USING GROUNDWATER CONCENTRATIONS TO ESTIMATE THE PROXIMITY OF RESIDUAL
MULTICOMPONENT DNAPL MIXTURES, WHERE THE INITIAL COMPOSITION IS KNOWN
A remediation project was being planned for a site that had contained a metal degreasing facility. The degreaser solution that was used consisted of 70 wt% trichloromethane, 15 wt% trichloroethylene, and 15 wt% tetrachloroethylene. A matrix of monitoring wells was drilled to try to locate subsurface source zones of DNAPL releases. A water sample from well SW-4 contained 88 mg/L trichloromethane (MW = 119.37), 1.6 mg/L tetrachloroethylene (MW = 165.82), and 4.2 mg/L trichloroethylene (MW = 131.37). Is this well likely to be close to an upgradient DNAPL source zone? Use data from Table 7.3.
TABLE 7.3 Values for Important Properties of DNAPL Contaminants Commonly Found at U.S. Superfund Sites | ||||||||
Chemical Compound | Density (g/cm^{3}) | Water Solubility (mg/L) | Vapor Pressure (torr) | Henry’s Law Constant (atm m^{3}/mol) | Dynamic Viscosity^{a} (centipoise) | Kinematic Viscosity^{a} (centistokes) | ||
water | 0.9991 (15°C) | _ | 12.8 (15°C) | _ | 1.145 (15°C) | 1.146 (15°C) | ||
(for comparison) | 0.9982 (20°C) | 17.5 (20°C) | 1.009 (20°C) | 1.011 (20°C) | ||||
Halogenated semivolatiles | ||||||||
Aroclor^{b} 1242 | 1.3850 | 0.45 | 4.06\times 10^{-4} | 3.4\times 10^{-4} | ||||
Aroclor^{b} 1254 | 1.5380 | 0.012 | 7.71\times 10^{-5} | 2.8\times 10^{-4} | ||||
Aroclor^{b} 1260 | 1.4400 | 0.0027 | 4.05\times 10^{-5} | 3.4\times 10^{-4} | ||||
Chlordane | 1.6 | 0.056 | 1\times 10^{-5} | 2.2\times 10^{-4} | 1.104 | 0.69 | ||
1,4-Dichlorobenzene | 1.2475 | 80 | 0.6 | 1.58\times 10^{-3} | 1.258 | 1.008 | ||
1,2-Dichlorobenzene | 1.3060 | 100 | 0.96 | 1.88\times 10^{-3} | 1.302 | 0.997 | ||
Dieldrin | 1.7500 | 0.186 | 1.78\times 10^{-7} | 9.7\times 10^{-6} | ||||
Pentachlorophenol | 1.9780 | 14 | 1.1\times 10^{-4} | 2.8\times 10^{-6} | ||||
2,3,4,6-Tetrachlorophenol | 1.8390 | 1,000 | ||||||
Halogenated volatiles | ||||||||
Carbon tetrachloride | 1.5947 | 790 | 91.3 | 0.020 | 0.965 | 0.605 | ||
Chlorobenzene | 1.1060 | 490 | 8.8 | 3.46\times 10^{-3} | 0.756 | 0.683 | ||
Chloroform (trichloromethane) | 1.4850 | 7,920 | 160 | 3.75\times 10^{-3} | 0.563 | 0.379 | ||
1,1-Dichloroethane | 1.1750 | 5,500 | 182 | 5.45\times 10^{-4} | 0.377 | 0.321 | ||
1,2-Dichloroethane | 1.2530 | 8,690 | 63.7 | 1.1\times 10^{-3} | 0.840 | 0.67 | ||
cis-1,2-Dichloroethylene | 1.2480 | 3,500 | 200 | 7.5\times 10^{-3} | 0.467 | 0.364 | ||
trans-1,2-Dichloroethylene | 1.2570 | 6,300 | 265 | 5.32\times 10^{-3} | 0.404 | 0.321 | ||
1,1-Dichloroethylene | 1.2140 | 400 | 500 | 1.49\times 10^{-3} | 0.330 | 0.27 | ||
1,2-Dichloropropane | 1.1580 | 2,700 | 39.5 | 3.6\times 10^{-3} | 0.840 | 0.72 | ||
Ethylene dibromide | 2.1720 | 3,400 | 11 | 3.18\times 10^{-4} | 1.676 | 0.79 | ||
Methylene chloride | 1.3250 | 13,200 | 350 | 2.57\times 10^{-3} | 0.430 | 0.324 | ||
1,1,2,2-Tetrachloroethane | 1.6 | 2,900 | 4.9 | 5.0\times 10^{-4} | 1.770 | 1.10 | ||
1,1,2-Trichloroethane | 1.4436 | 4,500 | 0.188 | 1.17\times 10^{-3} | 0.119 | 0.824 | ||
1,1,1-Trichloroethane | 1.3250 | 950 | 100 | 4.08\times 10^{-3} | 0.858 | 0.647 | ||
Tetrachloroethylene (PCE) | 1.620 | 200 | 14 | 0.0227 | 0.890 | 0.54 | ||
Trichloroethylene (TCE) | 1.460 | 1,100 | 58.7 | 8.92\times 10^{-3} | 0.570 | 0.390 | ||
Trichloromethane (chloroform) | 1.4850 | 7,920 | 160 | 3.75\times 10^{-3} | 0.563 | 0.379 | ||
Nonhalogenated semivolatiles | ||||||||
2-Methyl naphthalene | 1.0058 | 25.4 | 0.0680 | 0.0506 | ||||
o-Cresol | 1.0273 | 31,000 | 2.45\times 10^{-1} | 4.7\times 10^{-5} | ||||
p-Cresol | 1.0347 | 24,000 | 1.08\times 10^{-1} | 3.5\times 10^{-4} | ||||
2,4-Dimethylphenol | 1.0360 | 6,200 | 0.098 | 2.5\times 10^{-6} | ||||
m-Cresol | 1.0380 | 23,500 | 1.53\times 10^{-1} | 3.8\times 10^{-5} | 21.0 | 20 | ||
Phenol | 1.0576 | 84,000 | 5.293\times 10^{-1} | 7.8\times 10^{-7} | 3.87 | |||
Naphthalene | 1.1620 | 31 | 2.336\times 10^{-1} | 1.27\times 10^{-3} | ||||
