Chemosphere 87 (2012) 932–937

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Structural and optical characterization of dissolved organic matter from the lowerAthabasca River, Canada

Céline Guéguen a,⇑, Darcy C. Burns b,1, Alison McDonald a,2, Brooke Ring a,2

a Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario, Canada K9J 7B8b Water Quality Center, Trent University, 1600 West Bank Drive, Peterborough, Ontario, Canada K9J 7B8

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 October 2011Received in revised form 21 December 2011Accepted 23 January 2012Available online 18 February 2012

Keywords:1H NMRRiver DOMParamagnetic ionAromaticEEM

0045-6535/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.chemosphere.2012.01.047

⇑ Corresponding author. Tel.: +1 705 748 1011.E-mail address: celinegueguen@trentu.ca (C. Guég

1 Present address: Department of Chemistry, UniverStreet, Toronto, Ontario, Canada M5S 3H6.

2 Present address: Environmental and Life Science PrWest Bank Drive, Peterborough, Ontario, Canada K9J 7

Dissolved organic matter (DOM) is a ubiquitous constituent of natural waters and is comprised of avariety of chemically heterogeneous molecular structures and functional groups. DOM is often consid-ered to be a major ligand for metals in most natural waters and its reactivity is thought to be stronglydependent on its chemical composition and structure. In this study, a combination of UV/visible, emissionexcitation matrix fluorescence (EEM) and 1H NMR spectroscopies were used to characterize DOM fromthe Athabasca River (Alberta, Canada). The chemical characterization of river DOM showed that the mostupstream samples located in agricultural areas were blue-shifted and less aromatic and contained morehydrogens connected with oxygen functional groups than those in the wetland dominated area in theAthabasca oil sand deposit region. The presence of paramagnetic ions (Fe and Al) was not found to sig-nificantly affect the structural composition of DOM as revealed by 1H NMR. Such change in the qualityof DOM may have a profound impact on metal binding in the Athabasca River watershed.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Dissolved organic matter (DOM) is a heterogeneous mixturecomprised of the decay products of biomass in aquatic andterrestrial environments. DOM is known to play an important rolein the mobility, bioavailability, fate, and toxicity of organic andinorganic contaminants (Campbell, 1995). The reactivity of DOMin these roles is determined at the molecular level and, as such,there is a growing interest in better understanding its chemicaland structural composition.

DOM characterization may be divided into two broadcategories; functional analysis (dissolved organic carbon concen-tration, size and polarity, functional group content, etc.; Leenheerand Croué, 2003), and structural analysis (elemental composition,primary conformation, secondary conformation, and tertiary con-formation; Hayes et al., 1989). The structural analysis of DOM isparticularly challenging and cannot be isolated to purity. Thus,structural characterization is restricted to a coarse level and oftenentails using a variety of spectroscopic techniques. The opticalproperties of DOM, and particularly its absorbance and fluores-cence properties, have been used to distinguish compositional

ll rights reserved.

uen).sity of Toronto, 80 St. George

ogram, Trent University, 1600B8.

characteristics and discriminate between autochthonous(produced by biota) and allochthonous DOM (from the terrestrialenvironment) (Coble, 1996; Blough and Del Vecchio, 2002). Recentdevelopment of excitation–emission matrix fluorescence (EEM)can be used to discriminate among different fluorophore classesof terrestrial, autochthonous and anthropogenic origins accordingto their excitation/emission maxima (Coble, 1996). Fulvic-,humic- and protein-like substances are the major components ofthe fluorescent DOM in natural waters. The aromaticity and molec-ular weight of DOM can also be addressed using the specific absor-bance (SUVA; Blough and Del Vecchio, 2002) and spectral slope ofDOM (S275–295; Helms et al., 2008), respectively. In terms of func-tional groups, the recent advances in nuclear magnetic resonance(NMR) spectroscopy allow its application to DOM (Simpson andSimpson, 2009; and references therein). Unlike other spectroscopicmethods (fluorescence and UV/visible absorption), NMR hastypically required a large amount of material for analysis(�75–100 mg for solid-state NMR). The low DOM concentrationin natural waters (0.5–30 mg C L�1; Thurman, 1985) requires pre-concentration techniques to isolate sufficient amounts of material.Various techniques have been used to isolate DOM from naturalwaters, including ultrafiltration (Guéguen et al., 2002; Guo andSantschi, 2007) and resin adsorption (Thurman and Malcolm,1981). Those procedures can be laborious and may expose DOMto potentially harmful conditions. For instance, the isolation of ful-vic acid from soil and aquatic DOM involves several acidificationand neutralization steps (Thurman and Malcolm, 1981). Suchrigorous treatments may alter the structural composition of DOM

