Minor and trace element export from a glacierized Alpine headwater catchment (Haut Glacier d'Arolla, Switzerland)

HYDROLOGICAL PROCESSESHydrol. Process. 15, 3499–3524 (2001)DOI: 10.1002/hyp.1041

Minor and trace element export from a glacierized Alpineheadwater catchment (Haut Glacier d’Arolla,

Switzerland)

Andrew Mitchell,1,2* Giles H. Brown1 and Ron Fuge2

1 Centre for Glaciology2 Centre for Research into Environment and Health, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth,

Ceredigion SY23 3DB, Wales, UK

Abstract:

Major ion concentrations in meltwaters draining glacial environments have been widely reported. However, concentra-tions of minor and trace elements have received scant attention. This study presents trace and minor element variationsin bulk meltwaters draining Haut Glacier d’Arolla (Switzerland) based on twice-daily sampling throughout the 1999ablation season, which represents the most detailed meltwater quality dataset to date. In order to assess the mode ofexport from the catchment, these elements are partitioned into (i) ‘dissolved’ and (ii) ‘particulate-associated’ minorand trace element components.

A computer-based speciation model (PHREEQCi) was applied to the bulk meltwater data, suggesting that Ba, Be,Cd, Cu, Li, Rb and Sr exist primarily as mobile monovalent or divalent dissolved cations, which may be involvedin interactions with suspended sediment surfaces. Conversely, the model predicts the precipitation of Fe, Al, Mn andCr (oxi)hydroxides, suggesting these species may be predominantly transported as colloids, which may remove otherminor and trace elements from solution by co-precipitation reactions.

Laboratory leaching experiments on suspended sediments and fresh rock powder suggests that minor and traceelement concentrations may also be influenced by (oxi)hydroxide precipitation and adsorption–desorption reactionswith suspended sediment surfaces. The quantity and transport mode of trace and minor elements may influence theirbioavailability downstream of glacierized headwater catchments. Further, the enrichment of many dissolved minorand trace elements in meltwaters compared with world stream-waters, coupled with the timing of water and sedimentdelivery during the summer months, may have implications for downstream aquatic environments. Copyright 2001John Wiley & Sons, Ltd.

KEY WORDS minor and trace elements; chemical weathering; meltwater chemistry; suspended sediment;adsorption–co-precipitation; glacier hydrology; PHREEQCi

INTRODUCTION

Major ion concentrations (e.g. Ca2C, Mg2C, NaC, KC, HCO3�, Cl�, NO3

�, SO42�) in meltwaters draining

glacierized alpine and polar environments have been widely reported (Brown, in press). These have been usedto determine chemical denudation rates (e.g. Sharp et al., 1995a; Anderson et al., 1997; Hasnain and Thayyen,1999; Hodson et al., 2000), solute provenance and chemical weathering processes (e.g. Raiswell, 1984; Tranteret al., 1993; Fairchild et al., 1994; Brown et al., 1996a; Anderson et al., 2000), and the configurations andevolution of subglacial drainage systems (e.g. Sharp et al., 1995b; Brown et al., 1996b; Tranter et al., 1996).However, concentrations of minor and trace elements (e.g. Fe, Al, Ti, Mn, Co, Ni, Cu, Zn, As, Rb, Sr, Cd,Ba, Pb, U) in meltwaters are rarely reported (Brown and Fuge, 1998a,b), despite their potential as indicator

* Correspondence to: A. Mitchell, Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth,Ceredigion SY23 3DB, Wales, UK. E-mail: [email protected]

Received 4 May 2001Copyright 2001 John Wiley & Sons, Ltd. Accepted 13 September 2001

3500 A. MITCHELL, G. H. BROWN AND R. FUGE

species in solute provenance studies, and their importance as bio-limiting nutrients or toxic metals in aqueoussystems (Chester, 1990; Manahan, 1991).

Dissolved minor and trace elements in natural waters may be derived from rock weathering (e.g. Holland,1978), recycled marine salts (e.g. Chester, 1990), and anthropogenic sources (e.g. Langmuir, 1997). Glacierizedheadwater catchments offer ideal environments in which to study solute provenance associated with rockweathering (Brown et al., 1996b), since (i) within-catchment anthropogenic activity is often minimal, (ii) theyare often remote from major sources of chemical pollutants derived from urban or industrial centres,(iii) winter atmospheric deposition may be readily estimated from the pre-melt seasonal snowcover, and(iv) chemical denudation rates are high, resulting from large meltwater fluxes and an abundance of freshlyground, geochemically reactive rock flour. However, simple rock–water chemistry relationships may beconfounded by particulate-associated metal transport, which includes chemical sorption–desorption reactions,the formation of Mn and Fe hydroxides and associated co-precipitation of other metals. Particulate-associatedmetal transport has been widely investigated in temperate fluvial systems, particularly in relation to floodplaincontamination (e.g. Brugmann, 1995), and the use of particulate-associated metals as long-term and large-scale sedimentological tracers (e.g. Lewin and Wolfenden, 1978; Macklin, 1996). Particulate-associated metaltransport has largely been ignored in glacio-fluvial systems (Brown, in press). However, alkaline conditionsand large volumes of fine-grained, chemically reactive suspended sediments suggest sorption and hydroxideprecipitation, and thus particulate-associated metal transport may be significant in these environments.

The timing of major ion and suspended-sediment export from alpine glacierized catchments is controlled bythe dynamic nature of the subglacial hydrological system (Rothlisberger and Lang, 1987; Nienow et al., 1996;Richards et al., 1996). Investigations of alpine subglacial hydrology suggest that meltwaters are transportedin two principal flowpaths, classified by flow velocity (Hubbard and Nienow, 1997). ‘Delayed-flow’ watersare transported in a distributed hydrological system, which may include linked cavities, permeable sediments,film flow and broad canals (Hubbard and Nienow, 1997). This component is fed largely by snowmelt, and isthe dominant mode of subglacial drainage at the end of the winter. As the melt season advances, a dendriticsystem of channels develops, evolving to accommodate the increasing volume of meltwater generated duringthe summer months. These arterial channels expand up-glacier following the retreat of the seasonal supraglacialsnowpack (Nienow et al., 1996) and drain so-called ‘quickflow’ waters, which are comprised predominantlyof icemelt. These channels form the dominant mode of meltwater drainage later in the ablation season. Inalpine glacierized catchments the annual hydrograph is concentrated into a 4–5 month period, during which¾90% of the annual discharge is generated (Rothlisberger and Lang, 1987), and ¾90% of dissolved major ions(Collins, 1983; Metcalf, 1986; Sharp et al., 1995a) and suspended sediments (Gurnell, 1987) are exported.

This paper presents data on the magnitude, chemical behaviour and export of minor and trace elementsfrom a pristine glacierized headwater catchment. The authors (i) present dissolved minor and trace elementconcentrations and fluxes throughout the 1999 melt season, (ii) implement geochemical speciation modelsto estimate the chemical speciation of metals and the saturation states of potentially precipitating minerals,and (iii) present concentrations and fluxes of the labile fraction of minor and trace elements associatedwith suspended particulates (i.e. that fraction including (oxi)hydroxide colloidal precipitates, sorbed matterassociated with silts, clays and colloids, and the dissolution of microparticles, which is likely to be easilyavailable from sediment surfaces during water–rock interaction (e.g. Negrel et al., 2000)). This representsthe most detailed meltwater quality dataset to date, and allows for the first time the mode of export of major,minor and trace elements to be examined.

