Sampling Soil Pore Water

Spaces as large as these are Transmission Macropores Macropores Channel flow 10000 -0.015 

Pores of about this size and Fine macropores Mesopores Drainage hysteresis 1000 -0.15 

Pores larger than about 15 tm Mesopores Micropores Evapotranspiration 10 -15 

Water in pores of about this Residual pores Ultramicropores 

Field methods for sampling pore water are generally grouped under the general term lysimetry. This definition usually comprises a range of types of samplers (Wolt, 1994)  

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As more sensitive analytical methods for pesticides are developed, greater care must be taken to avoid sample contamination and misidentification of residues. For example, in pesticide leaching or field dissipation studies, small amounts of surface soil coming in contact with soil core or soil pore water samples taken from further below the ground surface can sometimes lead to wildly inaccurate analytical results. This is probably the cause of isolated, high-level detections of pesticides in the lower part of the vadose zone or in groundwater in samples taken soon after application when other data (weather, soil permeability determinations and other pesticide or tracer analytical results) imply that such results are highly improbable. 

Ross et al. [6] analysed samples of soil leachates from laboratory columns and of soil pore water from field porous cup lysimeters for aluminium by atomic absorption spectrometry under two sets of instrumental conditions. Method 1 employed uncoated graphite tubes and wall atomisation method 2 employed a graphite furnace with a pyrolytically coated platform and tubes. Aluminium standards were prepared and calibration curves used for the colorimetric quantification of aluminium. Method 1 gave results which compared favourably with method 2 in terms of both sensitivity and interference reduction for samples containing 1-15 uM aluminium. 

The validity of these assumptions depends on the way that soil pore water is conceptualized, that is, defined and sampled, and how that concept is translated into an operational method or model whereby soil pore water can be obtained and its... 

For the soil pore water sampling, the procedure described by Knight et al. (1998) and Tye et al., (2003) can be normally followed. Samplers are inserted into soil containers and soil pore water extracted by connecting a syringe to each sampler and applying a suction. 

The time required for sampling depends directly on the actual unsaturated hydraulic conductivity (k) of a soil. Soil pore water will be extracted when k > 10-3 m day-1 and when there is a good hydraulic contact between the soil and the sampler. 

Di Bonito, M. (2005). Trace Elements in Soil Pore Water A Comparison of Sampling Methods. Ph.D. thesis, University of Nottingham, Nottingham (http //etheses nottingham.ac.uk/archive/ 00000123/). 

Morrison, R., and Szecsody, J. (1985). Sleeve and casing lysimeters for soil pore water sampling. Soil Sci. 139(5), 446-451. 

In aquatic sediments or soils, there are also a range of trace elements species ranging from ions exchanged to particles, to those bound to organic matter or in various inorganic forms (e.g., oxides, carbonates, sulfides) or as more inert crystalline mineral phases. As in waters, speciation studies in soils and sediments are generally undertaken to better understand the bioavailability of toxic substances and to investigate transport pathways to and from other parts of the ecosystem. Sediment and soil pore waters (soil solutions) are of particular interest because they are in equilibrium with the solid phase and are the medium for contaminant uptake by plants and many other biota. The techniques used for speciation analysis in these aqueous samples differ little from those for waters. 

Natural waters encompass a wide variety of sample matrices including rainwater mineral spring waters groundwaters surface waters (river, stream, lake, and pond waters) soil pore waters runoff waters snow, hail, and sleet ice and ice cores and well and bore waters. 

Empirical data describing the extent of chemical uptake by plants roots are generally expressed as ratios of chemical concentrations in the plant compartment of interest (e.g., shoots, roots, xylem sap) to that in the exposure medium (soil, soil pore water, hydroponic solution) measured at the time the samples are collected. These ratios are generally referred to as bioconcentration factors (BCFs) but they may or may not reflect equilibrium conditions. Plant BCF values are widely used to provide direct and approximate estimates of plant tissue concentrations from measured exposure... 

In both types of extraction, it is not certain that all the water or even a representative sample of the water is removed from soil. Water in small pores, cracks, or held at greater pressures that those applied to remove water will not be removed and their constituents will not be included in the analysis. However, both these methods find wide use in soil analysis. 

A second approach is to obtain (extract) water as it naturally occurs in soil pores. Typically, a porous ceramic cup (other materials are available) is placed in an unsaturated soil, either in the field or laboratory. A vacuum is applied (slightly more negative pressure than the water in the soil pores see also Chapter 5) to the ceramic cup via tubing to move water into a receiving container. This water can be analyzed for all its constituents. A reason for obtaining this type of soil water sample is to analyze it for one specific constituent such as a herbicide, insecticide, or pollutant. Water extracted in this way may also better represent the concentration of the analyte of interest to which plant roots are exposed. 

As protozoa and nematodes live in pore water in the soil, most of the methods are adapted from toxicity tests designed for aquatic samples. Among the protozoa the tests with cihates Tetrahymena pyriformis, Tetrahymena thermophiia, Colpoda cucullus, Colpoda inflata, Colpoda steinii, Paramecium caudatum, and Paramecium aurelia have been developed [ 102,112-117]. It is the opinion of some authors that the sensitivity of infusorians is higher than that of microorganisms [115,116]. 

Studies on water pollution by POPs can be categorized according to the water bodies studied, such as rivers, seas, and oceans harbors, lakes, and reservoirs and groundwater. They can also be categorized according to sample types, e.g., surface water, deepwater, surface micro-layer, and pore water in sediments. In China, extensive monitoring of pesticide POPs has been carried out in rivers, bays and harbors, and lakes. The results show that the spatial differences of pesticide concentrations in water are larger than that in air, but smaller than that in soil. 

In general, the relationship between the concentration of a contaminant in the pore water and the total concentration in soil varies with soil depth due to variations in soil characteristics. Because soil samples in most experiments are taken in the upper soil layer, where at least the organic matter content is relatively high, the calculated available fraction of contaminants generally underestimates the actual average available fraction of contaminants for the whole unsaturated soil profile. 

Samples collected with these devices may inadequately represent the pore water in its natural occurrence because of problems inherent in the technique (Litaor, 1988). This limitation may be additionally influenced by the complex nature of the soil, whose heterogeneity highly affects the chemical concentrations in pore water. Hence, Rhizon samplers, with their small cross-sectional area, may not adequately integrate for spatial variability (Amoozegar-Fard et al., 1982 England, 1974 Haines et al., 1982), and may represent point samples with qualitative rather than quantitative attributes (Biggar and Nielsen, 1976). 

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