Ground Water Monitoring/Environmental Wells
With these contamination problems has come the need for ground water monitoring and remediation wells. It is extremely important that in the installation of these wells, the conditions below the surface are not distorted or made worse by products used in the well installation. Care needs to be taken in choosing products that will accomplish the most good and cause the least amount of problems.
Listed below are a number of drilling and grouting products that, when used properly, can ensure the success of an environmental well. These products have been designed to be used to facilitate the drilling and grouting of these wells and not further contaminate or cause problems with the ground water or soil chemistries in subsurface formations.
In the selection of the proper materials for the drilling of these wells, the main goals should be that the products selected:
- 1. will not alter the ground water or soil chemistries in the well
2. will not negatively affect the recovery of samples or contaminant
3. will aid in protecting the drilling equipment by lubrication and cooling
4. will maintain stability in the borehole and address problems that occur in the drilling process such as lost circulation, fluid loss, and subsurface pressures
Key requirements are necessary in the selection of grouting and sealing materials for these types of wells:
- 1. will not affect ground water or soil chemistries
2. will not hamper recovery efforts
3. will properly seal the well to prevent migration of water or contaminants from subsurface sources
4. will protect the well from the filtering of surface water or contaminants into the well
Below is a list of various products that have proven to be popular in recent years to accomplish these requirements. No individual product is perfect or will work in every situation, but many of these products will work for the majority of environmental applications. Some of them have been specifically formulated for these applications.
Drilling fluid products for ground water monitoring/environmental wells fall into two basic categories:
- 1. polymer-free bentonite drilling fluid
2. clay-free/polymer drilling fluid
Polymer-free Bentonite Drilling Fluids: These products are simply high quality sodium bentonite that has not been extended with polymers as are the higher yield products used in water well drilling. The yield of these products is approximately 90 barrels per ton. These products provide characteristics similar to treated products in that they provide lubrication and cooling as well as stabilization of the borehole, without creating contamination or interfere with ground water or soil chemistry.
Clay-free/Polymer Drilling Fluids: Certain drilling applications do not allow the use of any bentonite product in the drilling process. One instance of this is in horizontal remediation where there is concern that bentonite drilling fluid might hamper recovery efforts. In these cases, biodegradable polymers such as natural or modified natural polymers are recommended. These products provide viscosity, gel strength, improved borehole stability and a decrease in fluid loss without the use of bentonite drilling fluid. These polymers have a unique characteristic in that they break down chemically by adding calcium hypochlorite or enzyme breakers.
Bentonite materials for grouting in ground water monitoring/environmental wells fall into two basic categories:
- 1. Sealing products
2. bentonite tablets
3. coated bentonite tablets
4. bentonite chips
5. Grouting products
6. High-solids bentonite grouts
Bentonite Tablets: This product simply is compressed, pure sodium bentonite with no additives. Tablets provide an excellent low permeability seal. They can be used alone as a well seal or used in conjunction with high-solids bentonite grout. Tablets are an excellent choice for environmental wells because of their purity. They are available in 1¿inch, 3¿inch and 1¿inch sizes.
Coated Bentonite Tablets: These tablets have been covered with a coating in order to delay hydration during placement. Improvements have been made to the coating process which has made them more environmentally acceptable. They are available in 1¿inch and 3¿inch sizes.
Bentonite Chips: These products are natural sodium bentonite that require very little processing. They contain no additives and provide high solids and a low permeability seal when placed properly. They usually come in two sizes, which are medium (3¿inch) and coarse (3¿inch). They also are stable chemically and are used widely in monitoring wells and environmental wells.
Grouting products are mixed with water and placed in the well through a tremie pipe. They normally range from 20 percent to 30 percent solids.
High-Solids Bentonite Grouts: These products are formulated to be a one-sack mix. By adding water, these products form a slurry that easily can be pumped and placed. After placement, this slurry sets up to peanut butter or putty-type consistency and forms a low- permeability seal that does not affect ground water chemistries.
These products provide several advantages over cement grouts in monitoring wells:
- 1. There is no heat of hydration.
2. They have a low pH.
3. They form a very flexible seal with bonding capabilities.
4. They do not contaminate or interfere with ground water or soil chemistry.
5. They are easier to mix and place.
6. They cause less wear on grouting equipment.
Paddle mixers, venturi systems or recirculation are common mixing equipment for high-solids bentonite grouts. Positive placement pumps such as a Moino or piston pump usually are recommended for high solids bentonite grouts.
The latest versions of these products are more forgiving than earlier versions. Most manufacturers have both a 20 percent and 30 percent version of this product. The highest "active solids" grout that can be pumped is recommended. They are available in both granular and 200 mesh powder.
Other grouting materials are available such as cement, cement/bentonite or bentonite polymer mixes, but these products have some real disadvantages when used in monitoring/environmental applications. For this reason, they have not been discussed at length in this article.
The products discussed above usually are accepted for use in ground water/environmental wells. Please check with the manufacturers or refer to technical information for contaminant testing on these products.
If you have any questions or need more information on this subject, please contact me through National Driller.