STUDY AREA

Fieldwork was undertaken at Haut Glacier d’Arolla, Valais, Switzerland. This temperate glacier is ¾4Ð2 kmlong, has a maximum ice thickness of ¾180 m, occupies ¾6Ð3 km2 of a ¾12 km2 catchment, and lies onunconsolidated sediments that are between 0Ð05 and 0Ð26 m thick (Sharp et al., 1993; U. Fischer, unpublished

Copyright 2001 John Wiley & Sons, Ltd. Hydrol. Process. 15, 3499–3524 (2001)

MINOR AND TRACE ELEMENT EXPORT FROM A GLACIERIZED ALPINE HEADWATER CATCHMENT 3501

Figure 1. Haut Glacier d’Arolla; location and geology, after Brown and Fuge (1998a)

data) (Figure 1). Studies of major ion and suspended sediment export suggest that chemical denudationconstitutes <1Ð5% of physical denudation, but is significantly higher than the continental average (Sharpet al., 1995a). Dye tracing studies indicate that a hydraulically efficient dendritic channellized subglacialsystem develops during the melt season as surface ablation and subglacial discharges increase (Nienow



et al., 1996), concentrating ¾90% of the annual discharge into the period between May and September. Theweathering of suspended sediments within water-filled subglacial channels contributes the majority of solutepresent in bulk meltwaters emerging from the snout of Haut Glacier d’Arolla at the height of the ablationseason (Brown et al., 1994, 1996b).

The bedrock consists largely of igneous and metamorphic rocks of the Arolla series of the Dente BlancheNappe (Dal Piaz et al., 1977; Mazurek, 1986). The Arolla series is comprised mainly of Arolla gneisses,hornblende–biotote–tonalites, hornblende–biotite–quartz diorites and gabbros (Tranter et al., 1997), and ischaracterized by weakly metamorphosed Arolla granites and chloritic schist facies (Figure 1). The glacier isunderlain mainly by schistose granites in the northern half of the catchment, with a transition to units of gneissin the southeastern corner of the catchment. Amphibolites, gneisses and a dolorite–gabbro intrusion dominatethe western side of the catchment. These major units contain trace amounts of geochemically reactive traceminerals, such as calcite (up to 0Ð58%) and pyrite (up to 0Ð71%) (Tranter et al., 1997). Brown et al. (1996b)recorded concentrations of carbonate up to 12% and pyrite up to 5% in the fine fraction (<5 mm diameter)morainic matrix.

METHODOLOGY

Meltwater and suspended sediment collection

Bulk meltwater and suspended sediment samples were collected twice daily ¾200 m from the glaciersnout in the proglacial channel at 10 : 00 and 17 : 00 local time (approximating to minimum and maximumdiurnal discharge respectively) between 16 June [calendar day (CD) 167] and 20 August (CD 232) 1999. Inaddition, supraglacial meltwaters and snow samples (pre-melt snowpack and fresh snowfall) were collectedperiodically throughout the study period. Meltwater samples were collected by hand in a 1 l Nalgene low-density polyethylene (LDPE) bottle. Snow samples were collected with a clean plastic scoop, and were storedand melted in new plastic bags. For meltwater and snowmelt samples, 250 ml of sample was immediatelyvacuum filtered through 0Ð45 µm Whatman cellulose nitrate (WCN) membranes using a Nalgene polysulfonefilter unit and hand pump. The filtrate was split between two pre-cleaned 60 ml Nalgene LDPE bottles. Onewas acidified with 0Ð1 ml of PRIMAR HNO3 for cation, minor and trace element analysis, and the other wasleft un-acidified for anion analysis. Samples were kept in cool conditions in the field, and refrigerated on returnto the laboratory. Samples were stored for up to 170 days. pH was determined in the field using an Orion250A portable pH/ISE meter, ROSS combination electrode and automatic temperature compensation probe.Calibration was performed using Orion low ionic strength buffers, and Orion low ionic strength adjusterwas added to each sample aliquot used for pH determination prior to measurement. Used 0Ð45 µm WCNmembranes and associated suspended particulates were removed from the filter unit with plastic tweezersand retained in individual plastic bags. Bulk discharge was monitored from a rectangular weir located ina hydroelectric power intake structure operated by Grande Dixence SA, located ¾1Ð5 km from the glaciersnout.

Water—sediment interaction of freshly produced suspended sediments

In order to simulate meltwater-suspended sediment interaction in subglacial channels, dissolution experi-ments were performed on freshly crushed rocks from the catchment, using boundary conditions approximatingto those at Haut Glacier d’Arolla in accordance with methods used by Brown et al. (1994, 1996a, in press).Compared with other environments, water-rock interaction in subglacial and proglacial channels is relativelyeasy to simulate because natural and laboratory systems are comprised of waters that are initially dilute,sediments that are freshly comminuted, fine-grained and geochemically reactive, dissolved organic carbonconcentrations are low, and biological influences are limited (Brown et al., 1996a).

Three dominant catchment rock types [meta-granites (gneissic), meta-gabbros and meta-pelites] from theHaut Glacier d’Arolla catchment were crushed and milled to produce synthetic suspended particulates.



Scanning electron microscope analysis of true and synthetic suspended particulates indicates that these methodsadequately represent the surface morphologies of true suspended particulate loads (Petrovic, 1981). For eachof the three rock types, 4 g of crushed sediment was added to an acid-washed plastic beaker containing 1 l(š2 ml) of air-equilibrated 18 M� deionized water and agitated continuously with a polyethylene-coveredmagnetic bar, which was rotated with a magnetic stirrer. These conditions are analogous to near maximumsuspended sediment concentrations (SSCs) measured in the field, mixing and reacting in turbulent subglacialchannels with free access to the atmosphere. The apparatus was housed in a refrigerator at a temperatureof ¾2 °C, analogous to the temperature of bulk meltwaters. pH was measured in the beaker as for the fielddeterminations. Aliquots (60 ml) of the reaction mixture were collected using acid-washed plastic tubingconnected to an acid-washed syringe, and immediately syringe filtered through 0Ð45 µm WCN membranesand stored in pre-rinsed 30 ml Nalgene LDPE bottles and refrigerated for between 12 and 30 days beforeanalysis. The duration of the experiments was 48 h, with aliquots of the reaction mixture collected at 0, 0Ð25,0Ð5, 1, 2, 4, 8 and 48 h (after Brown et al., 1996a). Reaction mixture volume loss due to sample extractionwas ¾40%, but suspended sediment weights derived from used filter membranes show water–rock ratiosremained approximately constant.

Acid leaching of suspended sediments

To assess the labile fraction of minor and trace elements associated with the suspended particulate load,leaching experiments were carried out on suspended particulates retained on 13 WCN membranes. Sedimentswere leached with 100 ml of 0Ð1% HCl to assess the labile fraction, in accordance with standard geochemicalprocedures (e.g. Negrel et al., 2000). This will release minor and trace elements that may easily be removedfrom the solid phase to solution (sorbed matter, colloids and metal–ligand complexed precipitates) withoutattacking primary minerals. The membranes were oven dried at 60 °C for 12 h to remove all moisture,and sediments were then transferred from the filter papers into acid-cleaned test tubes. The weight ofsediment from each sample ranged from 0Ð01 to 0Ð77 g, representing SSCs of between 0Ð02 and 3Ð1 g l�1,which approximates the range of SSCs measured in bulk meltwaters during the ablation season. The testtubes were agitated every 300 s over a 2400 s period. Each leachate was immediately filtered through apre-rinsed, acid-washed Nalgene filter unit using 0Ð45 µm WCN membranes. The filtered leachates werestored in pre-rinsed 60 ml Nalgene LDPE bottles, and refrigerated for between 12 and 30 days beforeanalysis.