C. Guéguen et al. / Chemosphere 87 (2012) 932–937 933

such that the isolated material may not be completely representa-tive of the original material. This is particularly true for solid stateNMR, where samples must be freeze-dried. Solution-state 13C NMRhas been used for DOM functional groups but the low naturalabundance of 13C nuclei is a significant barrier to performing anal-ysis with minimal sample preparation. Solution-state 1H NMR hasbecome increasingly more sensitive and this, coupled to the factthat DOM is soluble, makes it an attractive method for characteriz-ing DOM at natural abundance concentrations (Lam and Simpson,2008).

In this paper, we report the optical and structural characteriza-tion of river DOM affected by Athabasca tar sands. Since metal con-tents can be high in the Athabasca River (Guéguen et al., 2011) theeffect of varying paramagnetic ions (Al3+ and Fe+3) on 1H NMRspectra was examined using commercial freshwater fulvic acidsat environmentally relevant conditions. Finally the structural andoptical compositions of DOM were examined in relation to theclose proximity of Athabasca oil sands development.

Fig. 1. Map showing the sampling location in the Athabasca River watershed.

2. Materials and methods

2.1. Sample collection and preparation

Surface river water sampling was carried out in June/July 2008and August 2008 (Fig. 1). Six sites were chosen on the main stemAthabasca River (AR01-03, AR05, AR09-10) and 3 tributaries(Clearwater–AR04, Muskeg–AR06-07 and Mackay Rivers–AR08)were sampled. Muskeg and Mackay Rivers are fed by muskeg drain-age waters. Sites upstream of Fort McMurray (AR01-02) were desig-nated as upstream whereas the downstream sites (AR03–AR10)were designated as in Athabasca oil sands deposit. Surface waterswere immediately filtered through a pre-combusted glass fiber filter(GF/F Whatman) and stored in the dark at 4 �C until further analysis.

Nordic Lake fulvic acid (NLFA, 1R105F), Suwannee River fulvicacid (SRFA, 2S101F), and Pony Lake fulvic acid (PLFA, 1R109F) wereobtained from the International Humic Substance Society (IHSS; St.Paul, Minnesota) and used without further treatment. IHSS stocksolutions were prepared in ultra-pure water (18 M X, MilliporeCorp., Bedford, Massachusetts) and used within 24 h.

The effect of cationic metals on natural abundance DOM 1Hspectra was tested by adding either Fe3+ or Al3+ to NLFA solutions.400 lg L�1 represented the upper concentration limit that thesemetals would occur at most AR sites (Table 1).

2.2. Optical analysis

The optical properties of DOM were determined using UV/visi-ble absorbance and EEM fluorescence spectroscopy at natural pH

Table 1pH, conductivity, DOC and, Fe and Al dissolved concentrations in AR samples collected in

Sample ID Date pH Cond. (lS cm�1)

AR05 July-2008 8.2 300AR09 July-2008 8.0 203AR10 July-2008 8.4 235AR01 August-2008 8.5 250AR02 August-2008 8.5 238AR03 August-2008 8.2 184AR04 August-2008 8.1 228AR05 August-2008 8.3 243AR06 August-2008 8.5 244AR07 August-2008 8.0 223AR08 August-2008 8.0 189AR09 August-2008 8.2 205AR10 August-2008 8.3 265

n.a. data not available.

