Organic Chemical Groundwater Contamination

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Organic Chemical Groundwater Contamination
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Organic Contaminants

• Appear frequently in groundwater, found in...
• water supply wells
• monitoring or irrigation wells in agricultural
  regions
• monitoring wells in industrial areas.
• many sources, mostly anthropogenic
• industry TCE, PCE, BTEX, acetone, PAHs, PCBs,
  etc
• defense industry special fuels, explosives (TNT
  etc)
• domestic pesticides, fuels, cleaners, paint
  thinners, creosote etc
• urban waste disposal petroleum hydrocarbons,
  pesticides, solvents etc
• agricultural pesticides, herbicides

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Species

• can identify many organic contaminants in
  groundwater (several hundred compounds) -
  chromotography, mass spectrometery
• many not yet identified
• sum of identified organic compounds lt total
  organic carbon (TOC) of sample
• sum of identified halogenated organic compounds lt
  total halogenated organics (TOX) of sample
• many unidentified compounds are degradation or
  synthesis products of other organic contaminants
  in the environment

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Fate and Mobility of Organic Contaminants in Water

• Controlling Processes
• Solubility
• Soluble
• Qucikly distributed
• Dilute easily
• Insoluble
• Tend to adsorb on solids
• Bio accumulate
• Given as Ksp solubility product
• Solubility increases with increasing salinity
• Organic-Water Partitioning coefficient

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Organic Solubility

• Most organic compounds are polar whereas water is
  polar
• Therefore, low solubility in water
• Benzene 1780 mg.L
• TCE 1000 mg/L
• Toluene 515 mg/L
• Napthalene 33 mg/L
• DDT 0.12 mg/L
• Solubility generally increases with temperature
  (increase of 10C, increase in solubility of
  5-30)
• Solubility tends to decline as the molecular
  weight of the compound increases

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Fate and Mobility of Organic Contaminants in Water

• Controlling Processes
• Organic-Water Partitioning coefficient
• Used to predict partitioning between water and
  organic compounds
• Measured in lab
• Ratio of a chemical's concentration in the
  octanol (C8H18O) phase to its concentration in
  the aqueous phase of a 2-phase octanol-water
  system
• Kow concentration in Octanol / concentration in
  aqueous

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Octanol/Water Partition Coefficient

• Kow lt 10
• Compound in hydrophilic and dissolved readily in
  water
• Kow gt104
• hydrophobic and tends to have
• low water solubility
• Low vapor pressure
• large soil/sediment adsorption coefficient
• large retardation factor
• Soil contamination problem
• Benzene is moderately hydrophobic Kow 100
• DDT is strongly hydrophobic Kow 106
• Generally, Kow decreases with molecular weight

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Fate and Mobility of Organic Contaminants in Water

• Controlling Processes
• Sorption
• Adsorption
• Adheres to mineral/soil particle surface
• Absorption
• Entry into volume of material
• Factors affecting sorption
• Surface area
• Mineral surface properties
• Koc
• pH

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Fate and Mobility of Organic Contaminants in Water

• Controlling Processes
• Volatilization from soil and water
• Many organic compounds are volatile
• Produce an organic vapor in equilibrium with a
  NAPL
• Vapor pressure increases with increasing
  temperature
• Vapor pressure decreases with increasing
  molecular weight
• Benzene high vapor pressure
• Benzo-a-pyrene low vapor pressure

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Fate and Mobility of Organic Contaminants in Water

• Controlling Processes
• Hydrolysis
• Alkyl halides (R-X) spontaneously break down in
  water
• H2O R-X ? R-OH H X
• Examples CH3-Cl (Methyl Chloride),CH3-Br (Methyl
  bromide), C2H5-Br (Ethyl Bromide), C3H7-Br
  (Propyl bromide)
• Esters convert to parent organic acid plus
  alcohol
• H2O CH3COOCH2CH3 ? CH3COOH C2H5OH
• Water ethyl acetate ? acetic acid ethanol
• Biodegredation

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Fate and Mobility of Organic Contaminants in Water

