Hydrobiologia 348: 19–38, 1997. 19I. P. Muniz (ed.), The Høylandet Reference Area.c 1997 Kluwer Academic Publishers. Printed in Belgium.

Soil and stream water chemistry in a pristine and boggy site in mid-Norway

Rolf D. Vogt1 & Ivar P. Muniz2

1Department of Chemistry, University of Oslo, POB 1033, Blindern, N-0315 Oslo, Norway2 NINA, POB 736 Sentrum, N-0105 Oslo, Norway

Key words: bog chemistry, pristine stream water chemistry, water flowpath, PCA, salt episode

Abstract

Stream- and soilwater at the 18.7 ha pristine Ingabekken catchment, on gneiss bedrock at Høylandet, have beenstudied for three years, including intensive episode studies in spring and autumn. The site mainly consists of blanketbogs which are typical for these marine west coast climates. Water drains through the blanket peats by means oftwo major flowpaths. Each flowpath contributes to the stream with a distinct chemical fingerprint rendered by thesoil/soil water interactions along the flowpath, i.e. they may be regarded as end-members. The soil water fromthe upper peat layers is the end-member representative of stormflow discharge whereas baseflow originates mainlyfrom seepage of the other end-member, which is the mineral soil water from beneath the peat. The pHBaCl2 of thesoils that control the runoff chemistry during highflow conditions was as low as 2.6, allowing for a substantialpH drop in streamwater in the case of a seasalt episode. pH in the stream varied from more than 7 at baseflowto 5 or slightly below at stormflow. The lowest pH (4.8) was observed during early snowmelt due to release ofmeltwater highly enriched in seasalts. The fraction of exchangeable aluminum (AlS) was much higher in the surfacelayers of the lower reaches of the catchment than close to the water divide. This suggests a transport of Al, muchlike podzolisation, though downslope by a lateral flowpath. A Principal Component Analysis on the stream waterchemical data shows the importance of water flowpaths in addition to dilution or ionic strength and antecedentconditions as a factor in determining the water quality. On the plane of the two major principal components thebase cations (Ca2+, Mg2+, Na+, K+) were negatively related to [H+], and the total organic carbon (TOC) wasnegatively related to strong acid anions (Cl�, SO4

2�, NO3�). These relationships between the parameter loadings

along the two main principal components remained indifferent to the effects of both dilution and flowpaths.Under the present conditions of low acid deposition, this sensitive system is effectively buffered by its weak

acids and all released Al is complexed by natural organic acids. Similar boggy areas located in regions with heavyanthropogenic acid deposition may not be able to neutralize the mineral acids. A shallow water flowpath and a highH+ saturation of the ion exchanger in the soils controlling the highflow chemistry may lead to discharge episodeswhere strong mineral acids are allowed to pass through the system releasing elevated levels of toxic aluminum inthe stream.

Introduction

Surface water acidification is the result of complexinteractions between the naturally occurring biogeo-chemical processes and anthropogenic impacts onthese processes. To better understand the effects of thisanthropogenic deposition, we therefore need to firstunderstand the naturally occurring processes. Integrat-ed catchment studies (including monitoring of precipi-

tation, soil- and stream water, as well as measurementsof soil chemical characteristics) are important tools inassessing the dominant natural processes controllingstream water chemistry. However, most such integrat-ed field studies have been conducted in areas that havebeen anthropogenically acidified. Furthermore, to putdata from acidified sites into perspective, there is aneed for reference data from pristine but ‘mineral acidsensitive’ areas.

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Despite recent decreases in emission of SO2 inEurope and North America, ‘the acid rain still reigns’(Rodhe et al., 1995), and large, still pristine regions inthe northern hemisphere are receiving loadings of acid-ifying compounds that enhance the mobility of basecations out of the soil.

Blanket Histosols are common throughout a con-siderable part of a wide belt that runs through northernEurope and central Canada. Peat and Hydromor soils(i.e. permanently- and temporarily waterlogged super-posed humus, respectively) possess unique character-istics concerning both hydrology and soil chemistrydemanding special concern when attempting to delin-eate their hydogeochemical response factors. The His-tosol peat profile is by its nature generally waterloggedsince they develop in areas where the groundwater lev-el reaches the ground surface. The waterlogged condi-tions reduce the vertical hydrological conductivity sothat the infiltration capacity is often less than the rainintensity, leading to frequent sublateral- and overlandflow. Large amounts of excess precipitation combinedwith a distinct shallow sub-lateral water pathway ‘shortcircuits’ the geochemical buffering by bypassing thedeeper soils, often possessing greater bufferring capac-ities. These conditions render watersheds with a coastalboreal climate susceptible to acid episodes as thesewatersheds often comprise downslope and thin blanketbogs. Despite this, the Histosol soil type has receivedlittle attention in scientific literature (FitzPatric, 1983).Enhanced knowledge and understanding of the dom-inant processes controlling runoff chemistry in suchacid sensitive systems are required to determine cred-ible critical load estimates. This study is also particu-larly relevant for the further testing and developmentof the conceptual chemical relationships used in acid-ification models such as the Birkenes model (Christo-phersen et al., 1982; Seip et al., 1995), ILWAS (Gheriniet al., 1985), MAGIC (Cosby et al., 1985), PROFILE(Warfvinge and Sverdrup, 1992) and RAINS (Hordijk,1991).

This report summarizes the research on soil andstream water chemistry carried out at the 18.7 haIngabekken boggy catchment, situated within thebroader pristine Høylandet area (Figure 1). A morecomprehensive site description is found in the papersby Christophersen et al. (1990a, b), Mulder et al.(1995), Pijpers and Mulder (1990) and Fjeldal (1992).Effort is made to synthesize the data and findings pre-sented in these papers as well as information from oth-er studies in the Høylandet area (e.g. Anderson et al.,1996; Muniz, 1996; Blakar & Hongve, 1997). Based

on present understanding,new interpretations are madeon previous findings. Emphasis is placed on study-ing relationships between the chemical parameters, ingeneral, by means of advanced and simple statisticsas well as of detailed studies of episodic changes instream water chemistry. The chemical relationships areassessed in the context of water flowpaths and concep-tual soil/soil water chemical interactions (e.g. cationexchange, calcium over magnesium ratio (Ca/Mg) andaluminum (Al) solubility). The focus is set on highflowconditions, since the most adverse biological effects inacidified streams occur associated with peak discharge(Leivestad & Muniz, 1975).

Site description

The Ingabekken catchment, at an elevation of 280 to370 m (64� 390 N, 12� 60 E), is in the subalpine zoneof the greater Høylandet watershed in Mid Norway(Figure 1). This small subcatchment, situated on thesoutheastern slope facing Lake Storgronningen has asuboceanic climate. The Ingabekken catchment rep-resents a Histosol system being comprised mainly ofblanket bogs.

The discussion over the last decades regarding thecauses for soil and freshwater acidification has, asidefrom acid deposition, also involved land use changesin combination with naturally occurring organic acidsand seasalts (Rosenqvist, 1978; Overrein et al., 1980;Krug & Frink, 1983). The studied site has not been uti-lized for harvesting of animal fodder nor timber duringthe last century due to its inconvenient location, steeptopography and marginal productivity. Local effects ofland use changes have therefore not been considered.

