Evaluation of Various Monitoring Well Development Techniques

All well drilling methods alter the formation when penetrated by the bit.  Upon completion of the installation, all wells should be developed to remove as much of the fine grained materials adjacent to the producing portion of the borehole as possible.  Loose fines, silts, clays, and, colloidal size particles and drilling additives introduced during drilling activities can significantly impact the ability of the well to produce water and increase the turbidity of the water to the extent, the quality and analyses of water samples will be adversely affected.

 

Monitoring wells, in particular, are generally poorly developed by the drilling industry.  Small diameter monitoring wells limit the size and types of “tools” that can be lowered into the well to perform an adequate degree of well development.  Slotted screens also “deflect” energy necessary to remove clays and silts beyond the borehole.

 

Common methods employed by drillers for well development, include air lifting, bailing, pumping, and the surge block with check valve.  Lightly pumping or bailing a new well will often produce clear, low turbidity water in a short period of time.  The more aggressive development approach of the surge block, where the water in the borehole is forced in and out of the screen and/or adjacent formation, requires more time to obtain clear water; however, these wells will produce more water of lower turbidity for a much longer period of time.

 

Highly turbid water samples can adversely impact the levels of various metals and compounds analyzed.  Many state regulatory agencies and USEPA are beginning to require groundwater samples to meet minimum turbidity criteria (20 NTUs or less) before samples can be submitted for analyses.

 

Factors relating to the selection of the well development method include cost, development time, ease of use, impact on water quality samples and most importantly long-term effectiveness.