ABSTRACT
It is generally recognised that proper management of groundwater resources requires good quality monitoring data. There is consequently a need to obtain samples which are ‘representative’ of groundwater present within specified depth intervals in aquifers.
To obtain a sample which is representative of groundwater, requires flushing of either stagnant casing storage (in well or borehole) or water present in a sampling device (e.g. lysimeter). With boreholes, at least three well volumes need to be pumped before taking a sample from pump discharge, based on theory and experimental studies. Pumping rate can be optimised depending on requirements using a nomographic technique, given basic aquifer properties.
Concentrations of inorganic substances in pumped samples can be changed by reactions taking place as a result of pumping. Gas-lift pumping devices are particularly prone to this problem due to oxidation as a result of introduction of molecular oxygen, or changes in pH due to removal of carbon dioxide. Displacement pumps (bladder or diaphragm pumps) generally give the least problems because with these there is no direct contact with a gas-phase and no applied suction.
Volatile constituents of groundwater (organics (VOC’s) and dissolved gases) are also affected by gas lift devices, and particularly by sorption. Absorption of VOC’s and gases by flexible tubing used for pump discharge lines is perhaps the major difficulty. To avoid this, several schemes for obtaining in-situ samples or measurements have been devised. Further development of these devices is needed.
