GEOCHEMICAL CHARACTERIZATION OF BED SEDIMENTS OF THE RIO QUEQUÉN GRANDE CATCHMENT, ARGENTINA
L.B. Teruggi*, E. Marrocchino° D. Rapti-Caputo° & C. Vaccaro°
* Dipartimento di Ingegneria Civile, Università degli Studi di Firenze, via Santa Marta 3 - 50139 Firenze, Italia e-mail: lteruggi@dicea.unifi.it
° Dipartimento di Scienze della Terra, Università degli Studi di Ferrara, Corso E. I° d’Este, 32 - 44100 Ferrara, Italia
INTRODUCTION
Argentinian loess is the most extensive aeolian deposit in the Southern Hemisphere, spreading over a surface of 1.1 106 km2. The Argentinian Pampas is therefore an important region for the study of loess genesis (Teruggi, 1957; Zarate & Blasi, 1988) and the Pleistocene and Holocene palaeoclimatic events (Iriondo & Garcia, 1993; Nabel et al., 2000). Of particular interest in this context is the mineralogic - petrographic and geochemi- cal analysis of the Rio Quequén Grande catchment (Fig.
1), located in the southeast section of the province of Buenos Aires.
The Argentinian late Pleistocene and Holocene loess have recorded the palaeoenvironmental variations between glacial and interglacial events in a region that has not been affected by significant tectonical events.
Thus the petrographical, geochemical and sedimentolog- ical study of waters and sediment can contribute to clar- ify planetary climatic variations and to identify possible palaeoclimatic markers.
The aim of this study is to carry on geochemical analy- ses of mud bed sediments and superficial waters in the Rio Quequén Grande catchment. This is very important from a socio-economical point of view because it is part of one of the most productive plains in the world, and is characterized by intense agricultural activity.
Furthermore, the analyses of variations in the compo- sition of bed sediments makes it possible to assess the
hydrological and sanitary risk factors in an area consti- tuted by very erodible sediments. These sediments are more and more frequently related to flood events that have often compromised productive activities and have caused heavy economic losses.
Thus, the loess genesis process and the relationship between compositional, textural and petrographic sedi- ments variation have been performed.
STUDY AREA
The Rio Quequén Grande catchment is located in the southeast of the province of Buenos Aires, and is a typi- cal fluvial system draining the flat Argentinian Pampas (Fig. 1). The river catchment has a surface of about 9,940 km2. The main reliefs are located in the northeast- ern sector originated by some isolated Sierras of the Tandilia System (ortoquarzitic or igneous – metamor- phic crystalline basement) that make up the basin head- water and the main divides. Most of the catchment con- sists of a flat plain with a slope gradient of about 0.003.
The stream network develops its channels into aeolian deposits (loess and reworked loess); the basin shows a slightly entrenched morphology, almost as far down- stream as its outflow into the Atlantic Ocean, near the city of Necochea (Fig. 1). In some places the along- channel profile is stepped because of hard caliche layers in the bedrock.
ABSTRACT - The aim of this study was to conduct geochemical analyses of mud bed sediments and the super- ficial waters of the Rio Quequén Grande catchment, located in the southeast section of the province of Buenos Aires. The Rio Quequén Grande outflow into the Atlantic Ocean, near the city of Necochea. The river has cut its channel into aeolian deposits (loess and reworked loess). Most of the catchment consists of a flat plain.
Bed sediments and superficial water were sampled along the drainage network of the Rio Quequén Grande catchment.
The mud bottom sediments were analyzed to determine the major and trace elements, using the Ray-X fluore- scence (X RF) method and loss on ignition (LOI). Together with the analyzed samples, the composition of other samples of Argentinian loess and the composition of the Upper Continental Crust (UCC) were also plotted. They revealed considerable compositional variation.
The sample analysis also showed compositional differences as regards grain size. The samples from the Rio Quequén Grande are of sedimentary origin, as are the majority of loess deposits in the world.
The main cations and anions of the superficial water were analyzed. In some samples a remarkably high con- centration of toxic elements was found. Of these, the concentration of the arsenic content is higher than 100 µg/l in the northeastern sector of the hydrographic basin, principally due to the lithological composition of the Quaternary deposits, which are locally rich in grains of volcanic origin.
The integrated analysis of the geochemical results allows us to conclude that the geological setting has a strong influence on the geochemical composition of the sediment and water. The major zones with a pollution risk were identified.
KEY WORDS: Rio Quequén Grande, loess, geochemical, bed sediments, superficial water, pollution.
Mean annual precipitation in the basin is about 800 mm with the highest monthly values occurring in the summer (Varela & Teruggi, 2002). The results of the pro- cessing of available discharge data indicate a mean daily discharge of about 16 m3s-1, while the maximum and minimum discharges recorded are 1,700 and 1,6 m3s-1 respectively.
METHODS AND DISCUSSION
Twenty-four representative fluvial sections (Teruggi et al., 2003) were surveyed along the drainage network of
the Rio Quequén Grande catchment. In correspondence with these, bed sediments, superficial bars (when pres- ent) and superficial water were sampled (Fig. 1) at the end of 2001 and the beginning of 2002.