(Table 1). Absorbance measurements were made on a ShimadzuUV 2550 spectrophotometer in the 260–700 nm region with a1 cm quartz cuvette using Milli-Q water as a reference. The sam-ples were allowed to warm to room temperature prior to analysis.In order to correct for instrumental drift and light scattering an off-set correction was applied by subtracting the average absorbancebetween 650 and 700 nm from the Milli-Q absorbance spectrum.The measured absorbance at wavelength k was converted toabsorption coefficient a (m�1) according to

ak ¼ 2:303 Ak=L

where Ak is the absorbance and L is the path-length of the opticalcell in meters (L = 0.01 m).

Three dimensional emission/excitation matrices fluorescence(EEMs) were generated by measuring fluorescence intensity acrossexcitation wavelengths ranging from 260 to 460 nm and emissionwavelengths ranging from 270 to 518 nm. All measurements weremade at 20 �C using a thermostatted cell holder. If absorbance val-ues were higher than 0.05 (1 cm quartz cell) at the lowest excita-tion wavelength (i.e. 260 nm) the samples were diluted withMilli-Q water prior to fluorescence analysis to minimize reabsorp-tion and inner filter effects. This allowed us to maintain a linearrelationship between fluorescence intensity and concentration.

July 2008 and August 2008.

DOC (mg C L�1) Fe (lg L�1) Al (lg L�1)

46 158 840 172 1527 73 1910 11 915 19 1633 275 2324 222 1424 114 1742 475 437 n.a. n.a.43 355 2432 168 25426 n.a. n.a.

AR01 AR02 AR03 AR04 AR05 AR06 AR07 AR08 AR09 AR10

SUVA

260 [

L. m

g-1 . m-1

]

0

1

2

3

4

5

S 275-

295

[nm

-1]

0.012

0.014

0.016

0.018

0.020

0.022

0.024

0.026

Fig. 2. SUVA260 and S275–295 in the Athabasca River DOM samples in August 2008.

934 C. Guéguen et al. / Chemosphere 87 (2012) 932–937

Sample intensities were corrected for the dilution factor in thepost-processing. The recorded spectra were corrected for theinstrumental response characteristics. Raman scattering effectswere removed by subtracting a Milli-Q blank EEM. Finally the fluo-rescence intensity was corrected to the area under the Milli-Qwater Raman peak (excitation 350 nm) run the same day (Lawaetzand Stedmon, 2009). No significant variation in the integral of theRaman peak was observed during the investigation (<2%). The fluo-rescence intensity was reported in equivalent water Raman units(r.u.).

The humification index (HIX; Huguet et al., 2009) estimated thedegree of humification of DOM based on fluorescence analysis. HIXis the ratio H/L, where H is the fluorescence intensity recorded atexcitation wavelength 254 nm for emission spectrum integratedfrom 435 to 480 nm, and L the fluorescence intensity recorded atexcitation wavelength 254 nm for emission spectrum integratedfrom 300 to 345 nm.

2.3. Dissolved organic carbon and ancillary data

DOC concentration was measured by the high temperaturecombustion method on a Shimadzu TOC-VCPH analyzer. Three tofive replicate injections (100 lL each) were performed for eachsample, resulting in a typical coefficient of variation <2%.

Aluminum and iron concentrations were measured by high res-olution ICP-MS (Nu, AttoM) with indium and rhodium used asinternal standards. The accuracy of the ICP-MS measurementswas assessed using certified reference materials1640a and 1643e(National Institute of Standards and Technology, USA). Conductiv-ity, pH and temperature were measured using an AP85 Accumet(Fisher, Canada).

2.4. 1H NMR spectroscopy

The relative abundances of the main functional groups compris-ing DOM were measured using 1H NMR spectroscopy. 1H NMRspectra were obtained after diluting the DOM samples with 10%D2O (v/v) (99.9%, Sigma–Aldrich, Oakville, Ontario) containing tri-methylsilyl propionate (TSP) as an internal chemical shift reference(0.00 ppm). The diluted AR samples were transferred directly to a5 mm NMR tube (Wilmad, New Jersey) and were not altered inany way aside from passing them through a 0.7 lm pre-combustedglass fiber filter to remove particulate matter. pH Adjustments,fractionation and freeze drying were not used on AR samples. Eachof these steps may have inadvertently altered the structure andcomposition of DOM such that the material being analyzed wasnot representative of the original sample. The final DOC concentra-tions in the NMR solutions ranged from 8.6 mg C L�1 at AR01 to41.4 mg C L�1at AR05.

