Summary
Freshwater
During operation, flows downstream of the tailings embankment will be
reduced considerably due to the reduction in drainage area as a result of
runoff diversion in the upper basin. The expected changes along the
main stems of the streams are high (greater than 30%) in basins affected
by the development. The high impact extends through to the large
tributary that joins the Ambolona River system. Changes in downstream
flows for post-closure conditions are low.
Six water quality assessment scenarios were modelled, corresponding to
key project phases that represent baseline, operations (years 14, 20 and
27), post-closure (15 years after closure) and far-future conditions
(80 years after closure). The results of initial assessments showed that
groundwater seepage would result in elevated levels of mainly
manganese in downstream surface water, above the World Health
Organization drinking water guidelines. Based on these observations, a
groundwater interception system is proposed to collect seepage
immediately downstream of the tailings facility to minimize the
downstream impact to the surface water. The intercepted seepage will
be managed with the tailings facility effluent.
A stream below tailings area
The final modelling results predict that during operations all surface
water parameters of concern will be below WHO drinking water criteria.
Copper, manganese and zinc do exceed the South African Ecosystems
guidelines in some downstream basins, but only marginally for copper
and zinc. Baseline conditions for copper and zinc also exceeded these
guidelines. Therefore, manganese is identified as the critical substance
to manage in water below the tailings facility. For the post-closure
scenarios, parameters were predicted to be below drinking water
guidelines and there were marginal exceedences in some of the basins
of the South African Ecosystem guidelines for copper and manganese.
The Ocean
The marine outfall will be assembled on land, floated out to sea, flooded
and sunk onto the sea bottom. The construction activities will result in
increased turbidity around the pipeline which will have an effect on the
local marine resources. The effects will be short-term and on a local
scale. The effluent to be discharged into the marine environment
contains a variety of minerals, many of which are constituents of
seawater. However, the continuous discharge of large volumes of
effluent water will lead to the elevation of certain minerals to levels above
those which occur naturally in the Toamasina region.
The impact of tailings water discharge on marine water quality was
modelled based on predicted tailings water quality estimates derived
from laboratory testing of processed ore samples. The dilution and
settling modelling of the effluent, indicated that TSS, manganese and
sulphate were the constituents which would be above natural levels or
January 2006
Ambatovy Project — 39
