Low-Flow Purging and Sampling of Ground Water Monitoring Wells with Dedicated Systems

A field study was conducted to assess purging requirements for dedicated sampling systems in conventional monitoring wells and for pumps encased in short screens and buried within a shallow sandy aquifer. Low-flow purging methods were used, and wells were purged until water quality indicator parameters (dissolved oxygen, specific conductance, turbidity) and contaminant concentrations (chromate, trichloroethylene, dichloroethylene) reached equilibrium. Eight wells, varying in depth from 4.6 to 15.2 m below ground surface, were studied. The data show that purge volumes were independent of well depth or casing volumes. Contaminant concentrations equilibrated with less than 7.5 I. of purge volume in all wells. Initial contaminant concentration values were generally within 20 percent of final values. Water quality parameters equilibrated in less than 10 L in all wells and were conservative measures for indicating the presence of adjacent formation water. Water quality parameters equilibrated faster in dedicated sampling systems than in portable systems and initial turbidity levels were lower.     

The Effect of Three Drilling Fluids on Ground Water Sample Chemistry

Three monitoring wells were installed in borings that were constructed using water-based drilling fluids containing either (1) guar bean, (2) guar bean with breakdown additive, or (3) bentonite. These fluids were selected to observe their effect on the chemistry of subsequent water samples collected from the wells. The wells were installed to depths of 66 feet, 100.5 feet and 103 feet, respectively, in fine-to-medium sand and gravel outwash deposits near Antigo, Wisconsin. Drilling fluids were necessary to maintain an open borehole during well construction through strata containing cobbles and boulders.
The bentonite and guar drilling fluids caused temporarily elevated concentrations of chemical oxygen demand (COD) in ground water samples collected from the monitoring wells. Using standard development, purging and sampling procedures, elevated COD concentrations persisted for about 50 days for the well bored with the guar-with-additive fluid, 140 days for the bentonite well and 320 days for the guar well. Unfiltered ground water samples for all wells had greater concentrations of COD than samples filtered through a 0.45 micron filter. Sulfate concentrations also decreased with time in the guar-with-additive well and bentonite well, but not in the guar well.
The elevated COD concentrations are attributed to the large concentrations of oxidizable carbon present in the guar bean drilling fluid and in the organic polymers present in the bentonite drilling fluid. Well development and purging procedures, including borehole flushing, surging, bailing and/or chemically induced viscosity breakdown of the guar mud decreased the time before background conditions were achieved. Future research should evaluate the physical and geochemical interaction of different drilling fluid compositions with a variety of geologic matrices and drilling, well development and well purging techniques.     

An Integrated Ground Water Monitoring Program Using Multilevel Gas-Driven Samplers and Conventional Monitoring Wells

Hydrogeologic and ground water quality data obtained from a gas-driven multilevel sampler system and a polyvinyl chloride (PVC) monitoring well nest with the same aquifer communication intervals are compared. All monitoring points are in close proximity to each other. The study was conducted at an eight-acre uncontrolled hazardous waste site. The site is located in an alluvial valley composed of approximately 40 feet of alluvium overlying shale bedrock. The ground water at the site is contaminated with various organic constituents. A ground water monitoring network consisting of 26 conventional monitoring wells, nine observation well points, and six multilevel gas-driven samplers was established to characterize the hydrogeologic regime and define the vertical and horizontal extent of contamination in the vicinity of the abandoned chemical plant. As part of this study, a multilevel monitoring system was installed adjacent to a well nest. The communication zones of the multilevel samplers were placed at the same elevation as the sand packs of the well nest. The multilevel sampler system and well nest are located in a contaminated area directly downgradient of the site. A comparison of the vertical head distribution and ground water quality was performed between the well nest and the multilevel sampling system. The gas-driven multilevel sampling system consists of three gas-driven samplers that monitor separate intervals in the unconsolidated materials. The well nest, composed of two PVC monitoring wells in separate boreholes, has the same communication interval as the other two gas-driven samplers. Hydraulic head information for each multilevel sampler was obtained using capillary tubing. This was compared with heads obtained from the well nest utilizing an electric water level indicator. Chemical analyses from the PVC and multilevel sampler wells were performed and compared with one another. The analyses included organic acids, base neutrals, pesticides, PCBs, metals, volatile organics, TOX, TOC, CN, pH and specific conductance.     