Direct 1H NMR experiments were conducted at 25 �C using anarrow bore Varian Unity Inova spectrometer (Varian Inc., PaloAlto, California) operating at 499.764 MHz. The spectrometer wasequipped with a 5 mm variable temperature PFG 1H{13C/15N} indi-rect-detection probe for improved sensitivity. The W5 WATERGATEpulse sequence, as employed in the VnmrJ biopack set of pulse se-quences (Varian Inc., Palo Alto, California), was used to removewater resonance (Lam and Simpson, 2008). It should be noted thatthe direct 1H NMR analysis of DOM took �12 h to complete persample. A 12 h acquisition time offered the opportunity to assessDOM structure and composition at environmentally relevant con-centrations within a reasonable timeframe at a reasonable cost.All NMR spectra were Fourier transformed using 5.0 Hz line broad-ening and post-acquisition solvent subtraction. The post-acquisi-tion solvent subtraction algorithm was employed over a 100 Hzregion and frequency offset to the center of the residual H2O signal.Spectra were then phased, integrated, and baseline corrected.

Integral regions were selected as follows: 0.3 –2.05 ppm(unsubstituted aliphatic), 2.05–3.10 ppm (substituted aliphatic),3.10–4.20 ppm (carbohydrates), 4.4–5.40 ppm (residual water),5.60–6.10 ppm (alkenyl), and 6.10–9.0 ppm (aromatic). The preci-sion of each integral region was 6.1%, 2.8%, 3.6%, 0.1% and 0.4% asdetermined on triplicate measurement of 16 mg C L�1 NLFA 1HNMR. The lowest precision in the unsubstituted region could beattributed to intensity variations of the narrow resonances at1.27, 1.30 and 1.37 ppm.

3. Results and discussion

3.1. Physical chemical parameters

The pH, conductivity, DOC, and metal concentrations in AR sur-face waters are given in Table 1. The pH values ranged from 8.0 to8.5, with the highest values found at AR01-02, 06. Varying within anarrow range, pH did not account for the change in DOM quality. Incontrast, conductivity varied by a factor of �2 between sites withvalues ranging from 184 to 300 lS cm�1. Average values of DOCranged from 9.1 mg C L�1 at the most upstream station (AR01) to46.0 mg C L�1 in the Athabasca oil sands deposit (i.e. AR05). Strongdifferences were observed in dissolved metal concentrations be-tween sites. For example, a 42-fold difference in dissolved ironcontent was found between sites upstream and downstream sites(AR01 and AR06 respectively).

3.2. Optical properties

For all samples, the UV–Visible absorbance spectra of the sur-face river samples are featureless showing an exponential decreasewith increasing wavelengths, following a trend similar to that al-ready described for natural water samples. The spectral slope coef-ficient, S, describes how rapidly the absorption decreases withincreasing wavelengths and can be used to derive compositionaldifferences in DOM. Previous work on surface waters has suggestedan inverse relationship between DOM molecular weight (MW) andthe spectral slope coefficient (Helms et al., 2008; Guéguen andCuss, 2011). Higher spectral slopes indicate DOM with a lower aro-matic content (Blough and Del Vecchio, 2002). The average spectralslope determined over the range 275–295 nm for AR03-10(S275–295 = 0.015 ± 0.001 nm�1, n = 11; Fig. 2) was very similar tothe previously reported spectral slopes for DOM-rich river (Spenceret al., 2009). It is apparent that the lowest S values occur in con-junction with highest SUVA260 values (Fig. 2), revealing the pres-ence of high MW aromatic carbon sources (Spencer et al., 2009).Upstream samples (AR01-02) had significantly higher S275–295 val-ues (0.023 and 0.024 nm�1 respectively), indicative of lower MW

C. Guéguen et al. / Chemosphere 87 (2012) 932–937 935

(Guéguen and Cuss, 2011). This difference in spectral slopes inferscompositional differences between the DOM collected at upstreamand downstream sites.

