Journal of Hydrology, 135 (1992) 237-257 237 Elsevier Science Publishers B.V., Amsterdam

[21 Dissolved major elements exported by the Congo and the Ubangi rivers during the period 1987-1989

Jean-Luc Probst a, Renard-Roger NKounkou ~', G6rard Krempp ~, Jean-Pierre Bricquet b, Jean-Pierre Thi6baux c and Jean-Claude Olivry d ~Centre de G~ochimie de ia Surface (CNRS), I. rue Blessig, 67084 Strasbourg Cedex, France bCentre ORSTOM, B.P. 181, , Congo ~Centre ORSTOM. B.P. 893, . Central dfrican Republic OCentre ORSTOM, B.P. 2528. Bamako. Mali (Received 20 September 1991; accepted 6 October 1991)

ABSTRACT

Probst, J.-L., NKounkou, R.-R., Krempp, G., Bricquet, J.-P., Thi6baux, J.-P. and Olivry, J.-C., 1992. Dissolved major elements exported by the Congo and the Ubangi rivers during the period 1987-1989. J. Hydrol., 135: 237-257.

On the basis of monthly sampling during the period 1987-1989, the geochemis:ry of ~he Congo and the Ubangi (second largest tributary of the Congo) rivers was studied in order (I) to understand the seasonal variations of the physico-chemical parameters of the waters and (2) to estimate the annual dissolved fluxes exported by the two rivers. The results presented here correspond to the first three years of measurements carried out for a scientific programme (Interdisciplinary Research Programme on Geodynamics of Peri- Atlantic lntertropical Environments, Operation 'Large River Basins' (PIRAT-GBF) undertaken jointly by lnstitut National des Sciences de l'Univers (INSU) and Institut Fran~ais de Recherche Scientifique pour ie D6veloppement en Coop6ration (ORSTOM)) planned to run for at least ten years. The is more dPuted than the Ubangi (34mgl -I vs. 42mgl-I). For both rivers, the inorganic dissolved load is composed mainly of HCOf and SiO,. The chemical composition of the water does not change with time. in the Ubangi River, because of the presence of Precambrian carbonate rocks in its catchment, the proportions of HCOf and Ca2÷ are higher. On a seasonal scale, the concentration of dissolved cations and anions varies inversely with discharge, except silica. The comparison of the discharge-concentration relationship with a theoretical "zero dilution' shows that the evolution of the concentration of dissolved substances is a simple dilution by the surface waters, with, in the case of the Ubangi, a small supply of dissolved substances by the surface waters. Using three different methods of calculation, the estimated annual inorganic dissolved flux of the Congo ranges from 39 x 106 to 44 × l0t` tons (according to lhe year), with about 10% of this coming from the Ubangi drainage basin.

Correspondence to: J.-L. Probst, Centre de G~ochimie de la Surface (CNRS), 1, rue Blessig, 67084 Strasbourg Cedex, France.

0022-1694/92/$05.00 © 1992 -- Elsevier Science Publishers B.V. All rights reserved 238 J.-L. PROBST ET AL.

INTRODUCTION Until recently, the geochemistry of the Congo River has been poorly documented. Most of the data on dissolved solids quoted by different authors are based on one or few measurements, not taking into account seasonal variations. Clerfayt (1955) was probably the first to sample the Congo River water in January 1949 and to measure its chemical composition. Since then, some other data have been published by various authors (Symoens, 1968; Meybeck, 1978; Figu~res et al., 1978; Van Bennekom et al., 1978; Molinier, 1979). A longer series of measurements was carried out by Deronde and Symoens (1980) from December 1976 to November 1977 and allowed them to calculate an annual flux of dissolved solids of about 35.5 x 106tons. NKounkou and Probst (1987) have reviewed the available data on the Congo basin in a synthesis presenting hydrological features, dissolved and suspended materials transported by rivers, the carbon cycle, rock weathering and the erosion balance of the Congo basin. Since 1987, a scientific programme (Interdisciplinary Research Programme on Geodynamics of Peri-Atlantic Intertropical Environments, Operation 'Large River Basins' (PIRAT-GBF) undertaken jointly by Institut National des Sciences de l'Univers (INSU) and Institut Fran~ais de Recherche Scientifique pour le D6veloppement en Cooperation (ORSTOM)), has been carried out in the Congo River and its second largest tributary, the Ubangi River. The main scientific objective of this programme is to determine the interannual fluctuations of dissolved and suspended material carried by the river into the ocean, in relation to the hydroclimatic fluctuations (Olivry et al., 1988; NKounkou, 1989; Bricquet, 1990; Probst, 1990a,b: Giresse et al., 1990; Jouanneau et al., 1990; Moukolo et al., 1990; NKounkou et al., 1990; Mariotti et al., 1991). The project also aims to estimate the interannual fluxes of atmospheric CO2 consumed by rock weathering, which are drained into the ocean in bicarbonate form or in all other dissolved and particulate carbon forms (NKounkou and Probst, 1987; Probst, 1990b; Probst et al., 1992). The purpose of this paper is to present the initial results obtained concerning the water chemistry and the dissolved mineral fluxes exported by the Congo and the Ubangi rivers during the first three years (1987-1989) of the PIRAT programme.