Applications of Dual-Wall Reverse-Circulation Drilling in Ground Water Exploration and Monitoring

Dual-wall reverse-circulation drilling uses flush-threaded double-wall drill pipe and high-pressure air to provide continuous return of formation and water samples. Cuttings and formation waters are not contaminated with drilling additives or mixed with other borehole material. Up-hole velocity of about 70 ft/sec provides reliable logging of water, mineral or contaminant-bearing strata. Water samples representative of specific strata may be airlifted or bailed to the surface.
In the percussion hammer system, dual-wall drill pipe is advanced through chiefly unconsolidated material by the percussion action of an above-ground pile hammer. The borehole is drilled and temporarily cased in one pass. Wells or monitoring devices are installed as the drill pipe is hydraulically retracted during construction. A rotary head may be adapted as an option to allow dual-wall rotary drilling into consolidated or crystalline formations through a percussion hammer drill string temporarily left in place as a conductor.
The complex geology and variety of geoenvironmental problems in southern California has provided a testing ground for dual-wall drilling on hazardous material site investigations. Several case histories have demonstrated the capabilities and versatility of this method, including: (1) the installation of 4-inch and 6-inch diameter gasoline monitoring and recovery wells through gravels and cobbles at a filling station where hollow-stem auger drilling failed; (2) the confirmation of a dry borehole initially drilled by direct rotary at a landfill; and (3) multiple installations of monitoring devices through municipal refuse at a city of Los Angeles landfill.     

A Comparison of Sampling Mechanisms Available for Small-Diameter Ground Water Monitoring Wells

The objective of most ground water quality monitoring programs is to obtain samples that are "representative" or that retain the physical and chemical properties of the ground water in an aquifer. Many factors can influence whether or not a particular sample is representative, but perhaps the most critical factor is the method or type of sampling device used to retrieve the sample.
The sampling equipment available today ranges from simple to highly sophisticated, and includes bailers, syringe devices, suction-lift pumps, gas-drive devices, bladder (Middelburg-type) pumps, gear-drive and helical rotor electric submersible pumps and gas-driven piston pumps. New devices are continually being developed for use in small-diameter wells in order to meet the needs of professionals engaged in implementing elaborate ground water monitoring programs.
In selecting a sampling device for a monitoring program, the professional must consider a number of details. Among the considerations are: the outside diameter of the device, the overall impact of the device on ground water sample integrity (including the materials from which the sampling device and associated equipment are made and the method by which the device delivers the sample), the capability of the device to purge the well of stagnant water, the rate and the ability to control the rate at which the sample is delivered, the depth limitations of the device, the ease of operating, cleaning and maintaining the device, the portability of the device and required accessory equipment, the reliability and durability of the device, and the initial and operational cost of the device and accessory equipment. Based on these considerations, each of the devices available for sampling ground water from small-diameter wells has its own unique set of advantages and disadvantages that make it suitable for sampling under specific sets of conditions. No one sampling device is applicable to all sampling situations.     

Drilling and Constructing Monitoring Wells with Hollow-Stem Augers Part 1: Drilling Considerations

