8 Groundwater sampling methods for geotechnical purposes
8.1.1 Groundwater sampling methods shall be selected according to need. The quality of a groundwater sample is characterized by the extent to which it contains original constituents, such as suspended matter, dissolved gases and salts, or to which it has been contaminated during drilling. Groundwater can be sampled for the following purposes:
- a) to determine its aggressiveness to concrete;
- b) to determine its corrosive nature;
- c) to establish any risk to subsurface drainage systems and filters due to clogging and similar effects;
- d) to identify changes in groundwater quality resulting from construction work;
- e) to determine its suitability to be used as mixing water for construction material.
8.2.1 The number, location and the depth of sampling points shall be specified in advance on the basis of the engineering problems involved and the local geological and hydrological conditions (see EN 1997-2). If a group of aquifers is encountered, it can be necessary to collect separate samples from each aquifer.
8.1.3 If it is intended to take water samples for chemical analysis, only air and clean water can be used as flushing medium.
8.2.1 For groundwater sampling, the following minimal equipment is required:
- a) clean sample bottles with airtight stopper;
- b) pump;
- c) groundwater sampler;
- d) thermometer;
- e) thermally-insulated or refrigerated box for the transport of sample bottles.
8.2.2 Specific equipment and measures shall be defined by the purpose of the water sampling and laboratory requirements.
8.2.3 Water sample containers should be made from an inert material against the parameters to be determined (e.g. polyethylene, polypropylene or glass), should be clean and should be completely filled.
8.3 Techniques of groundwater sampling
The samples shall be taken from groundwater which has freshly entered into the horizon to be investigated, care being taken to ensure that any stagnant or contaminated water is pumped out prior to sampling. To ensure correct sampling from boreholes, measures shall be taken to preclude the following:
- a) inflow of water from the surface or from other aquifers (due to inadequately sealed pipe runs through aquicludes);
- b) ingress of air by the action of drilling tools;
- c) residue from the flushing medium or sediments.
8.3.2 Extraction by pumping
Where pumps extract water, the tube shall have a sufficient internal diameter to allow sampling. For extraction, one end of the hose shall be attached to the outlet cock or pump discharge pipe, the other end being introduced into the sample bottle so as to reach its bottom. If samples are to be taken from water flowing at a high rate (e.g. during pumping tests or groundwater lowering work), the extraction point shall be located immediately adjacent to the well. The in situ parameters (conductivity, pH-value, temperature) should be constant before sampling.
8.3.3 Extraction by water sampler
The sampler shall be lowered slowly to the prescribed depth so that the water enters through the bottom or side inlet without turbulence. Any contact of the water sample with air should be avoided during filling and extraction.
8.3.4 Extraction by vacuum bottles
In cohesive soils and other low permeable soils, water can be sampled by vacuum bottles.For this purpose, a special filter tip shall be installed at the actual sampling level beneath the groundwater surface into which the vacuum bottle is lowered and the sample sucked out (see Annex D).