Lateral and Vertical Distribution

In the vadose zone, liquid or dissolved contaminants exist in a complex environment that involves interaction among the chemicals, soil grains, water attached to soil grains (or between the soil pores), the atmosphere in void spaces, and numerous 

Ideally, the site characterization study has defined the vertical and horizontal extent of the contamination. Contoured site maps showing the (three-dimensional) distribution of the contaminants allow identification of areas that require extensive restoration, or may be allowed to be monitored to closure under natural attenuation. Knowledge of how much contamination exists and its location is the important first step in the remediation process. Evaluation of these data will permit consideration of the various remediation remedies available. Where the contaminant is contained within the shallow ( 6 m) unsaturated zone and is recalcitrant (not readily biodegradable), excavation for off-site treatment or disposal may be the most expeditious procedure. Alternatively, depending on the contaminant, a variety of in situ procedures, including bioremediation, air sparging, soil vapor extraction, and fixation, may be applicable. 

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One of the limitations of the portable field survey instruments in the measurement of americium is that their quantitative accuracy depends on how well the lateral and vertical distribution of americium in the soil compares with the calibration parameters used. These methods can provide a rapid assessment of americium levels on or below surfaces in a particular environment however, laboratory-based analyses of samples procured from these environmental surfaces must be performed in order to ensure accurate quantification of americium (and other radionuclides). This is due, in part, to the strong self absorption of the 59.5 keV gamma-ray by environmental media, such as soil. Consequently, the uncertainty in the depth distribution of americium and the density of the environmental media may contribute to a >30% error in the field survey measurements. Currently, refinements in calibration strategies are being developed to improve both the precision and accuracy (10%) of gamma-ray spectroscopy measurements of americium within contaminated soils (Fong and Alvarez 1997). 

The lateral and vertical distributions of these carrier-phase metals in estuaries are largely controlled by particle dynamics, as opposed to other metals (e.g., Cu, Zn, and Co) which will be more affected by biotic uptake processes. 

A case study is presented that demonstrates the importance of evaluating lithofacies distribution, or lateral and vertical heterogeneities, and depositional environment as a control on LNAPL occurrence and migration, and implementation of an effective and efficient remediation strategy. This is followed by a case history on the development of a long-term remedial strategy for LNAPL recovery and aquifer restoration from a regional perspective. 

Joly observed elevated "Ra activities in deep-sea sediments that he attributed to water column scavenging and removal processes. This hypothesis was later challenged with the hrst seawater °Th measurements (parent of "Ra), and these new results conhrmed that radium was instead actively migrating across the marine sediment-water interface. This seabed source stimulated much activity to use radium as a tracer for ocean circulation. Unfortunately, the utility of Ra as a deep ocean circulation tracer never came to full fruition as biological cycling has been repeatedly shown to have a strong and unpredictable effect on the vertical distribution of this isotope. 

The vertical distribution of oil products in a soil profile depends on the fluxes and ground level of soil-ground waters, which both temporally and spatially are very dynamic. In spite of being restricted due to low water solubility, migration of oil products with natural waters in the areas of oil pollution may significantly increase the distance of lateral pollutant transport with surface and sub-surface waters. 

Aerosol particles Table 3.13 shows the percentage change in the actinic flux calculated by Peterson (1976) and Demerjian et al. (1980) for two cases (1) a particle concentration of zero, corresponding to a very clean atmosphere, and (2) a total particle concentration doubled compared to the base case. The actinic flux is predicted to increase if the total particle concentration is zero and decrease if it doubles (note, however, as discussed later, the sensitivity to the vertical distribution of particles and the relative importance of light scattering compared to absorption). 

The use of the sun or moon as the light source allows one to measure the total column abundance, i.e., the concentration integrated through a column in the atmosphere. This approach has been used for a number of years (e.g., see Noxon (1975) for NOz measurements) and provided the first measurements of the nitrate radical in the atmosphere (Noxon et al., 1978). As discussed later in this chapter, such measurements made as a function of solar zenith angle also provide information on the vertical distributions of absorbing species. Cloud-free conditions are usually used for such measurements as discussed by Erie et al. (1995), the presence of tropospheric clouds can dramatically increase the effective path length (by an order of... 

Fig. 38. Substrate dependence of the PS/PMMA domain structure spin-cast from THF a-c on SiOx d-f on octadecyl mercaptane (ODM) [352]. The SFM images have lateral dimensions of 14x14 pm2. a,d as spin cast b after immersion in cyclohexane to remove PS. e after immersion in acetic acid to remove the PMMA-rich phase. The cross-sections (c,f), which were recorded along the lines in (a,b,c,d), reveal the vertical distribution of the PS (dark grey) and PMMA (light grey) phases. The error bar in (c) indicates the accuracy of the superposition procedure. PMMA preferentially adsorbs on the more polar SiOx surface to form a homogeneous layer next to the substrate. On the ODM a PS/PMMA bilayer is observed. Courtesy of U. Steiner...

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