dnapls drew lonigro. dense nonaqueous-phase liquids densities greater than water or specific gravity...

DNAPLs

Drew Lonigro

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.

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.

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.

Migration

• Hydraulic conductivity (K=kpg/u)

• K=hydraulic conductivity

• k=intrinsic permeability

• p=fluid density

• g=gravitational constant

• u=fluid viscosity

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

• 2.57/1.114=2.31

• Thus, TCE is 2.31 times more mobile than water.

Vadose Zone Migration

• DNAPLs move vertically in the vadose zone under the influence of gravity.

• Water=wetting liquid• DNAPL=nonwetting 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.

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 Hobson’s Formula.• h0=1/pore diameter• Depth of migration depends on amount of

DNAPL or the depth of an effective aquatard.


• “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)


• “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)

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-dip…even if the hydraulic gradient and ground-water flow are in the opposite direction.

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.


• Kueper and McWhorter, 1991• geometry:

Parallel aperture~


• 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. Rocks…5 to 15%.


• “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.

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.


• 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.


• 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.


• 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.

dnapls drew lonigro. dense nonaqueous-phase liquids densities greater than water or specific gravity...

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