Seasonal variability of total dissolved fluxes and origin of major dissolved elements within a large...

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Journal of South American Earth Sciences xxx (2013) 1e14

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Journal of South American Earth Sciences

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Seasonal variability of total dissolved fluxes and origin of majordissolved elements within a large tropical river: The Orinoco,Venezuela

Alain Laraque a,*, Jean-Sébastien Moquet a, Rana Alkattan a, Johannes Steiger b,c, Abrahan Mora d,Georges Adèle e, Bartolo Castellanos f, Christèle Lagane a, José Luis Lopez f, Jesus Perez g,Militza Rodriguez g, Judith Rosales g

aGET, UMR CNRS/IRD/UPS, UMR 5563 du CNRS, UR 234 de l’IRD, OMP 14 Avenue Edouard Belin, 31400 Toulouse, FrancebClermont Université, Université Blaise Pascal, Maison des Sciences de l’Homme, BP 10448, 63000 Clermont-Ferrand Cedex 1, FrancecCNRS, UMR 6042, GEOLAB – Laboratoire de géographie physique et environnementale, 63057 Clermont-Ferrand, FrancedCentro de Oceanología y Estudios Antárticos, IVIC, VenezuelaeHSM, IRD, Fort de France, BP 8006, 97259 Martinique, Francef IMF, UCV, Ciudad Universitaria, Los Chaguaramos, Caracas, VenezuelagCIEG/UNEG, Urbanización Chilemex, Calle Chile, Puerto Ordaz, Estado Bolívar, Venezuela

a r t i c l e i n f o

Article history:Received 2 December 2011Accepted 5 December 2012

keywords:HydrochemistryTotal dissolved fluxesHydrologyOrinoco River

* Corresponding author.E-mail address: [email protected] (A. Laraque).

0895-9811/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jsames.2012.12.011

Please cite this article in press as: Laraque, Aa large tropical river: The Orinoco, Venezuel

a b s t r a c t

Seasonal variations of total dissolved fluxes of the lower Orinoco River were calculated taking into ac-count four complete hydrological cycles during a five-year period (2005e2010). The modern concen-trations of total dissolved solids (TDS) of the Orinoco surface waters were compared with data collectedduring the second half of the last century published in the literature. This comparison leads to theconclusion that chemical composition did not evolve significantly at least over the last thirty to fortyyears. Surface waters of the Orinoco at Ciudad Bolivar are between bicarbonated calcic and bicarbonatedmixed. In comparison to mean values of concentrations of total dissolved solids (TDS) of world riversurface waters (89.2 mg l�1), the Orinoco River at Ciudad Bolivar presents mainly low mineralizedsurface waters (2005-10: TDS 30 mg l�1). The TDS fluxes passing at this station in direction to theAtlantic Ocean between 2005 and 2010 were estimated at 30 � 106 t yr�1, i.e. 36 t km�2 yr�1. It wasobserved that the seasonal variations (dry season vs wet season) of total dissolved fluxes (TDS anddissolved organic carbon (DOC)) are mainly controlled by discharge variations. Two groups of elementshave been defined from dilution curves and molar ratio diagrams. Ca2þ, Mg2þ, HCO�

3 , Cl� and Naþ mainly

come from the same geographic and lithologic area, the Andes. Kþ and SiO2 essentially come from theLlanos and the Guayana Shield. These findings are important for understanding fundamental geo-chemical processes within the Orinoco River basin, but also as a baseline study in the perspective of thedevelopment of numerous mining activities related with aluminum and steel industries; and the plansof the Venezuelan government to construct new fluvial ports on the lower Orinoco for the transport ofhydrocarbons.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The Orinoco River is the world’s third largest river(37 000 m3 s�1, i.e. 1.2 � 1012 m3 yr�1) after the Amazon and Congorivers in terms of discharge to the ocean for a catchment of only1 � 106 km2 (Pérez Hernández and Lopez, 1998; Silva Leon, 2005),

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., et al., Seasonal variability oa, Journal of South American

ranking it 20th in terms of catchment area (UNESCO, 1979). Eventhough the hydrological cycle of this large tropical river is wellknown in comparison with the adjacent Amazon River, only a fewstudies have investigated its sedimentary and geochemical dy-namics and fluxes. A relatively small number of authors havepublished results from biogeochemical studies of the surface wa-ters of the Orinoco since the pioneer studies of Gessner (1960,1965), Livingstone (1963) and Edwards and Thornes (1970).Gessner (1960, 1965) investigated electrical conductivity while theother authors presented water analyses of the Orinoco River

f total dissolved fluxes and origin of major dissolved elements withinEarth Sciences (2013), http://dx.doi.org/10.1016/j.jsames.2012.12.011

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A. Laraque et al. / Journal of South American Earth Sciences xxx (2013) 1e142

upstream the Puerto Ayacucho station (Fig. 1). Venezuelan data(1970e1973) of concentrations of major and minor dissolved ele-ments was published in an anonymous report cited by Meybeck(1979). Stallard (1985) and Vegas-Vilarrubia et al. (1988b) relatedthe chemistry of the Orinoco basin with the chemistry of theAmazon basin, and other authors (e.g. Depetris and Paolini, 1991;Rosales Godoy et al., 1999) compared the Orinoco River to furtherSouth American rivers. Right and left margin confluences of trib-utaries on the main Orinoco River channel were also analyzed inRosales et al. (2008). The results of first studies on the chemistry ofthe Orinoco River were published mainly in the 1980s and thebeginning of the 1990s (Lewis and Weibezahn, 1981; Lewis andSaunders, 1984, 1990; Milliman and Meade, 1983; Nordin andMeade, 1985; Stallard, 1985; Stallard et al., 1990). Also, Meade andco-authors studied essentially the hydrology and the dynamics ofsuspended sediment in the Orinoco River channel (Meade et al.,1983, 1990). More recently, Mora and co-authors studied the geo-chemistry of dissolved major and trace elements in the lower Ori-noco River and its tributaries, as well as the associated weatheringprocesses within the river catchment and the relationship betweenthese processes and the different geological regions (Mora et al.,2007, 2009, 2010a, 2010b, 2011).

Fig. 1. The Orinoco River, its catchment and tributaries. The data for the present study we2002).

Please cite this article in press as: Laraque, A., et al., Seasonal variability oa large tropical river: The Orinoco, Venezuela, Journal of South American

With the exception of Paolini et al. (1987) and Lewis andSaunders (1989), who presented the results of first studies of dis-solved element fluxes for two distinguished periods (1982e1983;1982e1985), and the works of Lewis and Saunders (1990) aboutthe primary production of organic carbon, phosphorus and nitro-gen fluxes, most of the studies about the concentrations of dis-solved elements were focused on major elements (Lewis andWeibezahn, 1981; Paolini et al., 1983; Stallard, 1985; Yanes andRamirez, 1988; Weibezahn, 1985, 1990), trace elements (Chenet al., 2006), or organics (Lewis and Saunders, 1990; Paolini et al.,1987; Paolini, 1990, 1991; Vegas-Vilarrubia et al., 1988a). Therates and processes of weathering were investigated (Edmondet al., 1995, 1996; Stallard et al., 1990), and several studiesfocused on the Orinoco main channel (Lewis and Saunders, 1989;Mora et al., 2009), its tributaries (Lewis et al., 1986, 1987; Saundersand Lewis, 1989; Mora et al., 2007, 2008, 2010a, b), or both (Paoliniet al., 1987; Depetris and Paolini, 1991; Chen et al., 2006; Mora,2011).

