Environmental Assessment
Volume E-3.3
Tailings Facility
Soils
Ambatovy Project
33
January 2006
3.3.3.2 Assessment Criteria
The criteria used to rate soil residual impacts are outlined in Table 3.3-2.
Table 3.3-2 Assessment Criteria for Soils
Direction
Magnitude
Geographic Extent
Duration
Reversibility
Frequency
positive
,
negative
or
neutral
for the
measurement
endpoints
negligible
: no measurable
effect (<1%)
low
: <10%
moderate
: 10 to 20%
high
: >20%
local
: effect restricted
to the LSA
regional
: effect extends
beyond the LSA into the
region
beyond
regional
: effect
extends beyond the
region
short-term
: <3 years
medium-term
: 3 to
30 years
long-term
: >30 years
reversible
or
irreversible
low
: occurs once
medium
: occurs
intermittently
high
: occurs
continuously
3.3.3.3 Mitigation
Soil Erosion
Soil erosion is the displacement of soil by wind or water action. The potential
amount of water erosion expected on land may be calculated by the Universal
Soil Loss Equation (Wischmeier and Smith 1961; Gee et al. 1976). Details are
provided in Volume B, Section 3.3.
The soil erodibility factor (K) is affected by organic matter content and texture.
Soils high in silt and very fine sand are more susceptible to water erosion than
other soils. Geotechnical testing suggests that the tailings area will have a high
erosion potential due to the fine-textured surficial soils.
Wind and water erosion risk in the LSA is low when there is a vegetative cover.
During disturbance, the risk of wind and water erosion will increase. The risk of
wind erosion depends on soil texture, moisture and organic matter content, with
sandy soils having a higher risk. Water erosion risk increases where slopes
exceed 10% and fine-textured subsoils underlay coarse-textured soils. The
preliminary tailings plan outlines that tailings pit slopes will be greater than 30%
(Volume E, Section 2).
To prevent soil erosion and potential sedimentation from occurring during
construction, soil exposure must be minimized and surface runoff controlled,
especially during wet weather and in areas close to watercourses.
