Dnapl Free Product Plume

Dense nonaqueous phase liquids, DNAPL, are nonaqueous phase liquids that are denser than water. If sufficient DNAPL is present, the free product sinks downward through the water table to the bottom of an aquifer. Only an impermeable obstruction, such as bedrock, stops the downward movement of DNAPL mobile free product. Some examples of DNAPL are

• Halogenated solvents (mostly chlorinated or brominated), either pure compounds or mixtures of solvents

• Wood preservatives

* Because of its greater viscosity, diesel fuel would move about one-third as fast.

figure 5.12 Comparison of DNAPL and LNAPL movement in the subsurface, after spills.

• Polychlorinated biphenyls (PCBs)

• Many pesticides

Free product from DNAPL spills moves downward through the vadose zone (leaving a trail of residual soil-sorbed DNAPL) until it reaches the capillary zone of the water table. To continue moving downward, the DNAPL must displace water held by capillary forces. Consequently, downward movement slows while DNAPL piles up and spreads laterally. If sufficient weight of DNAPL accumulates, it presses downward through the capillary zone and continues down through the saturated zone (see Figure 5.12).

Where there is a decrease in soil permeability — whether in the unsaturated or saturated zone — DNAPL behaves similarly, the downward movement slows, and more lateral spreading occurs. This leads to the formation of pools and fingers. Eventually, impermeable bedrock is reached where DNAPL collects in pools. If the bedrock is slanted, DNAPL will migrate down the physical slope, even if the direction is opposite to the groundwater movement.

Many DNAPL compounds, especially chlorinated hydrocarbons, are less viscous than water. They can penetrate small fractures and micropores and become inaccessible to in situ remediation. Chlorinated hydrocarbons interact with some clays, causing them to shrink and crack. Clay-lined ponds are not reliable containment for liquid wastes containing chlorinated hydrocarbons.

Aerobic biodegradation techniques that work well with many LNAPL hydrocarbons have not been very successful with chlorinated DNAPL. Although there have been recent reports of successful biodegradation procedures involving anaerobic steps and carefully selected microbes, biodegradation and weathering are slow and DNAPL may persist for long time periods.

Solubilities are generally low; therefore, DNAPL will continue to dissolve slowly into the groundwater without significant diminution. Even with a moderate DNAPL release, dissolution can continue for hundreds of years under natural conditions before all the DNAPL has dissolved or degraded. Once in the subsurface, it is difficult or impossible to recover all of the trapped residual DNAPL. DNAPL that remains trapped in the soil/aquifer matrix acts as a continuing source of groundwater contamination.

Rules of Thumb for DNAPL

1. Chlorinated hydrocarbons are generally more dense than water (DNAPL). They sink to the bottom of the water table.

2. DNAPL movement is affected by gravity far more than by groundwater movement. It moves with the slope of the bedrock below the aquifer, independent of the direction of groundwater movement, and forms pools in bedrock depressions.

Testing for the Presence of DNAPL

It is very difficult to locate DNAPL free product with monitoring wells. First of all, DNAPL remains in the bottom of the well and may go unnoticed. Second, because DNAPL free product generally collects in pools at the bottom of an aquifer and in locations unrelated to groundwater movement, there often are no obvious guidelines as to where a well should be placed or how it should be screened to collect free product.

For these reasons, dissolved concentrations of DNAPL-related chemicals are often the only evidence that DNAPL free product is present at a site. The EPA has recommended an empirical approach for determining whether DNAPL free product is near a monitoring well where dissolved DNAPL-related compounds have been detected.11 In order to use this approach, it is necessary to measure concentrations of DNAPL-related compounds dissolved in groundwater, to know the composition of the suspected DNAPL, and to calculate the effective solubility of the measured DNAPL components.

The effective solubility is the theoretical solubility in water of a single component of a DNAPL mixture. It may be approximated by multiplying the component's mole fraction* in the mixture by its pure phase solubility.

where: Seff(a) = effective solubility of component a in a DNAPL mixture, in mg/L. Xa = mole fraction of compound a in the mixture. Spure(a) = pure-phase solubility of compound a, in mg/L.

Conditions That Indicate the Presence of DNAPL

If any of the following conditions exist, there is a high probability that DNAPL free product is in the vicinity of the sampling location.

• Groundwater concentrations of DNAPL-related chemicals are >1% of either their pure phase solubility (Spure) or their effective solubility (Seff).

• Soil concentrations of DNAPL-related chemicals are >10,000 mg/kg (1% of soil mass).

• Groundwater concentrations of DNAPL-related chemicals increase with depth or appear in anomalous upgradient/cross-gradient locations with respect to groundwater flow.

• Groundwater concentrations of DNAPL-related chemicals calculated from water/soil partitioning relationships are greater than pure phase solubility or effective solubility.

* The mole fraction of compound a in a mixture of several compounds is written Xa.

moles of a

total moles of all compounds

For a mixture with 1 mole of CCl4 and 3 moles of CHCl3, XCCl4 = 1/4 = 0.25 and XCHCl3 = 3/4 = 0.75. Note that the sum of all mole fractions must equal unity. The mole fraction of any pure substance equals unity.

Calculation Method for Assessing Residual DNAPL

1. Calculate Seff(a) as above.

2. Find Koc, the organic carbon-water partition coefficient in Table 4.6, from published literature, or estimate it from log Koc = log Kow - 0.21.

3. Determine foc, the fraction of organic carbon (oc) in the soil by lab analysis. Values for foc typically range from 0.03 to 0.00017 (mg oc)/(mg soil). Convert values reported in percent (mg oc/100 mg soil) to (mg oc)/(mg soil).

4. Determine or estimate the dry bulk density of the soil (db). Typical values range from 1.8 to 2.1 g/cm3 (kg/L). Determine or estimate the water-filled porosity (pw) of the soil.

5. Determine Kd, the soil-water partition coefficient from Kd = Koc x foc, as in Equation 4.16.

6. Using Csoil(a), the measured concentration, in mg/kg, of DNAPL-related compound a in saturated soil, calculate the theoretical concentration of a in pore water, Cw(a) from

• If Cw(a) > Seff(a) there is a possible presence of DNAPL near the sampling location.

• If Cw(a) < Seff(a) there is a possible absence of DNAPL near the sampling location.

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