Best methods for groundwater sampling

Purging is the process in which stagnant water is removed from a well before sampling occurs to ensure minimal disturbance to the groundwater flow regime and ensure a representative sample is obtained. In most groundwater monitoring wells there is a column of stagnant water that remains within the well casing between the sampling rounds.

This stagnant water is generally not representative of formation water as it no longer part of the aquifer flow and may be in contact with the air which can cause chemical changes to the water. This stagnant water can have different parameters to those in the surrounding aquifer.

In Lesson 1 we learnt that most commonly used and reliable sampling methods to obtain a representative groundwater sample in conditions found in the South East of Australia are the following:

• Low flow sampling method: Designed to leave the stagnant water within the bore undisturbed, whilst obtaining a representative sample directly from the aquifer through the screened interval at the depth of the pump. For the low flow sampling method, groundwater will be sampled using a Bladder Pump or a Peristaltic Pump.
• Removing a number of bore volumes: Resulting in a representative sample by making sure stagnant water within the bore is removed. For this bore purging method, groundwater will be sampled using a Bailer or a Monsoon Pump.

There are a number of factors that go into deciding on a sampling method. Some of these factors are known before heading out to site by obtaining the well construction information or field sheets from previous sampling rounds. The following factors are what go into determining the sampling method:

• Construction (width, screened interval)
• Condition of the bore (obstructions etc.)
• Depth to groundwater
• Water column present
• Groundwater inflow rates (recharge)

Before sampling begins, the groundwater bore will be dipped to measure the water level within the bore as well as the total depth of the bore. These factors aid in the decision of which method is most suitable.

Sampling

Physical water quality parameters will be recorded using a water quality meter throughout the pumping process. Water quality parameters include:

• Electrical Conductivity (EC)
• pH
• Dissolved oxygen (DO)
• Temperature
• Redox potential (ORP)

Once groundwater physical parameters have stabilised, samples will be taken, labelled, stored in the required laboratory supplied bottles for dispatch to a NATA laboratory.