Groundwater and permeability
3.8 Groundwater chemistry
3.8.2 Field monitoring of groundwater chemistry
In principle, the most obvious way to investigate groundwater chemistry is to obtain a sample of groundwater, seal it into a bottle and send it to a lab-oratory for testing. It might be appropriate to test for major anions and cations, selected trace metals and any other substances of interest at the site in question.
Table 3.4 Major anions and cations in groundwater Major anions Major cations
For groundwater lowering systems, obtaining a sample is relatively straightforward. It may be possible to fill a sample bottle directly at the charge tank. However, when taking groundwater samples from the dis-charge flow the following factors should be considered.
i Try to minimize the exposure of the sample to the atmosphere. Try and obtain it directly from the discharge of the pump or dewatering system.
Totally fill the bottle and try and avoid leaving any air inside when it is sealed. If the pump discharge is ‘cascading’ before the sampling point the water will become aerated and oxidation may occur. The discharge arrangements should be altered so as the sample can be obtained before aeration occurs.
ii Samples may degrade between sampling and testing. The samples should be tested as soon as possible after they are taken and, ideally, should be refrigerated in the meantime. The bottles used for sampling should be clean with a good seal. However, the sample may degrade while in the bottle (e.g. by trace metals oxidizing and precipitating out of solu-tion). Specialists may be able to advise on the addition of suitable preservatives to prevent this occurring. The choice of sample bottle (glass or plastic) should also be discussed with the laboratory since some test results can be influenced by the material of the sample bottle.
iii Use an accredited, experienced laboratory.
Sometimes groundwater samples may be required from a site when there is no dewatering pumping taking place – perhaps for pre or post construction background monitoring. In that case a sampling pump will have to be used to obtain a sample from an observation well. The water in the well has been exposed to the atmosphere and is unlikely to represent the true aquifer water chemistry. Therefore, it is vital to fully ‘purge’ the well before taking a sam-ple. Purging involves pumping the observation well at a steady rate until at least three ‘well volumes’ of water have been removed (a ‘well volume’ is the volume of water originally contained inside the well liner). Specialist sampling pumps should be used in preference to airlifting, since the latter method may aerate the sample, increasing the risk of oxidation of trace metals and other substances.
If samples are taken to an off-site laboratory, it may be several days before the results are ready. Even if the tests are rushed through as priority work, some of the actual procedures may take a week or more. Comprehensive testing of samples is not cheap, either; a reasonably complete suite of testing may cost several hundred pounds per sample at 2001 prices. A good way of reducing costs, and obtaining rapid results, is to take and test water sam-ples on a periodic basis (perhaps monthly), but carry out daily (or, using a datalogger, continuously) monitoring of the ‘wellhead chemistry’.
Wellhead chemistry is a hydrogeological term used to describe certain parameters, which are best measured as soon as the water is pumped from
the well – that is at the wellhead. It is best to measure these parameters here as they are likely to change during sampling and storage, which makes lab-oratory determined values less representative. Typical wellhead chemistry parameters include:
(a) Specific conductivity, EC.
(b) Water temperature.
(c) pH.
(d) Redox potential, EH. (e) Dissolved oxygen, DO.
Perhaps the most commonly measured wellhead parameter for ground-water lowering systems is specific conductivity, EC. Specific conductivity is a measure of the ability of the water to conduct electricity and is a function of the concentration and charge of the dissolved ions; it is reported in units of microseimens per centimetre (S/cm) corrected to a reference tempera-ture of 25⬚C. EC is useful in that it can be related to the amount of total dissolved solids TDS of the water (Lloyd and Heathcote 1985):
TDS:keEC (3.7)
where: TDS is the total dissolved solids in mg/l; EC is the specific conduc-tivity in S/cm at 25⬚C; and ke is the calibration factor with values between 0.55 and 0.80 depending on the ionic composition of the water.
TDS and EC have been related to each other for various water classifi-cations in Table 3.5. Fresh water will have a low TDS (most water supply boreholes for potable use produce water with a TDS of no more than a few 100 mg/l). The higher the TDS, the less fresh the water. The term ‘saline’ is used for convention’s sake and does not necessarily imply a high TDS is the result of saline intrusion (see Section 13.3). A high TDS may be an indica-tor of highly mineralized waters that have been resident in the ground for very long periods, slowly leaching minerals from the soils and rocks.
Table 3.5 Classification of groundwater based on total dissolved solids (TDS) Classification of Total dissolved Specific conductivity, EC (S/cm) groundwater solids,TDS (mg/l)
Fresh 1,000 1,300–1,700a
Brackish 1,000–10,000 1,300–1,700 to 13,000–17,000a Saline 10,000–100,000 13,000–17,000 to 130,000–170,000a
Note:aApproximate correlation; precise value depends on ionic composition of water.
Daily monitoring of EC using a conductivity probe has proved useful on sites where there was concern that water of poorer quality (e.g. from saline intrusion or from deeper parts of the aquifer) might be drawn towards the pumping wells. Readings of conductivity were taken every day, and reviewed for any sudden or gradual changes in EC, which would have indi-cated that poor quality water was reaching the wells. In effect, the EC mon-itoring was used as an early warning or trigger to determine when more detailed water testing was required.
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