Hydrochemistry of the rimouski river, a tributary to the St. Lawrence estuary

Abstract

Temperature, pH, total alkalinity, dissolved oxygen, silicate, nitrate, phosphate, and the Mn, Fe and Al content of suspended particulate matter (SPM) were measured in the Rimouski River estuary as functions of chlorinity during the period of May–September 1980. At any given time, the total alkalinity (TA) in conservative within the estuary with respect to chlorinity. However, the TA of the freshwater end member is related to river flow. This is attributed to dilution of the river water with bicarbonate-poor rain water. Generally, pH follows the same pattern observed for TA. Dissolved oxygen is usually more concentrated in freshwater and decreases linearly with increasing chlorinity. Freshwater is always saturated or supersaturated with respect to O 2. Seasonal fluctuations are attributed to temperature variations. There is no evidence for removal of soluble silicate from the freshwater entering the estuary. The concentration of silicate in the freshwater is strongly influenced by rainfall. Soil leaching, conditioned by high rainfall, increases the concentration of soluble silicate in freshwater. Nitrate behaves similarly, whereas phosphate is complicated by the presence of sewage. Analysis of the Mn, Fe and Al contained in the SPM indicates dilution of river borne particles rich in Mn by others less rich in this element. A decrease in Mn content with increasing chlorinity and SPM concentration, as well as increasing concentrations of SPM with increasing chlorinity, indicate that the composition is controlled primarily by physical mixing of material from two sources rather than by chemical processes. Within the analytical precision, the Fe/Al ratio does not vary with the chlorinity.