Direct-push wells offer the potential to save significant amounts of money and time for environmental ground water monitoring. Equipment used to install such wells usually is smaller and lighter than conventional drilling rigs, so less damage is done to surrounding property. The quality of data from ground water samples taken from direct-push wells are comparable to those obtained from conventional wells.
Despite these positive attributes, most states’ regulations inadvertently prohibit their use for long-term ground water monitoring, and relegate their usage primarily to screening purposes. The basis of the prohibition on the use of direct-push wells involves their lack of the relatively large volume of annular space that is required for the proper construction of conventional wells. Installation of wells by direct-push technology is not new, and variances permitting the use of direct-push wells for long-term monitoring commonly are granted in many states.
In the infancy of environmental monitoring, regulatory agencies and advisory organizations relied upon established well drilling techniques when drafting regulations that would be protective of human health and the environment for the installation of environmental monitoring wells. Those drilling techniques primarily were developed to access water and petroleum resources, and subsequently utilized to obtain environmental samples from aquifers to determine the identity and levels of contaminants present. The standard for environmental monitoring wells typically was based on 2- and 4-inch diameter wells drilled and installed with a hollow-stem auger or other conventional installation techniques. The technologies used for installing monitoring wells have advanced significantly since most of the regulations were written.
Construction MaterialsFor most installations, the drive rod typically is constructed of steel with threaded connections. For some exposed-screen and protected-screen wells, the drive rod doubles as the casing. The drive rods are hollow to allow for protection of the well components and insertion of sampling de-vices.
Expendable drive points typically are made of steel or aluminum.
Typically, schedule 40 or schedule 80 polyvinyl chloride (PVC) threaded or flush-jointed casings are used and installed inside an outer protective metal casing. Other materials such as stainless steel, polytetrafluoroethylene (PTFE) or polyethylene can be used if needed. The internal diameter ranges from 1⁄2 inch to 2 inches.
Slotted PVC screens with slot widths of 0.01 inches and 0.02 inches commonly are used, although wire-wrapped and mesh stainless steel, polyeth-ylene, polypropylene and PTFE screens also are available.
Pre-pack filters consist of an outer stainless steel mesh screen, and slotted PVC, polyethylene or nylon mesh that contain the filter pack (graded sil-ica sand), which is held against the inner screen. Pre-pack filters typically are installed using an outer protective metal drive casing. Typically, the inner screen is slotted PVC, although all-stainless and no-metal pre-pack filters are available.
Several methods can be used to seal a direct-push well. The seal can be tremie-grouted as the casing is withdrawn, using a grout of bentonite or cement. Grout machines for small-diameter direct-push wells are available. Prefabricated bentonite seals also are available, and they consist of a sleeve of dry granular bentonite that is wrapped around the riser. The bentonite sleeve expands when hydrated, and these seals are designed to be used below the water table.
Fine to medium sand is used as a grout barrier for situations when it is possible to tremie material into place. When installing a direct-push moni-toring well using the protected-screen method, the grout barrier is installed on the casing prior to placing the well. These pre-installed grout barriers can take the form of foam bridges or plastic collars. The foam bridge expands after it leaves the drive casing, and can be used with a pre-installed bentonite seal or a tremmied grout seal. The plastic collar operates in a similar manner.
Well DevelopmentMonitoring well design, construction, and development procedures can greatly influence the quality and representativeness of ground water sam-ples. Proper well development is important to ensure that a good hydraulic connection exists between the well and the surrounding ground water system. Proper well development also is essential to avoid or minimize turbidity in ground water samples, and is essential for ensuring representa-tive samples. Well development removes artifacts created by the drilling process, such as disturbed fines near the borehole and silts and clays that have been smeared along the walls of the borehole. Unless these disturbed and smeared fines are removed by developing the well, the hydraulic connection between the well and the surrounding ground water system likely will be poor. Techniques for developing a direct-push well are similar to those used for a conventionally constructed well and include surge blocks, pumping and jetting. Ideally, development should continue until all fines have been removed and relatively clear water is obtained.
