Subsurface Transporters

Mobile solvents capable of transporting elements at significant distances are transporters. There are only three fluids that can be such transporters in the geologic medium. They are underground water, nonpolar solutions, predominantly of organic compounds (oil, bitumens, oil products, etc.) and undergroxmd gases. 

Migration forms of the same element differ primarily in their attitude to natural solvents. Polar compoxmds well dissolve in water, nonpolar - better in nonpolar solvents, volatile and gas - in the subsurface gas. Preferences of the migration forms towards different subsurface transporters may be evaluated by their distribution in various media imder identical thermodynamic conditions. Let us assume that in close to normal, for instance in the aeration zone, component i has to distribute between sweet-water, underground gas at a pressure 1 bar and nonpolar hydrophobic liquid, which have equal volumes, i.e., in equation (2.336) = 5 = 1. 

Saturated vapor pressure of mineral components (predominantly ions) is minuscule, and their partition coefficients value may be close to 0. That is why fractions of mineral components in the subsurface gas or organic solvent may be disregarded and their fraction in ground water may be assumed equal to 1. 

Distribution of organic and gas components depends to a substantial extent on their properties and on property of possible nonpolar solvent. For estimating the distribution of individual components relative to major transporters, it is possible to use octanol as standard nonpolar solvent. 

To determine the preference of gases and organic components relative to water, gas and octanol we used equation (2.334). The partition coefficient defined from equations (2.300) and (2.319) is equal to 0.0416 Hj., and instead of coefficient can be used the octanol-water partition coefficient.  

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R.C. Knox, D.A. Sabatini, and L.W. Canter, Subsurface Transport and Pate Processes, Lewis Pubhshers, Boca Raton, FL, Chapter 6, pp, 268-272 (1993). 

The two flux equations of importance to subsurface transport are Darcy s law for the advective flow of water and other liquids and Fick s law for the diffusive flow of molecules and gases. These laws are independently discussed below. 

McBride MB (1994) Environmental chemistry of soils. Oxford University Press McCarthy JF, Zachara JM (1989) Subsurface transport of contaminants. Environ Sci Technol 23 496-502... 

Benzofuran was among those chemicals selected as representative compounds of waste chemicals from energy production for subsurface transport research (Zachara et al. 1984). 

Zachara JM, Felice LJ, Riley RG. 1984. The selection of organic chemicals for subsurface transport research. Report to U.S. Department of Energy by Pacific Northwest Laboratory, Richland, WA. NTIS No. DE85 007876. 

The retardation of subsurface transport of TNT arises from this compound s absorption into NOM and adsorption onto mineral siloxane surfaces covered with weakly hydrated cations like potassium (but not sodium and calcium). While components of feldspars exhibit some siloxane surfaces, here we anticipate that most of the silox-anes occur in the aluminosilicate clay minerals (e.g., illite) because these particles have such high specific surface areas (Table 11.3). Hence, the total for TNT may be found at this site ... 

McCarthy, J. F. Zachara,J. M.(1989). Subsurface transport of contaminants. Environmental Science Technology, 23, 496-502. 

Failing to incorporate soil-gas advection induced by barometric pumping into gas-phase subsurface transport models may, under certain conditions, under predict contaminant flux to the atmosphere. As previously described, Smith et al. (1996) compared TCE vapor fluxes measured with a chamber device to TCE in groundwater being removed by a pump-and-treat system and discharge into a surface-water receiving body at the same site. These researchers found VOC removal rates by flux to the atmosphere comparable in magnitude to both of the other attenuation pathways. 

Gaynor, J.D., D.C. MacTavish, and W.L Findlay (1992). Surface and subsurface transport of atrazine and alachlor from Brookston clay loam under continuous corn production. Arch. Environ. Contam. Toxicol., 23 240-245. 

The other hydrophilic organics identified in the various water samples are generally much less abundant (ppb levels), and consist mainly of carboxylic acids. A trace of the organic groundwater tracer pentafluorobenzoic acid (added to the cap of experimental trench section 4) was detected in water from sump S4 in expermental trench section 4 (sampled 8/18/81). A trace of this compound was also detected in water from waste trench 27 (sampled on 4/7/81), as a result either of subsurface transport in... 

In an attempt to deal with such unwanted substances as radioactive and chemical wastes, disposal sites are often used that are hydraulically connected with usable water supplies via subsurface transport routes. To manage these wastes effectively, it is desirable to have the capability of predicting the course of solute transport along these connecting routes. 

Engesgaard P., Jensen K. H., Molson J., Erind E. O., and Olsen H. (1996) Large-scale dispersion in a sandy aquifer simulation of subsurface transport of environmental tritium. Water Resour. Res. 32, 3253-3266. 

The subsurface environment also shares many processes with surface waters. Microbially mediated redox reactions and biodegradation processes are significant in each medium, and much of what was discussed in Chapter 2 on these topics applies directly to the subsurface as well. The presence of particles, and their potential to absorb chemicals, also is common to both surface waters and groundwaters when modeling subsurface transport, the high ratio of solid material to water requires special recognition of even moderate sorbing tendencies. 

Saturated zones are common in porous material, and provide significant pathways for the subsurface transport of water and solutes. The term "ground water" commonly refers to continuously saturated zones of appreciable thickness. Saturated conditions also occur on a small scale or short term basis in association with the infiltration and drainage of precipitation or surface runoff. As indicated previously, the pressure and elevation components are the primary contributors to the total moisture potential in the saturated zone. These two are commonly combined into a "piezometric head", representing the addition of the water pressure head to the elevation at which the pressure head is measured. The hydraulic conductivity does not change significantly with... 

Subsurface Transport of Explosives, Technical Report IRRP-95-2, U.S. Army Corps of Engineers (August 1995). 

This book is a comprehensive mforonce on subsurface transport and fate processes. Topics covered include soli and contaminant properties affecting transport and fate, hydrodynamic processes, abiotic processes, biotic processes, physical modeis of contaminant transport, empirical modeis and vulnerability mapping, mathematical modeling of contaminant transport, and applications. 

Use of low and perhaps unrepresentative analytical values for total Al and difunctional CAA concentrations may be one reason why mass balance calculations and computer model simulations of formation waters produce results that de-emphasize the role of organo-metallic complexes in the subsurface transportation of Al and Si (1.2.72.79). In addition, the thermodynamic constants for organo-metallic complexation by these species are unknown at elevated temperatures, and van t Hoff-type approximations used in many models may not be valid as the temperature extrapolation is too large. Complexation and transportation of Al and Si by polar organic compounds remain the only viable mechanism to explain the widespread Al and Si mobility noted in many clastic hydrocarbon reservoirs (3.35.39). 

Table IV contains a selective review of contaminants from point and norqxrint sources with a general estimate of persistence and risk to human environmental health. The persistence and risk estimates are generalized on the basis of subsurface transport and fate and risks to surface water environments or drinking water.

Pepper, D. W., and D. E. Stephenson. 1995. An Adaptive Finite-Element Model for Calculating Subsurface Transport of Contaminant, Ground Water, vol. 33, no. 3, pp. 486-496. 

McCarthy JF, Degueldre C. Samphng and characterization of groundwater colloids for studying their role in the subsurface transport of contaminants. In Buffle J, Leeuwen HV (Eds.), Environmental Particles, Vol. n. Lewis Publishers, Chelsea, MI, 1993, p. 247. 

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