Appendices 5-13 - Nautilus Cares - Nautilus Minerals

3.5 Sedimentation Thickness
The 1:5,000 dilution ratio is based on the presence of fine particulate (< 8 micron) ore which is still
contained within the return production water at concentrations of 5,940 mg/L. Hence a 1:5,000
dilution of this suspended sediment equates to 1.2 mg/L threshold of total suspended solids.
However, even though the modelling conducted here demonstrated that the concentrations of this
fine material falls rapidly below this threshold after discharge in the water column, this sediment will
slowly continue to fall to the seafloor over time and beyond the extents of the plume. In order to
understand and quantify the sedimentation of this fine ore over time, additional model runs were
conducted to quantify the sedimentation of this ore over the full mining operation (20 months)
particularly since this fine material will settle very slowly.
Note that the background daily sedimentation rate for the area has been measured at 0.0965
mg/cm 2 /day, which equals to approximately 590 g/m 2 for the 20 months of mining operation (or
0.180 mm) assuming average density of 3,300 kg/m 3 . From Figure 3.5, it can be seen that the peak
bottom thickness from the settling fines is less than 0.1 mm for the same 20 month period. Indeed,
the model predicted that sedimentation level between 0.05 mm and 0.1 mm were possible, but
extremely patchy. Between these patches, sedimentation levels were below the model resolution
here and a few orders of magnitude less that the natural background sedimentation for the region.
4 CONCLUSIONS
In conclusion, from the CHEMMAP single release modelling, and stochastic modelling of the plume
concentrations (200 individual sample simulations) for each of the two discharge options using
expected maximum concentrations and percent probabilities, the mixing is slightly enhanced when
the return water temperature is at the higher end of its potential range in a comparison to when the
return water is at the lower end of its temperature range. This is expected, since the buoyancy of
the warmer plume will be greater after discharge, hence enabling it to rise higher through the water
column, entraining more ambient water as it ascends.
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