Sampling ConsiderationsDirect-push monitoring wells can be used to obtain information related to the potentiometric surface of particular hydrologic units, water quality parameters, organic and inorganic contaminants, and migration characteristics of contaminants. Contaminants of concern may drive the sampling methodology and should be considered.
Samples from direct-push wells can be collected with bailers, check-valve pumps, peristaltic pumps, or narrow-diameter bladder pumps. Because exposed-screen monitoring wells do not have an annular seal, caution is required for sampling contaminants if the well is pushed through non-aqueous phase liquids or significant soil contamination. In addition to the methods mentioned above, methods also have been developed that do not require any purging. The passive diffusion bag method is one such method, and is used for monitoring volatile organic compounds. The Depart-ment of Defense also is experimenting with small containers with spring-loaded tops that can be triggered at the depth of interest. These are re-ferred to as snap samplers. The passive diffusion bags and snap samplers rely on the assumption that ground water is sufficient through the well screen, and that the water in the casing is representative of the surrounding ground water. Actual site conditions should be verified prior to employ-ing sampling methods that rely on this assumption.
For non-passive sampling techniques, wells are purged prior to sampling so that standing water, which does not represent the true ground water chemistry, is removed from the well. Water in the stagnant portion of the well (above the screen) can interact with the atmosphere. This interaction can result in loss of volatiles, change in the dissolved gas content of the well water, and can cause oxidation of some metals. However, these changes most likely would be less in smaller diameter direct-push wells. As with any type of well, the stagnant water in the upper portion of the well casing also can be affected by sorption of analytes (although this is less of a concern because equilibrium will be reached in most cases (i.e., as long as the casing is not being degraded), leaching of constituents from the well casing (such as metals from stainless casings), corrosion and deg-radation of the casing. The degree to which these reactions may occur will depend on the casing materials, the chemical environment in the well, and any biological activity in the well.
Three of the most common well purging techniques include:
- Purging a specified number of well volumes.
- Low-flow purging and sampling using indicator parameters to determine when to sample.
- Well purging volume based on well hydraulics and aquifer transmissivity.
Performance of TechnologyThere are many advantages associated with using direct-push wells when compared with conventional monitoring wells; however, there also are limitations with using direct-push wells.
System AdvantagesInstallation of direct-push wells is minimally intrusive and causes less disturbance of the natural formation than standard installation techniques. Percussion-hammer direct-push systems are smaller and more mobile, and have more access to a site than traditional drill rigs. Sampling and data collection are faster, reducing the time needed to complete an investigation, and increasing the number of sample points that can be collected dur-ing the investigation.
Direct-push technologies produce minimal cuttings because very little soil is removed as the probe rods advance and retract. Consequently, the po-tential exposure to contaminated soils is greatly reduced, and there is minimal material that must be disposed.
Due to the smaller diameter of direct-push wells, development volumes are decreased or reduced significantly. For example, a 10-foot water col-umn in a nominal 2-inch monitoring well equates to a well casing volume of 1.63 gallons. Alternately, a 10-foot water column in a typical direct-push monitoring well (e.g., 1⁄2-inch inside diameter) equates to 0.10 gallons. Stated another way, a 10-foot column of water in a conventional 2-inch well contains 16 times as much volume as the same 10-foot column in a 1⁄2-inch direct-push well.
Direct-push wells can be installed much more rapidly than conventionally drilled monitoring wells. Based upon anecdotal evidence, direct-push wells have been installed at a rate that is two times to five times faster than conventionally drilled monitoring wells. In a heaving sand environment, cased direct-push wells can be installed easier than conventional augered wells.
In addition, the direct-push well installation step can be integrated into a comprehensive dynamic characterization plan using chemical and lithologic sensors within a single deployment via in-situ emplacement of geophysical and analytical instruments. Also, closed sampling systems and on-board analytical instruments allow samples to be analyzed in the field, avoiding laboratory turnaround time, remobilization time and associ-ated expenses.
Due to the decreased volume of development fluids and cutting wastes, and the decreased installation time, workers receive significantly less expo-sure to potentially hazardous environments.