Major ion and minor and trace element analysis

Major ion composition of meltwaters and leachates was determined by ion chromatography (DionexDX-100). Accuracy was š3% and precision š5%. Minor and trace element analyses were undertaken byinductively coupled plasma mass spectrometry (ICP-MS) on a VG Elemental Plasmaquad in semi-quantitativemode, using Ru as an internal standard. An acidified standard solution containing the analytes of interest wasalso used to perform quantitative single point calibration. Multi-element calibration over a wide range ofconcentrations may be achieved using ICP-MS with single point calibration because the response of signalversus concentration is typically linear over six to eight orders of magnitude (Jarvis et al., 1992). For meltwateranalysis, analytes in the standard solution were 10 µg l�1, whereas Fe and Ca were at 100 µg l�1 owing totheir generally higher concentration in natural waters (Langmuir, 1997). For leachate analysis, analytes in thestandard solution were 100 µg l�1, whereas Fe and Ca were at 1000 µg l�1. During each suite of analysis, thestandard solution was run periodically every ¾20 samples. At the same time, procedural blanks, containing18 M� deionized water and Ru, were also analysed. The semi-quantitative corrected data were then correctedrelative to the standard, and water blanks were subtracted. Accuracy and precision were determined by repeatanalysis of standards with concentrations of analytes at concentrations comparable to meltwater samples.Accuracy was š15% and precision š10% for most analytes.



Speciation calculations

Speciation calculations were performed using PHREEQCi software using the MINTEQ database (Plummeret al., 1983; Parkhurst, 1995) on a selection of bulk meltwater analyses. The software calculates the partitioningof elements between different inorganic species (free ions or complexed with ligands), and the saturation stateof minerals in the solution. These results are useful in demonstrating (i) the likely form of chemical speciesunder the range of temperature, pH, pε and element concentrations exhibited in a particular aqueous setting,(ii) metal–ligand complexes that may be involved in adsorption reactions, since adsorption may be affectedby the free ion or complexed nature of the element (Stumm and Morgan, 1996; Langmuir, 1997), and (iii) thepotential formation of minerals, and the direction of chemical reactions (i.e. precipitation or dissolution ofminerals) (Plummer et al., 1983). Organic speciation was not considered, since biological impacts in subglacialchannels are thought to be negligible.

RESULTS AND DISCUSSION

Dissolved species

The concentrations of major ions and selected minor and trace elements in snow, supraglacial and bulkmeltwaters are summarized in Table I. These data indicate significant solute acquisition as dilute supraglacialand snowmelt waters are routed through the subglacial hydrological system. For example, the median Ca2Cconcentration measured in the supraglacial environment is 320 µg l�1, compared with a median of 6600 µg l�1

in bulk meltwaters, an ¾20-fold increase. The median Al concentration is 6Ð6 µg l�1 in supraglacial meltwatersand 35 µg l�1 in bulk meltwaters, an approximate five-fold increase. Temporal variations in minor and traceelement concentrations in snow and supraglacial meltwaters were small over the sampling period.

Ca2C is the dominant metal in bulk outflow, with lesser quantities of Mg2C, NaC, KC and Fe (Table I).The other dominant metals are Al, Ti, Mn, Zn and Sr, which are present at median concentrations ofbetween ¾6 and 35 µg l�1 (Table I). Li, Cr, Co, Ni, Cu, As, Rb, Cd, Cs, Ba, Hg, Pb and U are presentat median concentrations below 1 µg l�1. The pH of bulk meltwaters is predominantly alkaline (median8Ð1), ranging from 7 to 9. The dominance of Ca suggests that calcite dissolution is a major source ofsolute in the hydrological system beneath Haut Glacier d’Arolla, reflecting the geochemical reactivity ofthis ubiquitous trace mineral (Tranter et al., 1993, 1997; Brown et al., 1996b). Mass balance calculationssuggest that carbonate-derived Ca2C is the dominant cation in other high-altitude catchments dominated byigneous and metamorphic bedrock (e.g. Raiswell, 1984; Drever and Hurcomb, 1986; Mast et al., 1990; Finleyand Drever, 1997). Brown and Fuge (1998a,b) explained the distribution of minor and trace elements inmeltwaters draining Haut Glacier d’Arolla on the basis of lithogenic provenance. They attributed much ofthe Sr and Ba, along with Mg2C, to the ubiquitous trace carbonate found in the catchment. Fe, Ni, and Cowere thought to derive primarily from the breakdown of pyrite, which is also widespread. Other chalcophileelements, including Cu, Pb and Zn, were hypothesized to derive from other sulfides in the catchment, includingchalcopyrite, galena and sphalerite, and silicate minerals. Of the other minor and trace metals, Al, Ti, Mnand Rb were attributed to silicate minerals, and U from the breakdown of trace minerals such as allanite andapatite (Brown and Fuge, 1998a,b).

Comparisons of dissolved minor and trace element concentrations with other glacierized and mountainouscatchments are limited by the paucity of data from these environments. However, comparisons with worldstream water median concentrations yield some interesting results. Li, Cu, Zn, As, Rb, Sr, Cs, Ba and Pb arepresent at concentrations lower than the median concentration of world stream waters, whereas Al, Ti, Cr,Mn, Fe, Ni, Cd, Hg and U are present at higher concentrations (Table I). For example, at the height of theablation season median concentrations of Al, Mn, Fe and U are ¾4, 2, 10 and 18 times world stream watermedian concentrations respectively. This is surprising, given the short flowpath lengths (3–4 km beneath HautGlacier d’Arolla) and water-rock interaction times (<3 h in major subglacial channels in August) (Sharp et al.,1993; Nienow et al., 1996), and suggests that river systems fed by glacierized headwaters may derive much


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of their minor and trace element load in the headwater region. Consequently, it is important to considerthe timing and magnitude of this minor and trace element delivery to downstream river systems. However,observed concentrations may not be sustained downstream as a result of dilution, decreasing suspendedsediment reactivity (due to the removal of microparticles and sediment surface sites of excess free energy,and the formation of protective surface coatings), sorption reactions, and the formation of (oxi)hydroxides[e.g. Mn2O3, Fe(OH)3] and associated co-precipitation of other metals (see below).

Discharge characteristics for the 1999 ablation season

Figure 2 shows the discharge and rainfall during the 1999 ablation season. The bulk discharge magnitudeand amplitude generally increased as the melt season progressed, reflecting variations in radiation receipts atthe glacier surface and the up-glacier recession of the seasonal snowpack (Brown et al., 1994; Hubbard andNienow, 1997). Precipitation-driven discharge peaks are superimposed on these radiation-driven variations(e.g. CD 187 and CD 209). Four hydrological periods (P1 to P4) have been defined during the 1999 studyperiod, based upon qualitative analysis of bulk discharge records (Figure 2). P1 and P4 exhibit low dischargesrelative to P2 and P3. SSC and suspended sediment flux (SSF) are positively related to discharge over thesampling period (Figure 2).

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Figure 3. Variations in the pH of bulk meltwaters draining Haut Glacier d’Arolla, 16 June–20 August (CD 167–232) 1999



Seasonal variations in the concentration of dissolved major, minor and trace elements in bulk meltwaters

Seasonal variations in pH, and major, minor and trace element concentrations in bulk meltwaters for twice-daily samples during the 1999 ablation season are shown in Figures 3, 4 and 5 respectively. Summary statisticsare presented in Table II. Concentrations of major, minor and trace elements are generally inversely relatedto bulk discharge. Qualitative consideration of Figures 4 and 5 suggests that most major, minor and traceelements exhibit similar seasonal variations, with declining concentrations during P1 as discharge magnitudeand diurnal variability increases, reaching the lowest seasonal concentrations in P2. There is a general increasein concentrations over P3 and P4, with some elements, including Al, Rb, Zn, Ba, Mn and Ni, experiencingsudden large increases in concentration during P3, apparently associated with precipitation events (Figure 2).Conversely, Cd, As and Mn show a general decline in concentration over P1 and P2, but do not increase inP3 and P4. Ni and U concentrations increase from CD 182 up to the beginning of P2, and decline during theremainder of P2.

