During the mid-1940’s to mid-1950’s, low-level radioactive wastes (primarily radium and uranium) were deposited beneath and mixed in with other wastes in a municipal landfill in Port Hope, Ontario. To address this problem a predictive groundwater flow and contaminant transport model was developed for the area surrounding the former landfill site. The model was used to simulate steady-state groundwater flow in the multi-aquifer system underlying the site and to predict offsite contaminant migration. The model was also used to evaluate various remedial alternatives and to provide direction for further field investigations.
The landfill was situated above a multi-aquifer system consisting of a perched water table, and a lower sand aquifer and a bedrock aquifer. Steep ravines bordered the study area to the north and south. The lower sand aquifer was in good hydraulic connection with the streams that ran through the ravines while the bedrock aquifer was hydraulically connected with a major river to the east of the site.
The radioactive materials were deposited in the unsaturated zone above the perched water table. Groundwater concentrations in the source area were primarily controlled by: 1) the distribution of the waste; 2) solubility constraints for uranium and radium; and 3) adsorption and decay of the solutes during transport through the unsaturated zone. The volume and distribution of source material was delineated during an extensive program of spectral gamma logging.
Geochemical analyses were made with the PHREEQE chemical equilibrium model to determine the rate at which the radioactive contaminants would leach from the waste. Results indicated that radium would be bound in the soil zone. Movement of uranium and arsenic would be retarded; but solutes would eventually enter the groundwater beneath the landfill.
A three-dimensional groundwater flow model (MODFLOW) was used to simulate groundwater movement through the three aquifers. Calibration of the model was done by adjusting aquifer properties until a match was achieved between observed and simulated water levels and discharge to local streams.
Groundwater velocity data from the MODFLOW model was then supplied to the MT3D solute transport model. Analyses were done to simulate off-site movement of contaminants over a 40 year period. Three remedial scenarios were also evaluated to demonstrate the ability of the model to evaluate the effectiveness of redial efforts. The remedial measures studied included: (1) capping of the landfill to reduce infiltration; (2) installation of purge wells to capture the contaminant plume; and (3) excavation of the waste material in the source area.