Dynamic mass balance model for mercury in the St. Lawrence River near Cornwall, Ontario, Canada

Abstract

A dynamic mass balance model was developed for the St. Lawrence River near Cornwall, Ontario that predicts and hindcasts mercury concentrations and fluxes in three forms, elemental Hg (Hg0), divalent mercury (Hg2+), and methyl mercury (MeHg), in a six compartment environment (air, water, porewater, sediment, periphyton, and benthic invertebrates). Our objective was to construct a dynamic mass balance model for mercury in the St. Lawrence River near Cornwall, Ontario based on the framework and results of a steady-state mass balance model developed previously for this site. The second objective was to estimate industrial mercury emissions based on mercury residues deposited in sediments prior to 1970, the year when regulations were implemented to reduce mercury pollution in the environment. We compiled mercury concentrations, fluxes, and transformation rates from previous studies completed in this section of the river (area of approximately 100km2) to develop the model. Estimated mercury concentrations in all media were similar to measured data (R2=0.99), with only minor exceptions, providing a satisfactory overall description of the mercury loadings and transformation rates of the different mercury species. The estimated historical emissions prior to 1970 from local industries along the Cornwall waterfront were approximately 400kgyear−1. A storm sewer discharge of 5000m3/day resulted in a significant increase in mercury concentrations, particularly in sediment (617ngg−1 to 624ngg−1; p=0.004). Model results suggest that discharges of mercury from sources such as local industries and storm sewers have an impact on mercury in media such as sediment and water. This model should provide a basis for predicting and hindcasting mercury concentrations in other river environments as well, because it considers three distinct forms of mercury, and contains environmental media common to all rivers, including some (e.g. periphyton) not typically included in previous mercury models.