Diurnal variations in the water chemistry of a river contaminated by heavy metals: natural biological cycling and anthropic influence

A chemical survey of the water of the Lot River (Southwestern France) was conducted for 26 h. Water temperature, pH, dissolved oxygen and zinc exhibited a diurnal cycle. These variations are caused by biological activity in the river (photosynthesis during the day and respiration during the night). The Zn diurnal cycle was related to the pH variations. The solubility of Mn seemed to follow a photoreductive diurnal cycling pattern. An increase in conductivity due mainly to higher dissolved Ca, Mg and sulfate concentrations is attributed to an increase in the solute input from a small tributary which drains smelter tailings. Even though the river sediments are significantly contaminated with Zn, Pb and Cd, the heavy metal dissolved concentrations are always low.