Environmental Assessment
Volume B-3.7
Mine
Hydrogeology
Ambatovy Project
90
January 2006
Table 3.7-4 Residual Impact Classification for Hydrogeology
Phase
Direction
Magnitude
Geographic
Extent
Duration
Reversibility Frequency
Environmental
Consequence
Issue: Changes in Groundwater Flows and Quality at the Mine Site
construction
neutral
negligible
local
short-term
reversible
low
negligible
operations
negative
low
local
medium-
term
reversible
high
low
closure
neutral
negligible
local
long-term
reversible
high
negligible
Issue: Changes in Groundwater Flows and Quality on the Mokaranana
and
Torotorofotsy Wetlands
construction
neutral
negligible
regional
short-term
reversible
low
negligible
operations
negative
low
regional
medium-
term
reversible
high
moderate
closure
neutral
negligible
regional
long-term
reversible
high
negligible
3.7.6.2 Prediction Confidence
The hydrocensus showed differentiation between the various groundwater
aquifers, and a linear relationship was established between topographic
elevations and static water level data. Further, the calculated transmissivities for
the ferralite and saprolite units are consistent with the findings of investigations
in 1998-99. Matching calculated groundwater levels in the numerical model with
observed groundwater levels during model calibration, however, proved difficult.
Thus, aquifer parameters (transmissivity and recharge percentage) used in the
numerical model were adjusted within realistic values to obtain the best possible
correlation between the calculated and observed groundwater levels. The extent
and depth of the expected groundwater drawdown is based on a lower
transmissivity value used to fit the numerical model, and a higher calculated
value for transmissivity would result in a larger zone of influence. The prediction
confidence of medium is attributed to the groundwater model and the effect of
mine dewatering on groundwater contributions to surface water baseflows and
ultimately the wetland areas.
Overall, groundwater quality analyses completed to date show consistent trends
between sample locations. However, based on evaluation of cation balance and
other general chemistry parameters, some results have been deemed unreliable
(GCS 2005, July 2005, Volume 1, Appendix 7.1). There is some uncertainty as to
whether or not chromium measured in groundwater is present in its hexavalent
form (Cr
6+
), owing to the inherent difficulties in delivering an unpreserved
groundwater sample to a laboratory for analysis of chromium (VI) as required by
analytical standards. Nevertheless, water quality estimates as well as mitigation
planned for the project has assumed conservatively that all chromium present is
in its more toxic hexavalent form. Groundwater quality results are attributed a
medium prediction confidence.
