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LAST LECTURE THIS LECTURE SOIL DEGRADATION FACTORS FOR SOIL

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LAST LECTURE THIS LECTURE SOIL DEGRADATION FACTORS FOR SOIL Powered By Docstoc
					                                                                               LAST LECTURE



                                                               Soil erosion and its control




                                                                               THIS LECTURE



                                                               Soil denudation and degradation
                                                               Soil chemical contamination and remediation




           SOIL DEGRADATION                                     FACTORS FOR SOIL DEGRADATION


The deterioration of soil properties to the extent that the   Have to consider the natural context.
soil is no longer productive.
                                                              Two main components:
Components considered.
   Human induced degradation.                                 1) Disturbance of plant communities.
   Water erosion.                                               Over-grazing, deforestation, inappropriate crop production methods, and
                                                                overexploitation.
   Wind erosion.
   Erosion by gravity.
   Chemical deterioration.
                                                              2) Deterioration of Soil
   Physical deterioration.                                      Erosion, pollution, weakening of structure, large changes in pH, nutrients,
                                                                chemical composition




      TOXIC ORGANIC CHEMICALS


Released from plastics, plasticizers, lubricants,
refrigerants, fuels, solvents, pesticides
and preservatives

Xenobiotics are often toxic to living organisms and
resistant to biological decay

Compounds are often very similar to natural organic
compounds:
•    insertion of halogen atoms (Cl, F & Br)
•    insertion of multivalent nonmetals (N and S)




                                                                                                                                              1
Soil toxins may:                                                     PESTICIDES AND HERBICIDES
•     kill or inhibit soil organisms
•     be transported to air, water or vegetation

Sources of soil toxins:                                       •   Pesticides are chemicals designed to kill pests
•    industrial and municipal organic wastes                  •   Quantity applied is decreasing
                                                              •   Potency is increasing
•    discarded machinery
                                                              •   Herbicides are designed to kill weeds
•    fuel and lubricant leaks
•    military explosives                                      Benefits
•    pesticides                                               • Pesticides provide mosquito control (malaria vector)
                                                              • Protection of crops and livestock against insects
                                                                (increases agricultural productivity)
                                                              • Reduction of food spoilage during transport
                                                              • Herbicides facilitate conservation tillage




PROBLEMS WITH PESTICIDES & HERBICIDES

• Contamination of surface and groundwater
• Negative effects on microbial & faunal communities
• May remove natural enemies of pest species
  (rendering its use less effective)
• Some fungicides cure fungal diseases, but also kill
  mychorrizal fungi
• May take time to determine that a product is harmful to
  humans or wildlife (DDT)
• A small proportion of chemical applied reaches target
  (terminates on plant, in air and in soil)

Desirable pesticide characteristics
 1. Low toxicity to humans and wildlife
 2. Low soil mobility
 3. Low persistence




         TYPES OF PESTICIDES                                                  INSECTICIDES

                                                            • Chlorinated hydrocarbons (eg. DDT) until 1970s
                                                                  - banned due to persistence and toxicity
•Insecticides                                               • Organophosphates:
                                                                  - easily biodegradable but very toxic to humans
•Fungicides                                                 • Carbamates:
                                                                  - low mammalian toxicity and readily biodegradable
•Herbicides
(weed killers)

•Rodenticides

•Nematocides




                                                                                                                       2
                   HERBICIDES     NON-TARGET EFFECTS AND ALTERNATIVES


                                   Non-target effects:
• Generally exhibit lower
                                   • Bioaccumulation and biomagnification up the trophic levels
  mammalian toxicity
                                   • Disruption of human endocrine balance by traces of pesticides
  - plants targeted

• Deleterious effects on           Alternatives to pesticides & herbicides:
  aquatic vegetation
  - plants that provide habitat    •   Organic farming
  for (shell)fish                  •   Crop diversification (reduces insect/weed infestation)
                                   •   Provision of habitat for beneficial insects or inoculation
• Variety of options available     •   Organic soil amendments (reduces weeds)
                                   •   Pest-resistant plant cultivars




                   PERSISTENCE                  INDUSTRIAL ORGANICS

                                    Contaminate soils by accident or neglect
                                  Gasoline: benzene, polycyclic aromatic hydrocarbons
                                  Solvents: trichloroethylene
                                  Explosives: trinitrotoluene (TNT)
                                  Lubricants, hydraulic fluids transformer insulators & epoxy paints:
                                         PCBs – cause cancer & hormone effects in humans and disrupts
                                         reproduction in birds
                                           *extremely resistant to decay*


                                       Examples of
                                       industrial
                                       contaminants




Abandoned
wood-preserving
facility in
Michigan, USA


Contaminants
In wood-
preservers:

polycyclic
aromatic
hydrocarbons
(PAHs),
chlorophenols,
dioxins,
furans and
arsenic
                                   Bioremediation of wood-preservative contaminated soil using
(inorganic)
                                   white rot fungi in North Carolina. Chemicals of concern include
                                   pentachlorophenol and lindane




                                                                                                        3
PCB and dioxin-
containing soils
covered with
tarp at a
superfund clean-up
site, Michigan, USA




   WHERE DO INORGANIC POLLUTANTS GO?                                                   SOIL REMEDIATION FOLLOWING
                                                                                     ORGANIC CHEMICAL CONTAMINATION
                                                                          1. Physical and chemical methods

