GEOCHEMISTRY OF QUEBEC NORTH SHORE SALMON RIVERS DURING SNOWMELT - ORGANIC-ACID PULSE AND ALUMINUM MOBILIZATION

The episodic acidification of three salmon rivers on the Quebec North Shore during spring snowmelt is shown to result mainly from increased inputs of organic acids rather than strong mineral acids. Appreciable mobilization of Al occurs during the spring pH depression, accompanied by marked changes in Al speciation. Saturation indices, calculated for each river with respect to microcrystalline gibbsite (K(so) = 10(-9.35)), approach saturation for samples collected before or after the snowmelt period, but during maximum discharge, marked undersaturation is observed. Seasonal differences in Al - dissolved organic carbon (DOC) interactions were also noted. Outside the snowmelt period, a linear logarithmic model proved successful in predicting concentrations of monomeric organic Al ([Al-org] = 3.55.10(-3).{Al3+}0.129.{H+}0.[DOC]0.91 ; n = 19, r2 = 0.92), but during snowmelt, this model greatly overestimated [Al-org]. The failure of the model during this critical period is attributed to the presence in the spring DOC peak of an appreciable nonhumic (and non-Al-binding) component. Although concentrations of monomeric inorganic Al (notably fluoro-Al complexes) increase during the spring pH depression, levels do not appear to attain the thresholds for Al and H+ toxicity reported for juvenile Atlantic salmon (Salmo salar).