon loss. For Loch Saugh, there is relatively little variation in silicon concentration through the year (average 4.55 mg-Si lK1 with a range of 4.1–4.9 mg-Si lK1). The silicon concentration in the eastern UK Rivers show a significant range with averages varying from 1.21 to 7.48 mg-Si lK1 (Table 2). The lowest averages occur for the rural rivers in Scotland and northern England and higher concentrations occur for the industrial rivers of the Humber region and the agricultural rivers of south-eastern England. This increase corresponds to progression to higher average concentrations north to south and a climatic gradient to increasing temperatures north to south. The range in silicon concentration is 0–13.04 mg-Si lK1 and there is no consistent pattern between baseflow and stormflow conditions. Rather, under baseflow conditions, the concentrations exhibit the widest scatter in silicon concentration. This latter feature links to two opposing factors. Firstly, there is the effect of dilution of diffuse inputs of groundwater and point source inputs from industry under stormflow conditions. Secondly, during the spring/summer months there can be major algal and diatom blooms that remove silicon from the water column. The temporal patterns of change are illustrated in Fig. 3 for a time series based on the LOIS dataset, where three datasets are plotted: one for a rural river system (the Tweed), one for an industrially impacted system (the Aire) and one for an agriculturally impacted system (the Derwent). The plots show that there are cyclical patterns of change which vary from river to river. For the Tweed and the Derwent, there are major changes with low silicon concentrations occurring during the spring/summer months. This is most clearly marked for the rural Tweed. In the case of the Aire, the cyclical pattern is much less distinct. For the agriculturally impacted Thames, with a much longer monitoring period, the cyclical pattern is shown to continue through the years, but in this case, the silicon drops which take

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Fig. 4. Time series plots for dissolved silicon in the Tweed, Aire and Derwent. The plots show the fluctuations in concentration for a rural, industrial and an agricultural system, respectively.

place each spring/summer, vary considerably (Fig. 4). For example in 1997, silicon concentrations dropped to less than 1 mg-Si lK1 while in more normal years the concentrations dropped about 2 or 3 mg-Si lK1 (set against an average silicon concentration of about 6 mg-Si lK1). The ‘silicon crash’ during the summer months of 1997 on the Thames is associated with particularly high biological activity when carbon dioxide saturation declined from typical values of around seven times atmospheric pressure to about atmospheric pressure (c.f. Fig. 5; Neal et al., 2000). Further, there is a strong link between chlorophyll-a

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Fig. 5. The variations in dissolved silicon, excess partial pressure of carbon dioxide (EpCO2) and chlorophyll-a for the Thames.

levels, temperature and silicon concentrations: ½Si mg K Si lK1  Z ð0:057G0:010Þ  ðTemperature 8CÞ K ½Chlorophylla mg lK1  C 4:2G0:8 Where r2Z0.438 and NZ261. Note that silicon concentration increases as temperature increases, but decreases as chlorophyll-a increases. This would be anticipated where silicon solubility increases as temperature increases, but silicon removal increases as the plankton levels increase. Examining the data for the Eastern UK Rivers monitored within the LOIS programme shows a similar feature with regards to Chlorophyll-a, but the correlations are often less

