The objective of the study was to compare the performance metrics of alternative closure actions for a smelter site and determine the environmental and financial impacts for each of the alternatives. The baseline closure plan was simulated using GoldSim to establish the expected performance measures for the strategy. These included the completion times for the various closure activities (e.g., soil remediation activities) and the associated costs. Uncertainty about the closure activities (e.g., start times) and the system conditions (e.g., TDS concentrations in the groundwater plume, depth and aerial extent of soil contamination) were explicitly represented in the model using probability distribution functions.
The model simulates the collection of several groundwater plumes that have been identified at the site. The baseline plan is to send the water from the interceptor wells to a central mixing tank where the source with the lowest TDS concentration will be diluted with industrial water to bring the TDS concentration down to 1000 ppm. The model tracks the total volume of source water diluted and continues to mix additional source waters until the maximum permitted discharge rate is reached. The remaining source water is diverted to four evaporation cells.
The model includes a climate model that simulates rainfall and evaporation based on 10 years of historical data for the site and reproduces the variability in climate during this period (cool wetter years and hot, dryer years). An example plot of the evaporation rate over a 10-year period is shown below. The model tracks the inflows, precipitation and evaporation in the evaporation cells as a function of time. The model tracks all capital and operating costs associated with the closure activities, including maintenance and replacement costs. Costs are discounted to net present values.
The simulation of the baseline closure plan indicated the interceptor wells would exceed the capacity of the passive water treatment system (dilution and evaporation) and require the brine concentrator to be operated for one-to-two months during a 10 to 20 year period. An alternative plan was developed that involved diluting additional water and constructing a pipeline to transport the water during the summer months to a nearby ranch for agricultural use. The volume of water diverted from the evaporation ponds was sufficient to eliminate the need for the brine concentrator, saving an estimated $20 million over the projected duration of the closure activities.