The Importance of Developing Monitoring Wells
Normally when we think of developing wells, we associate development with improving the well's ability to produce water. Proper development removes drill mud (if used in the drilling of the well), clays and silts which tend to plug the screen and adjacent filter pack.
If these "fines" are not removed, the water will likely be "milky" or "turbid" from these fine particles being suspended in the water. Monitoring wells, installed for environmental sampling of groundwater, do not need to produce large volumes of water; however, underdeveloped wells may adversely affect environmental samples in a number of ways. First, if metals are a site compound of concern, then turbidity can increase the concentration of metals detected since the suspended particles causing the turbidity often contain a wide range of "naturally occurring" metals which may not be associated with the operations of the site. Artificially high levels of metals may result in unnecessary and costly remedial actions at the site.
Although I don't have conclusive proof, I have noticed that volatiles may also attach to these suspended particles and cause concentrations to be higher than in non-turbid samples. I have observed this on sites where turbid samples were obtained by bailing the well verses filtered sample results and samples obtained by the low flow purge method. One possible explanation is that turbidity may contain organic carbon, which is known to be a magnet for organic compounds; therefore, reducing turbidity will lower volatile organic compound concentrations.
Also, if a well is not fully developed, the well may be bailed dry and water will cascade downward as it enters the screen to fill the well. Cascading of the water will cause some of the volatiles to escape and result in lower than representative water samples and produce erratic analytical results over time.
The most effective method of well development to remove the fines is by purging the inside of the well screen, which produces a back and forth action of water passing through the sand pack and screen. Purging breaks down the bridges formed when water is pumped only one way through the screen.
For more information on developing wells, consult the various water well handbooks available.