Environmental Assessment
Volume B-3.7
Mine
Hydrogeology
Ambatovy Project
81
January 2006
3.7.3.3 Results
Based on the hydrogeological and lithological conditions documented during the
study, four layers were delineated. These layers are:
Ferricrete
- This is a surficial layer that forms a few metres (average 4 m) thick
crust over the top of the deposit. The ferricrete is localized and does not extend
throughout the whole of the study area.
Ferralite
(
Laterite
) - The ferralite layer underlies the ferricrete and is a red-
brown clay with an average thickness of 40 m. The ferralite extends through the
whole of the study area, and is considered to contain large volumes of
groundwater. However, due to the low permeability nature of the ferralite, the
aquifer is low yielding.
Saprolite
- This is the transition zone between the ferralite and bedrock where
the rock is already weathered but not completely altered into the red-brown clay
layer that constitutes the ferralite. The alteration often takes place along fractures
and/or faults, which gives this layer an irregular appearance.
Bedrock
- This comprises ultrabasic primary rocks such as peridotites and
gabbros intruded by numerous intrusive dykes, and it has a secondary porosity
resulting from fractures, joints and faults.
A conceptual groundwater model for the mine area is shown on Drawing
No. 3.1.4 in Volume I, Appendix 7.2. The ferricrete generally has the highest
transmissivity and therefore 50 to 60 percent of the groundwater flows within the
ferricrete layer to discharge along the edges of the plateau to surface water
sources. Residual groundwater that infiltrates vertically through the ferricrete
into the ferralite eventually reaches the ferralite associated groundwater table.
The groundwater migrates downward within the ferralite to the saprolite
transition zone and then travels at a near horizontal gradient to discharge into
surface water bodies.
The Ambatovy and Analamay ore bodies are situated on a topographic high
which forms a surface water and groundwater divide. Therefore, groundwater
from the proposed pit areas would be expected to flow concentrically away from
the ore bodies. A relationship between groundwater levels observed during the
hydrocensus and the topography is shown on Drawing No. 3.1.2 in Volume I,
Appendix 7.2. The figure shows a linear trend when comparing the topographic
elevation versus static water level data for each of the aquifers. As shown, there
are distinctive water tables associated with each of the aquifers. A general