SAMPLING PROCEDURES AND ANALYTICAL TECHNIQUES

River water samples were collected at Bangui for the Ubangi and 40 km upstream from Brazzaville for the Congo (Fig. 1). At Brazzaville, the drainage area of the Congo is 3.475 x 106knl2 (the total drainage area of the Congo is 3.073 x 106 km 2), with an interannual water discharge of 41 000 m 3s- ~. The DISSOLVED MAJOR ELEMENT EXPORT: CONGO AND UBANGI RIVERS 239

, i | u 15"' IZ 210° 25 ° liT' tllg ,°-1

. 5 ° .

0 o 0 °

., ~,)

13l't.A 7+7_J, V I l .L I". KINSIIASA

/

..... )~ Io °

H

0 o * Sampling station 15 °

E1u % mR 0 5GO km

Scale Limits of the Ubang| basin

20 ° E 250 30 ° E mn ,,,, ! t

Fig. 1. General map of the Congo and Ubangi river basins. 240 J.-L. PROBST ET AL. drainage area and the water discharge of the Ubangi river at Bangui are 0.5 x 106km2 and 4300m3s -~ respectively. Water samples were collected monthly by the ORSTOM teams, taken from the river surface with a 101 tin, decanted into 200ml polyethylene bottles and then dispatched to the laboratory for analysis. Filtration through a 0.45/~m millipore filter was made before analysis and the following techniques were used to analyse the different physico-chemical parameters (Krempp, 1988). (1) pH was measured only in the laboratory with the Mettler DL 40 RC, with an accuracy of + 0.02 pH units. This equipment was also used for alkalinity (HCOF) measurement, by titration with HCI, with an accuracy of 0.001 mmol l -~ . (2) Water conductivity was measured with the Hanna HI 8633 conduc- timeter and expressed at 20°C. (3) The colorimetric method was used for the analysis of NH~ and H4SiO4 on an automatic Technicon colorimeter. The method is based on the photo- spectroscopy of the NH~-sodium salicylate-chloride and H4SiO4-ammonium molybdate complexes. The sensitivity of the equipment is 0.001 mmol 1- (4) The major cations (Ca 2~, Mg 2+, Na + and K + ) were analysed by atomic absorption, with an air-C2H2 gaseous mixture. La was added (0.5%) to the sample for Ca 2+ and Mg 2+ analysis, and Cs20 (0.2%) for K +. The measure- ment is made with an accuracy of 0.001 mmol 1-~. (5) For the major anions (CI-, SO42- and NO;), liquid chromatography was used on a Dionex chromatograph. The detection limit is 0.001 mmoll -~.

WATER CHEMICAL COMPOSITION

In spite of its high discharge, the Congo River is relatively diluted, if compared, for example, to the Amazon River (Stallard and Edmond, 1983). The total inorganic dissolved solids load averaged 34 mg 1-I for the period 1987-1989 (Table 1). The previous study of Deronde and Symoens (1980) based on monthly sampling during a full year gave a lower value of 28 mg 1- t. The Ubangi waters are more concentrated than the Congo, with an average dissolved load of 42 mg 1-t The ionic composition of the Congo and Ubangi waters is shown in Fig. 2. For both rivers, Ca 2+ and Mg 2+ are the dominant cations whereas HCOf represents the dominant anion. The Ubangi shows a greater proportion of Ca 2+ and HCO3. This can be related to a greater dissolution of calcium carbonate in its basin, in which the geological substratum is mainly composed of crystalline and metamorphic rocks which have been weathered in a thick ferricrete mantle. Furthermore, the presence of paleocryptokarsts in the Precambrian carbonate formations has been pointed out by Boulvert and TABLE I

Mean annual values of the physico-chemical parameters for the Congo and the Ubangi rivers

2+ Rivers Year Qa pH Cond Si0 2 NUt Na+ K+ Ca Mg2+ HCO; SO~-NO; Cl- TDS

Congo 1987 38708 6.61 36.15 11.19 0.12 2.08 1.54 2.40 1.38 12.64 1.44 1.59 1.63 36.05 1988 39116 6.92 31.24 9.67 0.01 1.86 1.26 2.31 1.33 12.97 1.07 0.12 1.34 31.97 1989 37741 7.11 31.61 10.21 0.00 2.03 1.40 2.40 1.45 14.70 1.01 0.07 1.23 34.54 Mean 38522 6.87 33.00 10.36 0.04 1.99 1.40 2.37 1.38 13.43 1.17 0.59 1.40 34.18