However, up to date, dissolved element fluxes within the lowerOrinoco River were determined for one or two hydrological cyclesonly, based on irregular and low frequency sampling. We analyzethe seasonal and interannual variability of total dissolved fluxes

re collected at the Ciudad Bolivar stream gauging station (modified from Warne et al.,


Fig. 2. Mean monthly rainfall for the period 1987e2010 at Ciudad Bolivar and meanmonthly discharges of the Orinoco River between 1926 and 2010 (continuous line) andthe study period (2005e2010 e dashed line) (source: INAMEH, Cordoba, 1999; www.ore-hybam.org).

A. Laraque et al. / Journal of South American Earth Sciences xxx (2013) 1e14 3

determined from data collected during the period 2005e2010 witha regular monthly sampling procedure. Our objectives are (i) tocharacterize contemporary the major element chemical composi-tion of the river waters and the seasonal variations of the dissolvedconcentrations during the study period; (ii) to calculate meanmonthly, annual and mean annual TDS fluxes for the lower OrinocoRiver for four hydrological cycles; (iii) to compare the recent con-centrations of dissolved elements with results from the literatureobtained during the last thirty to forty years; (iv) to better under-stand the contemporary spatial origin of dissolved elements causedby the different physiographic regions within the Orinoco Rivercatchment.

2. Study area

The Orinoco basin is located in the North-East region of SouthAmerica, between Venezuela and Colombia. The source of theOrinoco River is at 1047 m a.s.l. within the Sierra Parima at theCerro Delgado Chalbaud (Fig. 1) (Carbonell, 1998), its channellength is about 2140 km (Silva Leon, 2005) and mean annual dis-charge at the river mouth is estimated between 37000 m3 s�1

(Pérez Hernández and López, 1998) and 37600 m3 s�1 (Cordoba,1999). Along the main river channel on its left bank, averagefloodplain width is 9 km and the inundated floodplain area at low(LW) and high water (HW) are approximately 400 and 7000 km2,respectively (ratio of HW:LW: 17.5) (Hamilton et al., 1990). Thebasin comprises three major physiogeographic regions: (i) in theNorth and West the geologically young and orogenic Andes andCaribbean Coastal Ranges (35% of the basin) where whitewaterstreams originate; (ii) on the left bank in the North the Llanos (50%),a lowland alluvial floodplain area built by the tributaries flowingfrom the Andes to the Orinoco River main stem; (iii) on the rightbank in the South, the Precambrian Guayana Shield (15%) withessentially blackwater tributaries (Lewis and Saunders, 1989;Rosales Godoy et al., 1999; Warne et al., 2002; Mora et al., 2009)(Fig. 1).

Therefore, left bank tributaries are characterized by high sus-pended sediment concentrations and a neutral pH, while right banktributaries deliver only very little suspended sediment to the Ori-noco mainstem and have low pH values. These continuous andheterogeneous deliveries from one river bank to the other, inaddition to fluvial islands, are maintaining a lateral asymmetrywithin the Orinoco mainstream, such as the catchment does. Thisasymmetry, which was pointed out by many authors (Lewis andSaunders, 1984; Nordin and Meade, 1985; Stallard, 1987; Lewisand Saunders, 1989), can however be locally interrupted as forexample downstream of the eight major bedrock structural controlpoints (Warne et al., 2002) between the Meta confluence and theriver delta.

The Orinoco catchment is characterized by a peculiarity, theCasiquiare diffluence, which links the Orinoco to the Amazon River.This diffluence may divert nearly 30% of the upper Orinoco dis-charge to the Amazon River at the location called “TamaeTama”(Georgescu-Pipera and Georgescu-Pipera, 1993; Laraque et al.,2006).

The hydrological regime of the Orinoco River with a pronounceddry and wet season is controlled by the seasonality of precipitation(Fig. 2) which in turn is determined by the position of the persistentBolivian anticyclone and the oscillation of the intertropical con-vergence zone (Pérez Hernández and López, 1998; Lewis et al.,1995). Each physiographic region is also defined by its climaticcharacteristics (Lasso et al., 2010). On the slopes of the easternAndes, precipitations vary between 2500 and 4000 mm yr�1 witha dry period of two months. On the alluvial plains, rainfall variesbetween 1000 and 2000 mm yr�1 with a dry season of five to six


months. Within the Guayana Shield region, rainfall generally variesbetween 2500 and 3500 mm yr�1 without presenting a welldefined dry season (Cressa et al., 1993; Silva Leon, 2005). Highestregions in the South receive up to 6000 or 8000 mm (Ewel et al.,1976).

The hydrological cycle of the lower Orinoco at the gaugingstation of Ciudad Bolivar starts in April, and is characterized bya unimodal regime with high flows between August and Sep-tember, and low flows between February and March (Fig. 2). Thespecific discharge of the Orinoco is 37.6 l s�1 km2, which is morethan three times higher than the specific discharge (11.6 l s�1 km2)of the Congo River (Laraque et al., 2001) and about the same orderof magnitude (34.6 l s�1 km2) of the Amazon River (Callède et al.,2010). Thus, the Orinoco and the Amazon rivers, have the highestspecific discharges for large rivers with catchment areas of aboutone million square kilometers or more.

3. Methods

3.1. Study period, gauging station

Four complete hydrological cycles were studied from data col-lected at Ciudad Bolivar gauging station: (i) April 2005 to March2006; (ii) April 2007 to March 2008; (ii) April 2008 to March 2009;(iv) April 2009 to March 2010. Ciudad Bolivar (08�0803800N,63�3602800E, 8 m a.s.l.) is the main hydrological gauging stationwithin the Orinoco catchment (Fig. 1), with daily water stage re-cords and discharges since 1926, which has been recorded by theInstituto Nacional de Meteorologia y Hidrologia (INAMEH) of Ven-ezuela. Limnimetric observations were carried out once or twicea day and rating curves were elaborated by the INAMEH and by theCentral University of Venezuela (UCV), using traditional gaugingwith current meters. The discharge data for this studywas providedby the INAMEH data base and Cordoba (1999). The hydrologicalstation at Ciudad Bolivar controls 83.6% of the total catchment areaand is not influenced by oceanic tides.

3.2. Total suspended solids (TSS)

During the study period between August 2005 and December2010, a total of 180 water surface samples were takenwith a 10-dayfrequency in the middle of the river channel. The water sampleswere transported to the laboratory and filtered using 0.45 mm cel-lulose acetate filters to separate the TSS. The sample transect waslocated within a narrow channel reach and just downstream ofa bedrock control point (Fig. 1). This structural control point


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Table 1Measured features of water quality of the Orinoco river at Ciudad Bolivar during thestudy period (April 2005eMarch 2010).