Seasonal variations in the flux of dissolved major, minor and trace elements in bulk meltwaters

Although the solute concentrations in meltwaters draining Haut Glacier d’Arolla are generally inverseto discharge (Figures 4 and 5), the instantaneous flux (mg s�1), derived from the product of concentration(µg l�1) multiplied by discharge (m3 s�1) of dissolved major, minor and trace elements generally increaseswith meltwater discharge (Figure 6). Thus, the largest solute fluxes occur when the bulk meltwaters are mostdilute [P2 and P3 (mg h�1), Table III]. Increasing scatter is evident in the flux–discharge relationships athigher discharges, which may be related to large rainfall events (Figure 6).

Figure 4. Seasonal variations in the dissolved concentration (µg l�1) of selected major ions: (a) Ca2C; (b) KC; (c) total alkalinity; (d) SO42�



Figure 5. Seasonal variations in selected dissolved minor and trace element concentrations (µg l�1): (a) Al; (b) Ti; (c) Rb; (d) Zn; (e) Cu;(f) Sr; (g) Ba; (h) Cd; (i) Mn; (j) As; (k) Ni; (l) U



Figure 5. (Continued )

Associations (log–log) between the summed hourly flux (mg h�1) of each dissolved species and summedhourly meltwater discharge (m3 h�1) were determined by least-squares regression during the sampling periodfollowing methods applied in studies of global chemical cycling (e.g. Gibbs and Kump, 1994) and glacialchemical denudation (e.g. Sharp et al., 1995a). Regression relationships between load and discharge forselected minor and trace elements yielded r2 values of >0Ð8. Hourly fluxes of selected minor and traceelements (Al, Ti, Mn, Fe, Ni, As, Rb, Sr, Ba, U) were then estimated from hourly discharge data for whichdischarge data were available [5 May (CD 125)–7 October (CD 280)]. Cu, Zn and Cd fluxes were notestimated owing to poorly defined regression relationships. Predicted fluxes ranged between 0Ð6 and 1Ð9 timesobserved fluxes (though the mean was unity). During the ablation season, the fluxes of dissolved Fe and Alare greatest (13 000 and 1000 kg), with lesser quantities of Sr (460 kg), Mn (260 kg), Ti (210 kg), Ba, Ni,As, Rb and U (each 14–23 kg). These are small in comparison with ablation season fluxes of major cations,which range between 10 000 (KC) and 190 000 kg (Ca2C) (Table III). Analysis of previous annual dischargehydrographs from Haut Glacier d’Arolla suggests that water drains the glacier throughout the winter, whenmajor ion concentrations may be three to seven times summer values (Sharp et al., 1995a). However, thewinter period is characterized by low, relatively constant discharges of ¾0Ð2 m3s�1, which for the periodbetween October and April represents <10% of the annual discharge. For the nearby Gornegletscher, cationfluxes outside the May-September period constituted 4Ð5–8Ð4% of the annual total (Collins, 1983; Metcalf,1986), whereas Sharp et al. (1995a) suggest that annual major ion solute fluxes may be underestimated by upto 10% by neglecting winter fluxes. It is therefore acknowledged that the present estimates may underestimateannual dissolved minor and trace element fluxes by <10%.



Tabl

eII

.Su

spen

ded

sedi

men

tco

ncen

trat

ion

(gl�1

),pH

,di

scha

rge

(m3

s�1),

diss

olve

dm

ajor

ion

and

min

oran

dtr

ace

elem

ent

conc

entr

atio

ns(µ

gl�1

)fo

rth

efo

urhy

drol

ogic

alpe

riod

s(P

1–

P4)

duri

ngth

e19

99ab

lati

onse

ason

,H

aut

Gla

cier

d’A

roll

a

1999

QSS

CpH

KC

Ca2C

SO4

2�H

CO

3�

Al

Ti

Mn

FeN

iC

uZ

nA

sR

bSr

Cd

Ba

U

Per

iod

1M

edia

n1Ð9

50Ð1

68Ð1

370

7000

6200

1700

039

8Ð011

480

0Ð60Ð7

8Ð51Ð2

0Ð718

0Ð30Ð9

0Ð7M

ax4Ð9

61Ð2

58Ð4

490

9000

8600

2300

083

1415

1200

1Ð13Ð8

231Ð8

1Ð028

0Ð71Ð7

1Ð5M

in1Ð1

10Ð0

57Ð4

300

5000

2800

1400

025

4Ð66Ð2

440Ð3

0Ð23Ð1

0Ð50Ð4

110Ð1

0Ð30Ð3

Per

iod

2M

edia

n3Ð0

60Ð6

88Ð1

360

6000

4300

1600

032

6Ð99Ð1

379

0Ð60Ð5

6Ð00Ð7

0Ð413

0Ð20Ð6

0Ð7M

ax7Ð6

71Ð9

68Ð8

434

7600

6700

2100

061

1015

880

1Ð02Ð0

241Ð4

0Ð919

0Ð51Ð2

1Ð3M

in1Ð8

20Ð0

97Ð6

270

4400

2900

7800

244Ð1

5Ð010

00Ð3

0Ð22Ð5

0Ð40Ð3

9Ð40Ð1

0Ð40Ð3

Per

iod

3M

edia

n3Ð8

30Ð7

18Ð3

390

6800

5300

1700

048

7Ð59Ð0

360

0Ð50Ð9

5Ð70Ð6

0Ð516

0Ð20Ð8

0Ð4M

ax6Ð7

93Ð5

8Ð970

084

0091

0024

000

120

1322

1200

1Ð13Ð4

281Ð0

1Ð124

0Ð51Ð9

0Ð9M

in1Ð5

10Ð1

77Ð8

300

5800

3000

1500

024

5Ð75Ð6

270

0Ð30Ð6

3Ð50Ð5

0Ð313

0Ð10Ð4

0Ð2P

erio

d4

Med

ian

2Ð39

0Ð51

8Ð343

070

0058

0018

000

317Ð6

8Ð138

00Ð7

0Ð87Ð9

0Ð60Ð5

170Ð2

0Ð70Ð8

Max

4Ð96

6Ð41

9Ð050

010

000

8600

2600

056

1110

1100

0Ð84Ð2

201Ð3

0Ð923

0Ð51Ð0

1Ð1M

in1Ð3

00Ð1

17Ð5

300

6000

3300

1600

024

5Ð95Ð1

150Ð4

0Ð25Ð3

0Ð50Ð4

140Ð1

0Ð50Ð3



Tabl

eII

I.D

isso

lved

maj

orio

n,m

inor

and

trac

eel

emen

tflu

xes

(mg

h�1)

for

the

four

hydr

olog

ical

peri

ods

and

esti

mat

edto

tal

seas

onal

expo

rt(k

g)du

ring

the

1999

abla

tion

seas

on,

Hau

tG

laci

erd’