Palaeolimnological studies of sediments from thenearby lake Røyrtjønna indicated that the pH has beenremarkably stable (5.6–5.9) since about AD 650 (Bergeet al., 1990). Muniz (1997) conducted a regional surveyof the lake Storgronningen drainage area during highand low discharge periods and concluded that this area(Figure 1) may serve as a pristine reference to moreantropogenically acidified regions. The chemical qual-ity of the stream ‘Ingabekken’ generally spans overthe 95% interval of the spatial variation found in theregional survey. This great span in chemistry is main-ly due to the difference between temporal and spatialvariation and that the regional sites include fewer first-order streams and instead several lakes and ponds withvery different hydrological regimes. The large tempo-ral variability found in Ingabekken stresses the great

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Figure 1. Location of study site and soil map of the Ingabekken catchment including stations for soil and soil water sampling.

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Table 1. Volume weighted bulk precipitation chemistry (as �eq l�1)from the various monitoring sites in the Høylandet area. The amountof deposition collected during the study periods (PRE) are presentedin mm

IngabekkenA Stor GrønningenB HøylandetC

Monitoring Fall Summer Summer 1987-

period 1986 1987 1988 1988

PRE 1001 370 270 2115

H+ 10 6 9 10,2

NH+4 3 13 22 17

Na+ 90 50 38 66

Mg2+ 2 9 13 16

Ca2+ 5 9 6 8

K+ 4 6 6 3

SO2�4 19 38 41 24

NO�3 4 � � 9

Cl� 100 66 59 75

A Semb, 1987.B Anderson et al., 1997.C From 18.02.87 - 1.1.89; Tørseth, 1995.

importance of considering the prevailing hydrologicalregime during sampling of spot samples. Questioninghow representative the Ingabekken site is to the broad-er area, we note that the site is generally more pristineand is more strongly buffered with lower concentra-tion of mineral acids and higher concentrations of bothweak organic and inorganic acidity and base cationsthan most sites in the regional survey.

Other streams, including the main brook, Skiftesa,have been monitored for shorter intervals in orderto further test the temporal representativeness ofIngabekken. Stream water chemical composition wassimilar in all the studied streams, though the pH rangeobserved at Ingabekken (4.8–7.2) exceeded the rangeat the other streams. This was mainly due to its thin-ner soils, smaller catchment and lower stream ordermaking Ingabekken a good reference site for mineralacid sensitive areas virtually un-impacted by anthro-pogenic activity. The similarity of the highflow chem-istry at Ingabekken compared with the earlier pris-tine composition at currently acidified catchments likeBirkenes in southernmost Norway can,of course, neverbe ascertained. However, if the highflow end-memberat Ingabekken is assumed representative for preindus-trial stream water chemistry in southern Norway, thiscatchment has the potential for contributing to theunderstanding of the acidification processes.

Deposition

Chemical data on bulk deposition are sparse. Precipita-tion chemistry was measured on-site only during fourmonths in the most stormy season in the fall of 1986 and1987 (cf. Semb, 1987). These data may be somewhatbiased towards more seasalts and less of local pollu-tants. Additional data are available from a site close tothe shore of lake Storgronningen, only 0.75 km fromIngabekken – but at a lower elevation (165 m). Herethe deposition was monitored during the snowfree sea-sons in 1987 and 1988 (Anderson et al., 1997). Theclosest station with permanent monitoring exists since1987 at Høylandet, the local village 5 km from thesite (Tørseth, 1995). Data from these stations are com-prised in Table 1.

Precipitation at Høylandet is normally generatedfrom unpolluted air masses over the North Atlanticocean. Generally we see that the precipitation atHøylandet is dominated by seasalts, especially dur-ing the fall and winter months. Excess sulphate con-centration was only about 9 �eq l�1. Total annu-al sulphate deposition (assuming an insignificant drydeposition) during the monitored period was estimat-ed to be approximately 1.3 g SO4

2� m�2 of which30% can be classified as excess sulphate. These esti-mates agree well with long term measurements (1987–1995) of precipitation chemistry at Høylandet (Tørseth,1995). Based on precipitation chemistry at lake Stor-gronningen (assuming that snow chemistry approxi-mates to autumn deposition chemistry) Anderson et al.(1997) determined the excess deposition to 0.7 g SO2�

4m�2. The excess sulphur and the nitrogen depositionat Storgronningen reflect some contribution from localsources with relatively high excess sulphate duringthe snowfree periods being associated with ammonium(Anderson et al., 1997). In the following the summerrainfall quality from Storgronningen was used insteadof on site rain chemistry since this was a more compre-hensive dataset collected reasonably close to the siteand therefore still beleved to be representative for thein site rainfall quality.

As the amount of precipitation increases and evap-otranspiration decreases with elevation, the amountof precipitation at the site is assumed to be around2200 mm yr�1 (cf. Blakar & Hongve, 1997 and refer-ences therein).

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Table 2. Soil density and content of soil organic mat-ter of composite samples (each of 75 sub samples)collected in a 300 m2 grid of the hydromor Histosolin the Ingabekken catchment.

Horizon Volume weight Organic matter

kg l�1 %

Hi Histic 0.451 95.8

He Hemic 0.517 91.4

Ha Sapric 0.533 91.2

Edaphic conditions and hydrology

Ingabekken is drained by small streams, which con-verge only 50 m above the weir (Figure 1). The catch-ment lies above the marine limit on bedrock of gneiss.During the glacial retreat the drainage was towardsthe west, leaving a thin layer of glacifluvial materialin the area (cf. Blakar & Hongve, 1997). At relative-ly high elevations in the landscape a small section ofthe Ingabekken catchment consists of orthic Podzols(Figure 1) which due to the high drainage and a coarsesandy soil texture are well developed with sharp bound-aries between the different genetic horizons. Abouthalf the catchment (primarily the upper northwest-ern part) consists of rock outcrops with shallow soilpockets developed into rankers. The remaining catch-ment – the lower parts adjacent to the streams, and theless steep northwestern facing slope – consists largelyof blanket peats. The peat areas are generally under-lain by gley (Bg) mineral soils directly on the solidbedrock. Although the bedrock is gneiss, these mineraldeposits contain some easy weatherable amphiboliticminerals (cf. Anderson et al., 1989). These peatlandsare either of minerothrophic or ombrothrophic char-acter. The minerothrophic bogs, as fens and swamps,are found in flat downslope areas or in topographicdepressions receiving drainage from above. Isolatedfrom the influence of groundwater the ombrothrophic(rain-fed) hydromor bogs, are found in the infiltrationzone close to the water divide. Blanket bogs are asso-ciated with cold climate, low evapotranspiration andan evenly distributed precipitation over time, causinglow organic decomposition rates. The organic mattermay thereby accumulate and dystric blanket Histosolsare thus formed.

Once established the hydromor Histosols becomemore mature and saprist (i.e. more decomposed) thantheir peat counterparts since enhanced humification isfavored by continuous seepage of fresh precipitation,

saturated with oxygen (FitzPatrick, 1983). The densi-ties of these ombrothrophic saprist hydromor soils aretherefore particularly high (cf. Table 2; common valuesfor organic material are between 0.04 and 0.2 kg l�1;Grip & Rodhe, 1991). The high soil density allows forless hydrological permeability leading to even greateroverland flow during periods of high rain intensities.

