In the lecture I treated the main concepts of environmental contamination, with some examples from the real world. I give a point by point summary below:
Contamination: levels of elements or compounds above the naturally occurring levels in that specific environment.
Pollution: element or compound levels above natural background but with proven deleterious effects on the ecosystem.
Point source: a spatially defined point of emission of one or more contaminants (discharge pipe, chimney, etc)
Non-point source: contaminant flux from a large area, such as nutrients from fertilized agricultural fields, run off from city streets, emission of gases from wetlands etc.
Contamination occurs in air, water and soil, and from all three environments we will see uptake into the local ecosystems (air: inhalation, water:imbibation, soils: uptake through microorganisms). Deposition from the atmosphere to the land occurs for many substances (atmospheric deposition) and this is the main process that has created low level contamination over much of the globe.
Concentrations of contaminants in water and soil/sediment are usually given in ppm or ppb (1 ppb Hg = 10-9 gr Hg/gr of soil) whereas in air those are given in ppm (vol). More important than bulk concentrations are the concentrations of the specific chemical species that are present: Chromium can occur as the shiny metal (relatively harmless), as Cr3+ ions (green solutions, relatively harmless) and Cr6+ (hexachromium, orange, very carcinogenic). Instead of documenting a specific Cr concentration in soil, it is more useful to report the amount (or % of total) of the Cr6+ (remember Erin Brockovitch). Apart from the fact that different chemical species have different effects on organisms, we have to assess the bioavailability of the species: how easily is it taken up by a given organism, what is the pathway of transfer into the ecosystem etc. The typical example here is Mercury, where Hgo (the liquid metal) is not easily taken up by higher organisms, whereas the organic form (methylmercury or (CH3)Hg+) is very easily assimilated in cell lipids. Another issue here is biomagnification or bioaccumulation: imagine that a microbe assimilates Cd from a polluted soil, and another animal eats these microbes: the animal will end up with lots of Cd from all the microbes. The animal is eaten by another predator and this animal ends up with the body burdens of many of the first animal and so on. The highest part of the food chain (carnivorous predators) will have the highest concentrations of contaminants as a result of this bioaccumulation process.
Typical contaminant groups are:
Metals: Cu, Zn, Pb, Hg, Cd, Ni, Cr etc
Acids: HNO3 and H2SO4 from air pollution, oxidation of ore-sulfide minerals, aerosol droplets
Organic compounds: PCB's, dioxin, benzene, MBTE, pesticides like DDT, etc
Nuclear waste: 90Sr, 137Cs, 131I, 60Co, 239Pu etc
Oxygen consuming compounds (BOD or Biological Oxygen Demand): mainly human and animal waste (sewage). Microbes mineralize the Corg and use O2 from the water or air and make CO2 (or CO) from the sewage particles.
Nutrients: NO3-, HPO4=. These can cause excess productivity (algal blooms) which leads to increased BOD in the water column.
Other substances: asbestos, silica-fibers etc.
Main sources of contaminants in the environment are:
High temperature combustion processes, such as coal fired power plants, garbage incinerators, sewage sludge incinerators, and medical waste incinerators. Most of these constitute point sources for a slew of contaminants, but atmospheric dispersal, conversion processes in the atmosphere and rain out lead to very disperse deposition in the environment. Typical contaminants groups from these processes are CO2 (greenhouse gas), NOx (forms from N2 and O2 in the air during the combustion process), SO2 (from FeS2 in the coals), trace metals (Cu, Zn, Pb, Cd, Hg in FeS2 in coals) and organic compounds (such as dioxin from plastic burning). The NOx and SO2 are converted in the atmosphere to HNO3 and H2SO4 and form the main constituents of acid rain. The metals are adsorbed on aerosol particles that form in the air and rain out over land and water. This forms the dilute 'veil of contamination' that appears in soils and waters in even the most remote areas, from the high Rockies to the Greenland ice cap.
Discharge pipes from Waste Water Treatment Plants (WWTP): Sewage particles, metals, bacteria, nutrients, organic contaminants
Urban run off: grease, organic volatiles, nutrients, metals
Ore smelters: Cu, Pb, Zn, S, Se
Ore waste piles/ coal mine dumps: Acid Mine
Drainage from the oxidation of pyrite
FeS2 + 3.75O2 + 3.5H2O => Fe(OH)3
+ 2SO4=+ 4H+
Pyrite + oxygen+water=>limonite (rust) + sulfate+ acidity
Point sources: industry-specific effluents
such as Pb from battery factories, Silver from silver plating industries,
Hg from chlorine bleach factories etc.