Hollow-stem augers are a widely used drilling method for constructing monitoring wells in unconsolidated materials. The drilling procedures used when constructing monitoring wells with hollow-stem augers, however, are neither standardized nor thoroughly documented in the published literature.
Variations in drilling procedures and techniques may occur as a result of the: (1) type of auger drill equipment and formation samplers used; (2) hydrogeologic conditions at the site, especially where heaving sands occur; and (3) known or suspected presence of contaminated zones, where there is a potential for the vertical movement of contaminants within the borehole.
In a saturated zone in which heaving sands occur, changes in equipment and drilling techniques are required to provide a positive pressure head of water within the auger column. This may require the addition of clean water or other drilling fluid inside the augers.
When monitoring the quality of ground water below a known contaminated zone, hollow-stem auger drilling may not be advisable unless protective surface casing can be installed. Depending on the site hydrogeology, conventional hollow-stem auger drilling techniques alone may not be adequate for the installation of the protective surface casing. A hybrid drilling method may be needed that combines conventional hollow-stem auger drilling with a casing driving technique that advances the borehole and surface casing simultaneously.     

A Multilevel Ground Water Monitoring System for Casing Advance and Bedrock Drilling Methods

A single-hole multilevel sampling piezometer system (MLSPS) has been designed by the Geological Survey of Canada (GSC) to be installed using drilling systems that continuously core (e.g., Rotosonic) or continuously sample (e.g., hollow-stem auger, Becker hammer) overburden and that have the flexibility of allowing additional coring (diamond drilling) or sampling (hammer drilling) of bedrock. The GSC-MLSPS (under license to Solinst Canada Ltd.) uses a patented GSC dry injection system for accurate emplacement of filter packs and seals. This system permits (a) the use of variable screen lengths; (b) the complete evacuation of piezometers before introduction of new ground water (no bailing); (c) the use of a number of types of hydraulic tests (e.g., slug, withdrawal/recovery, vacuum, pressure-pulse); (d) ground water sampling under a nitrogen atmosphere; (e) dissolved gas sampling; (f) a great deal of flexibility in the use of design materials; and (g) the elimination of bridging and collapse of filter packs and seals.     

Drilling and Constructing Monitoring Wells with Hollow-Stem Augers Part 2: Monitoring Well Installation

The procedures used to construct monitoring wells with hollow-stem augers may vary depending on the hydrogeologic conditions at the site. In cohesive materials in which the borehole stands open, the auger column may be fully retracted from the borehole prior to the construction of the monitoring well. In non-cohesive materials in which the borehole will not stand open, the monitoring well may be constructed through the hollow axis of the auger column.
The techniques used to construct monitoring wells through the hollow axis of the auger column may vary depending on the specific site conditions and the experience of the driller. Selection of an appropriately sized diameter hollow-stem auger for the installation of the required size of well casing is necessary to permit an adequate working space between the casing and augers, through which filter pack and annular seal materials are emplaced. Assurance that the filter pack and annular seal are properly emplaced is typically limited to careful measurements taken and recorded during construction of the monitoring well.     

Micro Computers Applied to Ground Water Monitoring and Testing

Micro computers have been demonstrated to be a most valuable, cost-effective means of long- and short-term data acquisition for ground water investigations and installations. A system can readily be assembled for the cost of labor saved in the field, or for the same cost as other conventional instruments, which perform only a fraction of the functions of the computer system
Documented in this article are systems which have been developed and used for simultaneous monitoring of several wells during aquifer tests, and systems installed for long-term monitoring of piezometric surface fluctuations. Both systems can be contained in a small suitcase or insulated cooler. Specific features of the systems include multiple channel capacity, one-month maintenance period, variable computer controlled reading intervals, magnetic tape data storage, data reduction and analysis capabilities while maintaining monitoring, graphic display of time and measurements, hard copy capability and barometric pressure change corrections.
Other applications are examined for the complete control of pumping tests, including pumping rate; in situ permeability tests; monitoring changes; and even strain. The pressure transducer system may also be applied for river gauging and current measurement.     