Previous studies of the water flowpaths atIngabekken, by Mulder et al. (1995), using an endmember mixing technique (EMMA; Christophersenet al., 1990c), concluded that baseflow discharge isdominated by seepage from the mineral soils beneaththe bogs. This is likely to be the case consideringthe generally waterlogged conditions and a greaterhydraulic conductivity of the glacifluvial sand depositsthan in the overlying mature compact hydromor mate-rial. During highflow, the EMMA technique showedthat the runoff chemistry becomes controlled by thesurface hydromor soils in the bog close to the waterdivide. This is also likely to be the case since the lowerreaches of the bogs are saturated with water.

As a first approximation one might therefore con-sider the stream water chemistry as a mixture of twotypes of soil water: Groundwater- and Surfacewaterrunoff (cf. Seip & Rustad, 1984; Neal & Christo-phersen 1989; Christophersen et al., 1990c). Thegroundwater end-member will be representative forbaseflow whereas stormflow is originating from theupper, more acidic soil zones. Hydrogeochemicalmechanisms controlling runoff chemistry can then berevealed by considering each end-member separatelywith special emphasis on the acidic surface hydromorsoils as they determine the chemical quality of runoffduring periods of stormflow; i.e. acid episodes.

Methods

From October 1986 until August 1988 routine sam-pling of the brook was conducted on a weekly/twoweekly basis, combined with intensive sampling dur-ing episode studies. Campaigns of soil solutions sam-pling within the catchment were conducted by severalresearch groups by means of different types of ten-sion lysimeters (Christophersen et al., 1990b; Fjeldal,1992; Mulder et al., 1995). The lysimeters in the bogwere located either close to the main stream (stream-bank) or close to the water divide. Lysimeters werealso installed in all genetic horizons of a Podzol profilelocated on a small mound (Figure 1).

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pH was measured in all samples. Subsets of sam-ples were selected for fractionation of aluminum (intoinorganic monomeric Al (Ali) as well as monomer-ic organic complexed Al (Alo)), and for analysis of[Ca2+], [Mg2+], [Na+], [K+], [NH4

+], [SO42�],

[Cl�], [NO3�], total fluoride, total organic carbon

(TOC) and total inorganic carbon (TIC). An acidimet-ric titration to pH 4.5 was also undertaken to give anestimate of the partial acid buffering capacity of thewaters (PBC4:5).

The Al-fractionation followed the operationallydefined Barnes/Driscoll procedure (Sullivan et al.,1986). Major cations and anions were determinedaccording to standard procedures using atomic absorp-tion spectroscopy and ion chromatography, respective-ly. Total fluoride was measured potentiometrically afteraddition of TISAB buffer. TOC was calibrated fromE254nm absorbency, based on the optical densities of34 samples determined for carbon (mg C l�1). Sam-pling and transport routines were optimized to min-imize degassing of CO2 prior to analysis. Samplesfor TIC determination were taken in air tight glassbottles. The organic anion contribution to the PBC4:5

(PBC(org)4:5) is calculated as the difference betweenthe PBC4:5 and the sum of [HCO3

�] + H+, where H+

denote the increase of [H+] in solution from the orig-inal pH to 4.5. See Christophersen et al. (1990a) andMulder et al. (1995) for more detailed analytical andcomputational protocol.

Fjeldal (1992), Pijpers & Mulder (1990) and Mul-der et al. (1995) collected soil samples along two par-allel soil transects (Figure 1). Both transects wereperpendicular to the stream with sampling pointsat 20 m intervals. These air dried soils were ana-lyzed for the Effective Cation Exchange Capacity(CECE), including Al, Fe, H+ and base cations(BC = Ca + Mg + Na + K), according to an exchangemethod developed for forest soils by Hendershot &Duquette (1986) using an unbuffered solution of 0.1 MBaCl2. A thorough method protocol is given in thereferred papers. Composite samples (each of 75 sam-ples) of the Histosol profile were collected from a300 m2 grid of the peat. CECE and exchangeablecations in these air dried samples were determined byextraction with 1M NH4NO3. These data have not beenpreviously published. Soil samples were also collectedin the peat close to the water divide by Christophersenet al. (1990a) and approximately halfway up the bogslope by Anderson et al. (1997). These air dried sam-ples were determined for Potential Cation ExchangeCapacity (CECP); i.e. using an buffered extractant. For

exchangeable base cations (BC), one obtains approx-imately the same value using either unbuffered orbuffered extraction, but the total CEC, including H+

and Al, will be larger in the latter case, implying a low-er base saturation (BS, i.e.% BC of CEC). The contentof organic matter of the soil was determined by loss onignition at 500 �C.

A multivariate Principal Component Analysis(PCA) (see, e.g. Esbensen et al., 1987) was conduct-ed (using correlation matrix) with statistical softwarefrom Minitab c (1993).

Results and discussion

Soil chemistry

Despite its ombrothrophic nature the hydromor atthe water divide is found to have large amounts ofboth effective and potential exchangeable bivalent basecations (Table 3). This is due to a large deposition ofseasalts (rich in magnesium, Table 1) augmented bybiological cycling (of especially calcium) by a heathervegetation through the rather shallow soils (20 cm deeporganic layer overlying a 20 cm thick mineral soil; cf.Pijpers & Mulder, 1990). Furthermore, these organicsoils suffer seasonal drying. This desiccation causespolymerization and stabilization of stable humic com-pounds with high exchange capacity and marked affin-ity for bivalent cations (Duchaufour, 1982).

An even more striking feature of these soils was avery low pHBaCl2. Values down to 2.6 were common,suggesting a strong ability to cause acid pulses duringsalt episodes, despite their high base saturation (seebelow). The high amount of effective exchangeableH+ is possible due to the lack of source for exchange-able aluminum. A high potentially exchangeable acid-ity (i.e. total acidity) was also found when determiningthe potential CEC (CECP) on the same soil samples(Table 3); an average of 818 meq OH� kg�1was need-ed to bring the extractant to pH 7.

