Right Portable Sampling Devices

Comparisons of the different sampling mechanisms available are given by Barcelona et al. (1984), Stuart (1984), Nielsen and Yeates (1985) and Pohlmann and Hess (1988). A comprehensive literature review of the effects of sampling devices on water quality is given by Parker (1994). 

Portable sampling devices were categorised by Nielsen and Yeates (1985) as grab samplers, suction lift devices and positive displacement mechanisms. A brief review of each category is given below. 

Yeats s criteria for the ideal sampling tool. They can be dedicated to individual monitoring wells to avoid cross-contamination between boreholes, they are portable, simple to use and relatively easy to clean. They are, however, not suitable for purging large volumes of water, and it can be difficult even with double-ended bailers to determine accurately where the sample was collected. Compared with other sampling devices, the operator is also more at risk of coming into contact with contaminated sample, especially when emptying the bailer (Fig. 3.1). 

Rosen et al. (1992) compared downhole sampling using a helical rotor type of centrifugal pump with an absorption cartridge for volatile organic compounds. They found that this method of bringing water to the surface could be reliable in many circumstances but that care was needed with adsorption, cross-contamination and out gassing. 

Submersible centrifugal pumps are now available with integral packers that allow the screened interval of the well to be isolated from the stagnant column above, thus negating purging requirements and reducing sampling times and costs. 

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