Received: 3 August 2016 Accepted: 5 October 2016 DOI 10.1002/hyp.11051 RESEARCH ARTICLE Dynamics of dissolved major (Na, K, Ca, Mg, and Si) and trace (Al, Fe, Mn, Zn, Cu, and Cr) elements along the lower Orinoco River Abrahan Mora1 | Jürgen Mahlknecht1 | Juan Carlos Baquero2 | Alain Laraque3 | Juan A. Alfonso4 | Daniel Pisapia5 | Laura Balza5 1 Centro del agua para América Latina y el Caribe, Tecnológico de Monterrey, Av. Abstract Eugenio Garza Sada Sur No. 2501, CP 64849 This study addresses the changes in dissolved major and trace element concentrations along the Monterrey, Nuevo León, México Orinoco River, including the mixing zone between the Orinoco and Apure Rivers. Water samples 2 Escuela Técnica Superior de Ingenieros de from the Apure and Orinoco Rivers were collected monthly in four sectors over a period of Minas, Universidad Politécnica de Madrid, 15 months. Auxiliary parameters (pH, dissolved oxygen, conductivity, and temperature), total Madrid 28003, Spain suspended sediments, dissolved organic carbon (DOC), and major (Na, K, Ca, Mg, and Si) and trace 3 GET–UMR CNRS/IRD/UPS–UMR 5563 du CNRS, UR 234 de l’IRD; OMP, 14 Avenue (Al, Fe, Mn, Zn, Cu, and Cr) element concentrations were measured in all sectors. The relative con- Edouard Belin, 31400 Toulouse, France tribution of both rivers after the Apure–Orinoco confluence was determined using Ca as a tracer. 4 Centro de Oceanología y Estudios Antárticos, Moreover, a mixing model was developed to determine whether dissolved species exhibit a con- Instituto Venezolano de Investigaciones servative behavior during mixing. The results indicate that DOC is removed from waters during Científicas (IVIC), Caracas 1020‐A, Venezuela the Apure–Orinoco mixing, probably due to absorption of DOC on mineral phases supplied by 5 Laboratorio de Fisicoquímica, Estación de Investigaciones Hidrobiológicas de Guayana, the Apure River. Dissolved Na, Ca, and Mg behave conservatively during the mixing processes, Fundación La Salle de Ciencias Naturales, San and their concentrations are controlled by a dilution process. The anomaly in the temporal pattern Félix 8051, Venezuela of K in the Orinoco is caused by the input of biogenic K originating from the Apure River during Correspondence the high‐water stage. The loss of dissolved Si during the low‐water stage can be explained by the Abrahan Mora, Centro del Agua para América ‐ Latina y el Caribe. Tecnológico de Monterrey, uptake of Si by diatoms. Dissolved Mn, Zn, Al, and Fe showed a non conservative behavior during Av. Eugenio Garza Sada Sur No. 2501, CP the Apure–Orinoco mixing. The removal of Mn and Zn from the dissolved phase can be explained 64849 Monterrey, Nuevo León, México by the formation of Mn‐oxyhydroxides and the scavenging of Zn onto Mn oxides. Dissolved Fe is Email: [email protected] controlled by redox processes, although the removals of Fe and Al due to the preferential adsorp- tion of large organometallic complexes by mineral surfaces after the Apure–Orinoco confluence can affect the mobility of both elements during transport. The conservative behavior shown by Cu and Cr can be related to the tendency of both elements to be complexed with small organic colloids, which are not preferentially adsorbed by clays. KEYWORDS major elements, mixing zone, organic matter, Orinoco River, trace elements 1 | INTRODUCTION tributaries have been classified based on their optical appearance and their physicochemical properties in “whitewater,”“clearwater,” and The Orinoco River ranks third in terms of water discharge to the “blackwater” rivers (Vegas‐Vilarrubia, Paolini, & Miragaya, 1988). oceans, with an annual mean discharge of 36.000 m3 s−1 (Lewis & Whitewater rivers originating from the Andes show near neutral pH Saunders, 1989). Like the Amazon River tributaries, the Orinoco River values and high concentrations of total suspended sediments (TSS) This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made. © 2016 The Authors Hydrological Processes Published by John Wiley & Sons Ltd. Hydrological Processes 2017; 31: 597–611 wileyonlinelibrary.com/journal/hyp 597 598 MORA ET AL. and major cations such as Na, K, Ca, and Mg. These characteristics are the processes controlling the content of dissolved trace elements in due to the reaction‐limited weathering regime and the diverse mixture these tropical environments. Moreover, the behavior of organic matter of silicates, carbonates, and evaporites in the Andes (Edmond, Palmer, and dissolved elements in mixing zones has been documented in large Measures, Brown, & Huh, 1996). Conversely, “blackwater” rivers orig- rivers such as the Amazon and Yangtze (Aucour et al., 2003; Ran, Yu, inating from the Guayana Shield have low concentrations of major ions Yao, Chen, & Mi, 2010). However, this issue has not yet been docu- and TSS and low pH values, mainly due to the presence of organic mented in the Orinoco. Because of the lack of this