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Nonaqueous Fluids in the

Vadose ZoneA brief overview of a messy topicA brief overview of a messy topic

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Nonaqueous Fluids in the Vadose ZoneMuch vadose study aimed at contaminant transport

One set of contaminates requires special treatment; those that are not miscible in water. referred to as Non-Aqueous Phase Liquids: NAPLs, low solubility in water. non-polar compounds which remain as separate liquid phase (as

opposed to alcohol or latex).

Subdivided into those with density lower than that of water (LNAPLs - Light; e.g., gasoline)denser than water (DNAPL - Dense, e.g., TCE, carbon tetrachloride).

Much vadose study aimed at contaminant transport

One set of contaminates requires special treatment; those that are not miscible in water. referred to as Non-Aqueous Phase Liquids: NAPLs, low solubility in water. non-polar compounds which remain as separate liquid phase (as

opposed to alcohol or latex).

Subdivided into those with density lower than that of water (LNAPLs - Light; e.g., gasoline)denser than water (DNAPL - Dense, e.g., TCE, carbon tetrachloride).

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Numerous sources - LNAPLsMost ubiquitous:

leaking underground storage tanks (LUSTs)

Gas stations:10% of single walled steel tanks leaked,

plumbing leaks in approximately 30% of these installations

lesson: don’t assume that the plume will be under the tank since most arise from delivery system failure (Selker, 1991).

Note: Most commercial single walled USTs have been removed in the U.S. due to tightened regulation.

Most ubiquitous:

leaking underground storage tanks (LUSTs)

Gas stations:10% of single walled steel tanks leaked,

plumbing leaks in approximately 30% of these installations

lesson: don’t assume that the plume will be under the tank since most arise from delivery system failure (Selker, 1991).

Note: Most commercial single walled USTs have been removed in the U.S. due to tightened regulation.

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Sources - LNAPLs cont.Major source of LNAPLs: household heating oil

tanks.

Long overlooked, there are a vast number of leaking buried oil tanks, (same proportions as old gas station tanks)

Household leaks rarely noticed until catastrophic failure, since there are no records of consumption.

The lower volatility of heating oil also limits the observation of leaks through vapor transport into basements etc.

Major source of LNAPLs: household heating oil tanks.

Long overlooked, there are a vast number of leaking buried oil tanks, (same proportions as old gas station tanks)

Household leaks rarely noticed until catastrophic failure, since there are no records of consumption.

The lower volatility of heating oil also limits the observation of leaks through vapor transport into basements etc.

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Sources - DNAPLsDNAPLs in the environment typically arise from disposal of cleaning

compounds.

Whereas LNAPLs are most commonly observed at points of delivery, DNAPLs are found at points of delivery, use, and disposal.

“Dry wells” and other ad hoc disposal sites represent a major portion of plume generators, often near the point of use, or at waste disposal sites.

Spills are typically of smaller volume than LNAPLs, but more serious due to higher toxicity and bulk penetration of aquifers

DNAPLs in the environment typically arise from disposal of cleaning compounds.

Whereas LNAPLs are most commonly observed at points of delivery, DNAPLs are found at points of delivery, use, and disposal.

“Dry wells” and other ad hoc disposal sites represent a major portion of plume generators, often near the point of use, or at waste disposal sites.

Spills are typically of smaller volume than LNAPLs, but more serious due to higher toxicity and bulk penetration of aquifers

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A typical scene

UST

Floating "Free Product"

Residual Saturation (1-5%)

Hydraulic Gradient

Aqueous Phase Plume

Vapor Phase Plume

Capillary Fringe

Source: Leaking Joint

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The Components of a Plume

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The Anatomy of a NAPL SpillPrediction of NAPL movement complicated by physical and

chemical processes making quantitative prediction generally impossible for field spills (Osborne and Sykes, 1986; Cary et al., 1989b; Essaid et al., 1993).

Most productive to understand the qualitative characteristics movement, rather than spend inordinate energy on quantitative prediction of NAPL disposition.

A key point: residual saturation can account for a large fraction of a spill.

Prediction of NAPL movement complicated by physical and chemical processes making quantitative prediction generally impossible for field spills (Osborne and Sykes, 1986; Cary et al., 1989b; Essaid et al., 1993).

Most productive to understand the qualitative characteristics movement, rather than spend inordinate energy on quantitative prediction of NAPL disposition.

A key point: residual saturation can account for a large fraction of a spill.

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Influence of Water Table

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Permeability

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Residual NAPLNAPLs tend to form small droplets (a.k.a. ganglia) in

the unsaturated zone

On the order of 5% of the volume of the region which experienced NAPL transport will remain NAPL filled with residual product (Cary et al., 1989c)

This important for planning in soil clean up, as well as understanding how much of the product may have reached the upper aquifer.

NAPLs tend to form small droplets (a.k.a. ganglia) in the unsaturated zone

On the order of 5% of the volume of the region which experienced NAPL transport will remain NAPL filled with residual product (Cary et al., 1989c)

This important for planning in soil clean up, as well as understanding how much of the product may have reached the upper aquifer.

