THE SAMPLING AND MONITORING OF GROUNDWATER QUALITY WD/89/37
Report No DWI0148
Accurate sampling and monitoring of groundwater quality is required for many purposes ranging from surveillance of potable water supplies to investigating the nature and extent of aquifer pollution. Groundwater sampling can be a time-consuming and expe nsive exercise, so care must be taken in the design of a sampling strategy to ensure that the optimum number of samples are collected in order to meet the objectives of the exercise.
A good knowledge of the hydrogeology and local groundwater flow regime is important when planning a sampling programme in order to enable the sensible interpretation of resultant analyses. Preferably, samples should be collected from dedicated monitoring boreholes which are only open to limited vertical sections of the aquifer. Results from these boreholes enable quality changes with depth to be investigated whereas a borehole penetrating the full thickness of the aquifer will only give an integrated sample.
The application of quality assurance programmes is recommended for all aspects of sampling and monitoring of groundwater quality. These programmes should be designed to be as comprehensive and to ensure well-documented lines of decisions, sampling protocols and quality control of sampling methods and analysis.
Techniques for collecting samples from both the saturated and unsaturated aquifers are many and varied. The techniques selected are controlled by the sampling point and the nature of the sample required. In the saturated zone samples of water are either pumped to the surface or collected from a standing column of water using a "grab device". Both these methods have their advantages and limitations which have resulted in the development of numerous specialized sampling devices that are designed to give greater control on what is being collected. These improved sampling methods range from the construction of dedicated sampling boreholes through a whole range of pumping devices to pore-water extraction and packer sampling. In the unsaturated zone, water is sampled using tensiometer and pore-water extraction techniques.
Sampling for different determinants requires specialized procedures to ensure the resultant analysis is representative of the in-situ values. As soon as a sample is collected it is subjected to physical and chemical changes which should be minimised. Some determinands are stable and require little treatment other than filtration of the sample whereas others need to be analysed on site on a flowing sample. The major and minor ions fall into the former category and redox and dissolved oxygen fall into the latter. Most samples need to be preserved by the addition of acid to ensure metals are kept in solution and care should be taken in selecting containers and storage methods that will not alter the nature of the sample. Specialized sampling methods are required for low-solubility determinands, volatile organic determinands, dissolved gases and microbiological populations occurring naturally or as the result of pollution. Sampling of the gaseous and solid phases of aquifers is also discussed as is the latest sampling technology currently under development.
Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.