Numerical simulation of salt water contamination in upper carbonate aquifer, Winnipeg, Manitoba

Groundwater quality issues have been experienced in several aquifers in Manitoba which leads to saline water intrusion in the fresh water portion of aquifers. The study deals with the salt water contamination in the upper carbonate aquifer due to old oil exploration activity in 1920’s. The objective of the study is the determination of salt water intrusion and its spreading for the recent 100 years. Groundwater quality was observed over a 24 years period from 1980 to 2004. We used an average hydraulic head boundary to account for the changes in the groundwater flow. We applied a constant density contamination transport modelling using MODFLOW-2005 to simulate the salt water contamination process and compared the simulated and observed salt concentrations at domestic wells located farthest from the contamination source. The spread of contamination is inferred by calculating the salt concentrations at the far most located domestic wells. In order to simulate the contamination phase (1920-1980) a calibrated contamination pumping source is applied which introduced a constant contamination to reach the observed concentrations at the domestic wells. The model also incorporated a remediation attempt carried out in 1981 which removed some saltwater from the freshwater aquifer. The actual attempted pumping rates were applied to simulate the remediation phase of the problem. The simulation results showed a good agreement between the simulated and observed salt concentrations at the domestic wells during the contamination phase and also during the remediation phase. All simulated salt concentrations were within a 5%-difference limit to the observed salt concentrations. The maximum spreading of saltwater was identified at a distance of 500 m from the contamination source. Maximum salt concentration was found to be 1600 mg/L at the position of the farthest domestic well located at north-east of the contamination source. The results indicate necessary requirement to remediate the carbonate aquifer and to maintain safe pumping limits.