Stressors and Soil Contamination - Sustainable Practices in Geoenvironmental Engineering - page 48

and dichromate (Cr 2 O 7 ) anions will not only render the previously nontoxic trivalent chro-
mium toxic but will also make it more mobile. The major form of Cr(VI) is CrO 2− at pH
greater than 6.5 and HCrO 4 − at pH less than 6.5. Both ions are very soluble.
Other than the natural sources, chromium found in the geoenvironment can be traced
to waste discharges and tailing ponds associated with chromium mining. Principal uses
for chromium (Cr) and its compounds include (a) use of chromium as alloys, with iron and
nickel-stainless steel and super alloys as probably the best known alloys, (b) chromium
compounds used in metal plating, tanning of hides, wood preservation, glass, and pottery
products, and (c) production of chromic acid. Chromium in the land and aquatic compart-
ments of the geoenvironment can be the result of production and waste discharges associ-
ated with the industries and from the tailing ponds associated with mining activities.
2.4.1.4 Copper (Cu)
Copper is found naturally in sandstones and in other copper-bearing oxidized and sulide
ores. These include such ores as malachite [Cu 2 (CO 3 )(OH) 2 ], tenorite (CuO), cuprite (Cu 2 O),
and chalcopyrite (CuFeS 2 ), with chalcopyrite being the most abundant. In addition to its
natural occurrence in the land environment, contributions of Cu to the geoenvironment
come from (a) deposition of airborne particles from mining of copper and combustion
of fossil fuels and wastes, (b) discharges from industrial processes utilizing copper as a
metal, and copper compounds (production of electrical products, piping, ixtures, and dif-
ferent alloys), and (c) industrial and domestic discharge of wastewater.
Copper deposited on the surface of the land environment from the various sources dis-
cussed in the preceding will initially be attached to organic matter and clay minerals, if such
are present in the landscape. Degradation of the organic matter through anaerobic or aerobic
means will release copper in its monovalent or divalent form, respectively. However, if the
subsurface soils contain reactive soil particles, the released copper will be bound to these
particles. Environmental mobility of copper in the substratum is not generally a big factor
when the soil substratum is composed of ine soil fractions consisting of clay minerals and
other soil fractions with reactive particles. Presence of copper in the receiving waters is most
often conined to the sediments since the copper will attach itself to the ine particles in water.
In terms of health considerations, copper is considered to be an essential trace element in
both human and animal nutrition. The amounts required, however, are extremely small.
Threshold limits for human ingestion of copper vary between different countries and
jurisdictions, with values of about 1.3 ppm for drinking water and 0.1 mg/m 3 for airborne
concentrations being reported.
2.4.1.5 Lead (Pb)
Lead in nature is found in sulide, carbonate, and oxide forms. These are galena (lead sulide,
PbS), anglesite (lead sulfate, PbSO 4 ), cerrusite (lead carbonate, PbCO 3 ), and minium (lead
oxide, Pb 3 O 4 ). Although it has three valence states (0, +2, and +4), the most common state
is +2. Compounds of Pb(II) have ionic bonds whereas the higher valence state, Pb(IV) com-
pounds, have covalent bonds. Lead found in the land compartment of the geoenvironment
will most often be bonded to reactive soil particles. It is a nonessential element.
Lead is used to a very large extent in the manufacture of lead-acid batteries and in the
electronics and munitions industries. Other lesser uses for lead are in production of crys-
tal glass, lead liner material, weights, insecticides, and in construction. Lead found in the