At the request of Rosemont Copper Company, Engineering Analytics (EA) has completed a postclosure analysis to determine the impact of pit backfill on predicted groundwater level drawdown. The post-closure groundwater flow model developed by Tetra Tech (2010) was used for the analysis.
The predicted long-term (1,000 year) pit-lake level was 4,279 feet and backfill was simulated at an elevation of 4,279 feet. This backfill elevation maintains the same hydraulic sink condition as without backfill present. The pit-lake water balance illustrates that the pit lake reaches the top of the backfill after approximately 52 years and then equilibrates to a long-term lake stage of 4,286 feet (Figure 1). Drawdown predicted by the base model, without backfill, was compared to the backfill simulation to determine the impact of a backfilled pit. There was minimal change in predicted drawdown at 20 and 1,000 years and a small decrease in the extent of drawdown propagation at 50 and 150 years (Figures 2-5). There was no simulated decrease in the extent of the 5-foot drawdown contour in Davidson Canyon.