Nonaqueous Fluids in the Vadose Zone A brief overview of a messy topic

Nonaqueous Fluids in the Vadose Zone 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).

Numerous sources - LNAPLs 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.

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.

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

A typical scene

The Components of a Plume

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.

Influence of Water Table

Permeability

Residual NAPL 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.

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. 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.

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).

Geologic Effects

Real Data…(Kueper et al., 1993)

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 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

Rate of spill effects

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

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.

LNAPLs vs DNAPLs

DNAPL Migration

DNAPL Migration

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 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

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

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

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!

Example the height of the capillary fringe 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.

Data from experiments Observed Actual in well free product

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

DNAPLs in Wells

Movement and Retention 1. Initial emplacement 2. Soluble losses 3. Aging

Initial Emplacement We have already discussed the over-riding issues. A few more remarks: Movement strongly affected 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 changes slowly as local microbes put out surfactants Movement typically unstable. No codes handle this. Any predictions must be field validated

Textural Interfaces: Multiphase flow Let’s look at three oil spill cases no water flowing little water flowing lots of water flowing

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 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)

Partitioning of Common NAPLs

Skimming Free Product

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