Construction of Monitor and Observation Wells

Monitor and observation wells constructed in Delaware must conform to the requirements of the Delaware Regulations Governing the Construction and Use of Wells (7 DE Admin. Code 7301).

This page reiterates some of the requirements found in the regulations. It also establishes additional criteria for the design and construction of monitor and observation wells intended to assist applicants in the design and construction of wells in a manner that will allow the most useful information to be obtained from the well(s).

Well construction materials and methods must be compatible with the monitoring program objectives. Tailoring monitor and observation well construction to meet the goals of a groundwater monitoring study requires special knowledge. It is recommended that assistance be sought from a qualified professional whenever possible.


Permits to construct monitor and observation wells must be obtained from DNREC prior to construction. Permits to construct monitor and observation wells must be prepared by a water well contractor licensed in the State of Delaware. Wells of all kinds shall be constructed by a well driller who holds a Delaware well driller license. Monitor well applications require a permit application fee of $75 per project for any number of wells submitted for review at one time. There is no permit application fee for observation wells.

Geologic Logs

Monitor and observation wells must be drilled and constructed by a method that allows for the determination of characteristics of the geologic materials under the site unless otherwise approved by the Department. Wells that are used as part of a specific environmental program may have special requirements.

DelDOT Rights-of-Way

Monitor wells proposed in a location that may fall within a Delaware Department of Transportation (DelDOT) right-of-way (ROW) require specific construction specifications and a detailed application review. These wells must be mounted flush with the natural ground surface and constructed with a six-inch radius cement apron around the top of each well.

Guidelines for determining when a proposed well location may fall within the DelDOT “Zone of Interest” are below.

Applications for wells in these areas must have the words “Zone of Interest” clearly marked on the application form. The applications require a joint review by both DNREC and DelDOT staff under a memorandum of agreement effective on May 9, 1995. Coordination of the joint review is handled by DNREC after the application is received. A copy of the memorandum of agreement is available upon request.

Zone of Interest Identification Scenarios

The following indicators are to be used in determining when a proposed monitor well location may be within the highway right-of-way.

  1. The back slope of roadside ditches generally indicate the right-of-way.
  2. Telephone poles and DelDOT fences delineate the general edge of the ROW, as they are usually located in the right-of-way.
  3. Two (2) feet from the roadside edge of sidewalks is a good indicator of the general location of the right-of-way.
  4. Highway signs are usually located in the right-of-way.

Screen Settings

The screen interval settings of monitor wells to be sampled at the water table should be designed to assure that a sample may be collected at anytime. However, the objective of the monitoring program is critical to designing the setting and length of the screen interval. The following screen settings are recommended where the water table is to be sampled:

  1. January through April: Screen should be set from the observed water table to 10 ft. below the observed water table.
  2. August through November: Screen should be set from 5 ft. above to 5 ft. below the observed water table.
  3. Wells constructed at other times of the year should be screened from 2.5 ft. above to 7.5 ft. below the observed water table.

Knowledge of unique hydrogeological characteristics at a site, such as large water table fluctuations or uncharacteristically wet or dry seasons may warrant deviation from these recommendations.


At least schedule 40 PVC or stainless steel casing is recommended. Joining casing with glue or other solvent cements is not acceptable if the solvent is incompatible with the monitoring objectives. Us of other materials must be noted on the permit application. Well materials should be properly cleaned prior to placement in the drill hole.

Sample Integrity

No chemical(s) or substance(s) such as oil based drilling fluids, chlorine or PVC glue may be introduced into a monitor well during construction or development operations if the substance will interfere with the overall monitoring objectives of the project.

Screen Slots

Unless otherwise approved by DNREC, well screens should be machine slotted. Slots in PVC screens for monitoring hydrocarbon fuels (oil or gasoline) must have screen slot widths of no less than .020 inches. Screen slots for all other applications must have slot widths of no less than .010 inches.

Annular Space/Gravel Pack

The annular space of the screened interval must be at least 2 inches on both sides of the casing and must be gravel packed unless natural material is course enough to provide a good sand pack. Casing or screen centralizers should be installed at a minimum of three (3) points around the circumference and spaced to insure that the screen and casing are properly centered in the borehole. The gravel pack, where required, must be compatible with the screen slot size. Where practical, the gravel pack should extend a minimum of two feet above the top of the screen. This extra gravel pack is meant to prevent grout intrusion into the well screen and allow proper development. However, the gravel pack interval should not be so high as to allow vertical movement of water along the well annulus or between aquifers.

Bentonite Plug

A 2-foot bentonite plug is recommended provided it is in an interval near or below the water table. The plug is to be placed above the top of the gravel pack. Either a granular or pelleted bentonite is appropriate. The monitor/observation permit application should be revised to clearly show where the bentonite plug will be placed.


A grout seal must fill the annular space interval above the bentonite plug. In the unsaturated zone, a cement and bentonite mix generally provides the best seal. This type of grout is less likely to shrink and crack during drying than pure bentonite or cement. The grout must be prepared in accordance with section 4.07 of the Delaware Regulations Governing the Construction and Use of Wells (1997). Three to five pounds of bentonite per 94 pounds of cement mixed with 6.5 gallons of water yields the best grout mixture. Monitoring objectives must be considered when selecting grout so that the grout chemistry does not interfere with water quality. Placement of grout must be completed using an approved method that insures an even and complete distribution of grout.

Upper Terminus

The upper terminus of the casing shall be finished no less than 12 inches above finished grade unless otherwise approved by the Department. The casing must be protected from entry by contaminants, vandalism, accidental damage, etc. The following protection devices are suggested:

  1. For steel casing – a locking steel cap.
  2. For PVC casing – a locking well plug or a protective concentric steel casing set in cement or a cement mixture around the above grade portion of a well with a locking cap.

If the well is located in an area where a ground level installation is needed, a curb box or manhole and cover flush with the pavement surface must be used to protect the well. A locking well plug or other appropriate seal should be used to insure that wells installed at ground level are watertight. In addition, the curb box should have a drain hole to prevent pooling of surface run-off.

Well Development

To the extent practical, all monitor and observation wells shall be developed to remove fine sands, silts, clays and rock particles from the water bearing zone surrounding the well screen. The purpose of proper well development is to assure that the well will yield a representative and sufficient quantity of turbidity-free water for sampling purposes and allow accurate pumping test evaluation. Well development shall consist of cyclic or intermittent pumping and surging, either mechanically or by using water or air under pressure. Development shall continue until all formation cuttings, drilling fluids, and additives are removed from the well.

It is recommended that all monitor and observation wells be developed within 48 hours after construction of the wells. If cement or a cement and bentonite grout is used, well development should commence no sooner than 24 hours after well construction.


Monitor wells shall not be disinfected without prior written approval from the Department.


In the event that contaminants are encountered during the drilling process, the water well contractor shall ensure that adequate precautions are taken to decontaminate the drilling rig and related apparatuses to prevent the transfer of contaminants from the site.

Contaminant Disposal

Contaminated fluids and drill cuttings derived from drilling or pumping monitor wells must be disposed in a manner which does not result in significant threat to human health or the environment. The appropriate Department contact should be contacted prior to the beginning the drilling operation.

Upper Terminus Elevations

The elevations of the tops of monitor and observation well casings, excluding the caps, should be established with reference to mean sea level datum or a common datum, as appropriate. This is necessary to determine the direction of groundwater flow and relate groundwater elevations to other available elevation points. The surface used for this measurement must be permanently marked.

Well Completion Report

Well Completion Reports must be filled out completely and accurately. The must be submitted within 30 days of the completed construction.