NONAQUEOUS-PHASE LIQUIDS

7.7 NONAQUEOUS-PHASE LIQUIDS

Many organic compounds are only slightly soluble in water and exist in both the dissolved and insoluble (pure) phase in ground water. Pure liquids that are not dissolved are called nonaqueous-phase liquids (NAPLs). NAPLs have been identified at four out of five haz- ardous waste sites in the United States (Plumb and Pitchford, 1985) and are typically com- posed of either a single chemical or a mixture of several chemicals. NAPLs are further classified as light NAPLs (LNAPLs) that are less dense than water and tend to float on the water table, and as dense NAPLs (DNAPLs) that are denser than water and tend to sink to the bottom of the aquifer. Typical LNAPL and DNAPL spills are illustrated in Figure 7.11. Compounds with solubilities less than 20,000 mg/L are likely to exist as NAPLs (Prakash, 2004). The NAPLs commonly encountered at contaminated sites can be categorized into four groups on the basis of their similar chemical structures, fluid properties, and behavior in the subsurface (Adeel et al., 2000): (1) chlorinated hydrocarbons, (2) petroleum products, (3) tars and creosote, and (4) mixtures with polychlorinated biphenyls (PCBs) and oils.

Chlorinated hydrocarbons are low-molecular-weight compounds that are sparingly soluble in water, volatile in nature, and denser than water; hence they are DNAPLs.

Residual saturation of LNAPL in soil from spill

Vadose

Infiltration and

contaminant plume saturation in saturated zone Ground water flow

(a)

Dissolved contaminant

Low permeability stratigraphic unit

DNAPL pools

Sand Ground water flow

Clay (b)

FIGURE 7.11 Typical (a) LNAPL and (b) DNAPL spills.

GROUND WATER

TABLE 7.19 Densities and Solubilities of NAPLs

Solubility at 10⬚C Liquid

Density at 15⬚C

(mg/L) LNAPLs

(kg/m 3 )

Medium distillates (fuel oil)

3–8 Petroleum distillates (jet fuel)

150–300 Crude oil

Trichloroethlene (TCE)

1,070 Tetrachloroethylene (PCE)

160 1, 1, 1-Trichloroethane (TCA)

Dichloromethane (CH 2 Cl 2 )

Chloroform (CHCl 3 )

Carbon tetrachloride (CCl 4 )

20 Source: Schnoor (1996).

Chlorinated hydrocarbons are used predominantly as solvents and degreasers at industrial, commercial, and military facilities. Commonly encountered chlorinated hydrocarbons are tetrachloroethylene (PCE), trichloroethylene (TCE), and carbon tetrachloride. Petroleum products are typically low-molecular-weight hydrocarbons whose solubilities are similar to those of chlorinated hydrocarbons and are less dense than water; hence they are LNAPLs. Commonly encountered petroleum products are benzene, toluene, ethylbenzene, and xylene (collectively referred to as BTEX), which are among the most soluble con- stituents of gasoline. Tars are by-products of coke and gas production, and creosote is a widely used wood preservative. Tars and creosote are DNAPLs and are sparingly soluble in water. PCBs have been used in many industrial applications, including fire retardants in hydraulic oils and electrical transformer fluids. The production of PCBs is currently banned. PCBs are generally denser than water, and hence are DNAPLs. The densities and solubilities of several NAPLs are given in Table 7.19.

Movement of ground water past a NAPL trapped in the solid matrix of a porous medium results in the dissolution of soluble compounds and an associated downstream plume. In some cases the dissolved concentrations are sufficient to affect the density of the water significantly, inducing a vertical ground-water velocity, v z , given by (Frind, 1982)

K ᎏ z ᎏ ρ v z ⫽⫺ᎏ

where K z is the hydraulic conductivity in the vertical direction, n e is the effective porosity, ρ is the density of the dissolved mixture, and ρ 0 is the density of the native ground water. The relative magnitude of v z to the horizontal seepage velocity will give an indication of the extent to which the contaminant plume moves in the same direction as the ground- water flow.

NONAQUEOUS-PHASE LIQUIDS

Since LNAPLs do not penetrate very deeply into the aquifer and are relatively biodegradable under natural conditions, they are generally thought to be a more manage- able environmental problem than DNAPLs, which tend to be trapped deep in the aquifer (Bedient et al., 1994). Other factors that make DNAPL contamination more difficult to remediate are: (1) Chlorinated solvents do not biodegrade very rapidly and persist for long periods of time in ground water, in fact, products of microbial degradation of halogenated solvents are sometimes more toxic than the parent compounds (Parsons et al., 1984); and (2) chlorinated solvents have physical properties, such as small viscosities, that allow movement through very small fractures and downward penetration to great distances. The pattern of DNAPL penetration in aquifers is commonly referred to as viscous fingering. DNAPL pools on impermeable boundaries are often difficult to locate and remediate (Blatchley and Thompson, 1998). A detailed account of the fate and transport of DNAPLs in ground water can be found in Pankow and Cherry (1996).