Parameter Unit Mean Min Max Max/min

Qj m3 s�1 31 416 3324 76 290 22.95Temperature T�C 28.80 24.80 32.20 1.30pH 6.65 6.34 7.57 1.19CE mS cm�1 to 25 �C 29.90 18.40 53.90 2.93Ca mg l�1 3.26 1.89 5.86 3.11Mg mg l�1 0.77 0.48 1.38 2.86Na mg l�1 1.34 0.68 2.50 3.68K mg l�1 0.79 0.58 1.38 2.38Cl mg l�1 0.80 0.10 1.85 18.50SO4 mg l�1 2.78 0.92 7.24 7.87HCO3 mg l�1 11.26 4.75 22.33 4.70NO3 mg l�1 0.03 N.D. 0.96SiO2 mg l�1 7.44 3.95 17.58 4.45Al2O3 mg l�1 0.88 N.D. 6.99Fe2O3 mg l�1 0.77 N.D. 4.50TDS mg l�1 30.13 11.25 48.68 4.33COD mg l�1 4.00 1.70 9.30 5.47TSS mg l�1 74.24 3.00 206.00 68.67


provokes flow vortices and turbulences which cause a certain ho-mogenization of the channel waters. At the beginning of this studyit was shown through ten solid gaugings using the point samplingprotocol (cf. Filizola and Guyot, 2004) at different water stageswithin the sample transect, that one sample taken close to thewater surface in the middle of the transect is representative formean TSS concentrations of the whole channel section.

3.3. Total dissolved solids (TDS)

For the analysis of total dissolved solids (TDS), one surface watersample was collected in the middle of the river channel once everymonth and filtered in situ (0.45 mm filter). In total, 48 samples weretaken during the study period. During each sampling, in situ mea-surements of water temperature, pH and electric conductivity werecarried out using a pHmeter WTW PH 320 and a conductimeterWTW LF 318, which were previously calibrated for typical ranges ofthese waters. Water samples were analyzed in the laboratory inorder to determine concentrations of major elements. Cation con-centrations (Ca2þ, Mg2þ, Naþ, Kþ) were measured using an ICP-AESspectrophotometer, anion concentrations (Cl�, NO�

3 , SO2�4 , HCO�

3 )were measured using an IC-HPLC and a continuous flow spec-trocolorimeter was used for measuring Si concentrations. Fe and Alconcentrations were determined using an ICP-MS on a spec-trometer equipped by collision cell. The total concentration ofinorganic dissolved solids was the sum of the concentrations of themajor elements plus themain oxides like SiO2, Al2O3 and Fe2O3. Thesamples for dissolved organic carbon (DOC) determination werestored in glass bottles previously burned in an oven at 450 �Cduring two hours; they were acidified with ultrapure H3PO4. Theanalytical precision, evaluated using repeated standard referencematerials analysis, was generally better than 10% while reprodu-cibility was determined using replicate sample analysis and wasbetter than 5%.

The ion balance (IB) was calculated from inorganic ions (Equa-tion (1)).

IB ¼ ðcations� anionsÞ=ðcationsþ anionsÞ � 100 (1)

In this study, the mean ion balance is 7% positive. This dis-equilibrium can be explained through the presence of organic an-ions with a negative charge, which has been reported in the CauraRiver waters by Mora et al. (2010b).

3.4. Dissolved element fluxes

Monthly fluxes of dissolved elements were calculated using thefreeware software HYDRACCESS (http://www.ore-hybam.org/index.php/eng/Software/Hydraccess). First, monthly concentrationdata was linearly interpolated in order to obtain estimates of dailydissolved element concentrations. Second, daily dissolved elementfluxes (kg s�1) were calculated by multiplying daily concentrations(Cdmg l�1) with daily discharges (Qdm3 s�1) (Equation (2); Moataret al., 2009).

Flux ¼ K 00 X365

j¼1

Cintd Qd (2)

where K 00is the conversion factor taking into account the period ofinterpolation between measured data and the units in which thefluxes will be expressed; Cint

d is the daily concentration linearlyinterpolated between two measurements and Qd is the measureddaily discharge.

Annual fluxes (t yr�1) were calculated as the sum of monthlyfluxes.


4. Results and discussion

4.1. Measured features of water quality of the Orinoco River

The measured features of water quality of the Orinoco River atCiudad Bolivar during the study period are presented in Table 1. Themean monthly discharges and the mean hydrological regime of theOrinoco River at Ciudad Bolivar during the study period are rep-resentative for those observed during the period 1925e2010 with84 hydrological cycles (Fig. 2). Mean annual discharges are of thesame order of magnitude for these two periods, with 32560 and32660 m3 s�1, respectively.

4.2. Total suspended solids (TSS)

The suspended sediment regime is characterized by two totalsuspended solids (TSS) peaks (Fig. 3), as observed during earlierstudies (e.g. Meade et al., 1990). The first peak appears during therising stage, whereas the second one, slightly lower, appears duringthe falling stage of the annual flood. The TSS concentrationsmeasured every 10 days varied by a factor of 69. Channel surfacewater was slightly acid, with mean pH values of 6.65, water tem-perature varied between the dry and wet season from 24.8 to32.2 �C, and mean conductivity was 29.9 mS cm�1.

4.3. Seasonal variability of TDS concentrations

Fig. 4 shows the seasonal patterns of the mean monthly con-centrations of major cations, major anions and SiO2 in the OrinocoRiver at Ciudad Bolívar. Major anions and cations showedminimumconcentration values during high water (HW) and maximum con-centration values during low water (LW). However, SiO2 concen-trations did not show a clear seasonal pattern, with highconcentrations values during low water and a pick in June. Totaldissolved solids (TDS) concentrations measured once a monthvaried between 11.25 and 48.68 mg l�1, while measured DOC valuesvaried between 1.7 and 9.3 mg l�1 (Table 1). These mean values aresimilar with those of Lewis and Weibezahn (1981) and Lewis andSaunders (1990), of 6.5 and 4.4 mg l�1, respectively.

Data from Orinoco water samples taken monthly at CiudadBolivar between April 2005 and March 2010 and averaged for eachmonth during the study period is showed in Table 2. Mean TDS andDOC concentrations are respectively 29.3 and 3.9 mg l�1, with


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Fig. 3. Mean monthly discharges (line) and mean monthly suspended sediment con-centrations (histogram) for 2005e2010.


a mean annual total of 33.2 mg l�1 for total dissolved matter(TDS þ DOC). Mean monthly TDS and DOC concentrations variedrespectively between 18.3e39.4 and 2.7e5.8 mg l�1, showing a lowtemporal variability. In contrast, mean monthly discharges presenta high variability with an order of magnitude of 10.

During the averaged hydrological cycle, TDS concentrationsdecrease during the rising stage of the annual flood due to thedilution effect, and conversely, increase during the falling stage andlow flows (Fig. 5). Temporal DOC dynamics during the hydrologicalcycle are opposite to those of TDS. DOC concentrations increaseduring the rising stage of the annual flood and show only littlevariations during the falling stage and low flows. It is suggested thatthe increase of DOC during the rising stage is due to more impor-tant eluviations of organic matter under tropical rainforest, espe-cially on the Guayana Shield. The findings of Mora et al. (2010b)studying the Caura River which drains the Guayana Shield showthat due to washout processes, the organic matter accumulated insoils is transported to the river channel during the beginning of therainy season, i.e. during the rising stage of the annual flood. Fur-thermore, it was observed on the Apure River that within left bankriver systems, plant decomposition processes can explain the highDOC concentrations shown during the rising stage (Mora et al.,2010a), contributing also to the observed increase of DOC duringthe rising stage at Ciudad Bolivar (Fig. 5).