Ubangi" 1988 3152 7.22 39.02 13.21 0.00 2.57 1.78 3.23 1.42 19.47 0.80 0.66 0.86 44.02 1989 2528 7.36 36.94 13.18 0.00 1.58 1.38 3.38 1.47 18.58 0.78 1.27 0.76 42.41 Mean 2840 7.29 37.99 13.19 0.00 2.08 1.58 3.30 1.44 19.03 0.79 0.96 0.81 43.22 a For the Ubangi, the samples of September 1988 and March 1989 are lacking. I Qa = mean annual water discharge (m's "); Cond = conductivity at 20°C (pScm- ); TDS = total inorganic dissolved solids (mg l"); SiOz• cations and anions (mg l"). 242 J.-L PROBST ET AL.

a - Cations b - Anions

Ha'. K" HC0i on0o /,\ °oooo,

Ca** 50 Mg+* 50 Cl-

Fig. 2. Ternary diagrams t-epresenting the ionic chemical composition of the Congo and the Ubangi waters. The hatched and darkened zones represent the domains of monthly variations of (a) cationic and (b) anionic composition.

Salomon (1988), which allows an explanation of the relatively high proportion of Ca 2÷ and HCO; in the Ubangi waters, particularly during the low water period. Considering the variations of ionic composition during the study period, one can observe for both rivers that the cation composition is relatively stable whereas the proportion of each anion varies remarkably for the Congo (Fig. 2). Taking all ions and silica together, the discharge-weighted mean of the chemical composition expressed as weight percentages of the total inorganic dissolved load is represented in Fig. 3. Two-thirds or more of the dissolved load of the two rivers is composed of HCO.~ and SiO.,. The water composition does not change from one year to another. There is also no change in dissolved

a - Congo river b - l.lbangi river

~ Na"

D Mg=* 4% Q Ca=. 5% B SiO= 2% [] HCO=" 2% ~ NO3" 2% ~ CI" 0 SO-="

39% 44%

Fig. 3. Chemical compositions of (a) the Congo (mean values of the years 1987-1989) and (b) the Ubangi (mean values of the years 1988-1989), expressed as weigh / percentages of the total inorganic dissolved solids. DISSOLVED MAJOR ELEMENT EXPORT: CONGO AND UBANGI RIVERS 243 solids concentratien. For the Ubangi River, the chemical composition includes only monthly samples for the years 1988 and 1989 because of lack of data in 1987. However, the water compositions of the two rivers are not significantly different (except for HCO3 ~ and Ca 2+ ). The analytical results show an imbalance between cationic (Z ~ ) and anionic (Z-) charges. The ionic balance (Bi, in per cent) is calculated by Si= and shows a deficit of anionic charges. The B~ averages 22% for the Congo and 9.7% for the Ubangi. These missing anionic charges can be attributed to the presence of organic anions to which our interest will be devoted in the future. Indeed, the dissolved organic matter represents 30-40% of the total dissolved solids of the Congo water.

TEMPORAL VARIATIONS OF WATER DISCHARGF AND PHYSICO-CHEMICAL PARAMETERS

Variations of water discharge

The variations of the Congo discharge on seasonal and secular scales (1902-1983) have been considered by some authors, including Molinier and MBemba (1979), NKounkou and Probst (1987), Probst and Tardy (1987), NKounkou (1989) and Martins and Probst (1991). The general trend for the Congo discharge fluctuations is a significant increase during this century (Pearson's correlation coefficient between annual discharge and time, r = 0.27, whereas the Ubangi discharge variations show a decrease which is not significant during the same period (r = - 0.12). The humidification of the Congo basin noted in its discharge increase duririg this century has probably affected only areas located south of the equator, whereas those located north of the equator, mainly the Ubangi region, were drying out. On a seasonal scale the Congo annual hydrograph shows two peaks resulting from the mixing of equatorial and tropical hydrological regimes. The main peak (in December) occurs two months after the unique peak (in October) of the Ubangi (see thin curves on Figs. 4 and 5).

Variations of pH, conductivity, total dissolved solids ( TDS) and silwa concentrations

Considering the pH and SiO2 variation curves for the Congo River (Fig. 4), one can observe no significant evolution with respect to discharge variations. However, the pH curve (laboratory measurement) shows a significant increase 244 J.-L. PROBST ET AL,

'~4 'i I'ICO3" 7.4.,H h ^,.,_ I 3.0 N.+ " " . 2oi AA /~

7.0 2.0 12, 1.0 8 35 0.5j 4 ~ ~..-~,; '25

° [ iY. °'' o .~. ,,$,,,,j8, 0.4 0.4...... 2~ 4 "ll" ..... J IIII J Ii II a JIM J II II J MM J II N JIIi J | II J MM J S N J IIII J |W

