Before environmental engineers can sample a monitoring well, they must remove the water standing in the well to allow fresh water from the aquifer to enter the well. Purging is necessary because the stagnant water in the well is subject to chemical reactions from contact with well construction materials and the atmosphere for extended periods of time (Seanor and Brannaka 1983; Wilson and Dworkin 1984). The volume of water which should be removed from the well is based on the hydraulic characteristics of individual wells and geological settings (Gibb, Schuller, and Griffin 1981). A general rule is to remove three to five well volumes or to remove water until the water quality indicators, such as pH, conductance, and temperature are stable.
When purging a well, engineers should not allow the water level to drop below the level of the well screen to avoid aeration and loss of volatile or redox-sensitive compounds. In addition, the pumping rate should not exceed levels that might cause turbulent flow in the well and subsequent pressure changes and loss of dissolved gases (Mer-idith and Brice 1992). Overpumping can also dilute the sample or increase its turbidity because of the fine particles that may be drawn into the well.
Engineers should use the same equipment for purging and sampling to minimize the number of items that enter the well and therefore, the possibility of cross contamination. Furthermore, placing the purging device at the top of the well screen or at the top of the column of water ensures that all stagnant water is removed (Unwin and Huis 1983).
Preservation Field Blanks Standards
Was this article helpful?