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NHDES Technical Bulletin           WD-WS-1-10

Well Abandonment and Decommissioning

In New Hampshire abandoned wells are required to be sealed in an appropriate manner to prevent the entry of contaminants into the groundwater. The responsibility for sealing or (decommissioning) an abandoned well lies with the well owner.

RISKS POSED BY IMPROPER WELL ABANDONMENT

There are very good reasons for well owners to make sure abandoned wells on their property are properly decommissioned.

• Improperly abandoned wells threaten drinking water supplies by providing open conduits into aquifers that have potential to supply drinking water to public or private wells.

• Any contaminants entering an abandoned well from the surface can travel easily into different water bearing formations whether in coarse sand and gravel aquifers or in bedrock fracture zones and can cross-contaminate a number of water bearing formations within one well.

• If a drinking water well is being replaced because of water quality problems in the original well, the abandoned well is a direct threat to the new water supply if it is not properly sealed.

• Improperly abandoned wells can create a liability problem at the time of property resale or if the well causes contamination in neighboring wells.

• Shallow dug wells create a physical hazard simply because of their large diameter and the potential for animals or people to fall into them. Typically, the older fieldstone-lined wells are the most dangerous because many were finished flush to the ground surface and were covered with wooden covers which are now decayed or non-existent.

HOW TO DECOMMISSION AN ABANDONED WELL

The proper well sealing method depends on the type of well being decommissioned. The three basic well types used to supply drinking water are drilled bedrock wells, drilled or driven gravel wells and shallow dug wells

Groundwater monitoring wells are another type specifically designed and used for aquifer assessment purposes including groundwater flow and water quality observations.

Well Decommissioning Procedures - Prior to decommissioning, all wells should be investigated to determine their condition, the details of construction and whether or not any obstructions exist that will interfere with the filling and sealing process. Any obstructions should be removed by cleaning out the hole if possible.

If the well was constructed after January 1, 1984, a report describing its relevant characteristics should be on file at the office of the Water Well Board. Copies may be obtained by calling 603-271-3406.

• Abandoned drilled wells penetrating fractured bedrock should be sealed by filling or grouting the well from a minimum of 40 feet below the bottom of the casing to the top of the well. Coarse fill material such as gravel or pea stone may be used in the remainder of the well to support the seal material.

• Abandoned drilled wells penetrating unconsolidated materials should be sealed by filling or grouting the well from the bottom of the well to the top.

• Drilled wells that have been contaminated due to a construction deficiency or continue to cause an environmental hazard should be sealed by pressure grouting the well. This is done with a conductor pipe called a tremie pipe, starting at the bottom of the well and slowly raising the conductor pipe toward the top of the well at a rate no faster than the grout material fills and displaces water from the well and until the well is completely filled. The grout mixture used should be a Portland cement mixed with 2 to 10 percent high solids bentonite clay mixed according to the correct water-to-cement ratio.

• Abandoned shallow dug wells should be filled and sealed by placing clean fill material free of organic matter into the well. Often, locally available fill materials are adequate to complete the job. The upper 2 feet should be filled with impervious material such as clay or hardpan and slightly mounded to prohibit surface water runoff from entering the filled excavation.

• Abandoned monitoring wells should be sealed by pressure grouting the well through a tremie line from the bottom of the well to the top. If possible, the casing should be removed prior to sealing.

• Some monitoring wells are constructed with a filter pack which is an artificial gravel pack placed in the annular space between the borehole and the well screen and used to create a better hydraulic connection with the aquifer. In order to properly seal this type of well, the casing should first be removed and the filter pack should be drilled out. The well should then be filled by the pressure grout method from the bottom of the well to the top using a tremie pipe.

MATERIALS TO SAFELY SEAL A WELL

There is a variety of acceptable grout and fill materials used for sealing wells.

• Portland cement, otherwise known as neat cement, mixed with 5 to 6 gallons of clean water per 94 pound bag.

• Cement-Bentonite grout which is a mixture of Portland cement with 2 to 10 percent bentonite clay mixed according to the proper water-to-cement ratio depending on the percent by weight of bentonite added. This sealant is the recommended material to use when decommissioning a contaminated well because, unlike neat cement that shrinks and can crack upon curing, cement-bentonite grout swells and remains plastic when cured which in turn creates a superior seal.

• Bentonite chips can be used for filling and sealing wells or portions of wells by applying directly into the well through the top at a rate no greater than three minutes per bag. When hydrated, bentonite chips will swell up to 12 to 13 times their dry volume and effectively seal the well. If the chips are applied at a rate greater than three minutes per bag, bridging can occur within the well and the well will not be filled. For this reason it is not recommended that bentonite chips be used for the entire depth of the well or much greater than 50 feet below the static water level.

DECIDING WHETHER OR NOT TO DECOMMISSION A WELL

Wells are expensive commodities and are generally an asset to the property even if they are not currently in use as long as they are properly maintained. Proper maintenance may be as simple as making sure that the well head remains above the land surface where it is protected from flooding. The well must also be fitted with a sanitary well cap or concrete cover depending on the type of well so that contaminates cannot enter the well accidently.

If it is decided that a well has no useful purpose, has no potential future use or has no real value and may constitute a liability, then, the well is for all practical purposes abandoned and must be properly decommissioned.

The decision to abandon a well is generally the home owner's, however, the New Hampshire Water Well Board may make the determination on its own initiative in response to reasonable supporting evidence.

An improperly maintained or abandoned well is an accident waiting to happen.

For further information please call the N.H. Water Well Board at 603-271-3406 or DES Water Supply Engineering Bureau at 603-271-3504.