Tailings Disposal - Mining and the Environment: From Ore to Metal - page 758

submarine currents meet land masses, or localized, caused by wind shear effects, where
wind-driven surface water is replaced by deeper water.
Bathymetric Survey
Assuming that the results of the initial review are favourable, the next stage is to under-
take a bathymetric survey of the coastal zone to either side of the coastal location closest to
the mine. The length of coastal zone to be surveyed will be selected based on information
from the initial review. If a very large area is involved, the survey may be conducted in two
stages - a broad survey over the entire area, followed by more detailed surveys in selected
sub-areas. Sidescan sonar surveys can be conveniently conducted along with the detailed
bathymetric survey. If the initial review indicates that seabed slopes are likely to be very
steep (20º or more), a seismic rel ection survey can also be incorporated in the programme.
Data from all these surveys are used to assist in the siting of the pipeline route and outfall
location and to assess seabed stability.
Seasonal Studies
Again, assuming that the results of the preceding step coni rm favourable conditions for
DSTP, the next stage is to collect oceanographic data required to coni rm site suitability,
to assist in selection of design parameters, and to provide data for input to subsequent
numerical modelling studies. Typically, two areas will be selected for parallel study in case
a 'fatal l aw' is identii ed at one or the other. Data required include current velocities and
current directions, and salinity-temperature-density (STD) proi les through the water
column. These data are required for a complete lunar cycle in each season of the year.
Currents are measured either by deployment of current meters, of which several types are
available, or by means of Acoustic Doppler Current Proi ling equipment (ADCP). STD
information is generated by cable-mounted sensors that measure temperature, salinity
and pressure allowing temperature, salinity and density (computed from a combination of
these parameters) to be plotted against depth (computed from pressure). STD probes are
conducted at a variety of locations at different stages of the tidal cycle in each season, to
dei ne the variations in depth of the mixing zone throughout the year. This information is
used to determine the minimum depth of the tailings outfall. The investigations outlined
above should be sufi cient to evaluate the feasibility of DSTP, to identify the preferred site
and to provide design information such as a preferred pipeline route and outfall location.
The data obtained can also be used for modelling studies to assess tailings transport and
sedimentation, as part of impact assessment.
Environmental Baseline Studies
Environmental baseline studies are normally carried out in parallel with these inves-
tigations and it may be logistically convenient to conduct such studies at the same time
as vessels are used to deploy or re-deploy current measuring equipment, and to carry out
STD probes. The baseline studies specii c to DSTP include: (1) sampling of seabed sedi-
ments for physical and chemical analyses, and for laboratory assessment of benthic organ-
isms; (2) sea water sampling and analysis, and (3) i shing to identify species and relative
populations of deep water i shes, and to obtain tissue samples for laboratory analysis of
baseline metal concentrations.. In addition to these baseline studies, it is prudent to estab-
lish baseline conditions in the nearby shallow water coastal habitats. Although these
habitats should not be affected by DSTP, they are likely to be of major concern to local
people, and it may be necessary at some time in the future, to prove that DSTP has not
affected them.