The Cation Denudation Rate as a Quantitative Index of Sensitivity of Eastern Canadian Rivers to Acidic Atmospheric Precipitation

Abstract

A model has been developed that relates the cation denudation rate (CDR) of a watershed (the rate that cations derived from chemical weathering are carried off by runoff), the atmospheric load of excess SO₄ ^{− −}, and the pH of the river. Chemical and discharge data for rivers in Nova Scotia and Newfoundland were used to develop and test the model, which is based upon the common major ion chemistry of soft surface waters, and may be expressed by three statements:

• (1) CDR (meq m⁻² yr⁻¹) − Excess SO₄ ^{− −} load (meq m⁻² yr⁻¹) = HCO₃ ⁻ (meq m⁻² yr⁻¹),

• (2) HCO₃ ⁻ (meq m⁻² yr⁻¹)/Runoff (m³ m⁻² yr⁻¹) = HCO₃ ⁻ (meq m⁻³),

• (3)
  $$ pH\, = \,pK\, + \,pP_{CO_2 } \, - \,pHCO_3 ^ - $$

The model in concentration form applies well to lakes.

A detailed analysis of the data for the Isle aux Morts River, Newfoundland, is presented, showing that the CDR varies throughout the year, affected by both discharge and seasonal pattern.