Aro

lla

1999

KC

Ca2C

SO4

2�H

CO

3�

Al

Ti

Mn

FeN

iC

uZ

nA

sR

bSr

Ba

U

Per

iod

1M

edia

n2

600

000

5000

000

044

000

000

120

000

000

270

000

5600

080

000

390

000

043

0047

0066

000

8300

4500

130

000

6200

4700

Max

670

000

011

000

000

067

000

000

290

000

000

350

000

014

000

06

900

000

1400

000

011

000

3100

013

000

027

000

1000

023

000

014

000

2000

0M

in1

600

000

3300

000

032

000

000

7300

000

015

000

022

000

3800

080

000

700

1300

2600

017

0024

0034

000

160

650

Per

iod

2M

edia

n4

100

000

6700

000

048

000

000

170

000

000

340

000

7900

011

000

04

200

000

6900

5000

6700

092

0045

0015

000

072

0076

00M

ax9

100

000

140

000

000

8100

000

037

000

000

01

100

000

190

000

210

000

1600

000

014

000

3600

029

000

023

000

1700

031

000

022

000

2000

0M

in2

100

000

3500

000

037

000

000

6000

000

019

000

046

000

5100

098

000

033

0020

0028

000

3100

2500

110

000

3300

3000

Per

iod

3M

edia

n5

600

000

9500

000

073

000

000

240

000

000

560

000

9100

010

000

05

200

000

6900

1300

079

000

7800

5700

210

000

9600

5700

Max

1500

000

015

000

000

013

000

000

043

000

000

02

500

000

260

000

440

000

1700

000

023

000

7400

060

000

015

000

2300

036

000

037

000

9200

Min

220

000

046

000

000

4300

000

098

000

000

200

000

047

000

5200

072

000

026

0049

0041

000

3900

2700

120

000

4400

3700

Per

iod

4M

edia

n3

300

000

6100

000

043

000

000

150

000

000

210

000

5900

050

000

220

000

045

0069

0074

000

5400

4000

120

000

4300

5100

Max

810

000

011

000

000

093

000

000

280

000

000

100

000

014

000

018

000

012

000

000

9800

5300

018

000

014

000

1100

026

000

018

000

1600

0M

in2

100

000

4100

000

037

000

000

9000

000

011

000

032

000

3000

014

000

031

0013

0029

000

2100

2200

9600

029

0018

00

Sam

plin

gpe

riod

(kg)

a

6200

100

000

8000

026

000

063

012

016

069

009Ð9

—b

—b

138

240

1210

Abl

atio

nse

ason

(kg)

a

1000

019

000

016

000

050

000

010

0021

026

013

000

16—

b—

b23

1446

021

18

aE

stim

ated

from

asso

ciat

ions

(log

–lo

g)be

twee

nth

esu

mm

edho

urly

flux

(mg

h�1)

ofea

chdi

ssol

ved

spec

ies

and

sum

med

hour

lym

eltw

ater

disc

harg

e(m

3h�1

)ov

erth

eab

latio

npe

riod

.b

No

estim

ate

poss

ible

due

toin

adeq

uate

regr

essi

onre

latio

nshi

p.



45000

40000

35000

30000

25000

20000

15000

10000

5000

00 1 2 3

Ca2+

4

Discharge (m3s−1)

Ca2+

flu

x (m

g s

−1)

5 6 7 8

40000

35000

30000

25000

20000

15000

10000

5000

00 1 2 3

SO42−

4

Discharge (m3s−1)

SO

42− f

lux

(mg

s−1

)

5 6 7 8

800

700

600

500

400

300

200

100

00 1 2 3

Al

4

Discharge (m3s−1)

Al f

lux

(mg

s−1

)

5 6 7 8

0 1 2 3

Rb

4

Discharge (m3s−1)

Rb

flu

x (m

g s

−1)

5 6 7 8

7

6

5

4

3

2

1

0

4500

4000

3500

3000

2500

2000

1500

1000

500

00 1 2 3

K+

4

Discharge (m3s−1)

K+ f

lux

(mg

s−1

)

5 6 7 8

0 1 2 3 4 5 6 7 8

0 1 2 3 4 5 6 7 8

140000

120000

100000

80000

60000

40000

20000

0

Total Alkalinity

Discharge (m3s−1)

HC

O3- f

lux

(mg

s−1

)

80

70

60

50

40

30

20

10

0

Ti

Discharge (m3s−1)

Tl f

lux

(mg

s−1

)

0 1 2 3

Zn

4

Discharge (m3s−1)

Zn

flu

x (m

g s

−1)

5 6 7

100

90

80

70

60

50

40

30

20

10

08

Figure 6. The dissolved flux of selected major, minor and trace elements versus bulk meltwater discharge during the 1999 ablation season



0 1 2 3

Cu

4

Discharge (m3s−1)

Cu

flu

x (m

g s

−1)

5 6 7 8

7

8

9

10

6

5

4

3

2

1

0

0 1 2 3

Mn

Ba

4

Discharge (m3s−1)

Ba

flu

x (m

g s

−1)

5 6 7 8

7

8

9

6

5

4

3

2

1

0

180

160

140

120

100

80

60

40

20

00 1 2 3 4

Discharge (m3s−1)

Mn

flu

x (m

g s

−1)

5 6 7 8

0 1 2 3

Ni

4

Discharge (m3s−1)

Ni f

lux

(mg

s−1

)

5 6 7 8

7

6

5

4

3

2

1

00 1 2 3

U

4

Discharge (m3s−1)

U f

lux

(mg

s−1

)

5 6 7 8

6

5

4

3

2

1

0

0 1 2 3

As

4

Discharge (m3s−1)

As

flu

x (m

g s

−1)

5 6 7 8

12

14

10

8

6

4

2

0

0 1 2 3

Cd

4

Discharge (m3s−1)

Cd

flu

x (m

g s

−1)

5 6 7 8

2.5

2

1.5

1

0.5

0

0 1 2 3

Sr

4

Discharge (m3s−1)

Sr

flu

x (m

g s

−1)

5 6 7 8

120

100

80

60

40

20

0

Figure 6. (Continued )



Speciation modelling—soluble complexes and mineral saturation states

The fluxes detailed above allow the export of dissolved minor and trace elements to be estimated. However,trace and minor elements may also be exported in complexed forms [e.g. Fe(OH)4

�, Al(OH)4�], which may

influence their mobility and therefore their mode of export from the catchment. In order to evaluate thechemical speciation of metals and to indicate minerals that may potentially precipitate in the bulk meltwatersolution, calculations were performed using PHREEQCi (Parkhurst, 1995) (Tables IV and V). These datarepresent speciation calculations performed on relatively concentrated bulk meltwaters from P1 and relativelydilute bulk meltwaters from P3 to encompass the range of bulk meltwaters concentrations exhibited during the1999 ablation season. Results suggest that a mixture of soluble metal and non-metal ligand complexes and freemonovalent and divalent ions characterize the distribution of metals in the bulk meltwaters. Within the range ofpH (7–9), pε (estimated at between four and ten in these aerobic oxidizing subglacial channels) and dissolvedion concentrations exhibited by the bulk meltwaters, all redox-sensitive elements (As, Cr, Cu, Fe, Hg, Mn, N,S, U) exist predominantly (>99%) in an oxidized state [e.g. As(V), Cr(VI), Fe(III)]. The alkaline earth metalsand alkali metals (Ba, Be, Ca, K, Li, Mg, Na, Rb and Sr) exist predominantly as uncomplexed monovalentand divalent cations. Most remaining minor and trace elements exist predominantly as hydroxyanions [e.g.