The upper peat soil layers were found to havedecreasing BS downslope, though with less exchange-able H+, while instead the amount of exchangeable Al(AlS, i.e.% Al on the CEC) increased. Similar spatialtrends in BS and AlS has also been found elsewhere(e.g. Birkenes in southernmost Norway; Mulder et al.,1991) in regions with poorly weatherable bedrock(Vogt et al., 1994). Downslope the deep and constantlywaterlogged sphagnum peat lack the biological cyclingbut receive some minerogenic seepage. This seepage

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Table 3. Effective and potential cation exchange capacity and the composition of the soil exchanger indifferent samples collected from the surface horizons of the hydromor soils. The samples were collectedfrom the water divide (top), midslope (mid) and adjecent to the stream (streambank). CEC denotestotal cation exchange capacity; H+ and BC denotes exchangeable amount of protons and base cationsrespectively - all in meq/kg; HS, BS and AlS denote the fraction (in%) of the cation exchanger occupiedby H+, base cations and Al3+, respectively

pH in CEC H+ BC HS BS AlS

Comments Method BaCl2 meq kg�1 %

Top1 CECE 0.1M BaCl2 2.7 283 41 224 15 79 6

Mid1 CECE 0.1M BaCl2 3.3 134 11 64 8 49 43

Streambank1 CECE 0.1M BaCl2 3.6 113 5 29 4 24 71

Top2 CECP 1M NH4OAc – 1025 818 206 80 20 –

1 Average values from Pijpers & Mulder (1990); Top: stations A & AB, Mid: stations B, C and AC,Streambank: stations D & AD (Figure 1).2 Average values from stations A & AB (Figure 1).

allows Alo from the gley mineral soil layers below thepeat in the upper reaches to be transported to the upperpeat layers in the lower returnflow regions and becomeimmobilized there. The elevated levels of AlS in thesurface peats close to the streams (Table 3) were there-fore associated with elevated levels of Aloin the soilwater. Although lateral flow, this process has muchin common with podzolization (Mulder et al., 1991,1995). High flow at Ingabekken becomes increasinglydominated by solutions originating in the surface peatbog close to the water divide (Mulder et al., 1995). Thisimplies that the rainwater during peak discharge onlyhave contact with the soil in these upper regions beforeentering the stream. The shallow stormflow flowpathand the great acidity of this soil (i.e. the low pHBaCl2

surface Histosol layers at the water divide) suggest thatthis site may be susceptible to episodic pH depressions.Such a pH depression in the stream can in turn causea mobilization of labile forms of Al from the surfacelayers of the streambank peat being high in AlS, aswell as the streambed proper (see Norton & Henrik-sen, 1983; Henriksen et al., 1988). The large storesof readily available organically bound and exchange-able Al (AlS) in surface peat close to the brook maytherefore be a potential future source of Al if exposedto strong mineral acids or acid surges caused by saltepisodes; analogous to the leaching of the Bhs layerin regions with acid deposition (Mulder et al., 1989).Focusing on the low AlS values of the surface Hydro-mor layers close to the water divide and that these soilsare the main contributors to runoff during highflowperiods, Mulder et al. (1995) concluded nevertheless,that acid deposition at current levels is unlikely to resultin increased Ali levels in the stream at highflow. This is

likely the case when considering the low anthropogenicacid loading at Høylandet. But it should, however, benoted that the existence of bicarbonate-rich groundwa-ter is no guarantee against acidification of the highflowend-member; the Plynlimon catchments in mid-Walesproviding a good example (Neal et al., 1986).

Early findings by Christophersen et al. (1990a)were based only on soil samples collected from thepeat profile close to the water divide. As presented byPijpers & Mulder (1990) and Mulder et al. (1995) theselocations show relatively low levels of exchangeable Alrelative to regions further downslope. This lack of Al(only approx. 4 meq kg�1, or about 5% of the CECP) inthese pristine soils led the authors to hypothesize that,because of acid deposition, the exchange sites undergoa transition under which exchangeable H+ is replacedby aluminum. Clearly, the new information concerningthe spatial distribution of AlS does not support such ahypothesis. This example stresses the importance ofinsight into the spatial structure of soil data, particu-larly in case of modelling or comparative studies.

The gleyed mineral soil layers beneath the bog werealso sampled and theire exchange characteristics weredetermined by several research groups for either poten-tial or effective CEC (Table 4). The pH values, both inwater and salt (BaCl2) extract, were relatively similarand the span in spatial variation was only between 4.5and 4.9. The amounts of effective exchangeable basecations along the transects were generally low (from1.0 to 13 meq kg�1), though since the CEC also waslow the BSE differs considerably (from 5.6 to 52%).Also in samples determined by extraction with 1MNH4OAc/NaClAl (i.e. CECP), the BSP range from 2 to30% (n = 6; Christophersen et al., 1990b). The cause

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Table 4. Effective and potential cation exchange capacity and composition of deep mineralhorizons beneath the peat soils. CEC denotes total cation exchange capacity; H+ and BCdenotes exchangeable amount of protons and base cations respectively - all in meq/kg; HS, BSand AlS denote the fraction (in %) of the cation exchanger occupied by H+, base cations andAl3+, respectively.

CEC H BC HS BS AlS

Comm. Method meq kg�1 %

1 CECP 1M NH4OAc/NaClAl 35 27 6 77 17 6

2 CECP 1M BaOAc/NaClAl 22 15 6 68 27 5

3 CECE 0.1M BaCl2 28 2 4 9 20 73

4 CECE 1M NH4NO3 75 33 7 45 10 45

1 Median values from 6 samples (Christophersen et al., 1990 a,b).2 Bg-horizon of a midslope Histo-dystric glaysol from Anderson et al. (1996 this volume).3 Median values of 9 samples from the soil transects (stations A, D, H, J, L, AA, AB, AC, AI;Figure 1) from Fjeldal (1992).4 Average data on composite sample (of 75 sub samples from a 300 m2grid).

for this spatial variation may lie in an uneven distrib-ution of more base rich minerals (e.g. Hornblende) asfound by Bain et al. (1990) in the C-horizon of glacialmineral deposits in the area.

Soil water chemistry

The soil water at 5 cm depth (H-horizon) in the hydro-mor bog at the water divide corresponds closely to aslightly more concentrated precipitation (30% in termsof [Cl�]) (Figure 2). The [H+], [Na+], [Mg2+]and[K+] remain practically constant relative to chloride,while [Ca2+] was further enriched by a factor of 4.At the streambank the soil water from the same depth(5 cm H-horizon) was less concentrated by evapotran-spiration (20%; using the [Cl�] as a proxy) thoughgreatly enriched in base cations, for [Ca2+] by a factorof 11.5, and for [K+] by 6, and for [Mg2+] 2, as wellas [Na+] by 1.5. Even though concumption of H+ inexchange for Ca2+ may be an important process (seenext chapter) the relative small loss of [H+] comparedto precipitation does not contribute significantly to theobserved release of base cations. This leaching musttherefore mainly be due to the organic acids, providingboth protons for ion exchange and an anionic chargefor cation co-transport.

The chemical composition of the soil solutionremains stable down through the peat, except in theHistic (Hi) layer at the streambank (‘bog at stream-bank’ in Figure 2). Due to the watersaturated con-ditions and low hydrological conductivity of the Hi-horizon (see above) carbon dioxide from decompo-sition processes accumulated in this soil water. The

resulting high pCO2 and an average pH of 6.1 causedhigh bicarbonate levels, allowing for elevated releaseof calcium into solution. In the deeper sapric organicand gleyic mineral layers (Ha/Bg) of the peat bogs theconcentrations of Ca2+, Mg2+, and to a lesser extentalso K+, generally reach high levels. The samples hadfrequently pH values above 7, with Ca + Mg account-ing for more than 50% of the cationic charge, and theweak acid anions accounting for 75% of the anion-ic charge. This corroborates reasonably well with therunoff chemistry during baseflow (Figure 2).

