Environmental Assessment
Volume B-3.8
Mine
Hydrology
Ambatovy Project
99
January 2006
ponds are predominantly operated to provide flows that, at a minimum, meet
current average levels. Although a degree of excess runoff can be diverted to
supplement mine water supply, these volumes would be released to the receiving
streams if considered advantageous or not detrimental to the downstream
environment
Changes in flows during periods in the wet season could be more pronounced
due to high rainfall and large amounts of excess runoff. A portion of the excess
runoff is used to supply the mine while the remainder is discharged to the
receiving streams. During wet season periods and assuming all ore process plant
water needs (about 1,000 m
3
/hr) are met from the ponds, the flows immediately
downstream of the ponds are expected to increase by 0 to 5% in Year 4
depending on the basin, and increase by an average of about 21% in Year 20.
The exception is the Torotorofotsy basin where changes in flows in Years 15 and
20 are limited to 14% and 12%, respectively, in an attempt to minimize effects on
the downstream wetlands. The largest increases in flows occur in January in all
basins.
The estimated changes in flow are based on the model inputs and assumptions
given in Volume I, Appendix 8-2, Table 8.2-1. One of the key assumptions is the
amount of water diverted from the ponds to the ore preparation plant. This water
can be used to meet plant water requirements, and also includes runoff diversions
from mine pits and waste stockpiles in the Analamay area to maintain acceptable
water quality levels in pond release water. Throughout operations, downstream
flows and physical conditions will also be monitored, and feedback will be
solicited from downstream users to verify that any changes are considered
acceptable.
Table 3.8-7 Change in Flows at 500 m Downstream of Runoff Collection Ponds
Operation Scenarios
Basin
Season
Year 4
Year 10
Year 15
Year 20
Post-Closure
(after Year 28)
December - March
0 to 2%
12 to 17%
11 to 20%
12 to 21%
23%
(b)
Antsahalava
April-November
0%
0%
0%
0%
23%
(b)
December - March
0 to 5%
12 to 16%
16 to 20%
18 to 22%
20%
Sahaviara
April-November
0%
0%
0 to 1%
0 to 3%
(a)
20%
December - March
0 to 3%
9 to 17%
14 to 20%
16 to 22%
24%
(b)
Sahamarirana
April-November
0%
0%
0%
0%
24%
(b)
December - March
0%
5 to 10%
3 to 8%
10 to 12%
8%
Torotorofotsy
April-November
0%
0%
0%
0 to 5%
(a)
8%
December - March
0%
3 to 8%
12 to 20%
0 to 8%
12%
Sakalava
April-November
0%
0%
0%
0%
12%
December - March
1 to 3%
0 to 1%
0 to 9%
18 to 22%
11%
Ankaja
April-November
0%
0%
0 to 2%
(a)
0 to 3%
(a)
11%
(a)
Maximum changes occur in November; all other months are 0% change.
(b)
Reduces to < 20% change within 1,500 m of ponds.