The geochemical characteristics of the Orinoco water samples atCiudad Bolivar (Fig. 6) show that the surface water is mainly

Fig. 4. Seasonal patterns of major ions and SiO2 concentrations in the Orinoco River at Ciudmean monthly data.


dominated by Ca and bicarbonate. Ca dominates the cationic chargewith approximate equal amounts of Mg and (Na þ K). Ca, Mg and(Na þ K) contribute on average respectively 53.4%, 21.1% and 25.4%to the cationic charge. HCO�

3 dominates the anionic charge andsamples are distributed between SO4eHCO3 apex with a low andconstant Cl proportion (approximately 10%). The higher concen-trations of SO2�

4 were observed during the rising stage, specificallyin the months of May and June. On the HCO�

3eSiO2eSO2�4 þ Cl�

diagram (Fig. 7), the samples are mainly arrayed along thealkalinity-Si axis. The mean values corresponding to the samplestaken in May and June are distributed in the interior of the ternaryplot.

4.4. Fluxes of TSS, DOC and TDS and the evolution of mean monthlydata during the study period

TDS fluxes vary between 248 and 1900 kg s�1 during the hy-drological cycle (Table 3). In general, we can observe that the am-plitudes between values of element fluxes are higher than those ofthe concentration values for the same elements. This fact can beexplained by the discharge amplitude effect observed here.

The yearly evolution of TSS, DOC and TDS fluxes is inherentlysimilar to that of discharge (Fig. 8). During the study period, themean annual flux of TDS was 955 kg s�1, and the mean annual fluxof DOC was 134 kg s�1 18.5% of inorganic dissolved fluxes areprovided by cations and 49.7% are provided by anions. Theremaining 31.8% are provided by neutral species (SiO2, Al2O3 andFe2O3). For the study period, the annual matter fluxes in the Ori-noco at Ciudad Bolivar were 108 � 106 t yr�1. TSS represents 68.3%(i.e. 74 � 106 t yr�1), TDS 27.8% (i.e. 30 � 106 t yr�1) and DOC 3.9%(i.e. 4.2 � 106 t yr�1). Reported to the drainage area upstreamCiudad Bolivar, the specific total transport is near 130 t km2 yr�1,decomposed by 88.5 t km2 yr�1 for TSS, 36 t km2 yr�1 for TDS and5 t km2 yr�1 for DOC.

Total dissolved solid fluxes determined for the period 2005e2010 (30 � 106 t yr�1) at Ciudad Bolivar were similar to thoseobtained by Paolini et al. (1987) for the period 1983e1984(29 � 106 t yr�1) at the same station. These two different periodswere characterized by similar mean discharges of 32700 and34300 m3 s�1, respectively. Lewis and Saunders (1989) presenta flux of 38.69 � 106 t yr�1 at Barrancas downstream of the Caroniconfluence. However, this value seems high considering the inputof the Caroni estimated by Paolini et al. (1987) of 1.5 � 106 t yr�1. Inrespect to DOC, fluxes measured between 2005 and 2010 are

ad Bolivar. Data collected during the four year study period were averaged to represent


Table

2Dataco

llected

atCiudad

Bolivar

duringthefourye

arstudyperiodwereav

erag

edto

representmea

nmon

thly

data.Ca2

þ,M

g2þ,N

aþ,K

þ,C

l�,S

O2� 4

,HCO� 3,N

O� 3,S

iO2,Al 2O3,Fe

2O3co

ncentrations(mmol

l�1).To

tald

issolved

solid

s(TDS)

anddissolved

orga

nic

carbon

(DOC)co

ncentrations(m

gl�

1).Mea

n(m

ean)va

lues,m

inim

um

(min),max

imum

(max

)arecalculatedfortheen

tire

studyperiod,a

swelltheratioof

max

imum

values

over

minim

um

values

(max

/min)an

dthestan

darddev

iation

(STD

).

Ca2

þumol

l�1

Mg2

þumol

l�1

Naþ

umol

l�1

Kþumol

l�1

Cl�

umol

l�1

SO2� 4

umol

l�1

HCO� 3umol

l�1

NO� 3umol

l�1

SiO2umol

l�1

Al 2O3umol

l�1

Fe2O3umol

l�1

TDSmgl�

1DOCmgl�

1

April

107.55

41.70

83.76

21.39

33.06

39.09

263.21

N.D.

136.93

13.54

4.04

39.32

3.40

May

84.77

31.62

65.76

25.99

32.22

36.85

175.85

N.D.

114.52

7.32

2.29

30.10

4.01

June

70.61

26.59

42.89

19.92

20.30

35.42

129.12

1.80

142.47

19.64

7.80

29.16

4.15

July

56.38

22.34

32.81

17.35

14.62

19.52

119.85

N.D.

105.71

4.17

2.20

21.07

5.29

Augu

st54

.05

24.32

33.80

17.61

14.78

14.80

131.82

N.D.

122.54

11.77

12.11

24.71

5.77

Septembe

r61

.01

26.46

38.16

18.67

14.72

15.76

144.45

1.31

114.96

7.69

6.28

24.31

3.71

Octob

er65

.74

28.57

43.07

18.40

16.29

16.68

163.11

N.D.

123.51

7.60

7.76

26.60

3.84

Nov

embe

r72

.57

28.86

48.21

19.29

18.30

25.00

157.73

N.D.

119.83

9.45

6.19

27.29

3.29

Decem

ber

83.13

31.59

55.14

20.39

20.26

25.39

188.89

N.D.

130.97

9.27

3.81

30.27

4.32

January

78.04

30.11

63.94

19.17

20.01

22.75

198.19

0.63

138.42

6.80

1.26

30.31

3.27

February

97.66

37.80

77.86

20.79

27.35

30.05

239.57

N.D.

128.87

7.04

5.02

35.17

2.70

March

109.09

41.59

87.60

23.15

31.28

34.69

269.61

N.D.

153.92

6.95

2.92

39.61

2.87

Mea

n78

.38

30.96

56.08

20.18

21.93

26.33

181.78

1.25

127.72

9.27

5.14

29.28

3.89

Min

54.05

22.34

32.81

17.35

14.62

14.80

119.85

N.D.

105.71

4.17

1.26

18.29

2.70

Max

109.09

41.70

87.60

25.99

33.06

39.09

269.61

1.80

153.92

19.64

12.11

39.45

5.77

Max

/min

2.02

1.87

2.67

1.50

2.26

2.64

2.25

1.46

4.71

9.64

2.16

2.14

STD

18.74

6.36

19.44

2.46

7.12

8.74

51.75

0.59

13.63

4.09

3.09

6.11

0.92

Fig. 5. Mean monthly discharges (line), mean total dissolved solids (TDS) concentra-tions (dark histogram) and mean dissolved organic carbon (DOC) concentrations (greyhistogram). The DOC concentrations are multiplied by 10. All data are averaged for thefour hydrological cycles studied between April 2005 and March 2010.



slightly higher than between 1983 and 1984 (4.2 � 106 t yr�1 vs3.22 � 106 t yr�1, respectively), conversely to TSS fluxes (74 � 106

t yr�1 vs 93 � 106 t yr�1).At the global scale, the surface waters of the Orinoco are little

mineralized (concentrations of cations of 6 mg l�1) in comparisonwith mean concentrations of cations (23.2 mg l�1) of world’s riverwaters (Meybeck, 1979). Taking into account this study and theworld’s largest rivers data presented by Meybeck (1979), the Ori-noco supplies 3% of continental fluxes of inorganic dissolvedmatter(TDS þ SiO2) and 5% of world’s total discharge to the oceans, whileits catchment area represents 0.7% of the total continental surfaces.