-- eol ~s I 3.o 1 Ic~+ A 3.s l cr -tAA A t ,. / . 3.0 2.0 2.2 O 20 ~Ti.i.i.i.i.~i.i I i.e ,villivIjllilij, ' so, _co,t,,cti,i~ . 2.1. MI2+ ..~

(~ 1 .§ u VI~IV VV V1 1.2 21 4535

"i'i'i'i'i'i'i'i'i'i'i'i'i'i'iTi l, • i i m i i, a o. i i. a i m 1987 1988 1989 1987 1986 1909 1987 1988 1989

Fig, 4, Monthly variations of discharge (Ihin curves) and physico-chemical parameters (thick curves) for the Congo River during the period 1987-1989. with time (r = 0.65; significance level, S = 0.01%; number of observations, N = 36) during the study period. The reasons for this increase still have to be determined. For the Ubangi (Fig. 5), the pH (laboratory measurement) also shows an increase with time (r = 0.61; S = 0.07%; N = 26) and varies inversely with water discharge, whereas the variations of dissolved silica concentration follow that of the discharge, but with a time lag of about three months. For both rivers, water conductivity and the total dissolved solids (TDS) vary inversely with discharge, that is to say when discharge increases, TDS and conductivity decrease and vice versa. This phenomenon, common to most of the world rivers, expresses a dilution ce *he river waters by rain and surface waters and consequently a decrease ol" conductivity. So, conductivity (Cond) is a linear function of TDS (TDS = SiO2 + cations + anions) and DISSOLVED MAJOR ELEMENTEXPORT: CONGO AND UBANGI RIVERS 245

381 .9 7.8 p]B[ 4 .6 7.4 -3 pH 7.0: 2 -0 8.6 t'i'i'i'iTi'i'i'i'i"...... i'i 1 i~ ~'i'i'i'i'i'i'i'i'i'i'i "i';'" 141 sto2 ~ A , zs. K+ j 1.81 -.4 1.4 m "6 =0 12 " 1.8. 1"0 1 "3 O z 11 i. 1.4: 0.6

...... 0.2 Lo ~' -- 1.0 Slid liJ |ll JIIHJS ^. 9 -1"

m

3.5 °.._ ...... 0,4(i. i.k.i-i, i-i.i.k-;,.i.i, i

,6 08 ~ J 40: 1.8 , 3 1.4 0 30 i*i'J'i Yi'i'"i'i'iTi' 1'0 ~"*' Yi'i" F ;'k" Fi'i'i Ti I JEliJIII l|lJlild 1887 1980 1989 1087 198B 1989 1987 1998 1989

Fig. 5. Monthly variations of discharge (thin curves) and physico-chemica! parameters (thick curves) for the Ubangi River during the period 1987-1989. can be expressed by the following equations: for the Congo River

TDS (mgl- ~) = 1.02 x Cond (#S cm- ~ at 20°C) (2)

(r = 0.92; S = 0.01%; N = 36) and for the Ubangi River

TDS (mgl-~) = 1.08 x Cond (/~S cm- t at 20°C) (3)

(r = 0.94; S = 0.01%; N = 26)

These equations can be useful to determine approximately in situ, the concentration of the total mineral dissolved solids by measuring water conductivity. 24,6 J.-L. PROBST ET AL.

Variations of cation and anion concentrations

For both rivers, the evolution of the concentration of major cations (Ca 2+, Mg 2+, Na +, K +) during the study period appears clearly to be cyclic, following the hydrological cycle. This is contrary to Moukolo et al. (1990), who concluded in their study that during the period 1987-1988 there was no relationship between dissolved element concentrations and discharge. The concentration is at a minimum during high water discharge, increases during falling river stage and reaches the maximum value at low discharge. For the Congo, the lower concentration often occurs one month after the peak discharge of December. This behaviour expresses a simple dilution by rain waters of the cations released by rock weathering, i.e. no other process seems to affect their concentration variation. The dilution of concentration at high discharge is also observed for the major anions (HCOf, Cl-, SO42- and NO~-), especially for HCOj- and CI-. Note that NO3 concentration often equals zero and indicates values below the detection limit.

RELATIONSHIPS BETWEEN DISCHARGE AND CONCENTRATION

The dilution of dissolved substances at high water discharge can be understood by considering the relationship between water discharge and the concentration of dissolved substances. This allows the determination of the type of dilution and the calculation of the mathematical expressions of these relationships. The theoretical approach to understanding these relationships has been investigated by Hall (1970, 1971), who proposed various models to explain the evolution of the concentration of dissolved solids with respect to water discharge and river input of elements. Represented on Figs. 6 and 7 are plots of relationships for the parameters which show the best correlation, i.e. TDS, Cond, Na +, K +, Ca 2+, Mg2+, HCOj-, SO~- and CI-. In general, it appears that for the Ubangi, the points are less scattered than those for the Congo, e'.,en if some data are lacking for the Ubangi. This shows that the law which links river discharge and dissolved matter concentration is simple. For the Congo, one can observe a little scattering of the points, which indicates a more complex feature of the relationships. Indeed, the Congo water at Brazzaville results from the mixture of waters coming from three different hydrological regions: the equatorial region, and the north (Ubangi) and south tropical regions. When the north tropical region discharges little concentrated water, the south tropical region supplies a high discharge of diluted water. This explains the disturbance of the discharge--concentration relationship observed at Brazzaville. It also appears for both rivers that the scattering of points is less marked for the cations than DISSOLVED MAJOR ELEMENT EXPORT: CONGO AND UBANG! RIVERS 247

