ESTIMATING ORGANIC-ACID CONTRIBUTIONS TO SURFACE-WATER ACIDITY IN QUEBEC (CANADA)

Inorganic ion balances for lake water lying on the Canadian Shield in the province of Quebec, Canada, consistently show significant anion deficits (mean deficit 70 to 90-mu-eq L-1). Models taking into account organic anions have been tested for their ability to eliminate this anion deficit. The data set consisted of the chemical composition (pH, all major ions, major trace metals, dissolved organic and inorganic C, total alkalinity) of almost-equal-to 1200 lakes from four hydrographic regions. The relative performances of three models (the 1983 Oliver et al. model and two derived models) were evaluated by comparing the capacity of each model to compensate fully for the anionic deficit of the waters. The study showed that without calibration all models systematically underestimated the anionic deficit. The two derived models were recalibrated and validated by regional discrimination of the data; charge balances calculated with the recalibrated models were markedly improved. Based on calculated anion deficits, the contribution of natural organic acids to Quebec surface water acidity increases from southwest to northeast and greatly exceeds that previously reported.