4.5. Comparison of TDS loads observed during this study with datacollected over the last forty years

Table 4 and Fig. 9 compare the mean concentrations of dissolvedcations, anions, SiO2 and TDS obtained in this study with thosecollected over the last forty years by diverse authors. For the majorpart of the dissolved elements, our values are within the rangeobtained by Meybeck (1979) and Paolini et al. (1987) at CiudadBolivar; and Lewis and Saunders (1989) at Barrancas, as well asduring the beginning of the 21st century by Mora et al. (2009),80 km downstream of Ciudad Bolivar. The concentrations of Cl�

(2.9 mg l�1) and SiO2 (11.5 mg l�1) cited by Meybeck (1979) areclearly higher than those of the other studies. Conversely, SiO2(3 mg l�1) concentrations determined by Paolini et al. (1987), areclearly lower than the values of SiO2 from other studies (Table 4).

In order to compare TDS concentrations from our study withvalues from the literature, it was necessary to re-calculate the TDSconcentrations from the present study, taking into account thesame elements and the same number of elements as each differentauthor did (cf. legend of Table 4 and Fig. 9). The more similar valuesare those obtained in the most recent studies by Mora et al. (2009),with a TDS concentration (15 mg l�1) very similar to our calculatedvalue of 17mg l�1, which was obtained using Equation (6) in Table 4(Fig. 9). Even when comparing older studies from the 1970s withthis study, values do not change significantly.

Since none of the former studies present values which are sig-nificantly different from our present study, it is suggested that nomajor environmental changes, land use changes or climate changesinfluencing the production and supply of TDS to the Orinoco mainchannel occurred within the catchment. It could have been hy-pothesized that the sampling protocol (e.g. sampling location,sampling frequency) as well as the fact that some concentrationdata, e.g. those published by Lewis and Saunders (1989), are


Fig. 6. Ternary diagrams of dissolved major elements (expressed in eq % unit) of the Orinoco River at Ciudad Bolivar during the study period.


weighted by the discharge, may induce differences with the con-centration data obtained during our five-year study. However, Fig. 9does not show any significant influences between the different pastand present concentrations of TDS. Thus, it is suggested that theconcentrations of TDS do not show a high temporal variability overthe last decades.

4.6. Origin and production dynamics of TDS

The production dynamics of the total dissolved solid fluxeswhich will be discussed here are considered as temporal evolutionprocesses during the hydrological cycle of these fluxes. Thedescription of these production dynamics aims to identify (i) themain hydrological reservoirs which mobilize dissolved solids ofhydrological origin, (ii) the main geographic source region of thismatter, and/or (iii) their main sources (lithology, atmosphere,vegetation, or human activity). Hydrological origin, geographicorigin and the source may be overlaying in certain cases (e.g. theAndes e geographic source region, matter from water supplymainly superficial and corresponding to the main source of certainelements such as Ca or Cl because of an easily alterable subjected towater supply) (Edmond et al., 1996). Here, this description is real-ized using three different approaches. The first consists of evalu-ating the correlation between concentrations of different dissolvedelements and the hydrological variable (1/discharge, in order toconsider a possible dilution effect). The second approach consists ofidentifying the sources of dissolved matter to quantify their relative

Fig. 7. HCO3eSiO2eSO4 þ Cl diagram of the Orinoco River at Ciudad Bolivar during the2005e2010 study period.


contributions during the hydrological cycle. The third approachconsists of describing the relation between concentrations of dif-ferent dissolved elements and the hydrological variable (discharge).This last was quantified using a dilution index for each dissolvedelement whenever this is statistically possible.

4.6.1. Correlations between element concentrations and dischargeIn order to identify the elements which depend on same pro-

duction dynamics, a correlation matrix of element concentrationsand 1/Q (Q ¼ discharge) is presented (Pearson coefficient; Table 5).A positive correlation between element concentrations and 1/Qsignifies a dilution effect during the hydrological cycle. A correla-tion is estimated very good with (R > 0.85; R < �0.85), good with(R > 0.65), moderate with (�0.5 > R > 0.50).

It appears that the major part of the elements of the dissolvedload present relatively homogeneous production dynamics. Na, Mg,HCO3 and Ca have a very good significant R value (R > 0.90 and p-value <0.05). Cl is well correlated with this first association(R > 0.75). SO4 is slightly correlated with these elements(0.59 < R < 0.73) whereas K is lowly correlated (R < 0.61). Tem-perature, pH, TSS, Si and Al are not significantly correlated with thefirst group of elements. In spite of the low R value, DOC presents aninverse correlation with the first elements group and a positiverelationship with water discharge.

The good correlation between Ca, Mg, Na, HCO3 and Cl suggestsimilar dynamics of mobilization of the reservoirs which producedthese elements. This can be due to an equivalent lithologic sourceand/or geographical origin. Ca, Mg and HCO3 are generally pro-duced by carbonate and silicate weathering; Na is produced byatmospheric inputs and by the weathering of evaporites and sili-cates; Cl originates from atmospheric inputs and the weathering ofevaporites (halite). Themain source of dissolved load in the OrinocoRiver are the Andes (Saunders and Lewis, 1989; Edmond et al.,1996) where evaporites, carbonates and silicates are present. Thiswas also observed for the adjacent Amazon River (e.g. Gibbs, 1972;Stallard and Edmond, 1983; Moquet et al., 2011). The dissolved loadoriginating from the Andes dominate the Orinoco River geo-chemistry in comparison with inputs from the Guayana Shield andLos Llanos. But whenwater samples in both margins were collectedduring 2003, it was evident that along the longitudinal gradient, allresults for samples in the right margin were lower in conductivity,TDS and major elements than in the left margin, all of which weremore related to their Andean origin (Rosales et al., 2007; OrinocoCorridor Project).

The temporary evolution of DOC concentrations observed dur-ing the study period is in agreement with observations by Saundersand Lewis (1988) and Mora et al. (2010a), who reported a positive


Table

3Ave

rage

fluxe

sof

dissolved

elem

ents

(kgs�

1)an

ddisch

arge

(Q)(m

3s�

1)at

Ciudad

Bolivar

calculatedforea

chmon

thfrom

fourhy

drologicalc

yclesstudiedbe

twee

nApril20

05an

dMarch

2010

.

Qm

3s�

1Ca2

þkg

s�1Mg2

þkg

s�1Naþ

kgs�

1Kþkg

s�1Cl�

kgs�

1SO

2� 4kg

s�1HCO� 3kg

s�1NO� 3kg

s�1SiO2kg

s�1Al 2O3kg

s�1Fe

2O3kg

s�1DOCkg

s�1TD

Skg

s�1TD

Sþ

DOCkg

s�1TS

Skg

s�1To

talkg

s�1

April

9546

38.7

9.0

17.4

8.2

10.0

36.1

108.4

1.0

93.1

13.8

5.7

32.9

341

374

716

937

May

2147

873

.016

.730

.618

.818

.979

.024

0.3

3.6

185.0

24.8

10.7

82.3

701

783

1484

3294

June

3605

810

9.4

25.2

41.8

26.4

22.2

116.3

354.0

8.3

300.5

42.0

23.0

151.1

1069

1220

2290

5213

July

5487

214

3.5

34.6

56.0

37.2

26.5

130.6

546.0

11.8

412.4

39.7

37.7

272.5

1476

1749

3225

6446

Aug.