3.0| ',~ K+ \~ HCO3" (~ 19 87 2 51 ~,,~*~ 225: ~o',,0, ® ~9o8 2.0]• / •* "~J~.-~~_ • 17.5 ~®~ 01989

7.5.

252~ ~0 38 ~:5 54 62 7025:22 3~o3~0 3"8 i6 5"4 -(~2 7"0 ® Na + 2.75 s~- 2.0. ® ® 2.25. ~

t.5 1.75- ®® 1.25. E 1.0 0.75- ® ®0 Z O 0,5' 0.25. 0 70 m 22 3o :3e 4s 54 62 70 22 3o 35 46 54 62 I-, 3.5. < ® el"

I,- 3.75- ® c# + 2.5 Z ® UJ 2.75 U ="*~-k* - Z 1.5 1.75 O

0.75 ,___.__.,__.___~___ . -. 0.5 @ 22 30 38 45 54 ~2 70 22 3.0 3'~ 46 i, 62 7o 2.1 60 TDS ® 52' 1.7 44' 1.3 36"

--~,7"- 28' ~"- ~ O-'O m..._ 0.9 20"

.5 ~ . , 22 30 35 ,=6 54. 52 7.0 12 22 3"0 35 46 5~ i2 70 WATER D I S C H A R G E (103 m3.s "1)

Fig. 6. Congo River. RelationsMps between discharge (Q,) and concentration (C,) of dissolved substances. The continuous curves represent the relation C~ = f(Qi); the dotted curves are the theoretical dilutiot, 248 J.-L. PROBST ET AL.

HCO3" O 1987 3 K + 36 ¸ o ~® O ) 1988 • ® 2 24 ® @------® @ @ '~O0 @0'3 ;®'" 12 \ \ \ \ %,

0 0 0 " 3 S 9 0 3 6 5 Na + 2.0 SO4 2- ¢l . O ! 1.6 ® m ® , ~o o E '•'• • 1.2 z 2 ~\ ~ ~'~'--®'--$®-s-e----.-_ 0.8 O \\ 0 @~ @ @ m 1 \ 0.4 % < 0 0 0 S 9 o I- O 9 Z Ca:Z+ 2.5 Cl" LM

Z ~e~¢~o~ 2.0 4 @ 0 " 1.5 \ ® " ~ ~~ 1,0 \ \ ®® ~ -O" \ 0.5

0 o --~ 3 S "9 o 3 6 9 Mg 2+

\ \ 0 \ 20 \\

0 0 3 S 9 o 3 6 i W A T E R D ! S C H A R G E (103 m3.s -1) Fig, 7, Ubangi River. Relationships between discharge (Qi) and concentration (C~) of dissolved substances, The continuous curves represent the relation Ci = f(Qi); the dotted curves a~e the theoretical dilution. DISSOLVED MAJOR ELEMENT EXPORT: CONGO AND UBANG1 RIVERS 249

TABLE 2

Mathematical expressions adjusted to the relationships between instantaneous discharge (Qi, 103m 3s-') and concentration (C~, mgl -~) of the dissolved substances

Rivers Species Equations r N S(%) Excluded months

Congo Na + Ci = 50.24 Qi-°'g87 -0.70 35 0.01 Jan. 87 K ÷ C~ = -0.018 Qi + 2.15 -0.76 34 0o01 Sep. 87, Jan. 88 Ca 2+ Ci =-- 50.09 Qi-I + 1.07 0.79 36 0.01 - Mg 2+ Ci = 43 Qi-i + 0.27 0.87 36 0.01 - HCOf C~ = 555.01 Qi--1"016 -0.87 35 0.01 Jan. 87 SO 2- Ci = 83.51 Qi--1"178 -0.69 36 0.01 - CI- C~ = -0.025 Qi + 2.40 -0.61 35 0.01 Apr. 88 TDS Ci = 359.24 Q-0.u3 -0.79 36 0.01 -

