A Computer Simulation of Water Quality Change Due to Dredging of Heavy Metals Contaminated Sediments in the Old Harbour of Montreal

Abstract

Aquatic sediments accumulate heavy metals that are discharged into the environment. This study investigates possible water quality changes due to release of heavy metals such as Cd, Ni, Zn and Pb into the water column during dredging of anoxic sediments in the Old Harbour of Montreal. An environmental impact assessment of the sediment removal requires estimating the speciation of heavy metals in the water column with and without the solid phase. Chemical equilibria in the St. Lawrence River water are calculated using the program EQUILIB from the software FACT. Results show that the water is oversaturated with respect to CaMg(CO3)2 and Fe(OH)3. It is speculated that (ZnO)(Fe2O3) and (NiO)(Fe2O3) control the solubility of Zn and Ni in the water. The speciation of heavy metals in pore water of anoxic sediments is calculated, taking into account that the corresponding sulfides are solid phases and control their solubility. The impact of various scenarios on the area of concern is reviewed from an ecotoxicological perspective. Dredging might change the redox and acid-base conditions in the water column. Dredging can lead to an increase in the concentration of heavy metals in the water column and a change of metal speciation, but its impact will be less visible if the sediments contain high levels of calcium acting as a buffer.

Mobilization of Trace Metals and Inorganic Compounds during Resuspension of Anoxic Sediments from Trepangier Bayou, Louisiana

“…Similar to the findings of this study, many investigations (Calmano et al, 1993; Degtiareva and Elektorowicz, 2001; Dubrovsky et al, 1985; Lors et al, 2004) have shown a distinct pH drop after exposing the anoxic sediments to oxygenated overlying waters, and it was suggested that the oxidation of AVS and other iron sulfide minerals was the primary source of the increased acidity, according to the following reactions: As shown in Eq. [1] and [2], 1 mol of AVS can produce 2 mol of H + , whereas 1 mol of pyrite can produce 4 mol of hydrogen ions.…”

“…In agreement with our observations, Degtiareva and Elektorowicz (2001) and Lors et al (2004) demonstrated that the fate of toxic metals during resuspension depended on the calcium content in the sediment. In contrast, Calmano et al (1994) reported a continuous increase in dissolved Cu, Cd, and Zn concentrations after sediment resuspension at pH 7.5.…”

“…Similar to the fi ndings of this study, many investigations (Calmano et al, 1993; Degtiareva and Elektorowicz, 2001;Dubrovsky et al, 1985;Lors et al, 2004) have shown a distinct pH drop after exposing the anoxic sediments to oxygenated overlying waters, and it was suggested that the oxidation of AVS and other iron sulfi de minerals was the primary source of the increased acidity, according to the following reactions: Fig. 1c.…”

“…Previous resuspension experiments have been performed under controlled pH–redox potential conditions (Calmano et al, 1994; Hirst and Aston, 1983; Khalid et al, 1977; Simpson et al, 2000) or without pH control (Calmano et al, 1993; Degtiareva and Elektorowicz, 2001). It has been reported that resuspension of sulfide‐rich anoxic sediments tends to decrease the solution pH, primarily due to the oxidation of AVS and iron sulfide minerals not included in the AVS measurement (Burton et al, 2006).…”

“…A significant release of toxic metals to the aqueous phase and enhanced mobility and bioavailability of these metals, therefore, were expected with increased acidity. Degtiareva and Elektorowicz (2001) calculated the speciation of metals in the water column with and without solid phase before and after dredging using computer simulation based on an equilibrium speciation model. They concluded that the fate of toxic metals during resuspension depended on the calcite concentration in the sediment.…”