The bog soil water concentrations of Alo arebetween 4 and 10�M. The [Alo] is highest in the upperH-horizons, especially in the streambank bog, and low-est in the middle Hi-horizons, especially at the waterdivide. Vogt and Taugbøl (1994), studying soil waterin anthropogenically acidified sites, found that [Alo] insoil water may be modelled by a simple model usingthe [DOC], [H+] and [Ali] in solution, along with thecomplexation and protolysation constant for the DOCmaterial and the number of organic binding sites. Themobilization of Alo is therefore best studied using amultivariable approach. A principal component analy-sis (PCA) of the [Alo], [DOC], [H+] and [Ali] wasconducted on all bog soil water data irrespective of thesampling location. Along the first principal compo-nent (PC1), explaining 42% of the data variation, theAlo was negatively related to the DOC (i.e. loadingsof �0.458 and 0.257 respectively), and positive relat-ed to H+ and Ali (�0.541 and �0.657 respectively).In the second principal component (PC2), explaining26% of the variance, the DOC showed high loading(�0.889) along with Alo (�0.429). The third principal

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Figure 2. Chemical composition of precipitation, soil water and stream water. Weak acids comprise both organic anions and bicarbonate.Top graph shows absolute concentrations, bottom graph shows relative composition. Letters in bars denote soilwater datasets: data from(a) Christophersen et al. (1990b); (b) Fjeldal (1992), Pijpers and Mulder (1990) and Mulder et al. (1995).

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Table 5. Volume-weighted average as well as median high-flow and baseflow values for stream water chemistry (seasalt episode data excluded). Note that the volume-weightedstream water values will be biased towards the highflowsituation due to an over representation of samples duringepisodes. Sum(cat.-an.) does only include inorganic species

All samples

volume Highflow Baseflow

weighted (> 8 l/s) (< 8 l/s)

H+ 8.0 6.0 0.4

Na+ 97 86 159

K+ 6 5 6

Ca2+ 19 22 69

Mg2+ 28 25 51

NH+4 �eq l�1 <1 <1 3

NO�3 2 <1 1

SO2�4 25 20 29

Exc. SO2�4 12 9 13

Cl� 124 96 144

HCO�3 3 3 51

Alo �M 1.7 2.0 1.7

Ali <1 <1 <1

TOC mg l�1 4,7 5,9 6,0

TIC �M 76 72 136

PBC4:5 0.05 0.04 0.15

PBC(org)4:5 meq l�1 0.02 0.00 0.02

Sum (cat.-an.) 0.01 0.02 0.04

component (PC3) explained only 20% of the variation,though had a strong Alo loading (0.707) which wasnegative related to all the other variables, especial-ly H+ (�0.675). From this we may speculate that themobilization of Alo appears primarily controlled by themobilization of Ali in these acid soil waters (cf. PC1),and secondly by the (production and leaching of) DOC(cf. PC2), especially when pH is high (cf. PC3). Anyreleased Ali was complexed in solution by organic lig-ands on DOC so that Ali in solution remained low.Also at the pristine HUMEX site in mid-western Nor-way, with only minor [Ali], Vogt et al. (1994) foundthat the variation of [Alo] in the peat soil water waspoorly determined by the [DOC]. Here including the[H+] into a linear model improved the correlation atonly some locations.

A puzzeling feature in our data was that there waspractically no sulphate found in all soil water sam-ples from the organic bog layers. This is a paradoxconsidering that in the runoff, the sulphate although

low never decreased below 4 �eq l�1 during high-flow and 6 �eq l�1 during baseflow. In fact, usuallythe sulphate concentrations were about 20 and 29 �eql�1 during highflow and baseflow respectively (medianvalues; Table 5). A sink of sulphate in the soils may beenvisaged through (bio)chemical reduction processeswhere the S becomes bound to the organic matter orreduced to sulphide. In its reduced form the sulphurmay have been lost from the sample by volatilizationas H2S, especially in samples with low pH. In sampleswith high pH the oxidation to sulphur is more likelyto be a dominating process in the sample vessel. Sig-nificant amounts of sulphate were in fact found wherethe pH was high; i.e. in the mineral layers beneaththe peat close to the streambank (average pH was 6.2).A low amount of iron in the peat (as inferred by lowamounts of exchangeable Fe on the soil ion exchanger;see Pijpers & Mulder, 1990) could permit such a trans-port of sulphur in reduced form, either as sulphide or asbound to dissolved organic matter, through the deep-er soil layers. Upon entering the stream, the mixingwith aerobic water would serve to rapidly oxidize thesulphide compounds to sulphate so that hydrogensul-phide is not remitted to the atmosphere. It is currentlynot possible to assess the amount of sulphur being re-emitted from the catchment to the atmosphere.

Stream water chemistry – a general picture

When studying stream water chemistry in general,the results from a seasalt episode (i.e. samples with[Na+]>200 �eq l�1) during the snowmelt of 1987 areexcluded and discussed separately below in the seasaltsnowmelt section. Volume-weighted average as wellas median highflow and baseflow concentrations ofstream water chemistry are presented in Table 5 and inFigure 2. While the highflow resembles the precipita-tion in terms of ionic strength and chemical composi-tion, except for higher [Ca2+] and [TOC], the baseflowis twice as concentrated due to weathering, productionof bicarbonate, and evapotranspiration. The solute lev-el during highflow lies between the low concentrationin precipitation and the greater levels found for soil-water. This may only be explained by a strong dilu-tion of the soilwater by rainwater. The leaking of ionsto streamwater or accumulation in the catchment isreflected by a change in the concentration ratio withrespect to rainwater (summer rainfall; Table 1) andis expressed by the median fractionation factor (i.e.([X]/[Cl])streamwater/([X]/[Cl])rainwater) in Table 6. Thesite is leaking base cations, especially calcium during

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Table 6. Median fractionation factors for rainwater components in streamwater dur-ing highflow and baseflow. Fractionation factors are given relative to Cl�, i.e.([X]/[Cl])streamwater/([X]/[Cl])rainwater). Less than unit values denote excess leaching,while values greater than 1 denote accumulation within the catchment

Flow regime H+ Ca2+ Mg2+ Na+ K+ NH+4 SO2�4 NO�3

Highflow 0.5 2.0 1.7 1.2 0.7 0.0 0.4 0.1

Baseflow 0.0 5.0 2.3 1.4 0.7 0.1 0.4 0.1

baseflow, and accumulating nitrogen. Despite a largeproton production within the soils by organic- and car-bonic acid, there is a net neutralization of the precipita-tion within the catchment. An apparent accumulationof sulphur may eather be due to a loss by volatilizationof H2S and/or the use of summer rain quality, with ele-vated deposition of sulphate. Using instead the autumnrainfall chemistry as reference we find a insignificantaccumulation of sulphate.