Ubangi a Na ÷ Ci = -0.348 lnQi + 2.08 -0,93 25 0.01 Oct. 88 K ÷ C~ = -0.209 InQ~ + i.64 -0.50 25 1.03 Oct. 88 Ca 2+ Ca = 4.397 Qi--°2s3 -0.93 26 0.01 - Mg 2+ Ca = 2 Qi-°'29° -0.94 26 0.01 - HCOj- Ca = 25.53 Qi--°'3°9 -0.95 25 0.01 Oct. 88 SO~- Ci = -0.169 InQ~ + i.05 -0.43 26 2.53 - CI- C~ = 0.428 Q~ + 0.66 0.87 26 0.01 - TDS Ci = 51.11 Q-O.185 -0.95 24 0.01 Aug, Oct. 88

For the Ubangi, the samples of September 1988 and March 1989 are lacking. r = Pearson's correlation coefficient; N = number of observations; S = significance level.

for the anions. On the plots, one can sometimes see that there are one or two points far from the others. Therefore these outliers were not taken into account in the regression procedure. This applies to Na +, K +, HCO~- in both Congo and Ubangi, Cl- for the Congo, and TDS for the Ubangi. These excluded points are listed in Table 2. The mathematical expressions of these relationships are graphically rep- resented by the thick curves on the plots. Various functions were fitted (Table 2) and the best Pearson's correlation coefficients (r) were found for the following relations: Ci = a " Q~-I/. (4) Ci = a'Qi -I/" + b (5) Ci = -a'lnQi + b (6) where Ci is concentration in mg 1- ' ; Q~ is water discharge in 10 3m 3s- I; and a, b and n are constants. These relationships are also used to calculate the annual dissolved fluxes exported by each river (next section). Many authors fitted similar equations to the relation C~ = f(Q,) f:_,, 250 J.-L. PROBST ET AL. dissolved material (Ledbetter and Gloyna, 1964; Hart et al., 1964; Ineson and Downing, 1964; Gunnerson, 1967; Steele, 1968; Probst, 1983; Etchanchu, 1988; Kattan, 1989; Orange. 1990). This theoretical approach is based on the following considerations. (1) The total discharge (Qt) of the river results from the sum of the discharges of the various sources. As a simplification, these sources are surface water (Qs) and ground water (Qg): Qt = Qs + Qg (7) (2) There is mass conservation: Ct'Q, = Cs'Qs + C~.Qg (8) where Ct, Cs and C~ ;~.~'e respectively the concentration in river water, surface water and ground water. One can consider now the dilution of the dissolved solids. At low water discharge (Q~ = Qm~,), the concentration of the river water is maximum (C~ = Cmax) and the dissolved flux is

Fd = Cmax" Qrain (9) If one dilutes this flux by an increasing river discharge (Qi) with an assumed null concentration, the resulting evolution of the concentration (Cth) in river water can be expressed as Cth = (Cm~x" Qmin)/Qi (10) This theoretical dilution or 'dilution zero' is represented by the dotted curves on the plots (Figs. 6 and 7), first proposed by Kattan and Probst (1986). The comparison between the two curves (observed and theoretical) shows that for the Congo, the evolution of the concentrations follows more or less the theoretical dilution for most elements, even if the clusters of points are relatively scattered owing to the contributions of the different hydrological regions. On the contrary, for the Ubangi, the concentrations measured deviate markedly from the theoretical dilution when the discharge increases. However, when the discharge is higher than about 3000 m 3 s-~, the two curves tend to be parallel. Except for sulphate, chloride and potassium concen- trations, the clusters of points are less scattered for the Ubangi than for the Congo. These two different behaviours mean that for the Congo, the river concentration is simply diluted by surface waters which supply no significant amount of dissolved substances, and this dilution is only disturbed by the contribution inputs of the different drainage areas. In contrast, for the Ubangi, the surface waters supply significant inputs of dissolved substances to the river, even if its concentration decreases. However, the discharge DISSOLVED MAJOR ELEMENT EXPORT: CONGO AND UBANG1 RIVERS 251 variations are so important that, when the discharge increases, only dilution affects the river concentr.ation behaviour.

ANNUAL DISSOLVED RIVER FLUXES

According to the literature, the export of dissolved minerals by the Congo River varies between 35 x 106 and 50 x 106 tons year-~: 37 × 106 tons year -I (Livi:gstone, 1963); 46.5 x 106 tons year -I (Symoens, 1968); 35.4 x 106 tons year -~ (Deronde and Symoens, 1980); 36.6 × 106 tons year -t (NKounkou and Probst, 1987); 72 x 106 tons year -i (including organic matter, Olivry et al., 1988); and 50 x 106 tons year-~ (NKounkou, 1989). The variability of the above estimations can be explained by the temporal and spatial representativeness of the measurements used for flux calculation (point or long-term measurements), and by the analytical tech- niques used to measure the dissolved elements. The variability of the estimations is due also to the variability of the annual discharge and, lastly, to the method used in the calculation of the annual flux. The dissolved fluxes presented below are based on monthly measurements during the years 1987, 1988 and 1989. Because of lack of data, annual fluxes for the Ubangi were calculated only for the years 1988 and 1989. Three methods were used to calculate the annual fluxes: two stochastic methods (no. 1 and no. 2) and one deterministic method (no. 3).

