Environmental As
Mine
sessment
Volume B-3.7
Hydrogeology
Ambatovy Project
82
January 2006
groundwater contour map of the study area has been compiled and is shown on
Drawing No. 3.1.3 in Volume I, Appendix 7.2. The figure shows that
groundwater flow direction in the area of interest varies due to the hilly nature of
the site. In general, groundwater level is dependant on the topography.
The groundwater is a Magnesium-Bicarbonate type water, and is typical of
groundwater that has been recently recharged by rainfall. The low electrical
conductivity (EC) and total dissolved solids (TDS) also indicate water that is
recently recharged. The relatively high magnesium, bicarbonate and silica
concentrations are indicative of interaction of the groundwater with the ultrabasic
bedrock.
Table 3.7-1 summarizes the groundwater sampling results to date. Some data
have been deemed questionable, as discussed in Volume I, Section 7.1,
Attachment 2. Attempts were made to determine the proportion of chromium
that is present as hexavalent chromium, but due to sampling and analytical
difficulties the results remained inconclusive. Regardless, the groundwater
quality data show that total chromium and nickel in the groundwater are both
elevated in the natural condition, at concentrations above the WHO provisional
drinking water guidelines of 0.05 mg/L and 0.2 mg/L, respectively.
The presence of the elevated chromium can be attributed to the natural
occurrence of chromium in the ultramafic lithologies underlying the site, and the
weathered saprolitic and lateritic soils derived from the ultramafic rocks.
Chromium naturally occurs almost exclusively in its trivalent state (Cr
3+
),
however, and the occurrence of Cr
6+
is normally attributed to industrial activity.
Cr
6+
is known to occur naturally as well, and while the mechanism for its
occurrence is not fully understood (Gray, 2003), a prominent hypothesis is that
manganese oxides in the soil can act as oxidising agents to convert Cr
3+
to Cr
6+
(McBride, 1994).
