The Apure River: geochemistry of major and selected trace elements in an Orinoco River tributary coming from the Andes, Venezuela

Abstract

Abstract:The analysis of physicochemical variables and selected dissolved elements was performed on the Apure River waters for 15 months. The variables pH, alkalinity, dissolved O 2 , conductivity and Na, Ca, Mg and Cd concentrations showed maximum values during low water, whereas K, Si, Fe, Al, Mn, Zn, Cu, Cr and dissolved organic carbon (DOC) showed maximum concentrations during rising and high water. Five important factors were found to control the amount and temporal variability of the dissolved elements: lithology, hydrology, vegetation-floodplain processes, redox conditions and organic complexation. Weathering of silicates, carbonates and evaporites in the Andes provides most of the proportion of Na, Ca, Mg and HCO 3 to waters. The temporal variability of these ions is controlled by a dilution process. Although Si can be taken up by the biomass, Si and K can be leached from the floodplain by weathering of clays. Microbial decay of the submerged plants in the floodplain during the inundation periods provides DOC and K to river waters and changes the redox conditions in water. The changing redox conditions control the solubility of Mn, Zn and Fe. Dissolved Mn is a function of pH-dependent redox process, whereas Zn solubility is controlled by scavenging of Zn during the oxidation of Mn 2C to MnO 2 . Positive relationships between Al, Fe, Cu, Cr and DOC suggest that these elements are complexed by organic colloids generated in the floodplain. Moreover, the binding capacity of Fe with DOC increases under reducing conditions. Although Cd seems to be provided by weathering in the Andes, several processes can affect the mobility of Cd during transport.

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

“…The results of the measured variables in all selected sectors of the Orinoco River throughout this study are provided in Table S1. Data of the Caura and Apure Rivers have already been published in separate papers on the geochemistry of these rivers (Mora et al, ; Mora et al, ). These results, together with the studies performed in the Orinoco, Caura, and Apure Rivers in the 1980s (Lewis, Hamilton, Jones, & Runnels, ; Lewis & Saunders, ; Saunders & Lewis, ), represent a unique time‐series of data of major and trace elements measured along the Orinoco River and its tributaries, and they show the changes in the chemical properties of the Orinoco downstream of the Apure–Orinoco confluence.…”

“…However, this element only showed high concentrations during the low‐water stage at sites OR1, OR2, and OR3, with little variation along the hydrological cycle at sites OR4, OR5, and OR6 (Figure b). Studies carried out in the Apure River have shown that a high proportion of K comes from biogenic sources (Mora et al, ). The Apure River's deltaic floodplain (internal delta type) covers a large extent of lowlands areas.…”

“…During the rainy season, shallow flooding occurs in these large expanses of savanna (Saunders & Lewis, ; Hamilton & Lewis, ), and a vast area with high vegetation density is inundated, promoting the decomposition of large amounts of submerged vegetation. During this process, the K content in plants is leached from the vegetation to the river waters (Chaudhuri, Clauer, & Semhi, ), increasing the content of dissolved K in the Apure River (Mora et al, ), which is provided later to the Orinoco. However, this process is not extensive in the Orinoco River due to the differences in the morphology of its floodplain.…”

“…(d) 3D graphic showing the dissolved oxygen and pH dependence of Mn concentrations. Data on the Caura and Apure rivers are from Mora et al () and Mora et al (), respectively…”

“…The Caura River was also sampled during this period. However, although data of the Caura and Apure Rivers have been published previously (Mora, Alfonso, Baquero, Balza, & Pisapia, ; Mora, Baquero, Alfonso, Pisapia, & Balza, ), we used those data in order to carry out comparisons between the Orinoco and its tributaries. The water samples were collected 50 cm below the surface in prewashed polyethylene bottles (4 L), which were placed in a cooler with ice for transportation.…”