A field investigation of commonly-used monitoring well purging techniques was conducted under different conditions including type of pump, pump inlet location, and the use of packers. Tracers including deionized water, fluorescent dye, and lithium chloride, were used to define the amount of stagnant water at any given time in the pump discharge. Tests were conducted in shallow 5 cm (2 in) diameter wells. The effects of drawdown were examined.
Tests conducted in the absence of drawdown with the pump inlet in a fixed position at or above the screen showed a highly variable and unpredictable inclusion of stagnant water. The use of packers did not completely prevent the inclusion of stagnant water into the pump inlet. The inclusion of stagnant water into a sample was minimized by purging from some distance above the screen followed by relocation of the pump inlet into the screen for sample collection. In wells where drawdown occurred during purging, stagnant water inclusion was minimized by reduced pumping rates to allow for sample collection during periods of well recharge. Real time monitoring of indicator parameters such as pH, temperature and specific conductance was not generally successful in indicating when purging was complete.