Method no. 1

In this method, the annual flux (E, in tons) is obtained by multiplying the annual discharge-weighted mean of the concentration by the mean annual water discharge, as follows: Fa = (C~'Q,) ]#, (Qi) } "Q.'k. (11) where C~ and Q~ are respectively the instantaneous concentration (rag 1-~ ) and water discharge (m3s -~) measured each month, Q, is the mean annual discharge (m 3 s -j ) and ka is a time correction factor (ka = 31.536).

Method no. 2

The annual dissolved flux results from an ex)rapoiation of the mean instan- taneous flux to the entire year:

LLi= I where Fa, C~, Qi and ka are as in method no. 1. 252 J.-L. PROBST ET AL.

Method no. 3

In this method, one first calculates theoretical mean monthly concentrations of dissolved solids using the equations Ci = f(Oi) calculated in Table 2. In these equations, one substitutes Qi (the instantaneous water discharge) by the mean monthly discharge (Ore) to obtain ltheoretical mean monthly concen- trations (Cm) of dissolved substances which permit calculation of the annual flux (Fd) as follows: 12 Fa = ~ (Cm" am" kin) (13) i=1 where Fd is the annual flux (tons), Qm is the mean monthly water discharge (m 3s- ~), Cm is the mean monthly theoretical concentration (mgl- ~, Cm = f(Qm)) and k m is a t:.me correction factor (kin = 2.628).

TABLE 3

Annual dissolved fluxes (106 tons calculated for the Congo and the Ubangi rivers using t.hree different methods

Rivers Year Method SiO2 NH2 Na + K + Ca z+ Mg 2+ HCOf SO~- NOr CI- TDS

Congo 1987 I 13.66 0.15 2,53 1,88 2,93 i,68 15.43 1.76 1.95 1,99 44.01 1987 2 13,67 0.1~ 2,54 !.88 2.93 i,68 15.44 !.76 1.95 2.00 44.03 1987 3 - 2.38 1,73 2,8~ 1,68 16.51 1.3'7 - 1.69 4i.38 1988 I 11.63 0,01 2.29 1,56 2.85 1,64 16,00 1.32 0,14 1.65 39.44 1988 2 11,92 0,01 2.29 1,56 2,85 1.64 15.99 1,32 0,14 1,65 39,41 1988 3 - - 2.39 1.73 2,89 1,68 16,51 !.37 - 1,68 41,67 1989 ! 12,16 0.00 2.41 1.66 2,86 I.'72 17,49 !.20 0,08 1.46 41.11 1989 2 12,29 0.00 2,44 1.68 2,89 1,74 17,68 1.21 0.08 1.48 41.55 1989 3 - - 2,38 1.70 2.85 1.67 16,52 1.38 - 1.67 41.16 Mean 1 12.51 0.05 2,42 1.70 2.88 1.68 16.32 1.42 0.72 1.70 41.54 Mean 2 12.63 0.05 2.42 i.71 2.89 !.69 16.37 1.43 0.72 1.71 41.67 Mean 3 - - 2.38 1.72 2.87 1.68 16.51 1.38 - 1.68 41.69

Ubangi a 1988 I 1,31 0.00 0.25 0.17 0.32 0.14 !.93 0.07 0.06 0.08 4.22 1988 2 1,06 0.00 0.20 0.14 0.26 0,11 1.57 0,06 0,05 0.06 3.43 1988 3 - - 0,15 0,13 0.30 0,13 1.63 0.07 - 0.07 3.87 1989 1 1.05 0.00 0.12 0.11 0.26 0.11 1.48 0.06 0.10 0.06 3.38 1989 2 1.04 0.00 0.12 0.11 0.26 0.11 1.47 0.06 0.10 0.06 3.36 1989 3 - - 0.13 0,11 0.26 0.11 1,42 0.06 - 0.06 3.27 Mean ! !.18 0,00 0,18 0.14 0.29 0.12 1.70 0.07 0.08 0.07 3.':0 Mean 2 1,05 0.00 0.16 0.12 0.26 0.11 1.52 0.06 0.07 0.06 3.,~0 Mean 3 - - OI4 0.12 0.28 0.12 !.53 0.07 - 0,07 3.:~

For the Ubangi, the samples of September 1988 and March 1989 are lacking. TDS = total inorganic dms,: e ~ solids. DISSOLVED MAJOR ELEMENT EXPORT: CONGO AND UBANGi RIVERS 253

a - Congo river b - Ubangi river

DISSOLVED SUBSTANCES

Fig. 8. Annual fluxes of dissolved substances exported by (a) the Congo and (b) the Ubangi during the years 1987-1989.

