Groundwater Pollution

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Groundwater Pollution

• 0404 Contaminant Transport

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• Notes with the green edges were derived from
• 1.34 Waste Containment and Remediation Technology
  , As taught in Spring 2004, by Dr. Peter
  Shanahan , MIT OpenCourseWare, Creative Commons
  License, http//ocw.mit.edu/OcwWeb/Civil-and-Envi
  ronmental-Engineering/1-34Spring2004/CourseHome/
• Notes with the pink background were derived from
• 1.72, Groundwater Hydrology, As taught in Fall
  2005, by Prof. Charles Harvey (Massachusetts
  Institute of Technology MIT OpenCourseWare),
  http//ocw.mit.edu License Creative Commons
  BY-NC-SA

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• Dissolved compounds can be toxic and
  carcinogenic.
• Need to have MCLs (maximum contaminant levels)
• ?? ????  maximum contaminant level 
• ?? ??? ???? ??? ?? ????? ?? ??? ??.

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• Dissolved Substances
• A solute is a substance dissolved in a liquid
• Example Chloride is a solute and water is the
  solvent
• Concentrations measure in Mass/Length3 (mg/L)

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Contaminant MCL
Trichloroethylene (TCE) 5 µg/L
Tetrachloroethylene (PCE) 5 µg/L
Vinyl Chloride 2 µg/L
Benzene 5 µg/L
Carbon Tetrachloride 5 µg/L
Copper 1 mg/L
Lead 0.05 mg/L
Mercury 2 µg/L
Arsenic 10 µg/L
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• In reality
• Plumes are not so perfectly shaped
• Even in homogeneous media they are distorted
• In heterogeneous media plumes can be complex, and
  may make more than one plume
• Dispersivity is greater with greater travel
  distance

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• Sorption a type of surface reaction in which
  the solute spends some of its time stuck to solid
  surfaces thereby delaying its arrival in a
  process known as retardation.

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Sorption
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• Retardation factor - the factor by which the
  non-reactive (nonsorbing) solute moves compared
  to the sorbing solute which is delayed.

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• source
• xylenes
• benzene
• MTBE

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• Different chemicals (benzene, toluene,
  ethylbenzene, xylenes and Methyl tert-Butyl Ether
  (MTBE)) were found to be separating from each
  other.
• One reason for this is that different amounts of
  sorption causes separation of the various plumes.

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A Real Plume
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• One good thing about sorption
• Some hazardous species havent migrated. Some
  spills involving plutonium show that it has only
  migrated a few meters at most (in unsaturated
  zone)

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• One bad thing about sorption
• Even if you pump out a contaminant plume,
  contaminant will still be stuck to the solids.
  This contaminant will get into the liquid.
  Therefore, it takes a long time to cleanup a
  contaminant plume if there is sorbed solute.

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• Multi-phase Flow in Porous Media

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• Compared with Aqueous Phase Liquids

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• LNAPL lighter-than-water NAPL (floaters)
• Examples fuels gasoline, diesel fuel
• Plume forms on surface of water table
• Migrates in direction of water table
• Must be skimmed

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• DNAPL denser-than-water NAPL
• (sinkers)
• Examples chlorinated hydrocarbons
• TCE (1.46 sg), TCA (1.34),
• carbon tetrachloride (1.59)
• Can sink to bottom of aquifer to form pool
• Can migrate down dip on aquifer bottom
• Recovery difficult to impossible

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• The problem
• Easy to contaminate
• Low concentrations are bad
• Substances can migrate with flowing groundwater
• Hard to remove

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• Possible Movement of DNAPLs

B. L. Parker, J. A. Cherry, S. W. Chapman, and
M. A. Guilbeault(2003) Vadose Zone Journal
2116137 (2003).
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• Residual contamination in unsaturated soil acts
  as long-term source of contaminants to dissolve
  into infiltrating recharge

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• Solvent may be vaporizing. Solvent vapor is
  denser than air, and will sink in unsaturated
  zone and can contaminate ground water.

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• After NAPL passes through soil, fraction remains
  behind as a residual.

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• DNAPL pools are very hard to find.
• Other indicators must be used.
• The concentration of contaminant in the water
  phase is the most commonly used indicator
• C gt 1 solubility
• For chlorinated solvents C gt 10,000 ug/L

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• Light non-aqueous phase liquids

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• LNAPL floats on water-saturated soil of capillary
  fringe. Accumulation of LNAPL will depress
  capillary fringe and eventually penetrate through
  fringe and into water table. Oil has its own
  capillary fringe.
• Rise and fall of water table will create a smear
  zone of residual saturation.

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• The first classroom test will be on Thursday
  April 11.