Table IV. Results of aqueous speciation calculation performed using PHREEQCi (Parkhurst, 1995) on the most (P1) andleast concentrated (P3) bulk meltwaters draining Haut Glacier d’Arolla during the 1999 ablation season

Element(redox state)a

Early ablation season bulk meltwaters(Period 1)b

Middle ablation season bulk meltwaters(Period 3)b

Al Al(OH)4� (95%), Al(OH)3 (5%) Al(OH)4

� (96%), Al(OH)3 (4%)As(V) HAsO4

2� (97%), H2AsO4� (3%) HAsO4

2� (98%), H2AsO4� (2%)

Ba Ba2C Ba2C

Be Be2C Be2C

Ca Ca2C (98%), CaSO4 (1%) Ca2C (99%), CaSO4 (1%)Cd Cd2C (64%), CdCO3 (32%), CdHCO3

C, CdSO4

(1%), CdOHC (1%)Cd2C (64%), CdCO3 (33%), CdHCO3

C, CdSO4

(1%), CdOHC (1%)Cr(VI) CrO4

2� (98%), HCrO4� (2%) CrO4

2� (99%), HCrO4� (1%)

Cu(II) Cu(OH)2 (98%), CuCO3 (2%) Cu(OH)2 (99%), CuCO3 (9%)Fe(III) Fe(OH)4

� (55%), Fe(OH)3 (30%), Fe(OH)2C

(15%)Fe(OH)4

� (65%), Fe(OH)3 (26%), Fe(OH)2C

(9%)Hg(II) Hg(OH)2 Hg(OH)2

K KC KC

Li LiC LiC

Mg Mg2C (98%), MgSO4 (1%) Mg2C (99%), MgSO4 (1%)Mn(II) Mn2C (98%), MnSO4 (1%) Mn2C (99%), MnSO4 (1%)Na NaC NaC

Ni NiCO3 (93%), Ni2C (6%) NiCO3 (93%), Ni2C (6%)Pb PbCO3 (88%), PbOHC (8%), Pb2C (3%),

Pb(CO32� (1%), Pb(OH)2 (1%)

PbCO3 (85%), PbOHC (11%), Pb2C (2%),Pb(CO32

� (1%), Pb(OH)2 (1%)Rb RbC RbC

Sr Sr2C Sr2C

U(VI) UO2CO322� (88%), UO2CO33

4� (7%),UO2CO3 (4%)

UO2(CO3)2 (89%), UO2(CO3)3 (6%), UO2CO3

(4%)Zn Zn2C (50%), ZnCO3 (20%), Zn(OH)2 (18%),

ZnOHC (9%), Zn(CO322� (1%), ZnSO4 (1%)

Zn2C (42%), ZnCO3 (17%), Zn(OH)2 (29%),ZnOHC (10%), Zn(CO32

2� (1%), ZnSO4

(1%)

a Values in parenthesis indicates dominant (>99%) redox state.b Main stable complexes listed (values in parentheses indicate moles of species as a percentage of the total moles of that element in solution.Only species values >1% are listed).



Table V. Results of mineral saturation state calculations per-formed using PHREEQCi (Parkhurst, 1995) on the most (P1)and least concentrated (P3) bulk meltwaters draining HautGlacier d’Arolla during the 1999 ablation season. Saturation

states presented for all minerals at or near saturation.

Mineral Formula P1 P3

Gibbsite(C) Al(OH)3 0Ð43 �0Ð11Diaspore AlOOH 2Ð33 1Ð78Boehmite AlOOH 0Ð62 0Ð08Ba3(AsO4)2 Ba3(AsO4)2 3Ð77 1Ð63CupricFerrite CuFe2O4 16Ð47 15Ð06CuprousFerrite CuFeO2 7Ð47 6Ð59Fe(OH)2Ð7Cl0Ð3 Fe(OH)2Ð7Cl0Ð3 7Ð16 6Ð34Ferrihydrite Fe(OH)3 3Ð37 2Ð7Hematite Fe2O3 20Ð53 19Ð19Maghemite Fe2O3 10Ð14 8Ð8Magnetite Fe3O4 15Ð83 13Ð68Goethite FeOOH 7Ð76 7Ð09Lepidocrocite FeOOH 6Ð89 6Ð22Mg-ferrite MgFe2O4 11Ð58 10Ð19Bixbyite Mn2O3 5Ð8 5Ð76Hausmannite Mn3O4 4Ð52 4Ð31Pyrolusite MnO2 4Ð98 5Ð1Nsutite MnO2 3Ð34 3Ð46Birnessite MnO2 2Ð75 2Ð87Manganite MnOOH 2Ð83 2Ð81Calcite CaCO3 �0Ð89 �1Ð13Aragonite CaCO3 �1Ð03 �1Ð27

Fe(OH)4�, Al(OH)4

�], oxyanions (e.g. HAsO42�, CrO4

2�), or as uncharged hydroxides [Cu(OH)2, Hg(OH)2].U(VI), Ni and Pb occur as carbonate complexes [e.g. UO2CO32

2�, NiCO3, PbCO3] (e.g. Levinson, 1980).Cd is distributed between uncomplexed divalent cations and carbonate complexes, whereas Zn is distributedbetween uncomplexed divalent cations and hydroxides. These calculations suggest that dissolved Mn existsprimarily as an un-oxidized and un-complexed divalent cation [Mn(II)2C] (cf. Stumm and Morgan, 1996).

Saturation index (SI) calculations suggest the precipitation (positive SI) of a range of Al, Fe and Mnoxides and hydroxides (Table V). These include diaspore (AlOOH), ferrihydrite (FeOH)3, haematite (Fe2O3),magnetite (Fe3O4), goethite (FeOOH), manganite (MnOOH) and a range of MnO2 minerals. Cupric ferrite(CuFe2O4) and magnesium-ferrite (MgFe2O4) are also suggested as precipitates. Common rock-formingferromagnesian minerals and calcite, common in the catchment, are all undersaturated in bulk meltwaters.However, calcite (CaCO3) is only slightly undersaturated in concentrated waters from the beginning of theablation period. Fe, Al and Mn (oxi)hydroxide supersaturation suggests that these elements will exist primarilyin a solid colloidal form, and not as dissolved species. Other minor and trace elements are likely to be removedfrom solution by co-precipitation, as these (oxi)hydroxide solids form in solution (Stumm and Morgan, 1996;Langmuir, 1997). (Oxi)Hydroxides are highly sorbent materials and soluble (oxi)hydroxide–metal complexesare preferentially adsorbed onto Fe, Al and Mn (oxi)hydroxide solids (Langmuir, 1997), which suggestsmany of the minor and trace elements (present as hydroxide complexes, but not saturated in solution) inthis system may be adsorbed onto (oxi)hydroxide precipitates in subglacial and proglacial environments.Conversely, carbonate–metal complexes are weakly adsorbed by solids (Langmuir, 1997) and remain highlymobile (Aiuppa et al., 2000) suggesting U, and to some extent Cd, will not be readily adsorbed. Alkali metalsand alkaline earth metals present as monovalent or divalent cations with low ionic potentials should remain



mobile, and unadsorbed. However, it should be borne in mind that the equilibrium calculations derived byPHREEQCi are based upon thermodynamic principles, and do not consider kinetic processes.

Labile minor and trace elements associated with suspended sediment surfaces

Speciation modelling suggests that Fe, Al and Mn may be readily removed from solution by the precipitationof (oxi)hydroxide solids (e.g. FeOH3, AlOH3, Mn2O3), and it is likely that other minor and trace elementsare removed from solution by co-precipitation in these solids as they form (Stumm and Morgan, 1996;Langmuir, 1997). Adsorption might be expected to be important given (i) the predominance of soluble(oxi)hydroxide complexes that readily adsorb to (oxi)hydroxide solids [e.g. Al(OH)4

�, Zn(OH)C] (Stummand Morgan, 1996; Langmuir, 1997), (ii) the ubiquitous supply of silt and clay-sized particles in subglacialand proglacial environments (e.g. Richards, 1984; Fenn and Gomez, 1989), which have been shown tobe important for the adsorption of metals in temperate fluvial systems (e.g. Ongley et al., 1981), and (iii)the alkaline conditions of the bulk meltwaters, which favour the adsorption of minor and trace elements(Langmuir, 1997).

