Subsurface-site population

A brief outline as to how this chapter is organized may be helpful. In our attempt to consider H bonding and related effects on and at metal surfaces we will largely exclude the aforementioned complications such as surface reconstruction, subsurface-site population or hydrogen sorption effects, since they may obscure the essential H transfer and bonding phenomena. 

The lowest energy atomic site for H chemisorbed on Cu(l 11) is the fee hollow site with W = 2.3 eV. Smaller and different W exists at the other high symmetry sites. Thus, the PES is very laterally corrugated, both in energy and geometry. In addition, there are metastable subsurface sites inside the surface plane, e.g., one site exists below the fee hollow with W 0.9 eV above that of the most stable surface adsorption site [140]. This is made metastable by abarrier of 0.4 eV relative to the bottom of the subsurface well. Bulk octahedral absorption sites have essentially the same stability as the subsurface sites, with presumably similar barriers to migration into the bulk. Thus, populating the subsurface site represents the initial step in bulk absorption of H. 

In the present paper we have once more shown the active role of defects in the dynamics of gas-surface interactions. In particular we have analysed the case of 02 and C2H4 adsorbed on Ag surfaces, either flat or with a high density of open steps, finding that some processes are enabled by the presence of defects. For both gases open steps were indeed proved to remove the adsorption barriers for chemisorption. For 0/Ag(21 0), moreover, a pathway leading to population of subsurface sites was also found. 

Energetic analysis using temperature-programmed desorption (TPD), also called thermal desorption spectroscopy (TDS), on a well-defined surface makes it possible to determine the structure sensitivity and the heights of the energy barriers of the surfece processes. It was shown that the role of the substrate surface orientation in the population of subsmface sites is crucial. On the reconstmeted (110) faces of Ni and Pd or on more open faces [8,83,84], subsurface sites are populated at temperatures as low as 100 K and low H2 pressures (10 Pa), whereas higher temperatures and pressures are necessary with the more densely packed planes [8]. 

We need to make a decision related to the disposition of soil that has been excavated from the subsurface at a site with lead contamination history. Excavated soil suspected of containing lead has been stockpiled. We may use this soil as backfill (i.e. place it back into the ground), if the mean lead concentration in it is below the action level of 100 milligram per kilogram (mg/kg). To decide whether the soil is acceptable as backfill, we will sample the soil and analyze it for lead. The mean concentration of lead in soil will represent the statistical population parameter. 

The Louisiana sites so far ranked using draft guidance for data input to the model have not scored high, compared to many other sites in the nation. In general, two factors account for the relatively low scores (1) low population density (except in one case) and (2) subsurface geology and hydrology that tends to minimize potential for groundwater contamination. 

Exposure Levels in Environmental Media. Tetryl has been detected in seepage water, groundwater, and surface and subsurface soil at military installations (Army 1980, 1981b, 1986a, 1988, 1990b ATSDR 1987 HazDat 1994). More data are needed regarding levels of tetryl in surface water, groundwater, soil, and air in and around these sites. Quantitative information is needed to assess the potential for human exposure and to better identify exposed populations. 

Three of the near-surface data sets from Table 5-VIII are particularly convincing because the soil-gas measurements were made in basins that contained only one type of production. As shown by Fig. 5-20b, they are the dry-gas production of the Sacramento Basin (more than 450 sites), the gas-condensate production in the Alberta foothills (more than 650 sites), and the oil production of the Permian basin (more than 450 sites). Figures 5-20c, 5-20d and 5-20e show methane content (%C ), the methane ethane ratio (C1/C2), and the propane-.methane ratio (1000 x C3/C1) from the soil-gas populations over these three basins. These data clearly demonstrate that the chemical compositions of the soil gases from these three different areas form separate populations that appear to reflect the differences which exist in the subsurface reservoirs in these three basins. This correlation is particularly striking when compared with the data of Nikonov (1971), shown in Fig. 5-20a. 

Further studies have also demonstrated that thermophilic degradation of PAHs and nonvolatile hydrocarbons increased at temperatures likely to occur adjacent to the active treatment zone of an in situ thermal remediation site (Huesemann et aL, 2002). This observation is likely the result of shifts in the population of microorganisms from predominantly mesophUic to predominantly thermophilic. This type of community shift is usually associated with a reduction in the diversity of microorganisms. As subsurface temperatures cool after active thermal treatment, the consortia within the heated zone will again shift as conditions become less favorable for thermophUes and return to the optimum temperatures for mesophiles. The EPA Technology Innovation Office has pubhshed a more detailed review of this topic, available at the website www.clu-in.org. 

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