Sampling of Ground Water Monitoring Wells - Part 3
Low-flow and passive purging and sampling.
Studies have shown that ground water in the screened interval of a standard monitoring well can be representative of that in the formation, even though stagnant water lies above in the casing. This occurs when flow is generally horizontal and naturally purges the screened interval (See Figure 1). Unfortunately, the insertion of a sampling device, such as a pump, bailer or even a tube, can disrupt this equilibrium and cause mixing of the screened and cased interval waters (See Figure 2). The mixture of stagnant and screened interval water can even potentially be forced into the aquifer in all directions, including upgradient. This can result in unpredictable geochemical and microbiological effects that may affect data quality from the well over the short, and possibly long, term.