| Front page | | Contents | | Previous | | Next | Guidelines on remediation of contaminated sites Appendix 4.3 Water sampling The objective of sampling is to obtain water samples from borings which are representative of the part of the aquifer where screens have been installed as regards the parameters under scrutiny. Water samples are usually collected from screens that have been established in the borings. As regards localisation borings, only the upper saturated zone normally has screens installed, probably using a screen with a diameter of 63 mm or 25 mm. Screen-installation depends on hydrogeological conditions and the specific objective of the boring, cf. Appendix 4.1. A distinction should be made between well development and purging a well to removal of stagnant water. Wells should be developed immediately upon completion to ensure good performance. As regards wells, this development might be carried out using staged test pumping, cf. section 4.2.3 and /1/. The sampling process involves three stages: Purging Sampling Sample storage The groundwater of investigation wells is in contact with the air. This means, for instance, that the temperature and oxygen and carbon dioxide levels in the well and its immediate vicinity are significantly different from conditions in the aquifer. These differences may have the effect of causing differences in the content of contaminants in aquifer and well due to chemical and biological activity. In addition to this, volatile compounds may have evaporated from the water in the well. Removal of stagnant water must be carried out in the well to ensure that a water sample may be taken which is as representative of the groundwater aquifer as possible. Either submersible pumps or suction pumps are used for removal of stagnant water. The pumps can be divided into the following types: Suction pumps Centrifugal suction pumps Peristaltic pumps (tubing pumps) Vacuum pumps

Submersible pumps

	Centrifugal pumps (e.g. Grundfos submersible pumps)
	Positive pressure pumps, e.g. suction pumps
	diaphragm pumps
	piston pumps
	gear pumps
	vane pumps

Vacuum pumps and peristaltic pumps can be used in cases where the screen pipe has a diameter of no more than 125 mm and the water table is at a maximum depth of 6-7 m below ground level. The water is usually drawn up through 10-15 mm polyethylene (PE). In connection with vacuum pumping, the tubing used for sampling should normally be used as disposable equipment to avoid any transfer of contamination between individual wells (cross contamination).

Diaphragm pumps can be used with low-yielding wells with screen with a diameter of less than 125 mm. Usually; sampling is conducted using a 10-15 mm PE or Teflon tubing. Teflon tubing are often used with diaphragm pumps and are not disposable equipment. In these cases the tubing must be thoroughly flushed with clean water when used for a different well.

Centrifugal submersible pumps are used with pipe diameters of 63 mm and more. A range of tubing can be used; from 12 mm PE (PEL) tubing with the smallest submersible pumps up to 2" tubing when using the large submersible pumps /2/.

With every well submersible pumps must be properly cleaned before use. This is best done by pumping clean water through the pump and cleaning the outside, normally also using clean water for this purpose depending on the type and extent of contamination. Depending on size, the tubing should normally be used as disposable equipment. PE tubing (PEL tubing) are recommended /2/.

High-yielding wells may have primary pumping carried out in the following way:

In connection with primary pumping, the water should be made to pass a conductivity meter. When conductivity stabilises and becomes constant, the water is fresh aquifer water and not water from screen pipes and gravel packing (annulus water). However, quantities of water at least 10 times the water volume of screen pipes and casings must be pumped.

In low-yielding wells, removal of stagnant water may be carried out in the following way:

	If the well is pumped dry before pumping is complete, it should be pumped dry 1-4 times instead
	During pumping, pumping intake varies between the top and bottom of the water column. This particularly ensures the removal of any impurities on the water surface and at the bottom of the screen and water outside any casing is renewed.

The objective of sampling is to collect a water sample from the aquifer through the well. Sampling should be conducted immediately after removal of stagnant water. During this stage three factors are of special importance:

	Equipment must not give false positives (cross-contamination).
	Equipment must not be made of materials that adsorb or absorb substances.
	The method used must not mean that contaminant content of the sample is affected.

When vacuum and peristaltic pumps are used for water sampling, the sampling container is inserted between the well and the pump, so that the sampled water does not come into contact with the pump. In connection with pneumatic pumps and diaphragm pumps the water sample is pressed directly into the sampling flask through the tubing. In connection with submersible pumps the water is pumped directly through the pump and into the sampling flask through the tubing.

With every new consignment of sampling tubing, it is advantageous to routinely analyse a piece of the tubing in order to determine whether the tubing gives off undesired substances (and which substances, if any, are emitted).

Level specific water samples can e.g. be extracted by:

	Electrical log wells
	MPS
	In screens separated by packer and sealed casings.
	In long screens with watershed formation using several pumps, possibly controlled using heat-pulse flowmeters.

In addition to this, water samples may be extracted in connection with driven wells and cone penetration tests, etc.

When taking samples in sampling container, spraying the sample into the container must be avoided as this may occasion a considerable stripping of volatile substances from the sample.

Sorption of contaminants from the water may occur in the tubing. These effects may be minimised by shortening sample dwell time in the tubing as far as possible, and by using Teflon tubing where sorption must be completely avoided. Appendix 4.11 provides examples of forms for use in water sampling.

The packaging used for storing and during transport to the laboratory must ensure that the samples change as little as possible. Sample containers are supplied clean by the laboratory. Water samples that are to be analysed for organic parameters are kept in glass flasks with tight covers. Samples that are to be analysed for non-organic parameters such as heavy metals are often kept in plastic bottles. In connection with certain parameters of analysis the laboratory conducting the analysis will provide containers which have been specially cleaned or which contain fluids to instantly preserve samples on site.

The water samples must be stored in a cool, dark place (4^oC). The lapse of time between sample taking and analysis is minimised according to analysis parameters. The samples must be delivered to the laboratory conducting the analysis on the day of sampling. Whether the sampling flasks should be completely filled and whether preservation or screening should take place in the field should be arranged with the laboratory.

References

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