The great release of calcium within the catchmentcauses the Ca/Mg ratio in the stream to remain above0.6, even during extreme highflow. The ratio betweenexchangeable Ca2+ and Mg2+ in the bog, according tothe ammonium acetate method, increases from 1.1 inthe surface H-horizon layers to 3.3 in the Bg mineralsoils horizon beneath. The almost equimolar amountsof Ca2+ and Mg2+ only in the upper soil layers as alsoin streamwater during periods of high runoff (‘H’ inFigure 3) ([Ca2+] = 0.8 � [Mg2+]; R2 = 0.77, n = 46),fortifies the postulation that the high discharge chem-istry was mainly controlled by these upper bog layers.Similarly, during baseflow, when discharge water seepsfrom the Bg-horizon beneath the bog (i.e. with highCa/Mg ratio on the cation exchanger), the increasein streamwater [Ca2+] with increasing [Mg2+] wasmuch greater than unity ([Ca2+] = 2.2 � [Mg2+]�35.1;R2 = 0.83, n = 38) (‘L’ in Figure 3). This agrees with thepostulation that the discharge chemistry during base-flow is controlled by the Bg-horizon. During highflowthere was also a very good correlation between thesquare root of [Ca2+] and [Mg2+] vs. [Na+] (R2 = 0.8in both cases). Furthermore, the [H+] was mainly neg-atively related to the Ca/Mg ratio (R2 = 0.5) duringhighflow, suggesting that the consumption of protonsin the exchange of Ca2+ may be an important process.As also indicated in the soil water section these rela-tionships suggest that an ion exchange model controllthe mobilization of cations in the soil end-members.

During baseflow the Ca/Mg ratio was also nega-tively related to the low [H+] (R2 = 0.5), while posi-tively co-related with potassium, alkalinity and bicar-

bonate concentrations (R2 of Ca/Mg vs [K+] = 0.7;PBC4:5 = 0.7; HCO3

� = 0.6). This is due to all beingdependent on sufficient residence time allowing forenhanced weathering conditions. Negative, correla-tions are therefore found with increases in flow (i.e.decreasing residence time as well as dilution) (R2

of discharge vs. Ca/Mg = 0.5; [K+] = 0.3; Alkalini-ty = 0.4; HCO3

� = 0.4).Examples of the observed variations in pH, con-

centrations of chloride, sodium, and calcium with dis-charge, are shown in Figure 4. The pH at Ingabekkenwas, as often is the case (Rosenqvist, 1978; Henriksenet al., 1984; Christophersen et al., 1982), negative-ly correlated with flow; for the snow free season thepH ranged from about 5.0 at highflow to 7.2 duringbaseflow.

With decreasing flow the pCO2 and charge contri-bution of bicarbonate increased from about 2 and 2.5%at highflow, to 7 and 23% at baseflow, respectively(see Table 5). During baseflow the total concentrationof bivalent base cations was 120 �eq l�1. This highrelease of Ca and Mg is likely due to the weather-ing of the amphibole minerals (Bain et al., 1990) inthe gley soil beneath the bogs by the weak carbonicacid. A decreasing trend in the amount of excess sodi-um ([Na] – 0.85 �[Cl]) with an increase in discharge(marked with crosses in Figure 5) is therefore part-ly explained by decreased contribution of sodium fromweathering. During highflow conditions the runoff alsobecomes diluted by rain water through a lateral over-land flowpath, and excess Na in runoff does not differsignificantly from zero.

Even though pH values may drop to 5 the Al frac-tions remain low with Alo around 2 �M and Ali notexceeding 1 �M (Table 5). Other studies in pristineareas show similar features; for instance, the pristineKarvatn site, north-western Norway (SFT, 1987, 1988)and the HUMEX site in western Norway (Vogt et al.,1994). At these sites the organic anions have only amodest part of the ionic load while instead the influenceof seasalts dominated. At higher elevations in Jamieson

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Figure 3. The relationship between calcium and magnesium in stream water. ‘H’ and ‘L’ denote normal conditions, while discharge was greaterand less than 8 l s�1 (i.e. highflow and baseflow respectively). ‘S’ denote samples collected during the seasalt episode during spring melt.

Creek, British Columbia, a pH of 4.5 was observed,organic anions dominated and total monomeric Al wasin the range 2–4 �M, predominately as Alo (Driscollet al., 1988).

Relationship between chemical parameters in streamwater

It has been our intention to focus on the true multi-variate relationship in such hydrogeochemical systemsbetween the hydrophysical factors and the geochemi-cal mechanisms, as well as between the main chemi-cal variables. A PCA was therefore conducted on thestream water data (seasalt episode data were excluded)to identify the main forces governing the variability inthe data (45 cases) and the relationship between themain chemical parameters (8 variables).

The first principal component (PC1) describedmore than half (55%) of the variation in the data(Figure 6). Along this strong component parametersthat were positively related with discharge had posi-tive loadings, while negatively related parameters (e.g.Cl�) possessed negative loading. TOC had positiveloading along the PC1 (i.e. positive related to flow)because the major flowpath during periods of high-flow was through the surface layers of the bogs ofwhich the chemical fingerprint is a high concentration

of DOC. High negative loading of base cations coin-cides with scores of samples collected during baseflowconditions. This is due to the mineral soils beneath thepeat being the major source of runoff during baseflow.The chemical fingerprint of this end member is highconcentrations of base cations and sulphate. That thebase cations and sulphate were negatively related toTOC along the PC1 may therefore be explained by theshift from the upper bog horizons during highflow tothe deeper mineral soil layers as the main contributorto runnoff during baseflow. The fact that parameterswhich have similar concentrations in both soil waterend-members (i.e. no fingerprint; e.g. chloride) stillhave strong negative loading along the PC1 must beattributed to other factors than flowpaths. A direct con-tribution of dilute precipitation or meltwater duringhighflow may instead be a likely cause for this neg-ative loading. Chemical equilibrium effects of suchdilution (i.e. negative salt effect) would further serveto fortify the PC1 response of bivalent base cations,TOC and H+, by adsorption to the ion exchanger,dissolution and organic acid protolysation, respective-ly. This strong component reflects therefore that themain spread in runoff chemistry is found over a dis-charge gradient. This chemical variation is a combinedhydrological effect of flowpaths from geochemicallydifferent soil layers (end-members) and dilution by

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Figure 4. Variations in pH, concentrations of chloride, sodium, and calcium with discharge, for the late summer of 1987.

Figure 5. Excess sodium (i.e. Na ([Na]�0.85� [Cl])) relative to discharge in stream water. Crosses denote normal conditions, while ‘S’ denotessamples collected during the seasalt episode during spring melt.

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Figure 6. The first and second Principal Component (PC1 & PC2) in the stream water data. Letters in graph refer to chemical parameters andare positioned at theire respective variable loading. Numbers in graph at sample scores denote discharge in l s�1 during sampling.

Figure 7. The first and second Principal Component (PC1Cl & PC2Cl) in the stream water data corrected for dilution by dividing the concentrationsby the value for chloride prior to running the analysis. Numbers in graph at sample scores denote discharge in l s�1 during sampling.

rain or melt water fortified by the equilibrium responseto changes in ionic strength.