As seen in Table 3, the results obtained by 'the three methods are not very different. Among dissolved species, HCO3 and SiO2 show the greatest annaal fluxes (Fig. 8). The TDS flux (without organic matter) amounts to 40- 45 X 106 tons year-~, of which about 10% comes from the Ubangi drainage basin (Fig. 9). The contribution of the Ubangi region to the dissolved fluxes of the Congo River is low as compared with its drainage area, but slightly high relative to its water...... contribution (6-8°/.) tn the Congo discharge. For HC37~ j and Ca 2+, the contribution of the Ubangi is stil- higher. These ions, con~,lg

/ /

/ ~ _ 8 ~[~

Fig. 9. Contributions of the Ubangi River (in ,r~ercent} to the annual discharge (Q,) and the dissolved fluxes of the Congo River during the years 1988-~989. 254 J.-L, PROBST ET AL. probably from the drainage of the paleocryptokarsts, raise the contribution to a level higher than it would be without the carbonate dissolution. Indeed, the thick ferricrete mantle which covers the Ubangi basin is depleted in mobile elements. Furthermore, the Ubangi region is less watered than the remainder of the Congo basin, with a specific discharge of 5.71s -t km -2 versus 12.81 s -~ km -2. The 'disturbing effect' of the presence of carbonate rocks in the Ubangi drainage area has also been pointed out by Probst (1990b) who estimated that 25% of the HCO~- originates from carbonate dissolution.

CONCLUSIONS

The main objective of the scientific programme which supports this work is to follow the hydroclimatic fluctuations in the Congo basin and to analyse their consequences for river transport, that is, to understand the response of this second largest tropical-equatorial forest ecosystem to climatic variav:ons. Maybe the coupling of the time hydrological records of the Congo discharge and the study of the deposited materials in the adjacent Atlantic Ocean could give some indications of the past. The data collected during the first three years of this programme are not yet sufficient to discuss such a problem and, moreover, the three years studied do not exhibit much hydrologic contrast. However, some important results have been already obtained. (1) The Congo and the Ubangi rivers have diluted waters that mainly contain dissolved silica and bicarbonates and which always present anionic deficit, probably owing to dissolved organic materials. (2) The chemical compositions of the~e river waters are stable over time, i.e. the contributions of the different anions and cations to the total anionic and cationic charges do not vary over the hydrological cycle, whereas monthly concentration variations of most dissolved elements are cyclical and vary inversely with discharge. (3) Good relationships between concentration and discharge could be determined for most elements: for the Congo the concentrations are simply diluted by river discharge, whereas, for the Ubangi, onc can detect a signifi- cant supply of elements from the draining surface waters. (4) Three different methods used to calculate the total annual inorganic dissolved fluxes exported by the Congo give similar values, ranging from 39 × 106 to 44 x 106 tons according to the year, of which about 10% is supplied by the Ubangi (3.4 x 106 to 4.2 x 106 tons). (5) The annual fluxes of dissolved silica and bicarbonates have been estimated respectively at 11.6-13.7 x 106 and 15.4-17.5 x 106 tons for the Congo, and at 1-1.3 x 106 and 1.5-1.9 x 106 tons for the Ubangi. These early results also permit some preliminary observations. For the DISSOLVED MAJOR ELEMENT EXPORT: CONGO AND UBANG! RIVERS 255

a - Congo river b - Ulmagi river

• . . T.D.$. 1987 1988 IN9 1988 I~9 YEARS YEARS Fig. 10. Mean annual water discharge and fluxes of total inorganic dissolved solids exported by (a) the Congo and (b) the Ubangi during the years 1987-1989.

Congo River, the flux of the TDS in 1987 was slightly higher than that of the following years (Fig. 10), although the mean annual water discharge for this year is medium. However, in 1987 the flux of NOr seemed to be abnormally high relative to the two other years, which explains this high flux of TDS. On the other hand, in 1988, the most humid year of the study period, the flox of TDS was the lowest. For the Ubangi River, the two years of measurement contrast more climatically than for the Congo, and the dissolved fluxes follow the water discharge in the same proportion. For the continuation of this programme, which is planned to run for at least l0 years, the main question is "what will happen to the river transport, to the eros;ion balance and the consumption of atmospheric CO2, to the global biogeochemical cycle of each element, and finally to the respiration of this forest ecosystem, when this large drainage basin is affected by a drought or a very humid period?"

ACK NOWLEDGEM ENTS

This work was carried out with the support of INSU (Institut National des Scie:aces de l'UnLivers), ORSTOM (Institut Fran~ais de Recherche Scientifique pour le D6veloppement en Coop6ration) and CNRS (Centre National de la Recherche Scientifique). "/he authors are grateful to the ORSTOM hydrolo- gists of Brazzaville and Bangui, the Service des Voies Navigables du Congo and the Laboratory of Water Chemical Analyses (Centre de G6ochimie de la Surfa,ce, CNRS, Strasbourg). Contribution CNRS/INSU No. 4.

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