Collecting a groundwater sample from a borehole can be conducted by a variety of techniques, each with their own merits. Described below, are three commonly used sampling methods with their advantages and disadvantages.
Grab samples are collected using a steel or PVC/polyethylene bailer. These bailers are fitted with a ball valve which allows water to flow into the bailer while being lowered down the borehole and locks the water sample in the bailer as soon as it is lifted back out. The general rule of thumb, when the borehole construction is unknown, is to collect the sample five to ten metres below the water level. If the borehole construction is known however, the sample should be collected below the casing or within the screened (slotted) section of the casing. This method is low cost, quick and easy.
Purging is conducted using a pump to remove water from the borehole. During pumping of the borehole, the pH and electrical conductivity (EC) of the water are continuously measured. Only once both these parameters stabilise, the sample is collected. This generally occurs once three to five times the volume of the borehole is removed. This method ensures that the sample is collected from the aquifer and not from stagnant water in the borehole, as the chemistry of water remaining stagnant in a borehole may differ significantly from that of the surrounding aquifer. This technique however, requires expensive equipment, skilled labour and a lot of time. Additionally, when the groundwater is highly polluted or even naturally of poor quality, discharging this water on the surface becomes problematic.
Specific Depth Sampling
Specific depth sampling targets water strikes or flow zones within a borehole, in order to ensure the sampling of water flowing from the aquifer into the borehole, without having to purge. The sampling equipment for this technique can vary in price from the inexpensive bailers described above, to a moderately expensive pressurised sampler, depending on the monitoring requirements. Identification of these flow zones requires a groundwater specialist and specialised equipment; however this survey generally does not have to be repeated frequently.