Subsurface environmental conditions

Reversible and irreversible retention of contaminants on the subsurface solid phase is a major process in determining pollutant concentrations and controlling their redistribution from the land surface to groundwater. After being retained in the solid, contaminants may be released into the subsurface liquid phase, displaced as water-immiscible liquids, or transported into the subsurface gaseous phase or from the near surface into the atmosphere. The form and the rate of release are governed by the properties of both contaminant and solid phase, as well as by the subsurface environmental conditions. We consider here contaminants adsorbed on the solid phase. 

To illustrate these phenomena, we discuss relevant examples of volatile chemical products that originate mainly from agricultural and industrial-municipal practices. The rates and extent of volatilization of these chemicals are presented in relation to subsurface environmental conditions. 

Subsurface environmental conditions are suboptimal with low temperatures and low concentrations of growth nutrients. The decline of bacterial inoculae by protozoan predation is of major concern in soil (Acea etal., 1988 Acea Alexander, 1988 Casida, 1989) but may not be a factor in saturated subsurface environments. Immobilization of cells to carrier material may enhance microbial survival in the environment through control of predation and supply of nutrients and moisture. Stormo Crawford (1992) developed a cell immobilization technique for production of small beads (2-50 /rm) consisting of agarose and cells of PCP-degrading Flavobacterium sp. Microencapsulated Flavobacteria efficiently degraded PCP and survived for two years in soil columns at environmental conditions (Stormo Crawford, 1994). These results show that microencapsulation may be a very useful tool in in situ bioremediation. 

The major technical problem was the inability to define subsurface geohydrologic conditions with the initial data. Expertise in the area of geohydrology was clearly needed. A lack of specific analytical techniques precluded meaningful environmental and risk assessments. Cleanup efforts were complicated because poltiners are not regulated under RCRA but are regulated under state law. In the middle of the cleanup effort, the site became involved in Superfund activities, and to date this involvement has not been clarified. Project management has become very difficult because of the many players and laws involved. As a result, public confidence has been affected. 

It is now generally accepted that microorganisms are ubiquitous in the deep subsurface, although, as noted, not all strata are biologically active.85 Microorganisms have adapted to the complete range of environmental conditions that exist on and below the Earth s surface. They have been observed at pressures up to 1760 kg/cm2 (25,000 psi), temperatures up to 100°C, and salt concentrations up to 300,000 mg/L.86... 

Dissolution and precipitation in the subsurface are controlled by the properties of the solid phases, by the chemistry of infiltrating water, by the presence of a gas phase, and by environmental conditions (e.g., temperature, pressure, microbiological activity). Rainwater, for example, may affect mineral dissolution paths differently than groundwater, due to different solution chemistry. When water comes in contact with a solid surface, a simultaneous process of weathering and dissolution may occur under favorable conditions. Dissolution of a mineral continues until equilibrium concentrations are reached in the solution (between solid and liquid phases) or until all the minerals are consumed. 

Solubility equihbrium is the final state to be reached by a chemical and the subsurface aqueous phase under specific environmental conditions. Equihbrium provides a valuable reference point for characterizing chemical reactions. Equilibrium constants can be expressed on a concentration basis (/ ), on an activity basis (K ), or as mixed constants (K" ) in which all parameters are given in terms of concentration, except for H, OH", and e" (electron) which are given as activities. 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

Denitrification occurs only in the presence of oxidized nitrogen and in an environment with limited (whieh prevails in the subsurfaee). Beeause denitrifieation is an enzyme-mediated reaetion, the substrate concentration funetions as a rate-determining factor. The dominant denitrifying bacteria are heterotrophie. The favored environmental conditions for the growth of denitrifying baeteria include a neutral pH (6-8), a favorable water-air (oxygen) ratio, and a subsurface temperature between 20 and 30°C. 

Bennett and Barter (1997) discuss the effect of partitioning-dissolution in an aqueous phase of alkylphenol. Specifically, they show that the depletion of this crude oil component affects the chemical composition of the original pollutant. Partitioning at equilibrium can be considered the maximum dissolution value of a compound under optimal solvation conditions. Partitioning-dissolution is obtained by washing the crude oil with saline water at variable temperature and pressure conditions, similar to those in the subsurface. The data reported were obtained using a partition device able to simulate the natural environmental conditions of a crude oil reservoir. The alkylphenol partition coefficients between crude oil and saline subsurface water were measured as a function of variation in pressure, temperature, and water salinity. Preliminary trials proved that the experimental device did not allow alkylphenol losses due to volatilization. 

Enhanced natural degradation (END) is an in situ bioremediation technology for groundwater contaminated with hazardous organic compounds. By promoting the proper environmental conditions, the natural microorganisms in the subsurface soils multiply and transform the contaminants into nontoxic compounds, according to the vendor. 

The target population is a set of all samples for which the decision-maker wants to draw conclusions. It represents environmental conditions within certain spatial or temporal boundaries. For environmental projects, the target populations are usually samples of surface and subsurface soil, groundwater, surface water, or air, collected from a certain space at a certain time. 

For halorespiration to occur, Wiedemeier et al. conclude that the following conditions must exist 1) the subsurface environmental must be anaerobic and have a low oxidation-reduction potential (based on thermodynamic considerations, reductive dechlorination reactions will occur only after both oxygen and nitrate have been depleted from the aquifer) ... 

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