Benzo(a)Anthracene | 1.1740 | 0.014 | 1.16\times 10^{-9} | 4.5\times 10^{-6} | ||||
Fluorene | 1.2030 | 1.9 | 6.67\times 10^{-4} | 7.65\times 10^{-5} | ||||
Acenaphthene | 1.2250 | 3.88 | 0.0231 | 1.2\times 10^{-3} | ||||
Anthracene | 1.2500 | 0.075 | 1.08\times 10^{-5} | 3.38\times 10^{-5} | ||||
Dibenzo(a,h)anthracene | 1.2520 | 2.5\times 10^{-3} | 1\times 10^{-10} | 7.33\times 10^{-8} | ||||
Fluoranthene | 1.252 | 0.27 | 7.2\times 10^{-5} | 11\times 10^{-6} | ||||
Pyrene | 1.2710 | 0.148 | 6.67\times 10^{-6} | 1.2\times 10^{-5} | ||||
Chrysene | 1.2740 | 6.0\times 10^{-3} | 6.3\times 10^{-9} | 1.05\times 10^{-6} | ||||
2,4- Dinitrophenol |
1.6800 | 6.0\times 10^{-3} | 1.49\times 10^{-5} | 6.45\times 10^{-10} | ||||
Miscellaneous | ||||||||
Coal tar (45°F) | 1.028 | 18.98 | ||||||
Creosote | 1.05 | ~1.08 (15°C) | ||||||
Source: Adapted from USEPA, Dense Nonaqueous Liquids, S.G. Huling and J.W. Weaver, Ground Water Issue, Office of Research and Development, Office of Solid Waste and Emergency Response, Washington, DC, EP A/540/4-91-002, March 1991. | ||||||||
^a Dynamic viscosity measures a liquid’s resistance to flow. Kine matic viscosity is the ratio of dynamic viscosity to density, see Table 7.2. | ||||||||
^b Aroclor is the trade name for polychlorinated biphenyls (PCBs) manufactured by Monsanto. See Section 7.3.4. |
TABLE 7.2 DNAPL Properties Important for Predicting Mobility in Environment | ||||||
Properties of DNAPL/Soil | Definition/Typical Units | Comments | ||||
Density (d) | d = mass/volume
d = g · cm^{-3}; lb · ft^{-3} |
Density distinguishes between LNAPLs (d_{DNAPL} < d_{water}) and DNAPLs (d_{DNAPL} < d_{water}). It depends on temperature, pressure, molecular weights of components, intermolecular forces, and bulk liquid structure. |
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Dynamic viscosity (\mu) | \mu = fluid internal resistance to flow or shear. The CGS unit is poise (P); SI unit is N·s·m^{-2}. 1 P = 100 centipoise = 1 g/cm·s = 0.1 Pa·s |
Dynamic viscosity is a measure of the force required to move a liquid at a constant velocity. The common unit of μ is the centipoise (cP) because water at 20.2°C has a convenient viscosity of 1.000 cP. Viscosity decreases with increasing temperature (note water in Table 7.2). Intermolecular attractions are the main cause of viscosity. The lower the viscosity, the more fluid the liquid and the more easily it will flow through soils. The reciprocal of dynamic viscosity is called fluidity. |
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Kinematic viscosity (\nu) | \nu= dynamic viscosity/density The CGS unit is stokes (St) or centistokes (cSt); SI units are m^{2}·s^{-1}; stokes = poises/density 1 St = 100 cSt = 10^{-4} m^{2}·s^{-1} |
When the force causing a liquid to move is only due to gravity, as in NAPL movement in the environment, the fluid density, as well as the dynamic viscosity, affects the rate of movement. Using kinematic viscosity includes density in its definition and eliminates the force term (N or Pa). Kinematic viscosity is convenient for calculating hydraulic conductivity, which is inversely proportional to n. Since the density of water at 20.2°C is 0.998 g/cm^{3}, the kinematic viscosity of water at 20.2°C is, for most practical purposes, equal to 1.0 cSt |
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Solubility in water (S) | S = mass of dissolved substance per unit volume of water, in equilibrium with the undissolved substance. For environmental pollutants in water, the common units are mg/L or \mu g/L. |
Solubility measures a compound’s tendency to partition from the bulk compound into water. For a single-component NAPL, the solubility is the concentration of dissolved component in equilibrium with the NAPL. For NAPLs that are mixtures, each component of the mixture has its own characteristic solubility, which is generally lower than the solubility of the pure component (see Section 6.3.8). Thus, the overall solubility of an NAPL mixture is variable, depending on its composition, and changes with time as the more-soluble components leave the NAPL by partitioning into the water. Solubility can vary with temperature, pH, TDS, and the presence of cosolvents (e.g., detergents, EDTA, etc.). In general, the greater the molecular weight (high polarizability) and symmetry (low polarity) and the fewer hydrogen-bonding atoms, the lower the solubility, see Section 2.9. |
