Azura-Edo Independent Power Plant Environmental Impact ... - IFC

4.5.1 Soil Types
4.5.2 Soil Quality
The key soil types in Nigeria, as identified by the Food and Agriculture
Organisation (FAO) soil taxonomy legends are fluvisols, regosols, gleysols,
acrisols, ferrasols, alisols, lixisols, cambisols, luvisols, nitosols, arenosols and
vertisols (FAO, 2009). Within Edo State the soil type is chiefly a red-yellow
ferrasol on loose sandy sediments. Research from the neighbouring LGA of
Esan also illustrates ferrasol soils ranging in colour between red-brown in the
north of the region, to yellow-brown in the south, demonstrating a variation in
the leaching of the soil (yellow-brown soils indicate a higher level of leaching).
(Omofonmwan and Kadiri, 2007).
More specifically, Benin City is underlain by sedimentary formation of top
reddish clayey sand capping highly porous freshwater –bearing loose pebbly
sands, and sandstone with local thin clays and shale interbeds which are
considered to be of braided stream origin. Sands, sandstones and clays
vary in colour from reddish brown to pinkish yellow on weathered
surfaces to white in the deeper fresh surfaces. Limonitic coatings are
responsible for the brown reddish-yellowish colour.
The Study Area falls into the southern belt of forest soils. The soil types
within the forest soils zone are largely determined by the parent rock. The soil
type on the site is a mixture of sand, silt and clay with a predominance of sand
and hence can be defined as sandy loam (Royal Horticultural Society, 2011
and NIPP, 2010). The percent composition of silt and clay did however vary
considerably between seasons (Environmental Accord Limited Nigeria, 2011).
The soils sampled were predominantly friable in nature during the dry
season, with evidently higher moisture content during the wet season. Soils
were found to be fine grained with soil colour ranging between a reddish and
dark brown. Soils were lighter in colour to the south of the Project site
(southern portion) indicating poorer soils with less organic matter and a lower
nutrient content.
Northern Portion
Heavy Metal Composition. All heavy metals corresponded to levels that
naturally occur in soils (Allen, 1974) and fell within the limits tolerated by
plants and organisms indicating the low levels of pollution in the soil. The
highest concentration of heavy metals analysed across the Study Area during
the dry and wet season was iron, which was higher in the topsoil (864.10 –
20,000 mg/kg) compared with soil sampled at 15 – 30 cm depth (656.20 –
18,500 mg/kg). Nickel had the second highest concentration sampled during
the dry season (4.6000 - 10.150 in the topsoil and 5.6000 – 17.050 mg/kg in the
subsoil). The levels of nickel were less than 0.050000 mg/kg at all points
sampled during the wet season. Chromium recorded the second highest
AZURA EDO IPP
4-8
DRAFT EIA REPORT