Appendices 5-13 - Nautilus Cares - Nautilus Minerals

3.3 Affected areas and extents
In Table 3.1, from the stochastic modelling, a comparison of the expected maximum plume
concentration for the selected dilution ratios for the discharge given the two potential temperature
extremes and the affected areas (square kilometres) and maximum extents (kilometres) from the
discharge point are provided. The term “expected maximum concentration” is the mean expected
maximum concentration at each grid cell location (the mean value of the maximum from the 100
sample simulations per scenario). In Table 3.1 it can be determined that, on average, plume
concentrations exceeding 1/300 dilution was below the grid resolution of 60m x 60m for both
discharge temperature extremes. Indeed, a 1/600 dilution is just measurable at this resolution given
an average maximum extent of 0.085 km and area being 0.0144 km 2 .
Plume Concentration
PPB
Table 3.1 Comparison of Expected Maximum Plume Concentrations*
Plume
Concentration
Dilution factor
Area (km 2 ) & Extent (km)
of each division.
Discharge at 5.8°C
Page 15 of 24
Area (km 2 ) & Extent (km)
of each division.
Discharge at 11.4°C
200000 - 3333333 1/300 -1/5,000 0.47 km 2 & 0.60 km 0.30 km 2 & 0.49 km
3333333-1000000000 1/300 -1 < 0.0036 km 2 & 0.060 km < 0.0036 km 2 & 0.060 km
* Note. Area and extent calculations are cumulative
In Figure 3.2 the expected maximum concentration (over a 60m x 60m grid cell area) for the lower
temperature discharge, 5.8°C is shown. The colour coding represents the selected dilution ratios
from Table 3.1. In Figure 3.3 the expected maximum concentration for the higher temperature
discharge, 11.4°C, is shown. From Figures 3.2 and 3.3, it can be seen that the discharge
temperature creates some variability in the plume extent.
Stochastic modelling also provides a measure of probability for each cell with the model grid. The
term “probability of exceeding threshold” is the probability (as percent of samples) that the threshold
for the chosen threshold is exceeded at any time at each grid cell location. In Table 3.3 a
comparison of the probability of plume concentrations for each cell exceeding the 200,000 PPB
(1/5,000 dilution) is provided. Table 3.3 shows that the plume will only exceed the 1/5,000 dilution
threshold more than 50% of the time over an area of 0.81 km 2 at no more than 0.9 km from the
discharge site regardless of the temperature of the discharge plume. Table 3.3 also shows that the
extent of the plume at concentrations exceeding a 1/5,000 dilution is unlikely to extend more than