A Dry Injection System for the Emplacement of Filter Packs and Annular Seals in Ground Water Monitoring Wells

The reliability of filter pack and annular seal emplacements, and the degree of integrity of installed seals, are two of the most important factors to be considered when both installing and later utilizing ground water monitoring wells.
Numerous, and often costly, problems of using existing methods of installing filter packs and annular seals during the construction of ground water monitoring wells have led to the development of a technique of installing these monitoring well components using a dry injection system.
The dry injection system has been used to construct monitoring wells in extremely complex overburden/bedrock environments with a variety of drilling techniques. The system has shown that a high degree of reliability in the, construction of monitoring wells and greater confidence in obtaining representative ground water samples can be achieved over existing methods of filter pack and annular seal emplacement. The system has also been more cost effective than existing methods, especially for deep boreholes and multilevel monitoring system installations.     

The Rehabilitation of Monitoring Wells Clogged by Calcite Precipitation and Drilling Mud

Based on aquifer performance tests, 13 out of 15 wells situated at the Mixed Waste Disposal (MWD) area located at the Savannah River site. South Carolina, exhibited high skin factors and low well efficiencies indicative of severely damaged wells. The use of damaged wells in aquifer testing can lead to inaccurate determinations of aquifer properties, and such wells are unusable in future remediation programs. Moreover, damaged wells can go dry during purging, thus compromising sample collection. Pump tests, chemical analyses, and biological investigations revealed that the poor well performance at MWD was attributable to calcite precipitation on the well screen and drilling mud in the filter pack. The calcite problem resulted from improper well installation, and the drilling mud in the filter pack was due to inadequate well development.
Experimental rehabilitation procedures employed on two wells, MWD 5A and 1A, included acidification, swabbing, introduction of surfactants, and surging. Treatment of the wells substantially improved well yields, skin factors, and well efficiencies. Moreover, well rehabilitation was determined to be a reasonable alternative to drilling new wells at the MWD wellfield.     

The Application of Television Borehole Logging to Ground Water Monitoring Programs

Borehole television has been successfully utilized to gather in situ information on boreholes and wells in several ground water monitoring programs. Borehole television surveys are proposed as a viable alternative to other downhole instruments in the subsurface investigation stages of a ground water monitoring program.
The borehole television camera used by the authors was originally developed for use in the examination of nuclear reactor cores; the camera has since been modified for use in borehole investigations. The lens attachments are capable of looking sideward or downward and include built-in lighting assemblies. Use of the camera, lenses and various support equipment are discussed.
The in situ characterization of fractures that can provide pathways for contaminant migration poses a significant challenge. Borehole television inspection can provide information on the frequency, size and orientation of these fractures. Vertical correlations of rock cores in areas where voids are present (i.e. deep mining or karst topography) can also be simplified by this technique. In addition, borehole television can also be used to check monitoring well integrity. Casing inspections are especially useful where construction details are not known. Well screens may be inspected in place to determine if rusting has enlarged the screen openings or if screens have been damaged during emplacement or well development operations (i.e. surge block, air jetting, etc.). This information may prove to be very valuable in the decision to decommission a well. Examples of these successful applications in ground water monitoring programs at several Superfund hazardous waste sites are presented.     

Design, Installation, and Uses of Combination Ground Water and Gas Sampling Wells

Waste disposal sites with volatile organic compounds (VOCs) frequently contain contaminants that are present in both the ground water and vadose zone. Vertical sampling is useful where transport of VOCs in the vadose zone may effect ground water and where steep vertical gradients in chemical concentrations are anticipated. Designs for combination ground water and gas sampling wells place the tubing inside the casing with the sample port penetrating the casing for sampling. This physically interferes with pump or sampler placement. This paper describes a well design that combines a ground water well with gas sampling ports by attaching the gas sampling tubing and ports to the exterior of the casing. Placement of the tubing on the exterior of the casing allows exact definition of gas port depth, reduces physical interference between the various monitoring equipment, and allows simultaneous remediation and monitoring in a single well. The usefulness and versatility of this design was demonstrated at the Idaho National Engineering and Environmental Laboratory (INEEL) with the installation of seven wells with 53 gas ports, in a geologic formation consisting of deep basalt with sedimentary interbeds at depths from 7.2 to 178 m below land surface. The INEEL combination well design is easy to construct, install, and operate.