Subsurface contamination problem

Nonaqueous phase Hquids (NAPLs) present special problems for soil and ground water cleanup. Contaminant transport through ground water depends in part on the water solubiHty of the compound. Because NAPLs cling to subsurface particles and are slow to dissolve in ground water, they hinder cleanups and prolong cleanup times. Dense nonaqueous phase Hquids (DNAPLs) migrate downward in the aquifer and can coUect in pools or pockets of the substmcture. Examples of DNAPLs are the common solvents tetrachloroethylene (PCE) and trichloroethylene (TCE) which were used extensively at many faciHties before the extent of subsurface contamination problems was realized. 

Subsurface contamination by organic chemicals is a widespread and serious problem, restricted not only to oil spills, but also pertinent in former and still-operating industrial sites. Remarkably, chemical-enhanced oil recovery technology can be used to remove oily contaminants from soil see Chapter 16, p. 232 for further explanation. 

Subsurface drains function like an infinite line of extraction wells, and can be used to contain and remove a plume or to lower the groundwater table (Figure 16.12). They are more cost-effective than pumping for shallow contamination problems at depths of less than 12 m (40 ft). Depths may be increased if the site is stable, if the soil has a low permeability, and if no rock excavations are encountered. 

Yet, like surface waters, the subsurface environment has been a recipient of pollutant chemicals. To effectively solve contamination problems and mitigate threats of contamination, one must have a thorough understanding of the factors that govern the transport and eventual fate of subsurface chemi-... 

The applications of separation science to environmental restoration are centered on the cleanup of contaminated groundwaters and soils. The extent and complexity of the groundwater contamination problem continues to present formidable technological obstacles to cleanup. The most important factors that contribute to this complexity are the large number of contaminated sites, the wide diversity of the contaminants present in those sites, the Inherent complexity of the subsurface chemistry of the contaminants, and the difficulty in interpreting existing regulations to establish compliance and properly prioritize site remediation efforts. 

Characterization of the subsurface provides data for modeling transport of chemical contaminants. Research into modeling of fate and transport is itself an inqiortant aspect of dealing with the subsurface remediation problems. Inqirovements in the models will enable better understanding of how to deal with contamination at specific locations within the DOE complex. 

As is the case with surface waters, the subsurface environment plays key roles in natural chemical cycling, and is also a recipient of pollution. To understand the functioning of natural systems and to address contamination problems, it is necessary to describe and quantify the factors that govern the transport and fate of subsurface chemicals. This chapter begins with a generalized description of the subsurface environment within which these factors operate. 

Notwithstanding the natural heterogeneity of the subsurface, we can usefully consider homogeneous (bulk, effective) descriptions for at least some problems, especially for water flow (but less so for contaminant migration see Sect. 10.1). Therefore, two basic approaches to modeling generally are used to describe and quantify flow and transport continuum-based models and pore-network models. We discuss each of these here. 

Removal rates for the PetroClean bioremediation system are governed by the solubility of the contaminants and by the ability of indigenous microbes to degrade contamination. Additional problems that could affect the PetroClean system might include clogging, subsurface permeabilities, permitting problems, and regulations on the nutrients used in the process. 

The potential of washing the contaminant beyond the capture zone and the introduction of surfactants to the subsurface may cause problems. The technology should be used only where flushed contaminants and soil flushing fluid can be contained and recaptured. 

Serious environmental and technical problems may result from the improper application of flushing solutions. If chemicals precipitate in soils and sediments, they may clog pores, decrease soil/sediment permeability, and reduce the effectiveness of the treatment technique. Furthermore, hazardous solutions may leak from the treatment site and contaminate valuable groundwaters in surrounding areas. To prevent the spread of contaminants, barriers are used to control the subsurface flow of the flushing solutions (US EPA), 2002a, 8.1). 

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