Based on the linear regression between absorption coefficient at355 nm (a355) and DOC (r2 = 0.65, n = 14, p < 0.05), the non-absorb-ing DOC fraction was estimated for DOC concentration at a 0absorption coefficient. The calculated non-absorbing DOC fractionwas 835 ± 625 lM-C and represented 33% of the total averageDOC measured in the Athabasca River watershed.

The EEM fluorescence contour profiles of the Athabasca Riversamples are shown in Fig. 3. The river samples have similar fluores-cent features with two main fluorophores at the following excita-tion and emission wavelengths (Ex/Em): <260/440–460 nm (peakA; Coble, 1996) and 300–320/414–450 nm (peak C; Coble, 1996).These peaks were previously found in water samples collected inother aquatic environments (Guéguen and Cuss, 2011). Since emis-sion wavelength maxima were higher than 400 nm, both peakswere assigned to humic-like compounds. The absence of protein-like compounds (i.e. Em < 400 nm) may be overshadowed by thevery intensive humic-like bands. The maximum fluorescence inten-sity (Ex = 300–320 nm) in AR01-02 showed a significant deviationtowards lower emission wavelengths compared to AR03-10 (414vs. 420–450 nm). This blue shift can be explained by a reductionin the number aromatic rings, and a lower degree of conjugationof unsaturated bond systems (Coble, 1996). The significant shift ofthe fluorescence maximum towards shorter wavelengths associ-ated with higher S275–295 values suggests a reduction in DOMmolecular weight at AR01-02. Similar results were found in aquaticDOM studies (e.g. Guéguen and Cuss, 2011; and reference therein).

The humification index (HIX; Huguet et al., 2009) was stronglycorrelated with DOM aromaticity. Lower HIX values (<4) corre-spond to biological material whereas high HIX values (>6) corre-spond to highly humified material (Huguet et al., 2009). HIXvalues spanned from 6 at AR02 to 25 at AR04 with an average of14 ± 5. Similar HIX range was found in sediment and porewaters(5–20; Birdwell and Engel, 2010). High HIX combined with highSUVA260 and low S275–295 supports a strong aromatic character ofDOM in the Athabasca oil sands deposit (AR03-10).

3.3. Paramagnetic ions (Fe3+, Al3+) and structural analysis

DOM may act as a ligand for mono- and polyvalent ions and of-ten sequesters these ions in natural waters (Tipping, 2002).

Fig. 3. EEM fluorescence contour profiles of four AR samples (A-AR02, B-AR06, C

Binding, however, may activate additional NMR relaxation path-ways for those nuclei that interact with the metal. Under these cir-cumstances, metal ion binding may affect both the intensity andresonant frequency of the 1H DOM chemical shifts (Schmidtet al., 1997; Simpson et al., 2007), such that quantification of theratio of chemical constituents, i.e. aliphatic:aromatic ratio (ali:aroratio), may become skewed.

The addition of either Fe3+ or Al3+ to the NLFA did not signifi-cantly affect the measured ali:aro ratio (Fig. 4) with an averageof 7.79 ± 1.28 (n = 3, p < 0.005). Similarly, no significant differenceswere observed in the aromatic, alkene, carbohydrate, substitutedaliphatic, and saturated aliphatic regions. Consequently, the vari-able levels of metal contents observed in AR samples (Table 1) can-not fully explain the significant differences observed in H moietiesdistribution.