• Controlling Processes
• Biodegredation
• Aerobic
• C6H6 7.5O2 ? 6CO2 3H2O
• Denitrification
• C6H6 H 6NO3- ? 6CO2 3N2 6H2O
• Iron reduction
• C6H6 30Fe3 12H2O ? 6CO2 3OH 30Fe2
• Sulfate Reduction
• C6H6 7.5H 3.75SO42- ? 6CO2 3.75H2S 3H2O
• Methanogenesis
• C6H6 4.5H2O ? 2.25CO2 3.75CH4

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Non-Aqueous Phase Liquids (NAPLs)

• Contaminants that occur as a separate phases (ie.
  Not dissolved in water) in unsaturated and
  saturated zone
• Single phase organic liquids (eg. Dichloroethane
  DCA)
• More commonly, mixtures of organic liquids (eg.
  Benzene, toluene, ethyl benzene, and xylene,
  BTEX)
• May be either denser or lighter than water
• DNAPLs include many chloronated solvents eg.
  Trichloroethyene TCE, perchloroethylene PCE,
  Dichloroethane DCA, PCBs
• LNAPLs include petroleum products and other
  refined hydrocarbons such as BTEX

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Non-Aqueous Phase Liquids (NAPLs)

• NAPLs have a high surface tension ? non-wetting
  relative to water
• Leads to characteristic behavior
• NAPLs pool in the center of voids in unsaturated
  or saturated zone
• NAPLs preferentially enter large pores over
  smaller ones
• Once a pathway of continuous LNAPL has developed,
  it is preferentially used instead of new pathways
  developing
• LNAPLs are left behind as isolated blobs in the
  centers of pores (residual) when the source is
  cut off
• Can act as a long term source of aqueoud or vapor
  contamination

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LNAPLs

• Pool on the water table
• Generally flow along hydraulic gradient
• Migration affected by heterogenetity
  (preferentially enters coarser-grained sediment
  over finer-grained sediment)

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LNAPLs

• Plumes form near water table and extend from
  source
• Volatilization and dissolution are important
• Soil gas and soil water contaminated

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LNAPLs

• BTEX Compounds
• volatile monoaromatic hydrocarbons which are
  commonly found together in crude petroleum and
  petroleum products such as gasoline

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BTEX Properties
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LNAPLs

• Composition of gasoline

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BTEX BioRemediation

• Contamination of groundwater with the BTEX
  compounds is difficult to remedy because these
  compounds are relatively soluble in water and can
  diffuse rapidly once introduced into an aquifer.
• Techniques for in situ bioremediation of the BTEX
  compounds are used to eliminate or reduce
  contamination levels in an aquifer.
• Bio degredation is extremely complex, resulting
  in many intermediate chemical products.

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DNAPLs

• Pool on low permeability layers within or at the
  base of the aquifer or the unsaturated zone
• Flows along open fractures or bore holes downward
  in response to gravity, not the hydraulic
  gradient)
• Flows along the slope of the low permeability
  layer, not with the hydraulic gradient

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DNAPLs

• Solute plume develops away from source
• Difficult to predict plume migration
• Free phase migrates along contact with aquitard
• Dissolved phase migrates in groundwater
• Residual phases in saturated and unsaturated zones

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DNAPLs
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DNAPLs

• Common chlorinated hydrocarbons

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DNAPLs

• Chlorinated hydrocarbons

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Most Frequently Detected Organic Groundwater
Contaminants

• Trichloroethylene Dry cleaning, metal
  degreasing
• Tetrachloroethyene Dry cleaning, metal
  degreasing
• Benzene gasoline, manufacturing
• Toluene gasoline, manufacturing
• Methylene chloride degreasing, solvents, paint
  removal
• 1,1,1 Trichloroethane metal and plastic
  cleaning
• Chloroform solvents
• 1,1 Dichloroethane degreasing, solvents
• 1,2 Dichloroethene, trans- transformation
  product of 1,1,1 Trichloroethane
• 1,1 Dichloroethene manufacturing
• Vinyl chloride plastics and manufacturing
• 1,2 Dichloroethane metal degreasing, paint
  removal
• Ethylbenzene styrene and asphalt manufacture,
  gasoline
• Xylenes solvents, gasoline
• Phenol wood treatment, medicines

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