strong (Table 4). Thus, the reduction in silicon concentration across the sites largely links to the plankton activity as represented by Chlorophyll-a levels. In this context it is worth noting that the pH encountered on the Tweed can exceed 10 and almost reach 11 under periods of high biological activity. In this pH range there would be considerable enrichment in silicon within the water column if equilibrium even with quartz were established (2.3–7.1 mg-Si lK1 at pH 10, 10.7–46.0 mg-Si lK1 at pH 11, in the temperature range 5–25 8C). In fact, within this pH 10–11 range, five data points occur for the Tweed where temperature and silicon concentration data are available and the corresponding silicon concentration range is 0.2– 2.5 mg-Si lK1. These values certainly do not correspond to an enrichment associated with any silica solubility controls and indeed the waters are considerably undersaturated even with respect to quartz. In fact, these data for the Tweed subdivide into two groups. Firstly there are three data points for the spring summer months when temperatures are relatively high (19–22 8C) and silicon concentrations are low (0.23–0.98 mg-Si lK1). There are also two data points for the winter months (December and February) where temperature is much lower (4.3–4.8 8C) and silicon concentrations are higher (2.0–2.5 mgSi lK1): this may be due to freezing conditions reducing storm and soil drainage run-off and increasing longer residence time groundwater enriched in silicon, but a definitive statement cannot be made over this. With regards to the other Scottish rivers, data for the Harmonized monitoring scheme shows similar features to that for the eastern UK rivers monitored as part of the LOIS, although maximum concentrations observed are slightly higher in the former case. The range of silicon concentrations across the various river basins is 0.01–19.2 mg-Si lK1 with an average value of 5.3 mg-Si lK1: detailed presentation of the data is not given here because of the large number of sites considered.. The lowest silicon concentrations occur during the summer months in a sinusoidal manner. For the Taw, rainfall had concentrations less than 0.1 mg-Si lK1 while in the streams, the concentrations varied between 1.9 and 7.1 mg-Si lK1 with a mean of 3.83 mg-Si lK1. The two sites on the Taw had the lowest silicon concentrations (1.9–3.2 mg-Si lK1,

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Table 4 Multiple linear regression of silicon concentration against temperature and Chlorophyll-a across the LOIS Rivers: [Si]Za*{Temperature, 8C}Cb*{Chlorophyll-a, mg lK1}Cc

Swale: Catterick Br. Swale: Thornton Man. Ure: Boroughbridge Nidd: Skip Bridge Ouse: Clifton Bridge Ouse: Acaster Malbis Wharfe: Tadcaster Derwent: Bubwith Aire: Beal Bridge Calder: Methley Bridge Don: Sprotborough Br. Trent: Cromwell Lock Average

Temp (8C)

Ch-a (mg lK1)

Constant

r2

N

K0.046G0.019 K0.043G0.022 K0.005G0.020 K0.060G0.025 K0.032G0.026 K0.032G0.024 K0.060G0.017 K0.073G0.030 0.073G0.051 0.078G0.039 0.013G0.043 K0.043G0.036 K0.019G0.029

K0.023G0.019 K0.039G0.021 K0.008G0.005 K0.016G0.006 K0.019G0.007 K0.017G0.007 K0.004G0.011 K0.094G0.038 K0.074G0.044 K0.105G0.041 K0.031G0.008 K0.031G0.006 K0.038G0.018

1.9G1.1 2.3G1.4 1.5G1.1 3.0G1.7 2.4G1.6 2.2G1.6 1.8G1.1 4.0G1.8 3.3G2.8 3.4G2.0 4.2G2.1 4.4G2.4 2.9G1.7

0.216 0.239 0.109 0.296 0.257 0.232 0.285 0.338 0.069 0.153 0.354 0.490 0.253

150 154 151 157 123 156 162 150 170 152 158 152 153

with an average of 2.69 mg-Si lK1, for the upper, and 2.5–3.6 mg-Si lK1, with an average of 3.03 mg-Si lK1, for the lower sites). Correspondingly, there were higher concentrations for Den Brook (3.1–5.5 mgSi lK1, with an average of 3.96 mg-Si lK1) and Drewston sites had higher silicon concentrations (3.7–7.1 mg-Si lK1, with an average of 5.66 mgSi lK1). Clearly the granite area is providing less silicon due to the lower weathering rates compared to the Carboniferous sediments: Drewston has the highest silicon concentrations probably due to longer residence times (the Den Brook site, with its gleys, will provide higher throughput of short residence time water). 3.3. Thermodynamic analysis The Thermodynamic analysis reveals some very simple patterns of behaviour for the Plynlimon, acid waters network and the eastern UK rivers In general, the waters are undersaturated with respect to amorphous silica but close to saturation with respect to either quartz or chalcedony with scatter spanning between under to saturation (for chalcedony) or over saturation (for quartz). Note that in the following results, the saturation levels are presented on a logarithmic basis and that deviations from an equilibrium value of zero are exponential on an unlogged scale, values which we would normally class as close to saturation such as G0.1 correspond