(i) Water-rock interaction experiments. The interactions between complexed, particulate–associated anddissolved phases in meltwater solutions are highlighted during water-rock dissolution experiments (Figure 7).The rate of increase in the concentration of Mg2C (which is representative of species such as Ca2C,Sr, SO4

2�) decreases over time, following classical exponential dissolution kinetics (e.g. Brown et al.,1994, 1996a, 2001). Conversely, Al, Mn and Zn (which are representative of many of the minorand trace elements) exhibit complex behaviour during water–rock interaction. These elements appear

Figure 7. Temporal variations in the concentrations (µg l�1) of (a) Mg, (b) Al, (c) Mn and (d) Zn during laboratory dissolution experimentsbetween freshly crushed rock material from the Haut Glacier d’Arolla catchment and deionized water



to be rapidly released during the early stages (<3600 s) of water–rock interaction (Figure 7). There-after they are removed from solution to varying degrees. Aluminium concentrations decline rapidlyafter ¾3600 s of water–rock interaction, and remain low for the remainder of the experiment. Con-versely, dissolved Mn and Zn concentrations decline rapidly after the early stages of water–rock inter-action (>900 s). Their concentrations then increase markedly after ¾3600 s of water–rock interaction(Figure 7).

Adsorption and co-precipitation reactions must account for the decline in dissolved concentrations for manyminor and trace elements during the experiments, and indicate that temporal variations in the partitioningof minor and trace elements, under conditions analogous to those operating in subglacial and proglacialchannels, are complex. Elements will be removed from solution by sorption or co-precipitation, or released tosolution by de-sorption and mineral dissolution at different stages of rock–water interaction within subglacialand proglacial channels, reflecting temporal changes in solution chemistry (e.g. pH, pε, dissolved elementconcentrations, speciation, mineral saturations states) (Leckie and James, 1974), sediment parameters (e.g.concentration, mineralogy, particle size) and the duration of water–rock interaction (Brown et al., in press).The experiments presented indicate the potential of dissolved-particulate interactions in controlling solutionchemistry and solute transport. However, further experimental work is required to quantify the effects ofsolution chemistry and sediment parameters upon minor and trace element behaviour in subglacial andproglacial environments.

Table VI. Particulate-associated concentrations (µg g�1)derived from weak (0Ð1% HCl) acid leaches of suspendedsediments retained on 13 0Ð45 µm WCN membranes fromthe 1999 ablation season. Suspended sediment concentrations

ranged between 0Ð02 and 3Ð1 g l�1

Element/species Median Min Max

Li 4Ð2 1Ð1 99Be 0Ð48 0Ð28 4Ð6Al 1200 570 3000Ti 35 19 130Mn 140 110 530Fe 2600 1700 8800Co 3Ð0 1Ð7 6Ð7Ni 10 2Ð0 230Cu 7Ð0 4Ð2 53Zn 33 20 220Rb 3Ð0 1Ð5 15Sr 16 8Ð6 25Cd 0Ð49 0Ð11 14Cs 0Ð35 0Ð041 8Ð5Ba 29 16 190Hg 0Ð52 0Ð050 22Tl 0Ð21 0Ð11 1Ð6Pb 7Ð7 2Ð9 33Bi 1Ð5 0Ð49 3Ð5U 1Ð5 0Ð93 3Ð3SO4

2� 130 8Ð3 620NaC 380 90 4000KC 740 460 1500Mg2C 850 670 2500Ca2C 4700 2400 14 000



(ii) Sediment acid leaches. In order to quantify the importance of minor and trace elements export fromthe catchment associated with suspended sediment surfaces, dilute acid (0Ð1% HCl) leaches were performedat SSCs ranging between 0Ð02 and 3Ð1 g l�1. The dilute acid leaches yielded estimates of the mass of labilemetal (µg) per unit mass of sediment (g) (Table VI). Ca, Fe, and Al dominate the labile, sediment-associatedfraction (median 4700–1200 µg g�1). Labile Mg2C, KC, NaC, Mn and SO4

2� are liberated at concentrationsof 850–130 µg g�1, and the remaining elements (Li, Be, Ti, Co, Ni, Cu, Zn, Rb, Sr, Cd, Cs, Hg, Pb, Tl, Biand U) between 35 and 0Ð2 µg g�1.

(iii) Temporal variations in labile minor and trace element transport associated with particulate surfaces.Previous studies of meltwater chemistry have largely ignored dissolved-particulate interactions in subglacial

Figure 8. Linear associations between mass of particulate-associated minor and trace metals (log10 µg) and mass of sediment (log10 g)derived from weak (0Ð1% HCl) acid leaches of suspended sediments retained on 13 0Ð45 µm WCN membranes from the 1999 ablation

season

Figure 9. The log10 SSC (g l�1) versus log10 bulk discharge (m3 s�1) during the 1999 ablation season at Haut Glacier d’Arolla



channels. This study aims, for the first time, to provide an estimate of labile minor and trace elementsassociated with suspended particulate exported over the ablation period. Associations (log–log) betweenthe labile particulate-associated concentration (µg l�1) (derived from dilute acid leaching experiments) andSSC (g l�1) at the time of sample collection, exhibit a positive linear relationship (Figure 8). Associationswith hourly SSC [g l�1, from log–log SSC–bulk discharge relations (Sharp et al., 1995a; Figure 9], permitestimates of the mass of element (µg) per gram of sediment exported hourly over the ablation period. This is

Figure 10. Predicted minor and trace element particulate-associated and dissolved export (mg h�1) over the 1999 ablation season for (a) Feand (b) Al. N.b.: different y-axis scales used



Tabl

eV

II.

Susp

ende

dse

dim

ent

(kg

h�1)

and

pred

icte

dm

ajor

ion,

min

oran

dtr

ace

elem

ent

part

icul

ate-

asso

ciat

edflu

xes

(mg

h�1)

for

the

four

hydr

olog

ical

peri

ods,

and

tota

lse

ason

alex

port

(kg)

duri

ngth

e19

99m

elt

seas

on

1999

SSC

KC

Ca2C

SO

42�

Al

Ti

Mn

Fe

Ni

Cu

Zn

Rb

Sr

Ba

U

Per

iod

1M

edia

n18

0035

000

021

0000

059

000

600

000

2100

081

000

140

000

018

0049

0025

000

2000

6900

1600

078

0M

ax66

000

8400

000

099

000

000

07

500

000

130

000

000

310

000

016

000

000

300

000

000

6600

060

000

030

0000

023

000

02

300

000

240

000

02

2000

0M

in22

013

000

5400

033

0025

000

1100

3500

5800

022

028

015

0012

022

084

027

Per

iod

2M

edia

n61

002

200

000

1700

000

030

000

03

600

000

110

000

480

000

850

000

061

0024

000

130

000

9800

4800

086

000

5100

Max

8900

013

000

000

01

600

000

000

1100

000

020

000

000

04

600

000

2500

000

047

000

000

089

000

890

000

440

000

035

000

03

700

000

360

000

034

000

0M

in12

0018

000

01

000

000

3300

032

000

011

000

4300

073

000

012

0027

0014

000

1100

3400

8900

390

Per

iod

3M

edia

n98

004

500

000

3700

000

057

000

07

400

000

210

000

960

000

1700

000

098

0046

000

240

000

1800

010

000

017

000

011

000

Max

6200

076

000

000

880

000

000

690

000

012

000

000

02

800

000

1500

000

028

000

000

062

000

550

000

2700

000

210

000

210

0000

220

000

019

000

0M

in68

077

000

390

000

1600

014

000

052

0019

000

320

000

680

1300

6800

530

1400

4100

170

Per

iod

4M

edia

n18

0034

000

02

100

000

5800

059

000

020

000

7900

014

0000

018

0048

0025

000

1900

6700

1600

076

0M

ax24

000

1800

000

018

000

000

02

000

000

2900

000

077

000

03

700

000

6800

000

024

000

160

000

790

000

6200

046

000

059

000

045

000

Min

460

4200

020

000

091

0076

000

3000

1000

018

000

046

075

040

0031

072

024

0088

Sam

plin

gPe

riod

(kg)