 1. Vaporize into the atmosphere                                          Ex situ treatment
                                                                          •    Remove soil and incinerate    high T chemical decomposition
 2. Absorbed by soils                                                     •    Remove soil and apply vacuum extraction or leaching
                                                                          •    The treated soil is destroyed
 3. Percolate and leach through soil
                                                                          In situ treatment
 4. React chemically within soil                                          •    Removal by injection of surfactant (later pumped out)
                                                                          •    Water flushing, leaching, vacuum extraction, heating (similar to ex situ)
 5. Broken down by microorganisms
                                                                          Organoclays
 6. Wash into streams through surface runoff                              •    Surfactants such as quaternary ammonium compounds
                                                                          •    Can replace metal cations on soil clays
 7. Absorbed by plants & animals, becoming part of food chain             •    Clays then attract instead of repel nonpolar organic compounds
                                                                          •    Soil contaminants are immobilized, increasing the likelihood of
                                                                               decomposition before uptake by a plant or animal




           SOIL REMEDIATION FOLLOWING                                                  SOIL REMEDIATION FOLLOWING
         ORGANIC CHEMICAL CONTAMINATION                                              ORGANIC CHEMICAL CONTAMINATION

                                                                              4. Phytoremediation
 2. Bioremediation
                                                                                  Plant roots take up pollutants from the soil:
 • Enhanced plant and microbial action degrades organic contaminants
   into harmless products                                                     (i) Hyperaccumulation
 • Natural bacteria or bioaugmentation employed                               • Hyperaccumulating plants tolerate high contamination levels
 • In situ or ex situ treatment with bacteria: works on PAHs,                 • The toxin is removed through harvesting    DO NOT EAT!
   pentachlorophenol and trichloroethylene
                                                                              (ii) Enhanced rhizosphere phytoremediation
 3. Biostimulation                                                            • Plant roots excrete compounds that stimulate the growth of
                                                                                    rhizosphere bacteria that degrade the organic contaminant
 • Enhance naturally-occurring microbial populations with fertilization       • Transpiration by the plant causes contaminant-laden soil water to
   (sometimes combined with a surfactant)                                           move toward the plant roots, where rhizosphere reactions take
 • Can inoculate soils with more effective microbes                                 place

                                                                              Phytoremediation only suitable where large areas of soil are
                                                                                 only moderately-contaminated     often time-consuming




                                                                                                                                                           4
                                                                                              COST AND TIME PER
                                                                                            REMEDIATION TECHNIQUE




                                                                SORBED OR COMPLEXED CHEMICALS




                                                          Some organic chemical pollutants are complexed with soil
                                                          organic matter or sorbed by inorganic materials

                                                             very difficult to bioremediate soils with high complexation
                                                          or trapping of pollutants within internal structural layers of
                                                          clays

                                                          BUT: complexed pollutants are rather immobile
                                                                  unlikely to cause significant environmental harm




  THE EFFECT OF SOIL AGING ON CONTAMINANT                         SALTS FROM COAL BED METHANE
                AVAILABILITY                                              PRODUCTION
  Some pollutants become
  a) trapped in nanopores,                                Water used to apply pressure becomes high in sodium
  b) adsorped to minerals or complexated with humus, or
  c) buried under mineral coatings.                       Salts can slowly accumulate in the root zone

  These trapping processes increase with time
                                                          Impairs aggregation
                                                          and reduces hydraulic
                                                          conductivity
 ‘trapped’ pollutants
    are virtually un-                                     Increases osmotic potential
affected by microbes
                                                          Can be ‘washed’ from
isolated from living cells
                                                          well-drained soils with
   and their enzymes
                                                          limited success




                                                                                                                           5
          TOXIC INORGANIC SUBSTANCES                                             ELIMINATION OF INORGANIC CHEMICALS

   Also coined ‘HEAVY METALS’ (though not all are metals or heavy)
Mercury        Cadmium          Molybdenum       Fluorine            Boron
Lead           Arsenic          Manganese        Zinc                        1. Reduce application of toxins
Nickel         Copper           Selenium         Chromium

                                                                             2. Immobilize
                                                                                   Maintain pH > 6.5
                                                                                   Drain wet soils (oxidized forms are usually less soluble)
                                                                                   Heavy phosphate application (reduces availability)

                                                                             3. Removal by chemical, physical or biological remediation
                                                                                   Hyperaccumulating plants
                                                                                   Chelating compounds can solubilize lead (used in
                                                                                   combination with hyperaccumulators)




 INORGANIC POLLUTANTS IN SEWAGE SLUDGE                                              NATURAL ATTENUATION LANDFILL




                                                                                                                           REQUIREMENT:

                                                                                                                           >1.5 m clean soil to water table

                                                                                                                           Moderate permeability

                                                                                                                           High CEC

                                                                                                                           High adsorption

                                                                                                                           Daily and final soil covers




              CONTAINMENT LANDFILL




                                                                                                                                                              6
        RADIOACTIVE POLLUTION IN SOIL                                 READING FOR TUESDAY

Atomic weapon testing (Atmospheric fallout)
Radioactive waste and accidents (Sometimes kept secret)
Background radiation from rocks (Uranium rocks   Radon Gas)


                                                              Chapter 20: Global soil quality

                                                              = LAST NEW MATERIAL FOR FINAL EXAM




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