Ground Water Monitoring Well Development
October 1, 2011
Monitoring well development is necessary to ensure that complete hydraulic connection is made – and maintained – between the well and the aquifer material surrounding the well screen and filter pack. The appropriate development method should be selected for each project on the basis of the circumstances, lithology, objectives and requirements of that project.
Project-specific planning documents can identify the specific development method to be used. In general, most wells are developed by using surge block and bailing methods to draw the coarse and/or fine material out of the sand pack. Other development methods that may be used include jetting, airlift and submersible pump methods. Jetting typically is not used as a development method for environmental investigations, but is commonly used for water resource monitoring wells or production wells.
Well development should begin no sooner than 48 hours after well installation. However, if drilling muds are used during well installation, well development should occur approximately 24 hours following well installation so that the drilling mud does not set up in the well screen section. Generally, a phased process is used to develop wells, starting with a gentle bailing phase to remove sand, followed by a surging phase, and then a pumping phase after the well begins to clear up.
After a well is first installed, and, in fact, often before the bentonite pellet seal is set, gentle bailing is used to remove water and sand from the well. The purpose of this technique is used to settle the sand pack. After further well sealant materials have been added and allowed to set for approximately 48 hours, bailing is resumed as part of well development. The purpose of bailing is to remove any fine material that
may have accumulated in the well, and start pulling in natural material into the sand pack. Bailing often is conducted until the sand content in the removed water begins to decrease.
After the sand content begins to decrease, surging is conducted. A surge block is used to move sediments from the filter pack into the well casing. Surge blocks need to be constructed of materials that will not introduce contamination into the well. Surge blocks should have some manner of allowing pressure release to prevent casing collapse. The surge block is moved up and down the well screen interval and then removed, followed by a return to bailing to remove any sand brought into the well by the surging action.
Care should be taken to not surge too strongly, as the well screen interval often is the weakest part of a well. Surging should be followed by additional bailing to remove fine materials that may have entered the well during the surging effort.
After surging has been completed and the sand content of the bailed water has decreased, a submersible pump is used to continue well development. The pump should be moved up and down the well screen interval until the obtained water is relatively clear. Well development must continue until the water in the well clarifies, and monitoring parameters, such as pH, specific conductivity and temperature, stabilize. It should be noted that where very fine-grained formations are opposite the screened interval, continued well development until clear water is obtained might be impossible.
During well development, pH, specific conductivity, temperature and turbidity should be monitored frequently to establish natural conditions and evaluate whether the well has been completely developed. The main criterion for well development is clear water (Nephelometric turbidity units or NTU of less than 5). As mentioned above, clear water often can be impossible to obtain with environmental monitoring wells. A further criterion for completed well development is that the other water-quality parameters mentioned above stabilize to within 10 percent between readings over one well volume.
The minimum volume of water purged from the well during development should be approximately a minimum of three borehole volumes (wells typically will not reach stabilization of water-quality parameters before this condition is achieved, and may not have reached stability even after this threshold has been achieved). The above is a general guideline for difficult well development; project-specific planning should address any project constraints on well development.
Development water should be stored in 55-gallon Department of Transportation-approved drums and/or baker tanks, depending upon the total volume of purge water removed from the newly installed wells.
This article is provided through the courtesy of the U.S. Environmental Protection Agency; it is excerpted from the agency’s standard operating procedures for monitoring well installation.