Transport of major solutes and the relationship between solute concentrations and discharge in the Apure River, Venezuela

Abstract

The Apure River is a major white-water tributary of the Orinoco River in Venezuela. The Apure is rich in solutes; its contribution to dissolved inorganic solids in the Orinoco (24%) is proportionately much greater than its contribution to discharge (6%). About 40% of the calcium and bicarbonate at the mouth of the Orinoco originate in the Apure drainage. The relationship between discharge and the concentrations of major solutes in the Apure was characterized with a two-compartment hyperbolic mixing model. Previous applications of the two-compartment model have been based on separate determinations of the model parameter β, which is a constant describing watershed hydrology, for each solute from data on concentrations. The use of a weighted mean β for all solutes is proposed as a means of assessing the importance of processes other than mixing. The model, when used on the Apure data, shows that a strong dilution effect prevails for sodium, calcium, magnesium, sulfate, and bicarbonate, and that a strong purging effect (increase of concentration with increasing discharge) is characteristic of soluble silicon. Biological immobilization of soluble silicon by diatoms during the season of low discharge is sufficiently large to account for the positive relationship between discharge and the concentration of soluble silicon. Specific transport rates of solutes from the basin are generally higher than global averages. In contrast, specific transport of chloride is low. Atmospheric sources control chloride transport in the Apure watershed; the low transport rates of chloride are probably explained by the great distance between the Apure watershed and the oceanic sources of atmospheric chloride.