3.4. 1H NMR of DOM

The 1H NMR spectra of dissolved organic matter in the AthabascaRiver exhibit broad bands as would be expected from the 1H NMRspectra of heterogeneous mixtures such as DOM (Fig. 5). All AR spec-tra showed the strongest resonance in the aliphatic regions (0.6–3.1 ppm) (75.2 ± 5.2%). Strong resonances were observed in the0.6–2.1 ppm region (44.0 ± 5.1%), which is characteristic of hydro-gens bound to saturated carbon including proteins, and those inthe 2.1–3.1 ppm region (31.2 ± 4.3%) which are characteristic of pro-tons adjacent to functional groups such as carbonyl, ester and amide.Sharp resonance associated with polymethylene chains (1.3 ppm)was seen in most AR samples. The third most prominent signal arosefrom H–C bound to oxygen (i.e. carbohydrate H; 3.1–4.2 ppm), whichaccounted for 17.3 ± 3.2% of the total integral of the spectra. Weakerresonances were found in the alkene and aromatic regions(2.9 ± 1.8% and 4.6 ± 0.9%, respectively). Similar abundance of aro-matic protons was found in rainwater DOM (5%; Santos et al.,2009). Carboxylic hydroxyl groups are important in DOM and metalbinding (Perdue, 1985). The carboxylic hydroxyl groups were mostlikely deprotonated in all of the DOM samples in this study (pH8.0–8.5; Table 1) and therefore were not observed by 1H NMR. Over-all, the 1H NMR spectra of AR DOM, the fulvic acid samples (SRFA,PLFA and NLFA; Table 2) and rainwater DOM (Santos et al., 2009)all showed similar profiles in terms of functional group composition.

The peaks situated in the 3.1–4.2 ppm region (i.e. carbohydrate)were 6 times greater than that in the 5.6–6.6 ppm region (i.e.

-AR08, D-AR09). Fluorescence intensities are reported in Raman unit (r.u.).

aromatic alkene CHO subst.ali unsubst.ali ali:aro

% H

moi

etie

s

0

10

20

30

40

50

ali:a

ro ra

tio

0

2

4

6

8

10

12

Fig. 4. Distribution of H moieties of NLFA (black), NLFA + 200 lg L�1 Al (white) andNLFA + 200 lg L�1 Fe (black) as revealed by liquid-state 1H NMR spectroscopy. Thehatched bar represents 400 lg L�1 Al (or Fe).

936 C. Guéguen et al. / Chemosphere 87 (2012) 932–937

alkene) (17.3% vs. 2.9%). These results were in accordance with thepredominance of fulvic acid in aquatic DOM samples (Ertel et al.,1986) and the close resemblance of FA and DOM in metal binding(Cabaniss and Shuman, 1988; Xue and Sigg, 1999).

ppm0246810

ppm0246810

A

C

Fig. 5. Examples of 1H NMR spectra of Athabasca River

Table 21H NMR spectra of the athabasca river DOM samples and extracted fulvic acids (SRFA, PLF

Sample ID Date Aromatic Alkene

AR05 July-2008 4.0 0.9AR09 July-2008 4.3 1.9AR10 July-2008 6.2 6.1AR01 August-2008 3.5 6.9AR02 August-2008 5.0 4.0AR03 August-2008 6.1 2.0AR04 August-2008 5.1 2.0AR05 August-2008 5.0 2.7AR06 August-2008 3.4 2.1AR07 August-2008 3.5 0.9AR08 August-2008 4.8 1.9AR09 August-2008 5.1 2.8AR10 August-2008 4.0 3.3

SRFA 5.4 0.8PLFA 3.5 0.4NLFA 10.0 3.5

The spectral proportion of hydrogens connected with oxygenfunctional groups (i.e. alkene and carbohydrate; HR–O) averaged20% with the highest values (23–31%) at AR01-02 (August) andAR-10 (July). The lowest HR–O values (16.5–17.2%) were found atAR06-08 located in the Muskeg and Mackay Rivers, two wetland-dominated river basins. Existing explanations for this trend include(a) formation of metal-DOM complexes causes the decrease in HR–O

(Pajaczkowska et al., 2003) and (b) nature and source of DOM isdifferent. Carboxylate groups are considered to have the most sig-nificant effects on DOM-metal binding (Perdue, 1985; Tipping,2002). Consequently, a change in metal load and/or carboxyliccontent would be expected to influence the formation of metalcomplexes within DOM. The heavy metal concentrations andspeciation were relatively similar at all sites (Table 1; Guéguenet al., 2011) with metal-DOM complexes being the dominant metalspecies (Guéguen et al., 2011). Furthermore, the levels of paramag-netic ions observed at study sites were not high enough to inducesignificant change in 1H NMR spectra of DOM (Fig. 4). Taken to-gether, these results suggest that metal speciation cannot explainthe decrease in HR–O. On the other hand, AR01 and AR-02 are lo-cated in an agricultural area whereas the downstream sites(AR03-10) are located in the Athabasca oil sands deposition, wherethe landscape is dominated by boreal forest and muskeg (up to 90%

ppm0246810

ppm0246810

B

D

DOM samples (A-AR02, B-AR05, C-AR06, D-AR08).