The results from a number of case studies that compared analyte concentrations taken from direct-push wells with those collected from convention-ally drilled monitoring wells revealed there were relatively few instances where significant differences were found between the concentrations of analytes taken from direct-push wells vs. those from conventionally drilled monitoring wells. Analytes that were compared in these studies in-cluded organic contaminants, inorganic analytes present at the site, and purge parameters measured during the sampling process.
Direct-push wells often are installed using percussion-hammer rigs that are lighter than conventional drilling rigs. Consequently, there are fewer and shallower ruts created during off-road travel. Direct-push wells can be installed faster than conventional wells, so the crew spends less time occupying the site, and is less apt to disturb the surrounding ecosystem.
Direct-push wells are less destructive to property than conventional wells. They can be installed using smaller and lighter equipment than a conven-tional drilling rig; hence there is less damage from rutting. Because the equipment is smaller, it is more maneuverable than a larger drilling rig. Thus, wells can be installed in hard-to-access locations. This may allow for wells to be placed in out-of-sight locations that are requested by the landowner, when technically appropriate. Direct-push wells can be installed more quickly than conventional wells, thereby allowing the crew to vacate the landowner’s property sooner.
Direct-push wells can be installed at cost savings ranging from 23 percent to 65 percent, depending on the total depth, screen length, filter pack se-lection, well diameter, and type of geologic formation present at the site. An additional mobilization step can be avoided when direct-push wells are incorporated into a field analytical program. Additionally, direct-push wells also are less expensive in the event that abandonment or replacement is required.
Limitations and DisadvantagesThe primary physical limitation of direct-push wells is that they are applicable only to unconsolidated materials.
The depth of penetration is controlled primarily by the reactive weight of the equipment or the type of hammer used (e.g., vibratory, manual, per-cussion). Consequently, direct-push technologies are most applicable in unconsolidated sediments, typically to depths less than 100 feet. Penetra-tion is limited in semi-consolidated sediments, and generally is not possible in consolidated formations, although highly weathered bedrock is an exception for some equipment. Direct-push equipment also may be limited in unconsolidated sediments with high percentages of gravels and cob-bles, or in caliche and dense, fine, saturated sands. As a result, other drilling methods are necessary in site assessment and remediation activities where geological conditions are unfavorable.
Until relatively recently, there were no extensive, scientific case studies that evaluated the use of direct-push wells for long-term monitoring. Con-sequently, states have been reluctant to recognize direct-push wells in this context. In addition, most state regulations and/or guidance are written such that only conventional monitoring wells with a large annular volume are permitted for long-term monitoring. However, summaries of case studies related to the performance of direct-push wells for long-term monitoring indicate that they perform satisfactorily in this capacity.
Geologic strata can be characterized with direct-push technology, but it differs from the methods used for conventional wells. Standard cone-penetrometer sensors can be used to ascertain soil properties. Dual-tube direct-push systems allow for the collection of continuous soil cores to validate and confirm information generated with in-situ direct-push sensor technologies. Ground water samples can be collected using an exposed screen sampler.
Direct-push wells are limited to a maximum diameter of casing that can be pushed or hammered by the equipment. As a rule of thumb, if it is nec-essary to install a well using casing greater than 2 inches in diameter, conventional drilling equipment should be used.
Direct-push wells may be more apt to cause cross-contamination of aquifers by providing a vertical flow path if they are installed without using an outer casing. Also, it can be more difficult to install a proper seal above the screen in narrow diameter wells.
Some practitioners still use screens without filter packs, which can result in higher turbidity, and thereby compromise results. If a direct-push well is not properly developed, samples taken from the well also may have a higher turbidity than a properly developed conventionally-constructed well would. Because of their smaller diameter, direct-push wells require specialized tools to properly develop the well. However, properly installed and completed wells are capable of providing the same quality samples with reference to turbidity as conventional wells.
This article is provided through the courtesy of the Interstate Technology & Regulatory Council’s Sampling, Charac-terization and Monitoring Team. It is excerpted from the group’s technical and regulatory guidance publication, “The Use of Direct-push Well Technology for Long-term Environmental Monitoring in Ground water Investigations.” The complete report is available at www.itrcweb.org.