The second principal component (PC2), describ-ing 18% of the variation, reflects mainly the spread

in chemistry at a given runoff intensity (or PC1) andmay be interpreted mainly as a hysteresis factor, prac-tically indifferent of dilution (i.e. Cl� has insignificantloading). Within the catchment there is a continous

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Figure 8. The first and second Principal Component (PC1HF & PC2HF) in the stream water data collected while runoff was greater than 8 l s�1.Numbers in graph at sample scores denote discharge in l s�1 during sampling.

accumulation or depletion of the different chemicalparameters (Table 6). The intensity of this leaching andaccumulation is enhanced by an increased hydrologicalresidence time. During a dry period causing enhancedresidence time, variables that are generally depleted(fractionation factor >1 in Table 6 and TOC) are ableto accumulate causing an enhanced leaching under thefirst runoff episodes. Variables that are generally accu-mulated within the catchment (fractionation factor<1in Table 6) will on the contrary become less availableas the residence time increases, causing a diminishedleaching during the initial runoff episode stages. I.e.the PC2 component is found to reflect the intensity ofdepletion/accumulation controlled by the antecedenthydroclimatic conditions in the end-members (see Vogtet al., 1990). This postulation is fortified by the factthat the variable loadings along this component arewell correlated (R2 = 0.90, n = 10) with the fraction-ation factors in both high- and baseflow streamwater(Table 6) (Figure 9). An important exception is for thestrong leaching of Ca from weathering during base-flow.

An example of the effect of previous hydrologicalconditions may be seen in Figures 4 and 10. During theonset of the storm on august 22, after a fortnight of norain, the concentrations of TOC in the stream increaserapidly. The initial phases of stormflow reach especial-ly high [TOC] (see Figure 10) (due to wash out of accu-mulated soluble organic matter; see, e.g. Vogt et al.,1990), while during the succeeding stages of high-

flow with greater direct overland runoff intensities, thestream commonly experiences higher [H+] (see Fig-ure 4). During the following episodes, the [TOC] waslower despite greater runoff intensities. Similar streamchemical response patterns have also been found else-where, e.g. Birkenes in southernmost Norway (Seipet al., 1989; Vogt et al., 1990). During baseflow (PC1is negative) slow seepage of streambank mineral soilwater causes the base cations and sulphate to reachhigh concentration in the stream. There is therefore atendency for the samples to wander clockwise aroundthe origo of the Figure 6 through a series of dischargeepisode (i.e. hysteresis effect coused by the changes inantecedent hydrological conditions). This again sug-gests that organic acidity was most important in pro-viding H+ during the onset of an event, while duringlater stages the mineral acidity retains its role as mobilecounter ion to the H+. The lower pH during successiveepisodes is partly due to shorter residence time and amore shallow flowpath resulting in less base cations,and partly enhanced protolization of the organic acidsby dilution.

The parameter relationships superimposed on thePC1 vs. PC2 plane (Figure 6) reveal a general pat-tern often recognized in natural water samples. Thefollowing parameter pairs: Ca2+ & Mg2+, NO3

� &SO4

2� and Na+ & Cl� were closely juxtaposed, dueto the strong co-variation among these parameters (seee.g. Muniz, 1997) due to both mutual chemical depen-dency (especially Ca & Mg) and by originating from

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Figure 9. The second Principal Component (PC2) and the fractionation factor in stream water. Squares and crosses denote highflow and baseflowconditions, respectively. Fractionation factor vs PC2 loading for Ca at baseflow is not shown.

a common source: The base cations were negativelyrelated to the H+(in both the PC1 and PC2 dimen-sions). Furthermore, the acidity was governed eitherby TOC (i.e. a proxy for organic acids) (in the PC1)or by strong mineral acid anions (in the PC2). A neg-ative relationship between TOC and H+along the PC2during high flow may partly be due to protonizationof organic acids by protons provided by strong min-eral acids and subsequent precipitation. Finally, theantagonistic relationship between Ca & Mg and H+ isperpendicular to the antagonistic relationship betweenTOC and NO3

� & SO42�: see arrows in Figure 6. This

pattern reflects in a very simplistic way the combinedeffect of variation in present and past discharge on therunoff chemistry.

In an attempt to make the PC1 a more pure flow-path component the effects of direct dilution was partlyaccounted for by simply dividing all sample concen-trations by its chloride value prior to running a newPCA. This approch will also correct for moderate vari-ations in the seasalt loading, though it will not reflectthe equilibrium reactions adjusting to these differentsolute levels. The PC1Cl and PC2Cl now accounted foronly 38 and 23% of the variation in the data, respective-ly. The new PC1Cl became almost an analog to the oldPC2, while the new PC2Cl resembled the old PC1 (cf.Figures 6 & 7). The main effect was therefore that the

‘PC1’ and ‘PC2’ had swapped positions as the majorcomponent in the data, except for the loadings of H+

and Na+. This fortifies the postulation of direct dilu-tion by rain or meltwater being an important force onthe former PC1 and thereby on the stream water chem-istry. Devoid of the dilution and ionic strength relatedeffect, the influence of antecedent hydrological con-ditions appears to become most prominent. Ascribingthe loss in the percentage of the variation described bythe PC1 and PC2Cl (55�23 = 32%) to dilution factors,suggests that more than half (i.e. 32 being 58% of 55)of the variation along the PC1 was due to differencesin volume. The lack of concistancy regarding the load-ing of H+and Na+ could reflect equilibrium reactionsresponding to the different solute levels. That [H+]was found to be less sensitive or even positive relatedto dilution is discussed below. A decrease in [Na+] bydilution will be alleviated by a release of sodium fromthe soil ion exchanger (i.e. negative salt effect).

In order to uniquely study the chemical parameterrelationships of the important highflow end-member,without the influence by the other main flowpath, thesamples collected at discharge less than 8 l s�1 wereomitted from the data set. A PCAHF on the high-flow samples (29 cases) gave a PC1HF and a PC2HF

that accounted for 46 and 19%, respectively, of thetotal variation in these data. Supposedly devoid of

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Figure 10. Variations in [TOC], [Alo], [SO42�] and [PBCorg:4:5] with discharge, for the late summer of 1987.

the effects of water pathways, the interrelationshipbetween the variables still remained similar to thosefound for the original PCA (and the PCACl), thoughthe PC1HF became a more unique dilution component.The [TOC] was thereby diluted along with the [Cl�],leaving only the H+ with positive PC1HF loading (i.e.positively correlated to a dilution) (Figure 8). This pos-itive relation with dilution is believed to be partly dueto (1) lack of neutralization by the deeper soil layers,(2) negative salt effect on the ion exchanger releasingmonovalent cations, (3) the effect of dilution on theprotolyzation equilibria.

The PC2HF also bears a resemblance to the PC2,with the main exception that Na+and H+ now swappedposition so that Na+ is no longer juxtapositioned withCl�. A separate PCA of baseflow samples was notpossible due to lack of sufficient data.

At large the H+ was inversely related to Cl�, Na+,Ca2+, Mg2+ and K+. Usually perpendicular to this

relationship, SO42� and NO3

� were negatively relat-ed to TOC. Since this pattern persisted irrespectivelyof the influence from factors as concentration/dilutionand flowpaths, we believe that this is due to theion exchange/weathering (H+ vs. base cations) andadsorption (sulphate vs. DOC) interactions, respec-tively, between the soil proper and its soil water asdiscussed in the previous sections.