to about G26% deviation from saturation. The salient features are as follows. 3.3.1. Plynlimon All the stream waters average within G0.1 of saturation with respect to quartz (saturation levels for amorphous silica and chalcedony saturation average K1.3 and K0.5, respectively). The extent of saturation varies as a function of pH, with waters above pH 5.5 either being close to quartz saturation or oversaturated almost to the point of chalcedony saturation in the extreme cases. Below pH 5.3 the waters become progressively more undersaturated as pH declines. This feature is illustrated in Fig. 6a where dissolved silicon is plotted against pH (with lines for quartz saturation at 5 and 25 8C to illustrate the levels of saturation), and quartz and chalcedony saturation is plotted against pH in Fig. 6b. The pattern reflects the relative inputs of groundwaters which have contacted silica and silicate minerals in the bedrock, and water from the acidic organic rich soils that are low in silica and silicate bearing minerals. Thus, quartz undersaturation occurs when the streams are acidic due to the dominance of soil water inputs depleted in dissolved silicon. This feature is also observed for a baseflow survey of 40 sites in the Severn (forest and moorland) and the Wye (moorland), based on an analysis of data collected by Locks (1996). However, for the moorland areas of the Wye, the stream waters tend to be slightly undersaturated

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† For peat, the waters are always undersaturated (average SIZK0.78).

Fig. 6. The variation of silicon concentration and quartz/chalcedony saturation as a function of temperature for the Afon Hore.

with respect to quartz: quartz saturation for the Wye average K0.35, range K0.31 to 0.0. Table 3 shows the range in quartz saturation indices within the Plynlimon soils to add more detail to the soil water inputs. The salient features are: † For the moorland soils, the waters are usually undersaturated with respect to quartz (average SIZK0.3) except for occasional samples which approach saturation. This situation applies throughout the soil horizons, but the greatest degree of undersaturation (average SIZK0.65) occurs within the near surface organic horizon (Oh). † For the forest soils, the waters approach quartz saturation, from undersaturation, on average, with depth. Only the forest Oh horizon is markedly undersaturated on average (SIZK0.32).

More detailed analysis for the forest soil waters pre- and post-felling (based on water quality data provided in Reynolds et al., 1988a,b) indicates (a) podzolic and gley soils show similar silicon concentration values and similar responses, (b) silicon concentrations increase slightly post-felling, possibly due to more intense weathering under more acid conditions and (c) the waters become less undersaturated with respect to quartz post-felling compared to pre-felling. Correspondingly, for unimproved moorland, ploughed and limed moorland, surface limed moorland and surface cultivated moorland (water quality data taken from Hornung et al., 1986), the waters are undersaturated to saturated with respect to quartz, average ranges for the different soil horizons being K0.7 to K0.3, K0.2 to K0.3, K0.2 to 0.3 and K0.8 to 0.0, respectively. For the groundwaters, the average quartz saturation levels of 0.09 with a range of K0.4 to 0.61 and undersaturation occurs at low pH in a similar way to the streams. Some of the ground waters flow routes transmit silicon depleted acidic soil waters during periods when the catchments are wetted up and groundwater levels are closest to the surface: at these times silicon concentrations would be expected to be lower. When quartz saturations are at their highest, the waters are close to chalcedony saturation and this situation occurs for waters with the highest alkalinities which are indicative of the highest extent of weathering. 3.3.2. Acid Waters monitoring network, MI, IGER and CEH lake sites The streams mainly straddle quartz saturation in a very similar way and in a similar range to the Plynlimon streams. For example, the range in quartz saturations for the Acid Waters Monitoring Sites is K1.76 to 0.21 with an average of K0.41. However, in some cases the waters may be slightly oversaturated with respect to quartz: e.g. for the IGER sites in Devon, the range in SI is 0.05–0.62 with an average of 0.33. For the lakes, the waters are mainly undersaturated with respect to quartz (7 of the 9 lakes monitored within the acid waters monitoring programme and all

