Question 7.1: USING GROUNDWATER CONCENTRATIONS TO ESTIMATE THE PROXIMITY O......
USING GROUNDWATER CONCENTRATIONS TO ESTIMATE THE PROXIMITY OF RESIDUAL SINGLE-COMPONENT DNAPL
Analysis of a water sample from a monitoring well indicated 6.4 mg/L of tetrachloroethene (PERC). Tetrachloroethene was a target contaminant because a dry cleaning establishment had once been on the site near the well. Is residual tetrachloroethene DNAPL likely to be in the subsurface upgradient near the well? 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. |
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Step-by-Step
Since the observed DNAPL is a pure solvent (tetrachloroethene) and not a mixture, its mole fraction, X, equals unity and S_{eff} = S_{pure}. From Table 7.3, the solubility of pure tetrachloroethene is 200 mg/L. By the guideline in Section 7.3.1, if the measured concentration of a single-component DNAPL in a well is 1% or more of its purephase solubility, it is likely that a DNAPL source zone is near the well.
One percent of 200 mg/L is 2.0 mg/L. The measured concentration of tetrachloroethene in the well is 6.4 mg/L. Because this is significantly larger than 2.0 mg/L, it is likely that a source zone of tetrachloroethene DNAPL is quite close to the well.
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