ASTM D6724-04(2010)

The direct push ground method is a rapid and economical procedure for installing groundwater monitoring wells to obtain representative groundwater samples and location-specific hydrogeologic measurements. Direct push installations may offer an advantage over conventional rotary drilled monitoring wells (Practice D5092) for groundwater investigations in unconsolidated formations because they reduce disturbance to the formation, and eliminate or minimize drill cuttings. At facilities where contaminated soils are present, this can reduce hazard exposure for operators, local personnel, and the environment, and can reduce investigative derived wastes. Additionally, smaller equipment can be used for installation, providing better access to constricted locations.

Direct push monitoring wells generally do not extend to depths attainable by drilling. They are also typically smaller in diameter than drilled wells, thereby reducing purge water volumes, sampling time, and investigative derived wastes. Practice D5092 monitoring wells are used when larger diameters and/or sample volumes are required, or at depths to which it is difficult to install direct push wells. Direct push monitoring wells should be viable for monitoring for many years.

Prior to construction and installation of a direct push well or any other type of groundwater well the reader should consult appropriate local and state agencies regarding regulatory requirements for well construction in the state. A regulatory variance may be required for installation of direct push monitoring wells in some states.

To date, published comparison studies between drilled monitoring wells and direct push monitoring wells have shown comparability (1, 2, 3, 4, 5). However, selection of direct push monitoring wells over conventional rotary drilled wells should be based on several criteria, such as site accessibility and penetrability, stratigraphic structure, depth to groundwater, and aquifer transmissivity.

Typical penetration depths for installation of groundwater monitoring wells with direct push equipment depend on many variables. Some of the variables are the size and type of the driving system, diameter of the drive rods and monitoring well, and the resistance of the earth materials being penetrated. Some direct push systems are capable of installing groundwater monitoring wells to depths in excess of 100 feet, and larger direct push equipment, such as the vibratory sonic type drill (Guide D6286) are capable of reaching much greater depths, sometimes in excess of 400 ft. However, installation depths of 10 to 50 feet are most common. Direct push methods cannot be used to install monitoring wells in consolidated bedrock (for example, granite, limestone, gneiss), but are intended for installation in unconsolidated materials such as clays, silts, sands, and some gravels. Additionally, deposits containing significant cobbles and boulders (for example, some glacial deposits), or strongly cemented materials (for example, caliche) are likely to hinder or prevent penetration to the desired monitoring depth.

For direct push methods to provide accurate groundwater monitoring results, precautions must be taken to ensure that cross-contamination by “smearing” or “drag-down” (that is, driving shallow contamination to deeper levels) does not occur, and that hydraulic connections between otherwise isolated water bearing strata are not created. Similar precautions as those applied during conventional rotary drilling operations (Guide D6286) should be followed.

There have been no conclusive comparisons of effectiveness of sealing between drilled monitoring wells and direct push monitoring wells. As with drilled monitoring wells, sealing methods must be carefully applied to be effective.

Selection of direct push monitoring wells versus conventional rotary drilled monitoring wells should be based on many issues. The advantages and disadvantages of the many available types of driving equipment and well systems must be considered with regard to the specific site conditions. Specific well systems and components, as well as direct push driving equipment, are described in Section 7.

Advantages:

Minimally intrusive and less disturbance of the natural formation conditions than many conventional drilling techniques.

Rapid and economical.

Smaller equipment with easier access to many locations.

Use of shorter screens can eliminate connections between multiple aquifers providing better vertical definition of water quality than long well screens.

Generates little or potentially no contaminated drill cuttings.

Less labor intensive than most conventional drilling techniques.

Disadvantages:

Cannot be used to install monitoring devices in consolidated bedrock and deposits containing significant cobbles and boulders.

Small diameter risers and screens limit the selection of useable down-hole equipment for purging and sampling.

Difficulty installing sand pack in small annular space if gravity installation of sand pack is used.

Difficulty installing grout in same annular space unless appropriately designed equipment is used.

1.1 This guide describes various direct push groundwater monitoring wells and provides guidance on their selection and installation for obtaining representative groundwater samples and monitoring water table elevations. Direct push wells are used extensively for monitoring groundwater quality in unconsolidated formations. This guide also includes discussion of some groundwater sampling devices which can be permanently emplaced as monitoring wells.

1.2 This guide does not address the single event sampling of groundwater using direct push water samplers as presented in Guide D6001. The methods in this guide are often used with other tests such as direct push soil sampling (Guide D6282) and the cone penetrometer test (Guide D6067). The present guide does not address the installation of monitoring wells by rotary drilling methods such as those presented in Practice D5092. Techniques for obtaining groundwater samples from monitoring wells are covered in Guide D4448.

1.3 The installation of direct push groundwater monitoring wells is limited to unconsolidated soils and sediments including clays, silts, sands, and some gravels and cobbles. Penetration may be limited, or damage may occur to equipment, in certain subsurface conditions; some of which are discussed in 5.5. Information in this guide is limited to groundwater monitoring in the saturated zone.

1.4 This guide does not purport to comprehensively address all of the methods and issues associated with monitoring well installation. Users should seek input from qualified professionals for the selection of proper equipment and methods that would be the most successful for their site conditions. Other methods may be available for monitoring well installation, and qualified professionals should have flexibility to exercise judgement concerning alternatives not covered in this guide. The practice described in this guide is current at the time of issue; however, new, alternative, and innovative methods may become available prior to revisions. Therefore, users should consult with manufacturers or producers prior to specifying program requirements.

1.5 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgement. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word “Standard” in the title of this document means only that the document has been approved through the ASTM consensus process.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory requirements prior to use.