Monitoring wells are fundamental to every ground water investigation. In pollution assessment and remediation, ground water monitoring wells often provide the only means for collecting representative ground water samples at a fixed location over time. These traditionally have been installed using drilling techniques and approaches that were developed for water-resource and petroleum-extraction applications.
Over the past 10 to 15 years, several alternative installation techniques and devices have been developed. Most notably, direct-push systems such as the hydraulic ram-based cone penetrometer and hammer-type devices have been used for installing ground water monitoring wells.
Direct-push wells are defined as wells that are installed by either a static push or dynamic push force. Hydraulic rams typically are used to provide a static pushing mechanism and hammer devices are used to provide a dynamic force. The primary distinction between a direct-push well and a conventionally installed ground water monitoring well is that a direct-push well can be installed without first having to construct an open borehole. The primary advantage of direct-push wells is the cost savings that is associated with the speed and ease of installation. Due to their lower costs, faster installation, decreased contaminant exposure and decreased waste production, direct-push wells are a desirable alternative to conventionally drilled wells.
However, regulatory barriers exist in most U.S. states that prevent the use of direct-push wells for long-term environmental monitoring in ground water. The primary barrier is the existence of regulations that require a larger annular space than typically is possible for direct-push well construction. Another barrier is the misperception that ground water chemistry data from direct-push wells are inferior to that obtained from conventional wells.
Direct-push wells have been installed at numerous federal and state sites for uses that range from temporary wells for sample collection and ground water level measurement to permanent installation. Often, these wells are used in place of conventionally drilled monitoring wells for long-term environmental monitoring or monitoring of contaminant migration.
It has been noted that direct-push monitoring wells can lead to cost savings that range from 23 percent to 65 percent, depending on the total depth, screen length, filter pack selection, well diameter and types of material penetrated. In addition, because several types of direct-push systems are capable of installing wells, an additional mobilization step can be avoided when direct-push wells are incorporated into a field analytical program. For instance, sensors and tools used in direct-push explorations are capable of soil-type classification, chemical measurement, plume and lithology mapping, and can be used to collect soil and water samples. Operators can pre-select the number of monitoring wells desired and strategically incorporate these into the site delineation effort, leading to optimized well placement, while reducing the time and level of logistical support typically required for multi-phased, multi-contracted efforts. In other words, direct-push wells can be installed during the field characterization phase without requiring an additional mobilization. As an exploration tool, direct-push wells aid dynamic work strategies by enabling rapid ground water sampling.
Construction DifferencesThe main construction differences between conventionally drilled wells and direct-push wells depend on the installation method deployed, machinery involved, and available well construction materials. Direct-push wells tend to be smaller in diameter than their conventionally drilled counterparts, leading to differences in annular space, casing and sealing dimensions. Depending on screen depth, these differences can impact the sampling options available to the user. For instance, while smaller diameter direct-push wells may preclude the use of larger-diameter pumps, smaller-diameter pumps now are commercially available and can be used in all but the deepest of wells. The same basic construction approach can be used for both drilled and pushed wells. Both types of wells can contain a screened area consisting of slotted casing surrounded by a filter pack, an annular seal and a surface protection device. For drilled wells, a filter pack and annular seal typically are installed using gravity-fed tremie approaches. Pre-pack filter jackets and modular sealing devices have become available for direct-push wells, effectively reducing the potential for bridging resulting from tremie approaches.
Direct-push monitoring wells have evolved from their initial use as temporary monitoring points to usage as permanent monitoring wells. Detailed comparisons of ground water chemistry data from direct-push wells with data from conventionally drilled hollow-stem auger monitoring wells were not available until recently. Because little was known about the ability of direct-push wells to produce representative ground water chemistry samples and the longevity of such wells, the majority of existing state regulations do not specifically address direct-push technology.
Several studies have been conducted to determine the suitability of direct-push wells. These studies have been sponsored by both private industry and the U.S Department of Defense (DOD). Most recently, this has included a demonstration sponsored by DOD’s Environmental Security Technology Certification Program. This demonstration was conducted to determine the long-term performance of direct-push wells at five test sites in different parts of the country. Earlier direct-push well studies include a jointly sponsored study by the British Petroleum Corporation of North America and the U.S. Environmental Protection Agency, and a study conducted by the Naval Facilities Engineering Service Center at Port Hueneme, Calif. The general conclusion from these studies was that direct-push wells are comparable to conventionally constructed wells in regard to ground water chemistry samples.
Direct-push wells can be installed using either a static force system or a dynamic system. Static force systems consist of hydraulic ram units with a static weight of 20 tons to 30 tons, while dynamic systems consist of a percussion hammer and hydraulic rams mounted on a smaller truck or track unit.
Installation EquipmentDirect-push well equipment either pushes or hammers steel rods, sampling devices, geotechnical sensors, and/or analytical sensors into the subsurface. With this method, no soil is removed, and only a very small borehole is created. In most cases, the steel drive rod is retracted, and the riser pipe and screened section are exposed to the borehole. Filter packs either can be tremmied from the surface, or can be installed as pre-packs that surround the screened section and are emplaced with the screen and riser pipe materials.
The two major types of direct-push installation equipment are the static force and dynamic force systems. Static force systems sometimes are known as cone penetrometer (CPT) systems, and generally are the larger of the two. Static systems usually are mounted on a 10- to 30-ton truck. Unlike a dynamic system, which uses a percussion hammer, static systems use a static reaction force to advance steel rods, a sampler or an analytical device into the ground. The force used can almost be as great as the weight of the truck, which is supplemented with steel weights. Static systems that weigh 20 tons are common.
Dynamic systems typically consist of percussion hammer rigs mounted on pick-up trucks or tracks. A percussion hammer system uses a force generated both by the static weight of the vehicle on which it is mounted and a percussion hammer. Percussion hammer systems tend to be the most common and lowest-cost system available.
Research to date has concentrated on developing new sensor technologies to allow the systems to produce continuous information on the geologic strata and contamination as the drive rod is pushed through the formation. Techniques such as laser-induced fluorescence, soil gas sensors, and gamma radiation detectors represent a few of the technologies being integrated with direct-push that offer an alternative to traditional drilling and sample characterization techniques. While direct-push systems often have been used as an alternative to drilling for the screening phase of an environmental site characterization, drilling or augering still is the most common method used to install long-term monitoring wells.
Installation MethodsDirect-push wells can be installed using either an exposed-screen technique or a protected-screen technique.
With the exposed-screen technique (often referred to as a well point), the riser and screen can be driven or pushed directly, or the riser and screen can be assembled and placed around the drive rod (on the exterior) that is connected to an expendable metal drive tip. Because the well screen is exposed to formation materials while being advanced, proper well development is important to remove sediment from the screen slots. The exposed screen method is faster and less expensive than other direct-push installation techniques. Disadvantages associated with this method include:
- Concerns with contamination of the target zone resulting from dragging down of nonaqueous phase liquids, contaminated soil and/or ground water during the push.
- Clogging of the exposed screen by silts and clays.
- The need for additional purging because of contamination and clogging concerns.
- Fragility of the screen because of the perforated open area.
- The lack of an annular seal.
- Advanced within a protective outer drive rod that is driven to the target depth and then removed from the well.
- Advanced within a protective outer drive rod that is driven to the target depth and then retracted, but left in the well to form an annular seal.
- Lowered into the drive rod or outer casing once the target depth is obtained.