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20 lakes monitored by CEH Lancaster: average saturation about K0.6, range K1.63 to K0.02). While the data are scattered, the general trend towards lower-saturation/greater-undersaturation at lower pH is observed within the streams, there is no similar trend for the lakes as the lakes integrate the stream water responses across all flows. In the case of Loch Leven, the lake water is on average undersaturated with respect to both chalcedony and quartz (average K0.92 and K0.44, respectively). The range in chalcedony and quartz saturation is, respectively, K2.7 to 0.1 and K2.3 to 0.58. Thus, the waters straddle saturation to undersaturation with respect to these two minerals. There is a broad change in the extent of chalcedony and quartz saturation/undersaturation with the season as well as the pH. This is illustrated using data from Loch Leven in Fig. 7 for chalcedony saturation (quartz shows a similar pattern although the extent of under saturation is greater: to avoid undue detail, the quartz saturation plot is not provided—chalcedony has been chosen as the waters approach saturation with this mineral). Thus, during the winter months, the waters approach chalcedony saturation while during the spring much greater undersaturation occurs with increased biological activity. In the case of the CEH survey of 20 lakes in the Lake District, a similar feature is observed except that the waters do not reach saturations close to that for chalcedony, but are nearer to that for quartz. Considering quartz saturation only (the same pattern

occurs for amorphous silica and chalcedony saturation but there is a systematic pattern of greater undersaturation). Quartz saturation averages K0.47 with a range of K1.54 to K0.01. However, the levels of saturation change systematically through the year, with saturation levels closer to zero during the winter months and are more undersaturated during the spring and summer months. The patterns of change for the average saturations and range of saturations (in brackets) are: January K0.14 (K0.47 to K0.01), April K0.48 (K1.04 to K0.11), July K0.84 (K1.54 to K0.31) and October K0.42 (K1.03 to K0.21). The extent of the seasonal change in silica concentration and saturation level is linked to the trophic state of the lake: the 20 lakes cover a range from oligotrophic to eutrophic and the relation between silicon variations as a function of phosphorus concentration is shown in Fig. 8 to illustrate the differences. In the least productive lakes, estimated by the annual average concentration of total phosphorus, there is a relatively small change in silicon concentration, but in the productive lakes, 80–90% of the available silicon is removed as a result of uptake by diatoms. Lakes are known to be important in causing silicon-depletion, as a result of diatom growth, in river-lake systems (Conley et al., 2000). The slope of change, up to a TP concentration of 10 mg lK1, is about 12 mg TP mgK1 Si which is consistent with the known phosphorus and silicon requirements of diatoms (Foy et al., 2003). For Loch Saugh, the waters

Fig. 7. The variation in chalcedony saturation with respect to month for Loch Leven.

Fig. 8. Seasonal variation in silicon concentration (maximumK minimum concentration) in Cumbrian lakes (the English Lake District) in relation to annual average concentration of total phosphorus.

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remain approximately in equilibrium with chalcedony through the year 3.3.3. Eastern UK rivers The eastern UK Rivers show a wider range in saturation compared to the upland waters described above. In terms of quartz, the waters vary from under to oversaturated (Fig. 9) and there is an upper bound which lies close to equilibrium with respect to chalcedony. For these rivers, much of the riverine input is often from groundwaters with long residence times. Indeed, for the Kennet which has the highest groundwater inputs, the level of saturation index averages 0.17, with a narrow range of K0.04 to 0.29, with respect to chalcedony (the comparable figures for quartz are 0.64, and 0.43–0.77). Taking the dataset for the Eastern UK rivers as a whole, it seems that there is near equilibrium to

