All sampling quality assurance plans require monitoring wells be purged properly prior to obtaining a water sample for laboratory analysis. The purpose of purging is to obtain a water sample unaffected by conditions created by the well and/or borehole. Some of the factors that can influence the water sample include dissolved oxygen from the well itself, elevated pH from the lime in cement grout, occurrence of metals from bentonite used in well seals or increases in turbidity from naturally occurring clays, which can release metals into the sample to levels above the background of the water bearing zone being sampled.
Purging techniques have evolved considerably over the last 20 years as we have learned more about retrieving representative formation water samples. Without going into extensive history, the following are considered fairly standard operating procedures for collecting water samples. First, it is assumed the well has been properly developed at the time it was drilled. Development usually takes place after the well is completed and all cement and clays have hardened or set up. Development consists of pumping and surging of the well aggressively to remove drilling fluids and suspended fine particles not native to the formation waters.
Assuming the well is properly developed, monitoring wells still need to be purged before each sampling event. Water quality can be affected in and near the borehole in many ways. First, water adjacent to the borehole may be impacted by the pH of cement or metals in clays used to drill or seal the annular space. Secondly, water above the bottom of the casing or top of the screen is stagnant and not circulated as water compared to in the lower portion of the screen or open hole. This water should be removed by bailing or pumping during purging operations. Another reason for purging is to remove any water which may have preferentially migrated along the outside of the casing that has not been properly sealed, or down the inside of the casing of a surface flush mounted well not properly capped.
Gentle pumping (about a quart or less a minute) is now the preferred method of purging a monitoring well prior to sampling. Low flow purging helps to minimize turbulence in the well which tends to increase suspended solids and thus metals concentrations. Aggressive purging also tends to aerate and volatilize compounds, which may be analyzed.
The volume of water needed to be withdrawn to properly purge a well depends upon a number of factors. Generally three to five well volumes are recommended, however, the preferred way to tell if a well is sufficiently purged relies on water quality indicator parameters such as, conductivity, pH, temperature, dissolved oxygen (DO) etc. Purging continues until these parameters have "stabilized" within, say, 10 percent over the last three readings.
Purging is an important part of obtaining representative samples of site water quality conditions. A little extra care and time are worth the effort in the long run.