Analysis of the geochemical bed sediments and super- ficial water were conducted in the Earth Sciences Department of Ferrara University.
Mud bed sediments
The X-ray fluorescence (XRF) analysis of some mud bars and bed sediments was carried out using an ARL ADAVNT.XP spectrometer. Loss on ignition (LOI) was
Fig. 1 - Study area and location of samples.
measured by weighting before and after 12 hs of calcina- tions at 1050°C. LOI values vary from 3% to 18%, reflecting variable amounts of carbonates, hydrous phas- es (mainly clays) and organic matter.
In the binary diagrams (Fig. 2) together with the pres- ent samples the composition of other samples of Argentinian loess and the composition of the Upper Continental Crust (UCC) (Taylor & McLennan, 1981) are plotted. They show considerable compositional variation.
The variation in the CaO content, for example, could be related to the presence of carbonatic particles, coming from carbonatic formation called caliche.
The samples were grouped by the same grain size, highlighting that the composition is directly conditioned by grain size distribution.
The present samples also show compositional differ- ences relating to grain-size differences. It was possible to arrange them into two groups, the characteristic his- tograms of which are shown in Fig. 3; one of them con- sists only of bed sediments, while the other one mainly consists of superficial bar sediments.
In Fig. 4 the diagram Na2O/Al2O3 vs K2O/Al2O3 (Garrels & Mackenzie, 1971) showed that the irregular thick shaded line is the lower limit of most igneous rock compositions. All shaly rocks, regardless of their age or degree of metamorphism, are distinctly lower in Na2O/Al2O3 than any of the igneous rocks, because alteration of igneous to sedimentary rocks involves the formation of clays and carbonate minerals and removal of Na into the ocean, and retention of K in shale. An
Fig. 2 - Binary diagrams of SiO2(%), K2O (%), CaO (%) vs Al2O3and CaO (%),K2O (%), Rb/Sr vs SiO2for loess of the Rio Quequén Grande main branch, its affluents, data from Garrels & Mackenzie (1971) and Taylor & Mclennan (1981).
important implication is that all particles making up the loess deposits have experienced at least one cycle of the sedimentary differentiation process.
A popular hypothesis regarding the origin of loess par- ticles is that glacial grinding of bedrocks might have effectively comminuted continental rocks to silt-sized particles.
This enables us to conclude that loess particles, regardless of the scale of deposition and the proximities of provenance, must have undergone previous sedimen- tary differentiation. This is also true for the loess deposits of the Argentinian Pampas, where a significant amount of vulcanoclastic components (glass shards and plagioclase crystals) can be observed.
Superficial water
For the determination of the geochemical composition of the superficial water, electrical conductivity, tempera- ture and pH were measured in situ while the following parameters were carried out at the Geochemical Laboratory of the Department of Earth Sciences of University of Ferrara: a) Mg++, Ca++, Cl-, SO4- - and NO3- using spectrophotometer methods; b) Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Se, Mo, Ag, Cd, Pb, and U using a ICP-MS procedure; and c) HCO3concentration, using volumetric methods.
The chemico-physical characteristics of the water show high variations of the electric conductibility with values between 600 to 2,000 µS/cm (Martinez et al., 2004). The qualitative conditions of the waters along the south-western sector of the investigated area is particu- larly decayed due to the high concentration of Cl-
(550mg/l) and SO4- -(500 mg/l), with an electric con- ductibility of 30,000 µS/cm.
In some of these locations a remarkably high concen- tration of toxic elements was found. Among those, the arsenic content has a concentration that is higher than 100 µg/l in the north-eastern sector of the hydrographic basin, principally due to the lithological composition of the Quaternary deposits, which are locally rich in grains of volcanic origin.
CONCLUSIONS
The integrated analysis of the geochemical results allows us to conclude that geological setting has a strong influence on the geochemical composition of the sedi- ment and waters.
Sediment analysis shows that it is possible to group samples with the same grain-size, highlighting that the compositional distribution is directly conditioned by grain-size. Samples from the Rio Quequén Grande show a sedimentary origin, as is the case with most loess deposits in the world.
The major zones that are at risk of pollution have been identified. In particular, water analysis shows great vari- ations in the physiochemical characteristics. In some locations a remarkably high concentration of toxic ele- ments was found. Among these, there was a high arsenic concentration.
The availability of topographic and bathymetric pro- files, textural and geochemical bed sediments analysis and chemical water characterization will enable a com- plete characterization of environmental conditions and a greater understanding of the effects of pollution.
ACKNOWLEDGMENTS - The field work was funded by the Italian Ministry of Foreign Affairs, through the Office of Scientific and Technological Cooperation of the Italian Embassy in Buenos Aires. The authors would like to thank M.
Farenga and A. Ferrante for their assistance with the fieldwork and Prof. M. Del Monte and Prof. M. Barbieri for their helpful reviews.
Fig. 3 - Representative grain size distributions: a) Superficial bar sam- ples. b) Bed sediment samples.
Fig. 4 - Na2O/Al2O3vs K2O/Al2O3diagram showing the sedimentary nature of the Rio Quequen Grande loess.
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Accettato per la stampa: Giugno 2005