undersaturation with respect to chalcedony and mainly oversaturation with respect to quartz. The degree of undersaturation with respect to both chalcedony and quartz can be extremely high with saturation indices as low as K2.8 for chalcedony. These low values correspond to the low silicon concentrations described earlier in the paper, and they link to the periods of very high biological activity. 3.3.4. Scottish rivers The saturation levels for the Scottish Rivers as represented within the Harmonized Monitoring Scheme are similar to that for the Eastern UK rivers. The regional Scottish data show that the saturation index with respect to quartz is 0.39 with a range of K2.20 to 1.12, while the corresponding value for chalcedony is an average of K0.09 and a range of K2.69 to 0.61 (2766 data points). The highest degree of undersaturation occurs during the summer months and at higher pHs when the biological activity is at its highest (the high pHs are also linked directly to the biological activity due to incorporation of dissolved CO2 into the growing biomass and the change in the equilibrium state for the inorganic carbon–hydrogen ion system) and this feature is similar to that for the other UK rivers. The waters vary from under to oversaturated at low flows with respect to chalcedony and saturation is approached at higher flows. This differs in some ways from the eastern UK rivers monitored within the LOIS, but it must be borne in mind that there is a larger number of rivers analysed within the Scottish Harmonized Monitoring Scheme and that there will be an associated wider variability in geological types and bedrock reactivity. While there is large scatter, the waters follow a sinusoidal pattern with respect to chalcedony (and quartz) saturation, with values spanning saturation to undersaturation during the winter months while during the summer months the largest degree of undersaturation occurs and there are far fewer points which are oversaturated at this time. This pattern of behaviour is similar to that described above for the lakes.

4. Discussion Fig. 9. The variation in quartz and chalcedony saturation as a function of pH for eastern UK Rivers.

Surface water silicon concentrations are variable across the UK but cover a relatively small range of

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0–19 mg-Si lK1. This range is well within that found for many rivers globally (Meybeck, 1980; Berner and Berner, 1996). Atmospheric inputs have lower silicon concentrations than the streams, and as a result the major source of silicon comes from within the catchment. There is no clear equilibrium process within the streams/rivers. Rather, there seems to be variability in silicon concentration that links to the inputs of silicon bearing groundwaters that dilute with increasing flow, and biological uptake processes (plankton levels as represented by chlorophyll-a levels) which occur during the spring and summer baseflow periods, that can reduce silicon concentrations in to below the detection levels of measurement and certainly below any solubility controls associated with SiO2 minerals (even quartz). The main biological uptake of silicon is caused by a specific, abundant, group of benthic and phytoplanktic algae, the diatoms. They have a substantial silica requirement because they construct their cell wall from silica and the ratio of silica to chlorophyll can be as high as 30–40 on a weight basis (Reynolds, 1984). In many lakes and rivers, early algal growth in the spring is dominated by diatoms, leading to a potential

91

for silica uptake, to levels that restrict further diatom growth until the concentration increases later in the year. the importance of biogenic processes in regulating silicon concentrations at biologically productive times of the year is well recognised (Berner and Berner, 1996). Usually it is assumed that quartz is a relatively inert mineral in surface and groundwaters as the reaction kinetics are supposedly extremely sluggish. However, this assumption must be put within the context of the absence or presence of seeding surfaces and activation surfaces for increased solubility or exchange. Studies on groundwaters indicate that often there may be equilibrium with chalcedony for waters which are hydrothermal in nature or high in CO2 and low pH that promotes extremely high weathering rates (Appelo and Postma, 1999; D’Amore and Arno´rsson, 2000). For surface waters, as shown by Casey and Neal (1984), quartz surfaces can become activated when they contact waters that are oversaturated with respect to quartz. This can lead to rapid (less than a week) changes in dissolved Si concentration that plateau within a month or two. For example, in an artificial circulating stream channel being used to

Table 5 Variations in silicon concentrations (mg-Si lK1) for quartz (particle size range 0.1–0.4 cm diameter; sediment loading 2 kg lK1) as a function of initial silicon concentration and time (temperature 20 8C): data taken from Casey and Neal (1984) Initial Silicon concentration 0 10 30 0 10 30 0 10 30 0 10 30

Storage time (days) 1

2

Concentration 0.62 0.83 8.66 8.30 21.90 22.40 SiO2 saturation K1.91 K1.78 K0.76 K0.78 K0.36 K0.35 Chalcedony saturation K1.05 K0.92 0.10 0.08 0.50 0.51 Quartz saturation K0.60 K0.48 0.54 0.52 0.95 0.96