12ð

106

7300

7000

013

000

1100

032

015

0028

000

5567

340

2618

025

018

Abl

atio

nSe

ason

(kg)

16ð

106

9700

9300

018

000

1500

042

020

0037

000

110

8844

035

240

330

24

Abl

atio

npe

riod

%pa

rtic

le-

asso

cflu

xa

4833

1394

6788

7487

——

6939

9458

aPa

rtic

ulat

e-as

soci

ated

flux

expr

esse

das

perc

enta

geof

tota

lflu

x(d

isso

lved

and

part

icul

ate-

asso

ciat

ed).

bN

oes

timat

epo

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nose

ason

aldi

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flux

estim

ate

avai

labl

e.



expressed as an hourly flux (mg h�1) of labile minor and trace elements associated with particulate surfaces(for example Fe and Al, Figure 10a and b). The greatest particulate-associated minor and trace element exportis at the height of the ablation season (P2 and P3), when discharge and SSCs are highest. For example,median Fe and Al fluxes are 17 000 000 mg h�1 and 7 400 000 mg h�1 respectively in P3, compared with and1 400 000 and 600 000 mg h�1 in P1 (Table VII). Total ablation season export of minor and trace elements aregreatest for Fe (37 000 kg) and Al (15 000 kg), which are between one to two orders of magnitude greater thanTi (420 kg), Mn (2000 kg), Ni (110 kg), Cu (88 kg), Zn (440 kg), Rb (35 kg), Sr (240 kg), Ba (330 kg) andU (24 kg). These estimates are likely to account for >90% of the annual export of labile particulate-associatedminor and trace elements (Table VII).

(iv) Dissolved versus particulate-associated minor and trace element transport during the ablation season.The labile particulate-associated flux accounts for the majority of the total ablation season element exportof most minor and trace elements (Table VII), as shown for Fe and Al (Figure 10a and b). For example,>70% of the estimated export of Al, Mn, Fe, Ni and Ba is transported as particulate-associated speciesduring the ablation season. For more mobile dissolved species (e.g. Ca2C, Sr, U, SO4

2�), export in theparticulate-associated form is far less important.

Figure 11 indicates the percentage of particulate-associated flux for Al and Sr, during the 1999 ablationseason. Figure 11a shows that, at the beginning of the ablation season, the percentage of particulate-associatedAl accounts for <1% of Al export, due to negligible suspended sediment fluxes at this time. Particulate-associated transport accounts for ¾50% of Al export at the beginning of the sampling period, and <90% atthe height of the ablation season (P2) as sediment fluxes increase. This pattern is characteristic of other minorand trace elements that have a strong affinity for the particulate phase. Figure 11b similarly shows that, atthe beginning of the ablation season, the percentage of particulate-associated Sr accounts for <1% of export.However, at the height of the ablation season (P2) the particulate-associated transport commonly accounts for<50% of Sr export, and is characteristic of other mobile dissolved species (e.g. Ca2C, Sr, U, SO4

2�).The seasonal evolution of the subglacial hydrological system controls a range of parameters that affect

geochemical processes in meltwater solutions [e.g. meltwater residence time, suspended sediment mobilizationand availability, water–rock ratio, and the availability of atmospheric gases (Sharp, 1991; Tranter et al., 1993,1997; Brown et al., 1994, 1996a, 2001)]. Therefore, it is likely that the magnitude and mode of export ofmany potentially toxic or bio-limiting dissolved and particulate-associated minor and trace elements in bulkmeltwaters are similarly influenced by the dynamic nature of the subglacial hydrological system at the seasonaltime scale.

CONCLUSIONS

The enrichment of dissolved major, minor and trace elements between the supraglacial and proglacialenvironment in glacierized catchments indicates the voracity of solute acquisition in alpine subglacialenvironments. Many minor and trace elements associated with the breakdown of silicates are present atconcentrations higher than world stream-water averages. These data imply that river systems fed by glaciatedheadwaters may derive much of their minor and trace element load in the headwater region.

Temporal variations in solute concentrations indicate that the most dilute meltwaters issue from the glacierat the height of the ablation season. The most concentrated waters are exported outside the summer meltseason. However, since >90% of the annual meltwater discharge occurs during the period between Mayand September, the greatest flux of dissolved major, minor and trace elements is delivered at the height ofthe ablation season. Speciation calculations, suspended sediment acid leaches, and water–rock interactionexperiments demonstrate that while particularly mobile species (e.g. major ions and Sr) are predominantlytransported in the dissolved phase, >70% of most minor and trace elements are associated with the particulate(>0Ð45 µm) phase as colloidal (oxi)hydroxides or sorbed species. The labile particulate-associated flux



Figure 11. The predicted percentage of particulate-associated flux during the 1999 ablation season for (a) Al and (b) Sr.* Hourlyparticulate-associated flux expressed as a percentage of total flux (dissolved and particulate-associated)

accounts for the majority of the total ablation season element export of most minor and trace elements. Formore mobile dissolved species, export in the particulate-associated form is far less important, and dissolvedtransport dominates their export.

The seasonal concentration and flux dynamics of dissolved and particulate-associated minor and traceelements are controlled by catchment bedrock geology and mineralogy, solution geochemistry (e.g. pH, pε,mineral saturation states, sediment concentrations), and the seasonal evolution of the subglacial hydrologicalsystem. These data may have implications for (i) the temporal variation of water quality in rivers fedby glacierized headwater catchments, such as the Rhone, (ii) the transport, dispersal and accumulation of



metal-laden glacial suspended sediments over large areas in downstream environments, and (iii) the temporaleffect on biota in downstream environments.

ACKNOWLEDGEMENTS

This work was supported by a University of Wales Aberystwyth Studentship (AM), University of WalesAberystwyth Research Grants 56/95 and 36/96 (GHB and RF), and the NERC (Grant No. GR3/11216). Weacknowledge Grande Dixence S.A. for the provision of discharge data, and Ian Willis and Doug Mair formeteorological data. Field assistance was provided by Becky Goodsell, Sam Clemmens and Kaspar Arn.Thanks must also go to Lorraine Hill for assistance and guidance in the laboratory, and Bill Perkins foradvice and support with ICP-MS. Assistance for AM to attend to Canadian Geophysical Union–EasternSnow Conference 2001 was generously provided by University of Wales Aberystwyth Studentship TravelGrant, and the Canadian Geophysical Union. We extend our thanks to two anonymous referees and Ming-koWoo, who provided invaluable feedback on the original version of the manuscript and whose comments havesignificantly improved the paper.

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Documents

Minor and trace element export from a glacierized Alpine headwater catchment (Haut Glacier d'Arolla, Switzerland)