Random approach, sampling site

Because of the heterogeneity of soils in general and helds in particular, statistics alone cannot provide the best guidance regarding sampling for all situations. Instead some random samples can be taken and the results used in combination with statistical approaches to guide the selection of additional samphng sites. 

Moreover, as the system of Eq. (24) is a starting point for the cluster-type approach and the MC method, therefore in principle the two methods can be used in combination. It has been proposed by Hood et al. [297] to write down the kinetic equations for describing variations in the occupancy state of each lattice site, i.e., to abandon consideration of the lattice ensemble, and to solve the system of the equations with the dimension equal to the number of sites. The system of the connected equations has been solved numerically. In each time interval the desorption probability for a given molecule is determined by the random sampling and then the general adsorbate change found. The combination approach allows to trace the adlayer structure and to construct a correlation between the structural and the kinetic behavior of the process. Such an approach has been applied to the N2/Ru(001) system to obtain a qualitative agreement with experiment [298]. 

For certain minerals with multiple element substitutions on lattice sites, electron channeling experiments can provide estimates of site occupancy using a similar thin-film analysis technique. This latter approach, termed ALCHEMI, utilises an orientational dependence of X-ray emission from specific elements on crystallographic sites. Conventional thin-film analyses, which measure the concentration of elements in a sample, do not require specific, known orientations of a sample, and are best obtained from randomly-oriented or non-Bragg diffracting crystals and with a convergent beam which minimises channeling effects. 

This presentation provides an overview of sampling methods and tools suitable to address most site characterizations. The basic sampling types discussed are the systematic, random, and judgmental sampling approaches. 

Figure 2.3 Stratified random sampling. This approach divides the site into smaller zones. Samples are collected randomly within each zone, and could even be composited.

Figure 2.6 Stratified systematic sampling or systematic nonaligned sampling. This approach combines random and systematic sampling. The site is divided into regular spaced regions, and then a sample unit is chosen randomly within each of these regions.

Stratified sampling is commonly used in a location which is known to have contaminants heterogeneously distributed. This is therefore the most common approach to sampling. In this type of sampling, the site is sub-divided into smaller areas, each of which is fairly homogeneous, and thus more accurate sampling can take place. Each sub-area is then randomly sampled. The sub-dividing of the... 

Next, EPA screened the sites through field testing to determine actual 2,3,7,8-TCDD contamination. EPA employed a biased sampling approach. Instead of taking samples randomly,... 

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