A seasalt snowmelt episode

During the initial parts of the snowmelt in the spring of1987 this usually dilute water system became enrichedin all major anions and cations. A record low pH (4.8)was recorded in the early phase of the melting at medi-um flows but with high [Na+] and [Cl�]. The pref-erential leaching of salts from the snowpack (Johan-nessen and Henriksen, 1978) resulted in high levelsof [Cl�] (4.5), [Na+] (3.7), [Ca2+] (2.9), [Mg2+]

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(3.7), [NO3�] (15); numbers in parenthesis give times

of greater than median concentrations (median meltcons./median general conc.; cf. Table 5). Relative tothe chemical composition of the summer precipitation(Table 1) the runoff became depleted in all monova-lent cations, especially Na+, as well as sulphate andnitrate. Only the relative depletion of SO4

2� and NO3�

may partly be due to the usually lower winter deposi-tion for these constituents (cf. Table 1). An apperentincreasing trend in the retention of [Na+] with decreas-ing flow (S in Figure 5), contradictory to the gener-ally observed trend, may instead be attributed to anincrease in the ionic strength. As the solution becamemore concentrated during the extreme salt pulse theion exchanger would adsorb sodium and desorb calci-um and magnesium. In fact, Ca2+ and to lesser extentMg2+ as the only cations, showed an increased excessleaching with an increased retention of Na+ (relativeto Cl�) (Caexcess =�0.8 � Naexcess + 10.4, R2 = 0.76;Mgexcess =�0.2 � Naexcess + 4.0, R2 = 0.59). Duringgeneral conditions the leaching of excess Ca2+ wasabout three times greater than for excess Mg2+, whileduring this salt episode the export of excess Ca2+

became six times greater than that of excess Mg2+.Nevertheless, a lower absolute Ca/Mg ratio (<0.7)was generally observed (‘S’ in Figure 3) during thisepisode. The increased leaching of excess Ca2+isbelieved to be due to the ion exchanger reacting tothis low Ca/Mg ratio in the salt pulse when releasingbivalent cations in exchange for Na,while the generallylow Ca/Mg ratio in the streamwater illustrates the mix-ing of some direct meltwater having bypassed the soilsdue to frozen soil layers. The latter explanation wasalso argued by Christophersen et al. (1990a) referringto these samples as outliers from which sodium hadbeen much less efficiently exchanged than predictedby a cation exchange relationship.

Rather disturbing from an environmental viewpointwas the fact that the salt effect caused a record low pH(4.8) and the Ali to increase more than seven times(2.0 �M) compared with the normal level, while the[TOC] became half the normal level. Although remain-ing low, the Ali increase occured despite a constant lev-el of Alo. A constant [Alo] in spite of a drop in [TOC]implicates an increased (1.7) Al complexation to theorganic matter (�M/mg C). This may be warranted bythe increased Ali and an increase (2.9) in the organicanion charge density (as inferred from charge balanceover mg C l�1).

The role of weak acidity

As the median pH during baseflow was 6.4 both thebicarbonate system and organic anions contribute to thebuffering of pH. A high median alkalinity (0.15 meql�1 titrated to pH 4.5) relative to the bicarbonateamount (51 �eq l�1, as inferred from total inorgan-ic carbon (TIC) and sample pH) nevertheless suggeststhat the 6 mg C l�1 of TOC would be the major alkalin-ity buffer, i.e. 0.15–0.05= 0.10 meq l�1, during base-flow. In fact bicarbonate was the dominating weak acidonly at pH greater than 6.4. The median charge den-sity of the organic matter in the baseflow is more thantwice as large as during highflow (0.07 and 0.03 eq/g Crespectively). Similarly high charge densities were alsofound in the more acid (pH 4.9) mineral soil horizonsat the HUMEX site below Terric Histosols (cf. Ytte-borg, 1996). The complexation by aluminum or iron isnot great enough to cause a significant loss of organiccharge. The high charge density while passing throughthe Bg-mineral soil layers at higher pH may be dueto both foregoing preferential precipitation/adsorptionof lesser charged, more hydrophobic, organic mole-cules, and the higher pH causing protolyzation of thephenolic sites on the organic acids.

At highflow the runoff was more acid (pH 5.2) andthe pH buffering was low (median alkalinity 0.04 meql�1) relative to baseflow conditions.The weak acids arenow less important as thay are dominated by the stillmoderate amounts of [TOC] (5.9 mg C l�1), account-ing for 8 �eq l�1, while bicarbonate only account-ed for 3 �eq l�1. Instead strong mineral acid anionsaccount for more than 90% of the anionic charge. Anobserved lack of correlation between the [TOC] andPBC(org)4:5 is believed to be due to an protonation ofthe organic weak acids by the released H+ from thesoil in exchange for sodium. This is seen as a correla-tion between excess sodium and organic charge density(R2 = 0.6, n = 33). Note also that [TOC] correlated pos-itively with Alo (R2 = 0.69, n = 92).

The low level of organic anions during highflowat Ingabekken is noteworthy assuming the highflowend-member at Ingabekken as representative for prein-dustrial stream water chemistry in southern Norway.It has been suggested that fresh waters now acidifiedwere previously strongly influenced by organic anionswhich were then replaced by strong acid anions, pHnot being significantly altered (Krug & Frink, 1983).This picture does not seem particularly relevant forIngabekken.

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Conclusions

The data show that even in small headwater catchmentsthe terrestrial part of the catchment exerts a strong influ-ence on chemical species in stream water, even dur-ing highflow. Comparing the precipitation chemistrywith the highflow composition in stream, it is seen thatTOC and calcium, in particular, are leached under suchconditions. The five main forces on the runoff chem-istry are (1) the geochemical processes occurring in theend-members (e.g. ion exchange, adsorption, weath-ering), (2) the soil water pathways determining whichend-member control the runoff, (3) straight dilution byrain or meltwater by overland flow, (4) the antecedenthydroclimatic conditions and (5) the effects of ionicstrength on equilibrium reactions (i.e. salt effect). Asimplified cation exchange model is found to explainqualitatively the observed cation response to changes inthe ionic loading in both soil end-members in additionto weathering by weak carbonic acid in the baseflowend-member. In this watershed, mainly covered by ablanket bog, there are two important water flowpaths.One is through the mineral soil layer underneath theorganic peat layers, the other is through the surfacelayers of the bog. The mineral soil water dominatesduring baseflow conditions, while the soil water fromthe surface Histosol horizons close to the water dividepredominates during periods of highflow.

Despite the reduced trend in sulphur emissions theacid deposition levels remain above the critical loadlimit for large regions in the northern hemisphere. Thehigh amount of exchangeable H+ of the highflow end-member and the shallow water flowpath through theIngabekken catchment during episodes, render suchcatchments with blanket bogs susceptible to acidifica-tion. One can merely speculate about the future of thiscatchment as it is exposed to prolonged deposition oflow levels of excess strong acid anions. In the discus-sion on ‘critical loads’ – i.e. the amount of acid deposi-tion an ecosystem can tolerate without adverse effects– the Ingabekken study at least suggests that 0.5 g SO4

m�2 yr�1 in excess of seasalts (i.e. the present loading)is acceptable for sensitive systems like our site.

Acknowledgments

The field work and data analysis were funded by theSurface Water Acidification Programme. R. V. held adoctor scholarship from the University of Oslo. Wewish to thank H. M. Seip, N. Christophersen, N. Vogt,

J. Mulder, T. Larssen and H. Anderson for valuablecomments and criticism to the paper.

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