When assessing a site for ground water contamination or quality, a monitoring well is often the primary source of hydrologic data. Monitoring wells that are installed and function properly should be able to:
While achieving these objectives, the monitoring system should also preserve the integrity of the subsurface system that is penetrated but not monitored.
In order to meet the objectives listed above, monitoring wells should be designed and constructed in accordance to certain minimum standards in each of the following categories.
When sitting a monitoring well, it must first be decided which
type of drilling process will be used. The decision is based on
the requirements of the monitoring well, the hydrogeological
environment, the quantity of water required, the depth and
diameter need to satisfy the well design, and the overriding
budget of the project. Wells can be either dug by hand , driven
in using well points, bored by an auger, or drilled. Drilling
proves to be the only feasible method for deeper wells that
require more productivity in unconsolidated deposits, or go
through any 
type of rock zones. Wells placed in
shallow unconsolidated material requiring less productivity, can
be dug, bored, or driven .
The three main types of drilling equipment are the cable tool, rotary, and reverse rotary, The cable tool works by lifting and dropping a string of tools suspended on a cable. Located at the bottom of the tool string is a bit which rotates a few degrees between strokes so the cutting face of the bit strikes different areas of hole bottom with each stroke. Periodically drilling must be interrupted to bail out the cuttings. This method is capable of drilling over a wide range of geologic materials. However, it is not capable of drilling as quickly or deeply as rotary methods.
In the conventional rotary method, drilling fluid is forced down the inside of the rotating drill stem and out through openings in the bit. This drilling fluid travels back to the surface carrying the cuttings with it through the annulus, which is formed between the drill pipe and the hole wall. Reverse rotary method the direction of the circulation of the drilling fluid is reversed. This method works particularly well when drilling large diameter holes is soft, unconsolidated formations. The conventional rotary rig in considered the fastest, most convenient, and cheapest system to operate.
The direct rotary drilling method is heavily dependent on its hydraulic circulation system. Drilling mud, consisting of a suspension of bentonitic clay in water, is the most common drilling fluid used. This drilling mud coats the hole wall and with this coating action comes hole stability and a seal which prevents the loss of drilling fluid to permeable formations. If the drilling mud cannot prevent the caving in of the hole walls, a well casing must be emplaced as the drilling proceeds.
The primary materials commonly used in monitoring well construction are the drilling tools and equipment, casing, riser pipe, well screen, centralizers, annular sealant, filter pack, and drilling fluids or additives. 2- or 4- inch diameter schedule 40 PVC are standard in construction of well screens and risers. All PVC pipes should be threaded due to the volatile organic compounds contained in PVC glues, which could contaminate the system. In cases of monitoring for nonaqiueous-phase liquids (NAPLs) or contaminants of high concentration which react with PVC, stainless steel should be used in place of PVC. The integrity of these materials should be of sufficient size and strength in order to meet the objectives of the monitoring point. All materials used in the construction of a monitoring well must be free of any contaminants during storage, assembly, and installation that could affect the monitoring of the groundwater system.
The equipment and technique to
construct a monitoring well borehole must be able to provide a
stable environment for which to insert the well. The borehole
must be, open, and vertical, while still maintaining the
integrity of the surrounding zone to be monitored. To insure
proper installation a professional should be consulted or used.
The two major drilling procedures used in monitoring well
installation are hollow stem augers or rotary-drills. The well
screen and riser are lowered directly within the hollow stem
during a hollow stem auger installation. During a rotary-drilled
well installation, potable water must be employed to dilute
drilling mud to ensure ease of installation. When feasible,
drilling procedures that do not introduce water or other liquids
into the borehole should be used. If the introduction of drilling
fluids such as air, water, or gas cannot be avoided safety
precautions must be used to avoid contamination of the zone to be
monitored.
After the screen and riser have been positioned the filter pack is placed from the base of the well to between 1-2 feet above the top of the well screen. The filter pack should consist if well rounded silica sand. Filter pack grain size should be decided upon based on the distribution of the surrounding aquifer's grain size.
After the filter pack is set the well must be sealed to the ground surface to prevent contamination from the surface or water-bearing zones above the well screen down the borehole. Bentonite is placed on top of the filter pack 1-2 feet thick to protect the filter pack from invasion of grout. The grout completes the seal from the top of the Bentonite to the top of the ground surface.
Finally, protective casing and locking well caps must be installed to insure that the monitoring well is protected from vandalism and accidental damage.
picture adapted from Bedient et al.
Well development removes the fine-grained material to improve the hydraulic efficiency of the well. Hydrologic efficiency is achieved when a large fraction of the fine materials from both the filter pack and aquifer material adjacent to the borehole no longer clog the pump or well screen.
There are several different methods of well development which include mechanical surging with bailing or pumping, over pumping, air lift pumping, and jetting. This procedure should be slow and systematic in order to prevent the movement of more material than the development method can effectively remove. In most cases, well development is site specific and can vary based on soil property, well location, and well type. In general once the pH is stabilized the well can be used for monitoring the site.
Photo by Dr. William Reay
Send comments or suggestions to:
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