3

4

7

15

30

0.92 8.40 22.20

1.33 8.06 21.80

1.51 7.38 19.00

1.87 7.00 14.20

2.35 6.85 10.60

K1.73 K0.77 K0.35

K1.57 K0.79 K0.36

K1.52 K0.83 K0.42

K1.43 K0.85 K0.55

K1.33 K0.86 K0.67

K0.88 0.08 0.51

K0.72 0.07 0.50

K0.66 0.03 0.44

K0.57 0.00 0.31

K0.47 K0.00 0.19

K0.43 0.53 0.95

K0.27 0.51 0.94

K0.22 0.47 0.88

K0.12 0.45 0.76

K0.02 0.44 0.63

N.B. In the original study by Casey and Neal (1984) silicon concentrations as high as 300 mg lK1 were used. However at such high concentrations, the waters had to be highly alkaline to maintain the silicon in solution even without the addition of quartz. For this table, the high silicon concentration experiments are excluded as pH was not recorded—for the lower concentrations, the pH influence on silicon solubility will be much lower. For the solubility calculations, a pH of 8 was used.

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study stream ecology in relation to physical and chemical processes, Casey and Neal (1984) showed that adding quartz concretions of Cretaceous age (flint) to the channel (with no previous streambed or plant material) reduced the silicon concentration in the Chalk groundwaters feeding the channel from about 4.3–2.4 mg-Si lK1 within 7 days. These changes were both reproducible and independent of flow. Correspondingly, laboratory simulations showed near identical results for flints and that the change in silicon concentration varied as a function of temperature in a way indicative of some sort of solubility or kinetic control. However, the steady state values obtained in the laboratory by Casey and Neal (1984) did not correspond to a simple equilibrium with quartz and it seems that it quartz acts as seeding agents for silicon precipitation in a form different from the ‘lattice-held’ silicon. Table 5 shows one set of data produced by Casey and Neal (1984) where quartz is added to silicon solutions of differing silicon level and the concentration changes measured as a function of time. The results show that with quartz added to pure water, the silicon concentrations increased over time towards quartz saturation after about 30 days. Correspondingly, with quartz added to waters oversaturated with respect to quartz and chalcedony, the silicon concentrations declined towards chalcedony saturation after about 30 days. However, even after 30 days, equilibrium was not reached, but the waters that approached nearest to saturation after quartz addition were those nearest chalcedony equilibrium before the addition of quartz. For the waters studied in this paper, chalcedony equilibrium seems to be generally occurring as an upper bound within many UK lakes, streams and rivers: however, in some cases the upper bound may correspond to quartz saturation and in other cases where perhaps a high weathering rate is sufficient to take the waters above chalcedony and quartz saturation. Undersaturation occurs (a) for acidic and acid sensitive upland systems during periods of high flow when silicon depleted waters from acidic soil horizons enter the stream and there is insufficient time for equilibrium to be established and (b) in lowland environments when high biological activity at low flows for the major rivers and lakes and silicon uptake is sufficient to produce quartz/chalcedony undersaturated conditions.

5. Wider comment This paper provides compelling evidence against a common assumption that quartz is inert and that silicon concentration in surface waters behaves in a chemically conservative manner other than when weathering products such as the clays and biogenic processes come into play. Rather, the results described here mirror processes occurring within groundwaters where chalcedony equilibrium is approached when the biology does not induce significant silicon uptake. The results clearly have broad implications to issues such as the estimation of weathering rates based on silicon fluxes. The latter are attractive for catchment mass-balance studies of weathering rates because a silicon budget can be established relatively easily since atmospheric inputs are negligible. However assumptions that silicon is chemically and biologically conservative could clearly lead to erroneous estimates of catchment weathering rates, particularly over short timescales. The kinetics of approaches to quartz and chalcedony solubility controls may well be important within the wide range of UK rivers examined in this study, but biological processes also come into play.

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