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Example of residualA spill of 10,000 l of product 10 m above an unconfined aquifer. Assuming that the NAPL wetted area of 4 m by 4 m and a residual saturation of 5%, how much of this original spill makes it to the water table in liquid form?

Solution:

The residual volume in the vadose zone is:

10 m x 4 m x 4 m x 5% = 8 m3

= 8,000 l

therefore about 2,000 liters (20%) makes it to the water table.

Obviously our uncertainty exceeds +/- 20%, so we really have little idea of how much made it to the water table, but should assume that a significant amount did.

A spill of 10,000 l of product 10 m above an unconfined aquifer. Assuming that the NAPL wetted area of 4 m by 4 m and a residual saturation of 5%, how much of this original spill makes it to the water table in liquid form?

Solution:

The residual volume in the vadose zone is:

10 m x 4 m x 4 m x 5% = 8 m3

= 8,000 l

therefore about 2,000 liters (20%) makes it to the water table.

Obviously our uncertainty exceeds +/- 20%, so we really have little idea of how much made it to the water table, but should assume that a significant amount did.

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Geologic EffectsGeologic configuration key to disposition of NAPLs LNAPLs: the vadose zone is of primary importance, since

the bulk liquid does not penetrate the saturated zone, DNAPLs: the structure in both saturated and unsaturated

regions will have a major impact on disposition. Main issue: layers between media of different texture. In

particular, horizontal bedding features will cause the plume to spread laterally with a dominant down-dip movement (Schroth et al., 1997).

Geologic configuration key to disposition of NAPLs LNAPLs: the vadose zone is of primary importance, since

the bulk liquid does not penetrate the saturated zone, DNAPLs: the structure in both saturated and unsaturated

regions will have a major impact on disposition. Main issue: layers between media of different texture. In

particular, horizontal bedding features will cause the plume to spread laterally with a dominant down-dip movement (Schroth et al., 1997).

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Geologic Effects

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Real Data…(Kueper et al., 1993)

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Rate of introduction highly influentialRapid spillsrequire broader areas to carry the flowlarger residual saturation in the unsaturated zoneless free product on aquifersless susceptible to extreme lateral flow due to textural interfaces.

Slow leaksmore susceptible to lateral diversion along textural interfaceslikely follow more isolated paths of flowSlow leaks tend to contaminate a larger area, while still delivering a

greater fraction of the product to the aquifer

Rapid spillsrequire broader areas to carry the flowlarger residual saturation in the unsaturated zoneless free product on aquifersless susceptible to extreme lateral flow due to textural interfaces.

Slow leaksmore susceptible to lateral diversion along textural interfaceslikely follow more isolated paths of flowSlow leaks tend to contaminate a larger area, while still delivering a

greater fraction of the product to the aquifer

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Rate of spill effects

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Real Data (Kueper et al., 1992)

The upper plot is from

an instantaneous

release, while the lower

plot resulted from a

slow injection, which

penetrated further, and

spread more widely

The upper plot is from

an instantaneous

release, while the lower

plot resulted from a

slow injection, which

penetrated further, and

spread more widely

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LNAPLs vs DNAPLsIn the vadose zone DNAPLs and LNAPLs behave quite similarly if saturation not

encountered. Logical since the only distinction we have made between these is their relative

density in comparison to water. there are no buoyancy effects in vadose zonethe physics of flow is essentially the same

Once saturated regions encountered, migration differs dramatically for LNAPLs and DNAPLs. LNAPLs travel in direction of the slope of the water tableDNAPLs travel in direction of slope of the lower boundary DNAPLs move through aquifers in web like networks of pores (e.g., Held and

Illangasekare, 1995). this reduces residual saturation, thus increasing the free product available to

spread through the aquifer.

In the vadose zone DNAPLs and LNAPLs behave quite similarly if saturation not encountered. Logical since the only distinction we have made between these is their relative

density in comparison to water. there are no buoyancy effects in vadose zonethe physics of flow is essentially the same

Once saturated regions encountered, migration differs dramatically for LNAPLs and DNAPLs. LNAPLs travel in direction of the slope of the water tableDNAPLs travel in direction of slope of the lower boundary DNAPLs move through aquifers in web like networks of pores (e.g., Held and

Illangasekare, 1995). this reduces residual saturation, thus increasing the free product available to

spread through the aquifer.

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LNAPLs vs DNAPLs

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DNAPL Migration

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DNAPL Migration

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Observing LNAPLs in WellsOften the first indication of NAPL contamination is the

observation of the product in a well

The extent of a plume at a site is often then delineated by installing additional wells on the site

The extent of contamination is then delineated by obtaining core samples and observing the depth of "free product" in the wells

BE CAREFUL: The depth observed in wells is not the free product depth on the aquifer

Often the first indication of NAPL contamination is the observation of the product in a well

The extent of a plume at a site is often then delineated by installing additional wells on the site

The extent of contamination is then delineated by obtaining core samples and observing the depth of "free product" in the wells

BE CAREFUL: The depth observed in wells is not the free product depth on the aquifer

