IN SITU MONITORING OF THE DIURNAL CYCLING OF DYNAMIC METAL SPECIES IN A STREAM UNDER CONTRASTING PHOTOBENTHIC BIOFILM ACTIVITY AND HYDROLOGICAL CONDITIONS

The past decade studies performed in rivers and streams demonstrated that many trace metal and metalloid experience diurnal cycles, with total dissolved metal concentrations often changing one- to five-fold during a 24h period. However, most of these studies were performed in summer under low-flow conditions. Moreover, samples were collected hourly by hand for later laboratory analysis. This laborious sample collection and processing approach limits hourly sampling to 1–3 days at maximum. More frequent analysis is required to understand the seasonal occurrence and amplitude of diel metal cycles, and the processes controlling these cycles.

With this goal in mind, we applied an automated voltammetric analyzer to study, at time scale of hour and under contrasting bio-chemical and hydrological conditions, the diurnal evolution of Cd, Cu and Pb in the Riou-Mort river (France) impacted by polymetallic pollution resulting from former open-cast coal mining and ore treatment. This analyzer is based on a bioanalogic microsensor which allows in situ real-time monitoring of the dynamic fraction, i.e. the potentially bioavailable fraction, of the target analytes. In parallel, T, pH, dissolved oxygen and conductivity were monitored in situ and water samples were collected for complementary analyses of the water composition.

Here, we summarise the original results of this work [1]. The data revealed that, in the studied river, the diurnal cycles of the Cd, Cu and Pb dynamic species were controlled by redox and sorption effects induced by either: pH diurnal cycle linked to metabolic activity of benthic biofilms; photoreduction of colloidal Mn oxides; and/or biofilm exudation of extracellular polymeric substances. We also observed that: the dynamic fraction of a given cationic metal can show diurnal cycle with opposite trends depending on the bio-chemical conditions; the trends of the diurnal dynamic metal species cycles may be different that those reported for the dissolved metal species. The importance of these findings is discussed in the context of interpreting existing data banks, assessing metal ecotoxicity impact, and designing more appropriate monitoring control strategies.

[1] Tercier-Waeber M.-L., Hezard T., Masson M. & Schäfer J. Env. Sci. Technol., 2009, DOI: 10.1021/es900247y.