If the well is purged by pumping from the screen or at the water level in the casing, water in the casing column will not be removed. Care should be taken not to sample this stagnant water. The standing water above the well screen should be removed by bailing or by pumping from the top of the water surface so that water is displaced upwards from the screen to the water table.
If pumping is an acceptable method for sample collection, then the pump (or intake tube) should be placed just below the top of the screen. Pumping should continue until the field water quality parameters stabilize.
The parameters often used to determine if the water sample is representative of the formation water include temperature, pH, specific conductance, turbidity and dissolved oxygen. Generally, the water sample is considered to be at equilibrium if the measurements have not changed significantly (5%) over the last well volume or 10 minutes of purging.
Considering the fact that ground water generally moves very slowly (usually less than a foot per day), ground water samples generally should not vary greatly between sampling events. The goal of collecting ground water samples should be to remove all of the factors affecting the quality of the sample. Using the same personnel, sampling equipment and methods will help to provide sample collection consistency, which will produce data that can be used to monitor true changes in ground water.