Appendices 5-13 - Nautilus Cares - Nautilus Minerals

3 RESULTS AND DISCUSSION
3.1 Dilution Thresholds
Two dilution thresholds were used in this study to determine the plume extent from each discharge
option (scenarios).
Key dilution ratios used in the modelling were 1/300, 1/600 and 1/5,000. These key dilution ratios
were used in the discussion and presentation of modelling results. The discharge water undergoes
pre-dilution with ambient seawater at the surface by increasing the discharge from 758 m 3 /h to
909.6 m 3 /h, hence the model is set to an initial concentration of 833,333,333 PPB of production
water. Therefore the target dilution ratios equate to plume concentrations as follows:
• Pre-dilution is 833,333,333 PPB or 83.33% of the discharge stream
• 1/300 dilution is 3,333,333 PPB
• 1/600 dilution is 1,666,667 PPB
• 1/5,000 dilution is 200,000 PPB
The plume was modelled using a non-reactive tracer within the CHEMMAP system.
3.2 Plume dynamics
The discharge configurations tested using the modelling were two extremes in discharge
temperature and 5 parts returning production water and 1 part surface seawater. The orientation of
the end pipes are designed to face vertically upwards to maximize the initial mixing of the discharge
due to the turbulence associated with a large discharge stream. If the pipes were facing
downwards, this initial mixing zone would be limited by the close proximity of the bottom to the
outfall site. Two scenarios were conducted, one representing the potentially cooler discharge
stream, 5.8°C and the potentially warmer discharge stream, 11.4°C For each scenario, 100
simulations were conducted (using the CHEMMAP stochastic model) for both temperatures for
comparison purposes. From the 200 in total simulations conducted, some were selected and
examined in detail to determine the dynamics of the plume created by the two extremes in
discharge temperature as follows.
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