Environmental Engineering Reference
In-Depth Information
adsorption by soil particles (USEPA 1995, Pescod 1992). Clay soils remove pathogens
through adsorption whereas for sandy soils it is through filtration. Pathogenic and
coliform organisms generally do not penetrate more than several meters in medium-
grained sand or finer material, while in fractured rocks where groundwater velocities can
be high, pathogens can be transported hundreds of kilometers (Freeze & Cherry 1979).
Fine textured soils tend to adsorb viruses more than coarse textured soils and clay.
Studies have shown that in heterogeneous aquifers of sand or gravel, sewage borne
pathogens can be transported tens or hundreds of meters. Laboratory investigations reveal
that viruses are relatively immobile in granular materials with adsorption being the major
retardation mechanism (Freeze & Cherry 1979). Wright (1999) reports on gross bacterial
contamination in karstic limestone terrain. Unsaturated fine-grained soils reduce bacteria
to acceptable levels and contamination of aquifers underlying unconsolidated soils is
unlikely.
2.2.3 Metals
High concentration of cadmium, lead, iron, manganese, aluminum, copper and nickel
pose a potential health hazard to humans and animals, and copper, zinc and nickel are
phytotoxic (Pescod 1992). Some metals such as Cu, Fe and Ni are essential
micronutrients for plants but become toxic at higher concentrations. Factors affecting the
plant uptake of trace metals are soil characteristics, toxic elements in the wastewater, type
of crop grown and background toxic elements in the soil and their distribution. Metals are
also removed from water through adsorption, ion exchange, hydrolysis, complexation
with organics, and precipitation reactions. Most metals in water tend to form hydrolyzed
species by combining with inorganic anions such as (bi) carbonate, sulfate, fluoride or
nitrate. Organic complexes may also be formed. Thus, the mobility of metals in
groundwater depends on the concentrations of the most important complexes formed by
the element in the water. In non-acidic groundwater, with high concentrations of
dissolved carbonates, metals like cadmium, lead and iron are maintained at low
concentrations due to their precipitation as insoluble hydroxides or carbonates (Freeze &
Cherry 1979). Thus lime application on soils to increase pH can be an effective way of
reducing the mobility and uptake by plants of such metals (USEPA 1992). Physical
adsorption of metals in groundwater occurs via clays, organic matter and colloids.
Fixation of cobalt, nickel, copper and zinc in soils and freshwater sediments is controlled
by the presence of hydrous oxides of iron and manganese. In an oxidizing environment,
the oxides of these metals occur as coatings on grains enhancing the adsorptive capability
of the medium (Freeze & Cherry 1979).
2.2.4 Major cations
The major ions, which undergo ion exchange in soil solution, resulting in changes to soils
characteristics are Ca, Mg, K and Na. Ion exchange sites are found in all soils, though
they are predominant in clays and soil organic materials (Fetter 1994). The cation
exchange capacity is defined as the sum of exchangeable cations adsorbed per unit
weight of soil. The Na and Ca exchange reaction is of special importance when it occurs
in smectite clays because it can cause large changes in permeability (Freeze & Cherry
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