Environmental Assessment
Volume B-3.9
Mine
Water Quality
Ambatovy Project
134
January 2006
pond in the Torotorofotsy basin. Therefore, changes in sediment quality in the
Torotorofotsy Wetlands are expected to be less than the maximum predicted
sediment quality changes in the Torotorofotsy River. Similar to the other river
basins, negligible changes in sediment quality in the Torotorofotsy Wetlands
would occur during post-closure conditions due to negligible changes predicted
for the Torotorofotsy River.
3.9.6
Impact Analysis
3.9.6.1 Prediction Confidence
The main potential sources of uncertainties in the water and sediment quality
assessment include surface and groundwater flows, baseline water quality,
seepage quality and runoff water quality. The prediction confidence of the
surface and groundwater flows are medium and low, respectively, as discussed in
Volume B, Sections 3.8 and 3.7. Baseline groundwater quality is not well
understood in the mine area; however, the effect of this limited information on
predicted changes in water and sediment quality is estimated to be medium to
low. The prediction confidence of the runoff quality from the mine and waste
areas are low to medium as discussed in Volume B, Section 3.2.
The number of pooled data sets for one year of sampling from the watercourses
ranged between eight and ten for each of the two (wet and dry) seasons. During
the dry season, the baseline condition of general surface water quality in the LSA
is adequately understood, resulting in moderate certainty in the data. Measured
surface water quality in the wet season is based on high analytical detection
limits; therefore, many data sets are below the corresponding detection limits.
Thus, substance concentrations used in the water quality model to characterize
baseline conditions in the wet season are potentially higher than the actual
concentration levels, resulting in more conservative predictions of surface water
quality.
Conservative assumptions were used in the water quality assessment, including:
•
no attenuation of water quality substances, except suspended solids, in
the collection or clarification ponds; and
•
no geochemical transformations and settling in the receiving waters; and
•
total chromium occurring in the hexavalent form.
The prediction confidence is also dependent on the success of the effective
mitigations proposed, including erosion control, water management and
sustainable reclamation. The proposed mitigation methods, such as sediment