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Geometry of LNAPLs in wells

Typical observation well at an LNAPL spill site where Hoil is the “True” depth of free product, Hcap is the thickness of the capillary fringe, Happ is the “apparent” depth of free product, and Hd the depression of the water surface in the well

Typical observation well at an LNAPL spill site where Hoil is the “True” depth of free product, Hcap is the thickness of the capillary fringe, Happ is the “apparent” depth of free product, and Hd the depression of the water surface in the well

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Calculating some depthsAt the oil-water interface in the well, the total head is

the total head at all points in the aquifer is constant (assuming that we are not pumping from the well), so head at the interface is also given by

Equating these we obtain

At the oil-water interface in the well, the total head is

the total head at all points in the aquifer is constant (assuming that we are not pumping from the well), so head at the interface is also given by

Equating these we obtain

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Finishing the algebraFrom the set-up geometry

solving for Hd

We may rewrite this using the geometric result as

Solving for Hoil

NOTE: denominator small!

From the set-up geometry

solving for Hd

We may rewrite this using the geometric result as

Solving for Hoil

NOTE: denominator small!

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ExampleFor typical NAPLs oil/w) is about 0.8. Taking Hcap to be 50 cm (typical for a silt loam texture), and assuming the true depth of free product to be 2 cm, we can use [2.162] to calculate the “apparent depth” of NAPL in the well

almost 3 m of “free product” in the well!

Very sensitive to:

the height of the capillary fringethe density contrast of the liquidsdensity contrast easy, but the height of the effective capillary fringe

is difficult to measure.

For typical NAPLs oil/w) is about 0.8. Taking Hcap to be 50 cm (typical for a silt loam texture), and assuming the true depth of free product to be 2 cm, we can use [2.162] to calculate the “apparent depth” of NAPL in the well

almost 3 m of “free product” in the well!

Very sensitive to:

the height of the capillary fringethe density contrast of the liquidsdensity contrast easy, but the height of the effective capillary fringe

is difficult to measure.

Happ = 2 cm + 50 cm 1 - 0.8

= 520.2

= 260 cm

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Data from experimentsObserved Actual

in well free product

Observed Actual

in well free product

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DNAPLs and wells...

In the case of DNAPLs, wells present a more serious threat. If a well screen crosses an aquitard, the well

itself can become a pathway for transport, with a DNAPL draining off the aquitard, into the well, and out the well into the lower aquifer.

For LNAPLs, by creating a cone of depression about a well you may facilitate removal of the contaminant which will then flow to the well

In the case of DNAPLs, wells present a more serious threat. If a well screen crosses an aquitard, the well

itself can become a pathway for transport, with a DNAPL draining off the aquitard, into the well, and out the well into the lower aquifer.

For LNAPLs, by creating a cone of depression about a well you may facilitate removal of the contaminant which will then flow to the well

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DNAPLs in Wells

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Movement and Retention

1. Initial emplacement

2. Soluble losses

3. Aging

1. Initial emplacement

2. Soluble losses

3. Aging

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Initial EmplacementWe have already discussed the over-riding issues. A few more remarks:Movement strongly affected by surface tensionSurface tension is a function of TIME!!

changes rapidly in first hours as interfaces come to local equilibrium with fluids (on the order of 30% change)

changes slowly as the fluids age through partitioning losses

changes slowly as local microbes put out surfactants

Movement typically unstable. No codes handle this.Any predictions must be field validated

We have already discussed the over-riding issues. A few more remarks:Movement strongly affected by surface tensionSurface tension is a function of TIME!!

changes rapidly in first hours as interfaces come to local equilibrium with fluids (on the order of 30% change)

changes slowly as the fluids age through partitioning losses

changes slowly as local microbes put out surfactants

Movement typically unstable. No codes handle this.Any predictions must be field validated

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Textural Interfaces: Multiphase flowLet’s look at three oil spill casesno water

flowing

little water flowing

lots of water flowing

Let’s look at three oil spill casesno water

flowing

little water flowing

lots of water flowing

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Soluble losses and agingMany NAPLs are moderately soluble in waterSince there is much more water than NAPL, this

leads to significant losses (plume)Many NAPLs are mixtures of hydrocarbons etc.

(e.g., gasoline has 10s of major components)Each of the constituents will partition into the

water and gas phases according to its own solubility

As the NAPL sits, it changes it makeup becoming less soluble/volatile (aging)

Many NAPLs are moderately soluble in waterSince there is much more water than NAPL, this

leads to significant losses (plume)Many NAPLs are mixtures of hydrocarbons etc.

(e.g., gasoline has 10s of major components)Each of the constituents will partition into the

water and gas phases according to its own solubility

As the NAPL sits, it changes it makeup becoming less soluble/volatile (aging)

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Partitioning of Common NAPLs

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Skimming Free Product

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Summary on NAPLsUnderstanding the physics and chemistry of NAPL movement is helpful

Don’t expect to accurately predict disposition

This has only been a brief overview. Lots of very good work on these issues

Understanding the physics and chemistry of NAPL movement is helpful

Don’t expect to accurately predict disposition

This has only been a brief overview. Lots of very good work on these issues