Environmental As
Tailings Facili
sessment
Volume E-3.8
ty
Hydrology
Ambatovy Project
76
January 2006
Table 3.8-6 Drainage Areas Downstream of the Tailings Facility
Basin
Basin
Segment
Basin
Segment
Area (km
2
)
Total
Drainage
Area (km
2
)
Area Diverted by
Tailings Facility
(km
2
)
Non-Affected
Drainage Area
(km
2
)
Reduction in
Drainage Area with
Diversion
A
A(tailings)
4.0
4.0
3.8
0.2
94%
A1
1.8
5.9
3.8
2.1
64%
A2
1.7
7.6
3.8
3.8
50%
A3
1.1
8.7
3.8
4.9
44%
A4
0.8
9.5
3.8
5.7
40%
A5
0.5
16.1
7.1
10.6
34%
A6
0.8
24.4
10.0
14.4
41%
B
B(tailings1)
1.7
1.7
1.7
0.0
100%
B(tailings2)
0.6
0.6
0.6
0.0
100%
B1
1.2
3.6
2.4
1.2
66%
B2
1.1
4.7
2.4
2.3
50%
B3
1.4
1.4
0.0
1.4
0%
C
C(tailings)
2.7
2.7
2.9
0.0
100%
C1
2.2
5.0
2.9
2.2
55%
C2
1.1
6.1
2.9
3.3
45%
C3
1.4
7.4
2.9
4.7
37%
Changes in streamflow within the various tailings sub-basins were calculated for
average conditions based on an average monthly runoff of 142 mm, as derived
from a mean annual runoff of about 1,700 mm. A similar analysis was
completed for dry conditions using a monthly runoff of 50 mm, as observed
during the 2004-2005 dry season. Results of the analyses are presented in
Tables 3.8-7 and 3.8-8. The baseline, operational and post-closure flows
represent the total flow in a stream, which is derived from both direct surface
runoff and groundwater inflows.
During operation, reductions in streamflow for each reach are roughly the same
for both the average month and dry month scenarios. They are also comparable
to the percent reductions in drainage areas shown in Table 3.8-6, which indicate a
much higher sensitivity to changes in drainage area than to changes in
groundwater contribution. Although the groundwater flux will decrease due to
interception and reduced permeability in the tailings area, the groundwater
component to total flow appears to be small. The exception to this may be in the
dry season when a larger portion of flow is expected from groundwater.
However, baseline data collected in 2004 to 2005 indicate considerably larger dry
season flows than expected from baseline groundwater modelling. This suggests
that dry season flows may also be maintained by basin storage, or that the
available groundwater data reflect large uncertainties.