6676

217

5.2

44.1

68.2

45.2

31.1

138.4

724.0

15.1

508.0

63.7

89.2

325.3

1902

2228

4130

6206

Sept.

6201

216

3.5

41.6

64.0

44.5

28.2

132.6

721.4

16.6

443.5

43.6

56.7

236.9

1756

1993

3750

6117

Oct.

4530

612

4.2

31.3

49.1

32.5

22.4

102.8

549.2

11.7

319.5

30.8

46.8

166.9

1320

1487

2808

4809

Nov

.33

470

100.0

23.9

39.2

24.6

19.2

86.7

406.8

9.3

243.4

27.1

28.1

120.8

1008

1129

2138

3753

Dec.

2413

685

.620

.135

.320

.617

.164

.435

6.1

4.1

217.4

24.7

14.5

122.2

860

982

1842

2259

Jan.

1258

345

.010

.520

.310

.410

.332

.418

3.2

1.2

113.9

9.7

5.5

46.4

442

489

931

992

Feb.

8483

35.9

8.5

17.1

7.8

8.7

27.7

129.8

0.7

85.0

8.4

6.5

33.5

336

369

705

703

March

6218

26.5

6.2

12.3

5.4

6.7

21.5

99.1

0.6

59.3

6.4

3.8

18.7

248

267

514

502

Max

/min

10.74

6.61

7.08

5.56

8.37

4.62

6.42

7.30

27.20

8.57

10.00

23.76

17.39

7.68

8.36

8.03

13Fig. 8. Yearly evolution of monthly fluxes of TSS, DOC and TDS and monthly dischargeof Orinoco surface waters at Ciudad Bolivar during the study period.



relationship between the discharge and DOC concentrations withinthe Apure River, a left bank tributary. But, in contrast to thesestudies, the present data does not show any positive correlationbetween Si, K and DOC. It is suggested that this can be explained bythe fact that our water samples were taken in the Orinoco main-stream and not in one of its tributaries. Indeed, our data is repre-sentative of water which originated from the left bank tributariesdraining the Andes and Los Llanos, as well as right bank tributariesdraining the Guayana Shield. The water of different geographicalorigin mixes in themain channel and thus do not show any positivecorrelation between Si, K and DOC.

The highest concentrations of DOC during rising and high waterperiods can be associated with the production of organic acids byplant roots, microorganisms and principally by microbial decay ofthe submerged plants in the floodplain as suggested by Mora et al.(2010a) for the Apure River and by Junk (1984) for the AmazonRiver.

4.6.1.1. Atmospheric inputs. On the global scale, Cl can derive fromatmospheric inputs and halite weathering. We defined Clcycl valuesas the Cl concentrations in river waters deriving from marine at-mospheric inputs. Clcycl values tend to decrease with the distanceto the ocean (Stallard and Edmond, 1981). Little data exists todetermine the Clcycl in the Orinoco basin. A maximum Clcycl valueof 4.6e26 mmol l�1 was determined by Edmond et al. (1995) andMora et al. (2010b) for the Guayana Shield in the Orinoco catch-ment. In the Andean tributaries, the atmospheric inputs are con-sidered to be negligible in comparison with halite weathering. Ifwe consider this large range of values as representative of theOrinoco Clcycl concentration, atmospheric inputs would representbetween 20 and 100% of the Cl concentration in the river. Takinginto account the composition of sea salt waters (Berner and Berner,1987), atmospherics inputs would be negligible in comparison ofother sources in the Orinoco basin. Only Na would be affectedbetween 8 and w50% of the atmospheric inputs. This range ofvalue is large but does not affect significantly the silicate con-tribution for Na, nor the total dissolved load originating from sil-icates as we show below.

4.6.1.2. Silicates inputs: contribution on Na production. Na is con-sidered to be a reference element in order to discriminate the dif-ferent sources of the dissolved load (e.g. Garrels and MacKenzie,1972; Berner et al., 1983; Gaillardet et al., 1999). In river water,dissolved Na is provided by three main sources: atmospheric in-puts, evaporite dissolution (halite) and silicate dissolution.

River Na and Cl correspond to the following contributions(Equations (3) and (4)):


Table

4Mea

nco

ncentrations(m

gl�

1)of

dissolved

cation

s,an

ions,SiO2an

dTD

Sfordifferentstudyperiodsov

erthelast

fortyye

arsfrom

thescientificliterature

andthis

study.

Sample

numbe

rStudyperiod

Ca2

þmgl�

1Mg2

þmgl�

1Naþ

mgl�

1Kþmgl�

1Cl�

mgl�

1SO

2� 4mgl�

1HCO� 3mgl�

1NO� 3mgl�

1SiO2mgl�

1TD

Smgl�

1Referen

ceEq

uationn�

Samplin

glocation

1519

70e19

733.30

1.00

1.50

0.65

2.90

3.40

11.00

11.50

35.25

InMey

beck

(197

6)1

Ciudad

Bolivar

119

792.28

0.64

1.34

0.75

0.80

1.30

10.40

0.10

2.30

19.91

Lewis

and

Weibe

zahn,1

981

2Ciudad

Bolivar

1219

83e19

842.79

0.52

0.90

0.70

0.85

3.09

8.85

Paoliniet

al.,19

873

Ciudad

Bolivar

6219

82e19

852.59

0.66

1.47

0.66

0.86

2.31

9.99

18.54

Lewis

andSa

unders,

1989

4Barrancas

1Dec

2004

3.84

1.15

2.35

0.84

0.70

0.82

0.04

3.19

12.93

Chen

etal.,20

065

Ciudad

Guay

ana

26Fe

b20

04eMay

2006

3.28

0.78

1.26

0.70

8.70

14.72

Moraet

al.,20

096

Ciudad

Guay

ana

5520

05e20

103.22

0.77

1.30

0.79

0.80

2.78

11.26

0.03

7.44

28.40

This

study

7Ciudad

Bolivar

TD

S (m

g.l-1)

(Lewis et al.

et al.

, 1989)

(Paolini ,1987)

(this study)

(cited by

Meybeck, 1979)

(Mora

et al., 2009)

Fig. 9. TDS concentrations (mg l�1) from different studies during the last forty years.The different study periods and legend are indicated in Table 4.



Naþtot ¼ Naþevap þNaþatm þ Naþsil (3)

Cl�tot ¼ Cl�atm þ Cl�evap (4)

With Xtot (X¼ Naþ or Cl�) as the total river concentration, Xevap,the evaporite contribution (halite in this case), Xatm, the atmo-spheric inputs and Naþsil the silicate weathering inputs.

Naþevap¼ Cl�evap (5)

Naþatm ¼ Cl�atm�ðNa=ClÞ sea (6)

with (Na/Cl)sea ¼ 0.86 (Berner and Berner, 1987)

(Na/Cl)sea ¼ sea salt water ratio.

As a sensibility test we propose here to determine the Na silicate(Nasil) inputs following two extreme scenarios:

1) All Cl (and associated Na) comes from evaporites dissolution.2) All Cl (and associated Na) comes from atmospheric inputs.

