Nonaqueous Fluids in the Vadose Zone

1. Nonaqueous Fluids in the Vadose Zone Williams, 2002 http://www.its.uidaho.edu/AgE558 Modified after Selker, 2000 http://bioe.orst.edu/vzp/

2. Nonaqueous Fluids in the Vadose Zone • Much vadose study aimed at contaminant transport • One set of contaminants 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).

3. Numerous sources - LNAPLs • Most ubiquitous: • leaking underground storage tanks (LUST’s) • 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 UST’s have been removed in the U. S. due to tightened regulation.

4. A typical scene

5. Sources - LNAPLs cont. • Major source of LNAPLs: household heating oil tanks. • Long overlooked, there are a vast number of leaking buried oil tanks, (same numbers as old gas station tanks) • Household leaks rarely noticed until catastrophic failure, since there are usually no records of consumption. • The lower volatility of heating oil also limits the observation of leaks through vapor transport into basements etc.

6. Sources - DNAPLs • 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 (e.g dry cleaners). • Spills are typically of smaller volume than LNAPLs, but more serious due to higher toxicity and bulk penetration of aquifers

7. The Anatomy of a NAPL 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.

8. The Components of a Plume

9. Residual NAPL • NAPLs tend to form small droplets (a.k.a. ganglia) in the unsaturated zone • Rule of thumb: 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.

10. Permeability

11. Example of residual • 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. • QUALITATIVE ESTIMATE – don’t state that this amountreached water table, just that high likelihood that a significant amount reached water table.

12. Geologic Effects • 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).

13. Geologic Effects

14. Rate of introduction highly influential • Rapid spills • require broader areas to carry the flow • larger residual saturation in the unsaturated zone • less free product on aquifers • less susceptible to extreme lateral flow due to textural interfaces. • Slow leaks • more susceptible to lateral diversion along textural interfaces – perched OR capillary barrier flow • likely follow more isolated paths of flow • Slow leaks tend to contaminate a larger area, while still delivering a greater fraction of the product to the aquifer

15. Rate of spill effects

16. LNAPLs vs DNAPLs • 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 zone • the 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 table • DNAPLs 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.

17. LNAPLs vs DNAPLs

18. DNAPL Migration

19. DNAPL Migration

20. Observing LNAPLs in Wells • 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 sample sand 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

21. 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

22. Calculating some depths • 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 Pow Pow

23. Finishing the algebra • From the set-up geometry • solving for Hd • We may rewrite this using the geometric result as • Solving for Hoil • NOTE: • denominator • small!

24. Example • For typical NAPLs goil/gw) 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 fringe • the density contrast of the liquids • Density contrast easy, but the height of the effective capillary fringe is difficult to measure.

25. Data from experiments • Observed Actual • in well free product

26. Skimming Free Product

27. Gas phase important in remediation • Soil Vapor Extraction (SVE) - vadose • Gas is pumped through vadose zone stripping volatile fraction (Henry’s law). • Prediction of flow essential to design remediation • Air Sparging - saturated • Air is pumped into aquifers to strip contaminants which will be lifted to the vadose zone, and extracted in gas phase. • Gas movement very complicated due to effects of heterogeneity and fundamental instability of buoyant gas movement in porous media.

28. 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 in 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

29. DNAPLs in Wells

30. Movement and Retention - All NAPL’s • 1. Initial emplacement • 2. Soluble losses • 3. Aging

31. Initial Emplacement • We have already discussed the over-riding issues. A few more remarks: • Movement strongly effected by surface tension • Surface 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, including vapor partitioning, adsorbing onto soil organic matter (SOM) – pp. 205-209 • changes slowly as local microbes put out surfactants • Movement typically unstable. No codes handle this. • Any predictions must be field-validated

32. Soluble losses and aging • Many NAPLs are moderately soluble in water • Since there is much more water than NAPL, this leads to significant losses (plume) • Many NAPLs are mixtures of hydrocarbons etc. (e.g., gasoline has 10’s 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 its makeup becoming less soluble/volatile (aging)

33. Partitioning of Common NAPLs

34. Summary on NAPLs • 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