information, we acids originating from forest soils that have very low buffering capacity can formulate the following questions: Why do the concentrations of because of the igneous rocks that predominate in the blackwater K and Si in the lower Orinoco River not show a unimodal seasonal pat- drainages. Because of the marked difference in water chemistry tern, as shown by the other major elements? Could major and between both typologies of river waters, there is a chemical heteroge- dissolved trace elements show a conservative behavior during the neity between the left and right banks within the Orinoco mainstream mixing process between the Orinoco and a “whitewater” river originat- because “whitewater” rivers flow to the left bank and “blackwater” riv- ing from the Andes? Which processes could control the content of ers flow to the right bank. This lateral asymmetry, which has been dissolved trace elements in this riverine mixing zone? Considering that noted by many authors (Lewis & Saunders, 1984; Laraque et al., mixing is an important natural phenomenon that can affect the chem- 2013; Stallard, 1987), is maintained in the Orinoco from the conflu- ical composition of water due to different reactions taking place ence with the Guaviare River in the upper Orinoco to the river mouth between the dissolved and particulate phases, understanding the due to the continuous and heterogeneous deliveries from one river major and trace element dynamics in the Apure–Orinoco mixing zone bank to the other (Figure 1). during a hydrological cycle is in prime importance in understanding The temporal variability of dissolved major elements in the lower the biogeochemical cycling of these elements in tropical environments. Orinoco River has been well documented by several authors. Elements Therefore, the objective of this work was to investigate the tem- such as Na, Ca, and Mg are mainly controlled by a dilution process, and poral variation and behavior of dissolved organic carbon (DOC) and they show a unimodal regime, with high concentrations during low dissolved major (Na, K, Ca, Mg, and Si) and trace (Al, Fe, Mn, Zn, Cu, water and low concentrations during the high‐water stage (Lewis & and Cr) elements along the lower Orinoco River mainstream. We Saunders, 1989). However, elements such as K and Si have shown a emphasized the mixing zone between the Apure River (a “whitewater” non‐defined seasonal pattern, with little temporal variations along river originating from the Andes) and the Orinoco River in order to the hydrological cycle. Despite the numerous works regarding the geo- identify the diverse processes that may affect the geochemistry of chemistry of major ions in the Orinoco waters (Edmond et al., 1996; the studied dissolved elements. Thus, as a novel issue in comparison Lewis & Saunders, 1989, 1990; Stallard, Koehnken, & Johnsson, with other works carried out in the Orinoco, we compared the mea- 1991), the geochemistry of dissolved trace elements in the Orinoco sured concentrations of elements after the mixing zone with those pre- and its tributaries has been poorly investigated because most of the dicted through a conservative mixing model to determine whether the published works, based on one sampling campaign, do not describe dissolved elements show a conservative behavior during mixing. FIGURE 1 Map showing the sampling sectors in the Apure and Orinoco Rivers MORA ET AL. 599 2 | MATERIALS AND METHODS The sampling sites were chosen inside the four sectors distributed between the Apure and Orinoco Rivers (Figure 1). Sector 1 was 2.1 | Study area, hydrological settings, and sampling located in the Apure River, Sector 2 was located in the Orinoco before the confluence with the Apure River, Sector 3 was located in the Ori- sites noco after the confluence with the Apure River, and Sector 4 was 6 2 The Orinoco River basin covers an area of 1 × 10 km . This basin can be located in the lower Orinoco River after the confluence with the divided into three geological regions: (i) the Andes, which comprises oro- Caura River (a “blackwater” tributary originating from the Guayana genic mountain belts; (ii) the Llanos, an extensive region built by sedi- Shield) and before the Puerto Ordaz City. Given the chemical hetero- ments originating from the Andes; and (iii) the Guayana Shield, a deeply geneity shown by the Orinoco between the left and right banks, two eroded crystalline bedrock zone dominated by intrusive Precambrian sampling sites were chosen for each sector in the Orinoco River. igneous rocks (Figure 1). The Orinoco River’s main channel is fringed by These two sampling sites were located approximately 800 m from a network of permanent lagoons and fluvial wetlands called the flood- each bank. The approximate width of the Orinoco River’s main chan- plain, which has an average width of 9 km and covers an area of nearly nel was 3.7, 5, and 2.2 km for Sectors 2, 3, and 4, respectively.