In the first scenario, Naatm ¼ Clatm ¼ 0. On average during thehydrological cycle, Nasil would represent around 60% of the totaldissolved Na in the river.

Following the second scenario, Clevap ¼ Naevap ¼ 0. Nasil wouldrepresent around 64% of the total dissolved Na exported by theOrinoco River.

It is difficult to quantify precisely the relative contribution of theatmospheric and evaporites inputs for the Na production, butwhatever the respective value, dissolved Na�sil value is not drasti-cally affected. 60e64% of the total Na comes from silicatesweathering. Fig. 10 illustrates qualitatively these results: a largepart of Na is provided from silicate weathering, whatever the Clorigin.

4.6.2. Comparison with global riversIn order to point out the influence of silicates, carbonates and

evaporites on the river chemistry, the data was plotted on Mg/Navs. Ca/Na and HCO3/Na vs. Ca/Na diagrams, using the end-members(characteristic Na ratio for granites, basalts, carbonates and evap-orites) proposed by Gaillardet et al. (1999) as a reference frame-work of possible lithological contribution (Fig. 11). We reported on


Table 5Correlation matrix showing the Pearson coefficients for major ion concentrations, chemical variables and discharge on the Orinoco River at Ciudad Bolívar.

1/Qd T�C EC pH TSS Ca2þ Mg2þ Naþ Kþ Cl� SO2�4 HCO�

3 Si Al DOC TDS

Qd 1 e e e e e e e e e e e e e e e

T�C N.S 1 e e e e e e e e e e e e e e

EC 83% N.S 1 e e e e e e e e e e e e e

pH N.S N.S N.S 1 e e e e e e e e e e e e

TSS N.S N.S N.S N.S 1 e e e e e e e e e e e

Ca2þ 80% N.S 95% 55% N.S 1 e e e e e e e e e e

Mg2þ 72% N.S 92% 60% �30% 98% 1 e e e e e e e e e

Naþ 84% N.S 90% 52% �29% 92% 91% 1 e e e e e e e e

Kþ 50% N.S 52% N.S N.S 53% 50% 61% 1 e e e e e e e

Cl� 77% N.S 86% 56% N.S 77% 75% 80% 70% 1 e e e e e e

SO2�4 67% N.S 75% 54% N.S 72% 67% 59% 33% 63% 1 e e e e e

HCO�3 74% N.S 93% 42% N.S 90% 90% 88% 49% 75% 51% 1 e e e e

Si N.S 43% N.S N.S N.S N.S N.S 27% N.S N.S N.S N.S 1 e e e

Al N.S 34% N.S N.S N.S N.S N.S N.S N.S N.S N.S N.S 74% 1 e e

DOC �44% N.S �41% N.S N.S �46% �45% �47% N.S �32% �28% �46% N.S N.S 1 e

TDS 80% N.S 92% 53% N.S 90% 87% 90% 52% 75% 57% 92% 35% N.S 52% 1

EC ¼ electrical conductivity uS:cm�1 at 25�CQd ¼ daily discharge m3 s�1

concentration units mg 1�1


this graph values without atmospheric correction and values cor-rected to atmospheric input with a Clcycl value ¼ 10 mmol l�1 asreasonable estimation. To correct from atmospheric inputs weapplied the methodology presented by Moquet et al. (2011). Xcycl isthe concentration of the elements X (with X ¼ Ca, Mg, Na, K andSO4) in the river which derived from atmospheric inputs. Todetermine the Xcycl, we applied the following Equation (7):

Xcycl¼ Clcycl � ðX=ClÞ sea�mmoles:1�1

�(7)

The X/Cl sea ratio is calculated from Berner and Berner (1987)data from sea salt water composition.

With X0 the concentration of the element X in the river cor-rected from atmospherics inputs, X0 is calculated following theEquation (8):

X0 ¼ Xriver � Xcycl (8)

Whatever the scenario, it appears that the water of the OrinocoRiver is mainly influenced by the silicate endmember.

It is difficult to precisely show the difference between thesilicate-carbonate and evaporite-carbonate trends in these dia-grams. Nevertheless, certain tendencies may be identified. In theMg/Na vs. Ca/Na diagram, the Orinoco waters appear to be more

Fig. 10. Cl vs Na concentration (mmoles/l). The atmospherics inputs ((Na/Cl)sea) andhalite ((Na/Cl)evap) endmembers are reported as reference. The higher value for at-mospheric inputs was estimated to be 26 mmol/l (Edmond et al., 1996).


influenced by the silicate and carbonate endmembers. However inthe HCO3/Na vs. Ca/Na diagram, the evaporite endmember appearsto influence somewhat more the Orinoco hydrochemistry. Theseresults show that the Orinoco surface water hydrochemistry iscomposed of the three endmembers (silicates, carbonates andevaporites) which are present in the Orinoco basin, especially in theAndes.

4.6.3. Dynamics of the dissolved and solid loadThe concentrations of the main chemical species vary during

the hydrological cycle with a dilution effect during the high waterflow, but this variation is small in comparison with dischargevariation. Also, the concentration values were compared with thetheoretical dilution curves established by Kattan and Probst (1986)(Equation (9)).

Ci ¼ ðQmin�CmaxÞ=Qi (9)

Ci is the model concentration of the chemical species, Qmin is theminimal discharge observed during the studied period, Cmax is themaximal observed concentration of the chemical specie and Qi isthe discharge value observed during the sampling period.

In order to give a numerical value of the dilution comportmentof the dissolved concentrations in relation with the water dis-charge, a numerical value was calculated using the Equation (10)proposed by Meybeck (1979):

C ¼ aQb (10)

in which C is the concentration (mmol l�1), Q is the discharge(m3 s�1) and a and b are regression coefficients.

The b value can be a good first order index to qualify the com-portment of the dissolved elements production during the hydro-logical cycle. When b is near �1 value, the flux of the consideredelement is constant during the year, indicating that the mobi-lization of the source of this element is independent from hydro-logical processes and the hydrological regime, suggesting thatelement fluxes are constant despite variations of precipitations andrunoff. When b is near 0, the element concentrations are constantand the flux is controlled by discharge variations and the produc-tion of the dissolved elements are dependent of runoff variations.All b values coefficients range between �0.4 and 0. This suggeststhat discharge variations during the hydrological cycle controldissolved element fluxes variations within the Orinoco River atCiudad Bolivar.


Fig. 11. Mg/Na vs. Ca/Na and HCO3/Na vs. Ca/Na (in molar units) diagrams for monthly water samples from the Orinoco River. Granite, basalt, carbonates and evaporite world end-members are defined by Gaillardet et al. (1999). Triangles are the gross data without atmospheric correction and small squares correspond to the data with atmospheric correction(with Clcycl ¼ 10 mmol l�1).


In the case of the theoretical dilution curves (Equation (9)), theb value is equal to �1. We used this curve as a visual frame of ref-erence (e.g. Fig. 12).

Ca, Mg, Na, HCO3 and Cl tend to follow a dilution effect but donot follow strictly the theoretical dilution curves. Thismeans that inspite of a dilution effect, a part of the fluxes variation is controlledby the runoff variation (e.g. Ca and K, Fig. 12). All the dissolvedmajor elements present a significant relation following the relationexposed in the Equation (10). According to the b coefficient, theelements can be divided into two groups. Elements with b valuescomprised between �0.4 and �0.24 are Ca, Mg, Na, HCO3, Cl andSO4; and elements with b values of about 0 are K and Si. In contrastto the elements within these two groups, DOC, Al and Fe do notfollow this kind of relation (Equation (10)).