The movement of NAPLs in ground water is governed primarily by gravity, buoyancy, and capillary forces. At low concentrations, NAPLs tend to become discontinuous and immobilized by capillary forces, and they end up trapped in the pores of aquifers, as illus- trated in Figure 7.12. Under these conditions, the concentration of the NAPL is termed the residual saturation , which is defined as the fraction of total pore volume occupied by residual NAPL under ambient ground-water flow conditions. In the unsaturated zone, residual saturation values are typically in the range 5 to 20%, while in the saturated zone this range is typically higher and on the order of 15 to 50% (Schwille, 1988; Mercer and Cohen, 1990). Residual saturation appears to be relatively insensitive to the types of chem- icals that comprise a NAPL, but is very sensitive to soil properties and heterogeneities (USEPA, 1990a). The residual saturation, S r , of NAPLs give a good measure of how much of the contaminant will remain trapped in the soil after the free product has percolated through the soil, and the residual saturation is also a good measure of how much NAPL

FIGURE 7.12 Residual saturation in porous media: (a) unsaturated zone; (b) saturated zone. (From Abriola and Bradford, 1998.)

GROUND WATER

TABLE 7.20 Residual Saturation of Petroleum Fuels

Fuel Soil

Middle

Oils Coarse gravel

Gasolines

Distillates

0.025 Coarse sand

0.075 Fine sand/silts

0.05 0.10 0.20 Source: American Petroleum Institute (1989).

will remain in the saturated zone after all the free product is pumped out of the aquifer. The residual saturation of various petroleum fuels in soils are given in Table 7.20, and the

residual mass fraction, M f , can be calculated using the relation

where ρ f is the density of the NAPL (see Appendix B.2), n is the porosity of the soil, and ρ s is the density of the soil. In applying the residual saturations shown in Table 7.20, val- ues of ρ are typically 750 kg/m 3 for gasoline, 800 kg/m f 3 for middle distillates, and 900 kg/m 3 for fuel oils.

Example 7.12 Estimate the residual mass fraction in mg/kg when spills of gasoline in medium sand are cleaned up by pumping free product from the surface of the water table.

Assume that the porosity of the aquifer is 0.23 and the density of the sand is 2600 kg/m 3 . SOLUTION From the data given, n ⫽ 0.23, ρ s ⫽ 2600 kg/m 3 , and the density of gasoline

can be taken as ρ f ⫽ 750 kg/m 3 . Interpolating in Table 7.20 between coarse sand and fine sand gives S r ⫽ 0.035 for medium sand. Substituting the data given into Equation 7.80

gives the mass fraction, M f , at residual saturation as

n r M f ⫽ᎏ

ρ s (1 ⫺ n ) ⫹ ρ f nS r

⫽ ᎏᎏᎏᎏ 750(0.23)(0.035) ⫽ 0.0030 kg/kg ⫽ 3000 mg/kg 2600(1 ⫺ 0.23) ⫹ 750(0.23)(0.035)

Therefore, when all the free-product gasoline is removed from the contaminated soil, approximately 3000 mg/kg will remain trapped in the pores of the solid matrix. This trapped gasoline will eventually be removed by such processes as evaporation, dissolution, and biological and chemical degradation.

Even at residual saturation levels, NAPLs are capable of contaminating large volumes of water and cannot be removed easily except by dissolution in flowing ground water. The long time scales required for flowing ground water to remove residual NAPLs is illustrated in the following example.

Example 7.13

A cubic meter of aquifer has a porosity of 0.3 and contains TCE at a resid-

REMEDIATION OF SUBSURFACE CONTAMINATION

is 10 mg/L, and the mean seepage velocity of the ground water is 0.02 m/day, estimate the time it would take for the TCE to be removed by dissolution.

SOLUTION From the data given, n ⫽ 0.3, and the residual saturation, S r , is 0.20, hence

the residual volume of TCE in 1 m 3 of aquifer is given by volume of TCE ⫽ 0.20(0.3)(1) ⫽ 0.06 m 3

Since the density of TCE is 1470 kg/m 3 , 0.06 m 3 corresponds to 0.06(1470) ⫽ 88.2 kg of TCE. With a solubility of 10 mg/L ⫽ 1.1 kg/m 3 , the volume of water required to dissolve the 88.2 kg of TCE is given by

dissolution water required ⫽ ᎏ

80.2 m 3

Since the seepage velocity of the ground water is 0.02 m/day, assuming that the contami- nated volume is a 1 m ⫻ 1 m ⫻ 1 m block of aquifer, the time required for 80.2 m 3 of water to flow through the 1 m 3 of contaminated aquifer is given by

time ⫽ ᎏ 13,367 days ⫽ 36.6 years

0.02 n (1 ⫻ 1)

Hence the residual NAPL will generate a contaminant plume at saturation level (1100 mg/L) for 36.6 years! This result should be considered as somewhat approximate, since dissolution rates are highly dependent on the range and size distribution of NAPL blobs (Schnoor, 1996).