Environmental Assessment
Volume E-2
Tailings Facility
Project Description
Ambatovy Project
14
January 2006
2.7
WATER MANAGEMENT DESIGN
2.7.1
Operational Flows
A water balance analysis was completed to estimate operating water storage
requirements and excess water removal pumping rates for the return water back
to the plant. Based on the water balance, a total operating water storage
requirement of 4 Mm
3
has been estimated for the water basin to satisfy
operational water storage requirements for each phase. Such storage provision
will allow excess runoff from wetter months to be temporarily stored until it can
be removed from the tailings facility during dryer months. Water balance results
for average precipitation conditions indicate that the average return water
pumping rate will be about 2,500 m
3
/hr. To provide adequate excess water
removal capacity for more extreme periods, the return water system maximum
design flow has been estimated at 4,000 m
3
/hr for all phases.
Gravity decant weirs with stoplogs for controlling water levels will be used to
transfer water from the tailings basin to the water basin. This will require the
tailings basin to be partially flooded in the first few months of operations for each
phase to allow flow to the water basin by gravity. A barge type pump
arrangement will be used for excess water removal and return to the plant from
the water basin. The barge system will be relocated to the final return water
return pond at the start of Phase 3.
A network of groundwater collection wells will be established immediately
downgradient of the constructed embankments. These wells will be designed to
intercept seepage from the tailings facility in order to prevent downstream
surface water quality impacts. Water from these wells will be pumped to the
water basin and managed with the supernatant and storm water collected in the
tailings facility.
2.7.2
Effluent Treatment
The tailings area will offer an opportunity for the solids to settle and consolidate.
Before pumping to the tailings pond, the water is neutralized to pH 8. At this pH
most of the metals will be precipitated as solids and immobilized in the tailings
facility. Supernatant water, which will include rainwater, will be pumped from
the tailings facility back to the process plant where a portion of the water will be
reused; the balance of this water will be disposed of to the ocean via an outfall
pipeline. This effluent will be pumped out to sea at a maximum design flow of
4,000 m
3
per hour. The length of the section of pipeline out to sea may be about
1 km where water depth is approximately 20 m (Figure 2.1).
