DNAPLs

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DNAPLs

• Drew Lonigro

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Dense Nonaqueous-Phase Liquids

• Densities greater than water or specific gravity
  greater than 1.
• Typically chlorinated hydrocarbons, such as
  degreasers perchloroethylene (PCE),
  trichloroethylene (TCE), as well as coal tar and
  creosote.
• A number of organic compounds that have one or
  more chlorine, bromine or flourine atoms.

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Chemical Characteristics

• High relative solubility a spillage can cause a
  high level of contamination relative to a
  concentration considered harmful to health.
• Low absolute solubility the rate of dissolution
  is low enough to allow the DNAPL to sink to the
  base of the aquifer, forming pools which may
  remain in place for many years. This phenomenon
  makes a case for 'pump and treat' remediation
  methods.
• The low viscosities of chlorinated solvents allow
  them to migrate rapidly in the subsurface, where
  mobility is proportional to the density /
  viscosity ratio.

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Chemical Characteristics

• The low interfacial tension between water and
  chlorinated solvents allow the liquids to
  penetrate small aperture fractures and pore
  spaces, resulting in deeper penetration and a
  higher volume of DNAPL in a given amount of rock.
  
• Chlorinated solvents' high specific gravity (1.1
  - 1.7) relative to water means that only a small
  head (pool height, h0) is required to facilitate
  penetration of the water table.
• Low degradability by chemical or biologically
  mediated reactions, giving a DNAPL the potential
  for a very long subsurface lifetime.

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Migration

• Hydraulic conductivity (Kkpg/u)
• Khydraulic conductivity
• kintrinsic permeability
• pfluid density
• ggravitational constant
• ufluid viscosity

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Migration

• Relative mobility of a fluid depends on the ratio
  of fluid density and fluid viscosity. (p/u)
• (p/u NAPL) / (p/u Water (1.114))
• TCE is 2.57
• 2.57/1.1142.31
• Thus, TCE is 2.31 times more mobile than water.

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Vadose Zone Migration

• DNAPLs move vertically in the vadose zone under
  the influence of gravity.
• Waterwetting liquid
• DNAPLnonwetting fluid
• Water occupies the smaller pores and capillary
  channels. DNAPLs migrate through the larger
  pores.
• The DNAPL displaces the air b/t pores so they
  become filled with the small amount of water
  wetting the mineral surface and the DNAPL.
• True vertical migration will only occur in a
  completely homogenous environment. Structural
  differences (grain size) will cause horizontal
  movement.

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Vertical Movement in the Saturated Zone

• DNAPLs must start to displace water to migrate
  downwards.
• DNAPL vs. H2O capillary force
• Vertical stringers reach a critical height (h0)
  found by Hobsons Formula.
• h01/pore diameter
• Depth of migration depends on amount of DNAPL or
  the depth of an effective aquatard.

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Vertical Movement in the Saturated Zone

• Monitoring wells to detect DNAPLs should be
  places at the bottom of the aquifer, just at the
  top of the confining layer. DNAPL from the zone
  of mobile DNAPL and irreducible water flows to
  the monitoring well, as will both water and DNAPL
  from the zone where both are mobile. The water
  and DNAPL from this zone will separate in the
  monitoring well, with the DNAPL sinking and the
  water rising. (Fetter)

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Vertical Movement in the Saturated Zone

• The relative thickness of the various zones
  depends upon the grain-size distribution, which
  is reflected in the permeability of the saturated
  zone. A low permeability aquifer (small pores)
  will have a thin layer of DNAPL collect on the
  bottom, while a more permeable aquifer (large
  pores) will have a thicker zone of mobile NAPL on
  the bottom and a thinner zone where both DNAPL
  and water are mobile. (Fetter)

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Horizontal Movement in the Saturated Zone

• Discontinuous stringers of DNAPL will be
  displaced by lateral flow of groundwater.
• This time the water must overcome the capillary
  force of the DNAPL stringer to displace it
  sideways.
• Once these bodies of DNAPL reach the aquitard
  they begin to move laterally, down-dipeven if
  the hydraulic gradient and ground-water flow are
  in the opposite direction.

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DNAPL Flow in Fracture Systems

• If the aquitard which the DNAPL collects upon
  happens to be fractured, there is potential for
  the DNAPL to enter the fracture.
• Assuming the fracture is filled with water, the
  capillary pressure of the DNAPL at the entrance
  to the fracture must be greater than the
  capillary pressure of the water within.
• Essentially, the wider the fracture aperture, the
  lower the entry pressure for a DNAPL.

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DNAPL Flow in Fracture Systems

• Kueper and McWhorter, 1991
• geometry

Parallel aperture
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DNAPL Flow in Fracture Systems

• Once a DNAPL has invaded a fractured medium it
  will preferentially enter the larger fractures.
• Gravity is still a major component of migration
  but so is fracture size.
• Once in a fracture system, it can undergo
  molecular diffusion from the fracture into the
  ground water in the pores of the rock or clay
  matrix.
• Clayey deposits can have matrix porosity of 30 to
  60. Sed. Rocks5 to 15.

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DNAPL Flow in Fracture Systems
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DNAPL Flow in Fracture Systems

• Theoretical calculations suggest that the
  diffusive loss of DNAPL from fractures in the
  matrix of clay aquitards can be complete within a
  time frame of days to a few years. (Parker,
  Gillham and Cherry, 1994)
• This would be very difficult to deal with when
  there is no longer a liquid phase.

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Monitoring for DNAPLs

• Drilling methods that allow the sampling of water
  during the drilling process facilitate the
  collection of data on DNAPL concentration - aided
  by the addition of indicator chemicals, employing
  gas chromatography or simply a matter of
  observing the presence of oily residues in the
  water.
• The ability to detect the presence of DNAPL in
  drilling is vital if one is to avoid the
  disruption of a DNAPL zone, allowing the DNAPL to
  penetrate to even greater depths.

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Monitoring for DNAPLs

• A monitoring well should be constructed with a
  screen and penetrate to the very bottom of the
  aquifer.
• It is helpful to include a section of solid pipe
  as a sump at the bottom of the screen so that any
  DNAPL can collect and be sampled.
• A bottom-loading bailer is used to collect the
  liquid from the bottom of the sump prior to any
  well purging. The bailer is slowly lowered all
  the way to the bottom of the well, collects the
  sample and then is slowly raised.
• The sample can then be inspected for DNAPL.

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Monitoring for DNAPLs

• Since migration is so much more complicated in
  fractured bedrock systems many monitoring wells
  are often needed to find the portions of the
  DNAPL plume.
• Often it is impossible to identify the entire
  thing.

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Monitoring for DNAPLs

• Fluorescence of unsaturated aliphatic
  hydrocarbons (e.g. TCE, PCE) when exposed to
  ultraviolet radiation provides an efficient
  method of detecting the presence of these
  compounds in field samples.
• Measuring the organic vapor concentration in soil
  samples using a portable organic vapor analyzer
  is accepted as a reasonably effective way of
  detecting the presence of DNAPL. However,
  empirical studies suggest that results are
  subject to great variability, and only full-scale
  analyzer readings can be taken to indicate the
  presence of DNAPL.