A and NLFA).

Carbohydrates Substituted Unsubtitutec Ali:aro

12.1 24.7 58.3 20.816.6 35.8 41.4 18.023.0 26.0 38.8 10.523.9 27.2 38.4 18.618.8 24.2 48.1 14.516.2 32.5 43.2 12.417.3 31.8 43.7 14.917.5 33.0 41.7 14.814.4 32.6 47.5 23.516.0 36.0 43.6 22.915.3 35.9 42.1 16.216.0 34.8 41.4 15.017.6 31.5 43.5 18.7

17.9 37.1 38.8 14.29.9 34.7 51.6 24.7

21.6 31.6 33.4 6.5

C. Guéguen et al. / Chemosphere 87 (2012) 932–937 937

at AR06-08). A change to wetland-dominated areas has beenshowed to affect DOM composition. Greater contributions of highMW and more humified DOM were associated with an increasein wetland area in the watershed (Wilson and Xenopoulos, 2009;Williams et al., 2010). Such enhancement is consistent with lowerS275–295 values at downstream sites (Fig. 2). This suggests that highHR–O levels were likely due to change in DOM composition.

The amount of aromatic carbon present in AR samples was esti-mated using the C/H ratio of 1.2 (Aluwihare et al., 2002). Based onthis ratio, between 4.1% and 7.4% of the carbon was present inaromatic compounds at natural pH. This is consistent with the esti-mates of Clair et al. (1996), who found that aromatic C proportionof DOM was generally less than 10% in brown waters. In general,aromatic carbon does not represent a large fraction of DOM despitethe significant soil leachate input to the rivers.

Finally, the ali:aro ratio ranged from 11 at AR10 (July) to �23 atAR06-07, with an average value of 17 ± 4 (n = 13). These ratioscompared well with FA samples (15 ± 9; Table 2) and with thestructural composition of mangrove DOM (9; Dittmar et al.,2006) and rainwater DOM (16; Santos et al., 2009). HoweverScapini et al. (2010) found much higher ali:aro ratio in marineand river FA (32 vs. 15–17).

4. Conclusions

Dissolved organic matter from the Athabasca River consists of acomplex mixture of aliphatic and carbohydrate structures (88–96%) with a minor contribution of aromatic structures (3.1–6.1%).Significant changes were observed along the Athabasca River withthe most upstream samples being blue-shifted, less aromatic, andcontaining lower HR–O than those in the Athabasca oil sand deposit.Although paramagnetic ions have been showed to alter NMR sig-nal, the levels of Fe and Al measured in this study were not highenough to induce significant change in 1H NMR spectra of DOM.However, a change in land use from agricultural (upstream sites)to wetland (downstream sites) may be responsible for the changein DOM composition. Since the DOM has profound impact on thebiogeochemistry of natural waters, a change in its chemical com-position may affect the mobility and toxicity of trace metals inaquatic systems. This change could be further accentuated withincreasing oil sands exploration in the wetland-dominatedwatershed of the lower Athabasca River.

Acknowledgments

This study was funded by the Natural Sciences and EngineeringResearch Council of Canada (CG) and the Canada Research Chair pro-gram (CG). The authors gratefully acknowledge Antoine Perroud andIsabelle Lavoie for their assistance with the collection of the Athaba-sca River samples. We thank D. Ellis and C. McMurdo (Trent Univer-sity) for their help with NMR spectrum processing. We thank theEditor and two anonymous reviewers for helpful comments and sug-gestions that helped improve the quality of this manuscript.

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