Environmental Assessment
Volume B-3.8
Mine
Hydrology
Ambatovy Project
110
January 2006
help quantify and understand the suspended solids concentrations associated with
more extreme storm events.
3.8.6
Impact Analysis
3.8.6.1 Residual Impacts
During operations, the highest changes in streamflows are expected to occur
toward the end of the operational phase when active mining and stockpile areas,
as well as reclaimed areas, are greatest. The largest changes in flows occur in
January (worst case) and, for most basins in Year 20, the increases are expected
to be 15 to 20% above baseline conditions for the first 1.5 km downstream of the
embankments (increases in the Torotorofotsy and Sakalava watersheds are less at
about 7%). For mean conditions assessed on a monthly basis, the changes in
flows are on the order of 10% at 1.5 km downstream of the ponds. Based on the
data provided in Table 3.8-8, these impact magnitudes are considered moderate
for worst case conditions and low for mean flows. In general, increases in
January flows will be 5 to 10% at distances 5 to 7.5 km downstream of the ponds
(i.e., low impact), and are expected to be less than 5% (i.e., negligible impact)
beyond the 7.5 km point. January flow increases within the Torotorofotsy and
Sakalava drainages are slightly less with low impacts expected between 1 and 3
km downstream of the collection pond, followed by negligible changes beyond
that. In terms of mean flows, the impact magnitudes are less than 10% (low
environmental consequence) beyond 1.5 km and less than 5% (negligible
environmental consequence) beyond the following limits: 3 km for the Sahaviara
basin, 5 km for the Antsahalava and Sahamarirana basins, 1 km for the
Torotorofotsy basin, 500 m for the Sakalava basin, and 8 km for the Ankaja
basin.
For post-closure conditions (all months), increases in flows of 10 to 22% are
expected in the Ambatovy area (Antsahalava, Sahaviara, and Sahamarirana
watersheds) up to a distance 5 km downstream of the collection ponds. These
moderate impacts are reduced to negligible impacts at about 7.5 km from the
ponds. Impacts in the Analamay area (Sakalava and Ankaja watersheds) are
considered moderate immediately downstream of the ponds (12% increase), low
by 1.5 km, and negligible at about 5 km. Changes in flows in the Torotorofotsy
River are considered low impact for the first 1.5 km downstream of the pond and
negligible beyond that point.
Changes in water levels were assessed at five locations around the mine site.
Based on relatively small changes at these locations, effects within the local
study area are also expected to be small and therefore result in a low
environmental consequence.