The difference between Si and K with the other elements ofgroup one has also been found in the Apure River by Saunders andLewis (1989), Lewis and Saunders (1989) and Mora et al. (2010a).Mora et al. (2010a) explain that floodplains of the Apure River(Fig. 1) can act as a major source of Si and K produced through theweathering of clays within floodplain sediments (production of Kand Si) and the decomposition of organic matter (vegetation)during the rising stage and high water phases (production of K).Furthermore, if the dissolved SiO2 in the Orinoco River is generatedmainly from the weathering of silicates, the concentrations of SiO2should have been highest during low water. However, the seasonalpattern of SiO2 concentrations depicted in Fig. 4 shows an unclearbehavior. This fact can be due to two important processes, whichhave been reported in the Apure River by Lewis and Saunders(1989). First, the weathering of clays in the floodplain during high

Fig. 12. Ca2þ (a) and Kþ (b) concentrations vs dai


water can increase SiO2 concentrations during this period. Second,given that primary production in the Orinoco River reaches itshighest values during low water (Lewis, 1988), the increasing ofdiatom growth can deplete soluble Si in river waters during thisperiod, because the diatoms can uptake dissolved Si for the for-mation of their exoskeletons, reducing the SiO2 concentrationsduring this period.

The distinction between (Ca, Mg, Na, HCO3, Cl and SO4) and (Kand Si) groups can be explained through differences in geographicorigin of the elements or through difference in the origin of watersources (i.e. hydrological reservoir). Following the geographicalhypothesis, the elements of the first group come from the Andeanpart of the basin and are diluted by the plain and Shield rivers.The second group is produced by the whole basin following anhomogenous process, consequently there is not a dilution effectduring the hydrological cycle. Following the hydrological reservoirhypothesis, the first group would be most influenced by a constantproduction reservoir than the elements of the second group. Thesetwo hypotheses are not opposed and can be associated. Indeed, thegeographical origin of dissolved element can be linked to charac-teristic hydrological dynamics of these geographical regions.

Whatever the group considered, the dissolved load flux varia-tion during the hydrological cycle is mainly controlled by thehydrological variation. This observation signifies that the dilutioneffect is low in comparison to the action of mobilization of therunoff. Consequently the hydrological regime controls the sea-sonal dissolved load production whatever the geographical originand the source of the dissolved load within the Orinoco Rivercatchment.

ly discharges and theoretical dilution curves.



During the hydrological cycle of the Orinoco River, the presenceof hysteresis between discharge and TSS and the two peaks of TSS(Fig. 3) concentrations invalidates the use of relations (TSS ¼ f(Q))to calculate sediment discharge. Precedent authors (Meade et al.,1983, 1990; Weibezahn, 1990; Warne et al., 2002) have alreadycommented the sedimentological cycles of the Orinoco River. Thefirst TSS peak during the rising stage can be explained by theremobilization of TSS in the river channel and rill and sheet erosionon hill slopes during the first rain events.

The second TSS peak during the falling stage can be explained bythe backwater effect that the Orinoco River exerts over the mainAndean tributaries during the flood peak, causing the deposition oftheir sediment loads. As the water level starts to drop again in therecession part of the Orinoco hydrograph, the energy slope andflow velocity of the tributaries are increased and the previousdeposited sediment are suspended again and transported to themain flow of the Orinoco, thus producing the second peak that isobserved in the chart.

The TSS concentration decreases during the high flow waterperiod due to a dilution effect of the runoff during the end of therainy season. Also, TSS did not respond to the kind of logarithmicrelation presented in Equation (10).

These observations clearly show that the TDS load produced bychemical weathering and the TSS load produced by mechanicalerosion are not directly related during the hydrological cycle (Figs. 3and 5).

4.6.4. Assessment for the origin of the TDSOn the basis of these three approaches (correlation between

dissolved elements, identification of the potential sources anddescription of the C vs Q relation), we confirm the observation ofnumerous authors (e.g. Edmond et al., 1996; Lewis and Saunders,1989; Saunders and Lewis, 1989; Mora et al., 2010a, b):

- Ca, Mg, Na, Cl SO4 and HCO3 are mainly produced by Andeanpart by weathering processes (silicates, carbonates andevaporites).

- K, Si and DOC are mainly produced by floodplains of the Ori-noco and Los Llanos and the Guayana Shield. In these regions,the weathering of clays and the vegetation can act as a signifi-cant or dominant source of Si and K, respectively. Consideringthat, at the catchment scale, the uptake of K by plants iscommensurate with the K release through plant degradation,silicate weathering is the dominant source of K of the OrinocoRiver.

This study shows that, whatever the origin of the dissolved load,the dissolved flux variations during the hydrological cycle aremainly controlled by the hydroclimatology of the Orinoco River.

5. Conclusion

The comparison of the present datawith data from the literaturepermitted to analyze temporal tendencies over the last thirty toforty years in the Orinoco River. Total dissolved elements concen-trations and total dissolved solid fluxes did not show any significantdifferences over the last decades. The low differences observedbetween the current (this study) and earlier (literature) TDS dataindicates rather little changes of hydroclimatic characteristics orhuman impacts within the Orinoco catchment. However, it is notexcluded that these differences may to a certain extent be due toinconsistencies with the sampling protocols, the sampling sites, aswell as the laboratory analyses.

The mean interannual concentrations of matter transiting at thehydrological station at Ciudad Bolivar totalize 108 mg l�1, which is


the sum of respectively 74, 30, and 4 mg l�1 of TSS, TDS and DOCconcentrations, respectively. Expressed in annual fluxes, TSS, TDSand DOC represent respectively 68.3, 27.8 and 3.9% of a meanannual flux of 108 � 106 t yr�1 of matter exported to the AtlanticOcean by the Orinoco River.

As expected, dissolved element concentrations, with theexception of DOC, vary inversely to discharges during the hydro-logical cycle, i.e. an increase of dissolved element concentrationsduring low waters and a decrease during floods. However, allflowcharts of dissolved and solid matter are parallel to the hydro-graph since the former are controlled by the important variations ofdischarge dominating the lower variations of dissolved and solidmatter concentrations. Thus, the variation of matter fluxes duringthe hydrological cycle is mainly controlled by the hydroclimaticvariability.

The Orinoco surface waters are four times less mineralized thanthe world’s rivers in average. Ca2þ, Mg2þ, Naþ, Cl�, HCO�

3 areexported from the Orinoco basin according to similar dynamics.This suggests a same geographical origin for all these elements, theAndes. Kþ and SiO2 are produced in a different way and depend onanother geographic origin, the Llanos and the Guayana Shield.

Acknowledgments

This study is part of two bi-national cooperation projects be-tween Venezuela and France. The ORE/HYBAM project (www.ore-hybam.org) and the Orinoco Corridor Project (CIEG-UNEG)financed the collection of sediment and geochemical data withinthe Orinoco river. The ECOS/NORD/Venezuela (V07U02) inves-tigated hydro-sedimentary dynamics and exchanges of the lowerOrinoco river between the main channel and its floodplain and itsimplications for the functioning and the biodiversity of this tropicalecosystem. We wish to thank all Venezuelan and French partici-pants of these two projects who contributed to this study.

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