Mine Rehabilitation Handbook - Mining and Blasting

6.4 Alkaline and Saline Soils
Alkalinity and salinity commonly occur together in Australian soils.
Highly saline strata are often encountered at mining sites. As a general
rule, overburden closest to the surface will be less saline. It should be
selectively handled and used to sheet dumps prior to topsoiling. Where
local soils are saline or salt pans and scaids are common, upper levels
of overburden may also be saline. Highly saline material should be
treated in a similar way to acid forming overburden, to avoid adverse
effects on plant growth and downstream water quality.
6.4.1 Determining Alkalinity and Salinity of Soils
• As a guide, soils can be considered moderately saline when they
have an EC (1:5 soil:water suspension) of 100-300 mS/m and
chloride levels exceeding 0.15% and highly saline when the EC is
greater than 300 mS/m and chloride levels exceed 0.45%.
• Soils can be considered alkaline when they exceed pH 8.0 and
strongly alkaline at pH 9.5 or above.
• Sodic soils are soils where sodium ions constitute a significant
proportion of the exchangeable cations in a soil. Sodic soils often
have poor structure and may become strongly alkaline. Soils are
classified as sodic when they have an exchangeable sodium
percentage (ESP) of 6 and strongly sodic at ESP 15.
• Soils affected by salinity and sodicity commonly occur in arid and
semi arid areas but have been extended by agricultural practice,
particularly irrigation.
• These soils may be recognised by salt crusting, sparse and salt
tolerant vegetation or “scalds” of bare and cracking earth with highly
dispersive soils.
6.4.2 Treatment
• Saline soils that are not strongly alkaline can, in theory, be reclaimed
by leaching the salts from the rooting zone.
• Sodic soils can be reclaimed by exchanging some of the
exchangeable sodium with calcium. Gypsum is the most effective
ameliorant for sodic soils. Application rates for mildly affected soils
are 2.5-5.0 tonnes/hectare. Application rates of 20 tonnes/hectare
may be required in severe conditions.
61
• Prolonged irrigation will leach out sodium and other deleterious salts
from saline and sodic soils but is only practical when the soil remains
permeable, the water table is well below the root zone and an
adequate supply of suitable water is available.
• Forming water impoundments 300-400mm high to induce infiltration
and leaching of salts is widely used in agriculture. Several seasons
are required to achieve sufficient leaching in arid areas.
• Ridge-ploughing to form ridges 200-300mm high spaced at 1-2
metres apart enables a more rapidly leached root zone to be formed
for direct seeding and planting.
6.5 Heavy Metals and Toxic Mines Waste
• Heavy metals occurring naturally on the site may be mobilised and
cause a potential health or environmental problem in overburden,
soils and water. Acid conditions and acid drainage indicate a possible
heavy metals problem.
• Arsenic, cadmium, mercury, lead, nickel, manganese and
molybdenum are cations that are potentially harmful to human life
because they bio-accumulate and/or require relatively small dosage
to affect health.
• Elevated levels of radioactivity may be associated with some mine
wastes. Identification and appropriate disposal of these wastes is
essential to avoid impact through: - direct exposure, offsite
contamination, or animals browsing on vegetation that has
accumulated radioactive elements.
• Copper is the principal concern for aquatic systems; however, other
heavy metals may accumulate in sediments.
• Aluminum, manganese, copper, zinc and lead may become available
in acid soils and be toxic to plant growth.
• Sub-grade ore, waste rock etc, which has the potential to generate
acidity and mobilise metals, should be selectively placed within
waste dumps (refer section 6.3).
• Neutralise the upper 200mm of waste material with lime or similar
material (refer 6.3) to provide a buffer against possible mobilisation
by acid drainage.
62