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Vapor pressure (P_{v}) | P_{v} = pressure exerted by a vapor in equilibrium with the liquid or solid phase of the same substance. There are many different units for pressure. The more common units are millimeters of mercury (mm Hg), torr, and atmosphere (atm). The SI unit is pascal (Pa). 1 mm Hg = 1 torr = 760^{-1} atm = 1.333 mbar = 133.3 Pa = 1.934 \times 10^{-2} psi 1 Pa = 1 N/m^{2} = 10^{-5} bar = 7.50\times 10^{-3} torr = 1.450\times10^{-4} psi |
Vapor pressure indicates an NAPL’s volatility, or tendency to vaporize, at a given temperature. It depends only on the temperature and increases exponentially with increasing temperature. On a molecular level, vapor pressure is an indication of the strength of intermolecular attractive forces, see Section 2.8.6. The vapor pressure of DNAPLs ranges from very high to very low; for example, compare 1,1-dichloroethylene and chrysene in Table 7.2 |
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Henry’s law volatility | The Henry’s law volatility of a compound is a measure of the transfer of the compound from being dissolved in the aqueous phase to being a vapor in the gaseous phase. |
The transfer process from water to the gaseous phase in the atmosphere is dependent on the chemical and physical properties of the compound, the presence of other compounds, and the physical properties (velocity, turbulence, depth) of the water body and atmosphere above it. The factors that control volatilization are the solubility, molecular weight, vapor pressure, and the nature of the air–water interface through which it must pass. The Henry’s constant is a valuable parameter that can be used to help evaluate the propensity of an organic compound to volatilize from the water. The Henry’s law constant is defined as the vapor pressure divided by the aqueous solubility. Therefore, the greater the Henry’s law constant, the greater the tendency to volatilize from the aqueous phase, refer to Table 7.1. |
1. Convert the weight-percentages of each DNAPL component to mole fractions.
a. 100 g of solvent contains 70 g trichloromethane, 15 g trichloroethylene, and 15 g tetrachloroethylene.
b. 70 g trichloromethane =\left(\frac{70 g}{119.37 g/mol}\right) = 0.586 mol
c. 15 g trichloroethylene =\left(\frac{15 g}{131.38 g/mol}\right) = 0.038 mol
d. 15 g tetrachloroethylene = \left(15 \frac{g}{165.82 g/mol}\right) = 0.090 mol
e. Total moles DNAPL = 0.586 + 0.038 + 0.090 = 0.714 mol
f. Mole fractions: X(trichloromethane) = \left(\frac{0.586}{0.714}\right) = 0.821
g. X(trichloroethylene) =\left(\frac{0.038}{0.714}\right) = 0.053
h. X(tetrachloroethylene) =\left(\frac{0.090}{0.714}\right) = 0.126
i. Sum of mole fractions = 0.821 + 0.053 + 0.126 = 1.000
Calculate S_{eff} from Equation 7.2 and Table 7.3.
S_{eff}(a) = X_{a}S_{pure}(a) (7.2)
. S_{eff} (trichloromethane) = 0.821\times 7920 mg/L = 6502 mg/L
. S_{eff} (trichloroethylene) = 0.053\times 1100 mg/L = 58.3 mg/L
. S_{eff} (tetrachloroethylene) = 0.126\times 200 mg/L = 25.2 mg/L
By the guideline in Section 7.3.1, if the measured concentration in a well of a multicomponent DNAPL mixture is 1% or more of the effective solubilities of its components, it is likely that a DNAPL source zone is near the well. The measured concentrations
. C_{meas} (trichloromethane) = 88 mg/L
. C_{meas} (trichloroethylene) = 4.2 mg/L
. C_{meas} (tetrachloroethylene) = 1.6 mg/L
are all greater than 1% of their respective effective solubilities. Therefore, the sampled well is likely to be close to an upgradient DNAPL source zone.