Extraction and Characterization of Pore Water in Contaminated Soils

Chemical elements that are either present naturally in the soil or introduced by pollution are more usefully estimated in terms of “availability” of the element, since it is this property that can be related to mobility and uptake by plants. A good estimate of the immediately available fraction can be achieved by measuring the concentration, or activity, of chemical species in soil pore water. Current analytical techniques enable the application of this approach to trace elements, such as plant and animal micronutrients and those defined as potentially toxic elements (PTEs) in environmental studies. A complete chemical analysis of soil pore water represents a powerful diagnostic tool for the interpretation of many soil chemical phenomena relating to soil fertility, mineralogy, and environmental fate. This chapter describes some of the current methodologies used to extract soil pore water. In particular, five laboratory-based methods are described and discussed in detail: (i) high speed centrifugation-filtration, (ii) low (negative-) pressure Rhizon samplers and passive diffusion samplers, (iii) high pressure soil squeezing, (iv) equilibration of dilute soil suspensions, and (v) diffusive gradients in thin-films (DGT). A number of operational factors are presented: pressure applicable (i.e., pore size accessed), moisture prerequisites of the soil, pore water yield, efficiency, duration of extraction, materials and possible sources of contamination for micronutrient and PTE studies. There is also consideration of the advantages and disadvantages of the methods, including costs and material availability.