Ground water is important as a source of drinking water as well as for irrigation and industrial use. It is a major natural resource in the United States and is often more readily available than surface water. It was estimated in 1985 that more than 50 % of the population uses ground water as its primary source of drinking water.
Ground water is an integral part of the hydrologic cycle and as it moves through the cycle it interacts with the soils and the subsurface geologic formations and becomes contaminated through many natural and human activities. Contaminants entering the ground water system might range from simple inorganic ions to complex synthetic organic chemicals.
Recognition of groundwater contamination as a major environmental concern due to increasing public awareness, federal and state regulations has led to a technological improvement in groundwater monitoring. The most frequently used approach in ground water quality monitoring is the development of monitoring wells and subsequent collection and analysis of data from these wells for selected water quality constituents.
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The purpose of a ground water monitoring program is to learn enough about the pollution problems in order to design an appropriate restoration program.
There are a large number of factors to be considered when designing a monitoring program. An outline of relevant information to be evaluated is listed below.
In order to obtain the data necessary to design a monitoring program, it is essential to construct groundwater monitoring wells. The purpose of constructing a groundwater monitoring well is to collect geologic, hydrologic (depth of the water table, aquifer characteristics, etc.), and chemical data on soil and water and to provide for long-term monitoring capabilities.There are a large number of variables which affect the installation of wells and the interpretation of data collected from them. Prior to locating the wells and designing a monitoring program, it is important to have a preliminary understanding of the in situ groundwater flow characteristics and chemical and physical properties of contaminants and their interaction.
Drilling of the borehole for the well constitutes the first step in well construction. This requires the formulation of a boring program. Some boring programs may be divided into two phases. The first phase is the reconnaissance to assess the needs of a specific area and requires small boreholes to collect the geologic data. The second phase makes use of this data to determine the appropriate location of the final and more expensive groundwater monitoring wells. Physical disturbances, contamination during drilling or contamination caused by the materials used could lead to a bias in the results of the monitoring program. An illustration of a wet rotary drill, normally used to drill groundwater monitoring wells is shown in Figure 1.
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Selection of the drilling method will depend on the depth and the nature of the geologic formations under investigation. The following are several drilling methods commonly used:
Solid-flight Auger Drilling
A solid flight auger is a solid rod with a continuous spiral wedded around it, as shown in Figure 2. A series of cutting teeth at the end of the drill stem dig into the formation as the drill is rotated. The loosened soil rises to the ground surface through the spiral. This type of auger is useful for cohesive soils. However, soils below the water table are usually loose, and the borehole walls tend to collapse. For this reason, solid flight augers are only used at shallow depths to drill through a stable formation, and they are not very useful in well instalation.
Hollow-stem Auger Drilling
This type of auger (see Figure 3) utilizes a spiral flight around a hollow pipe. Additionally, a center rod inside the hollow pipe rotates with the auger during drilling. After a sample interval is reached, the center rod can be removed and a sampling tool can be introduced within the hollow stem of the auger, which stays inside the borehole to prevent walls from collapsing. Figure 4 shows how an actual auger flight looks.
Hand Auger Soil Borings
This is a very simple type of auger that is used for soil sampling as well as for drilling wells at shallow depths. A hand auger does not have a flight. As it is turned, it digs into the ground. After a few turns, the earth displaced can be pulled out and emptied from the auger. To see a film on how a hand auger is used to drill a monitoring well, follow this link.
Wet Rotary Drilling
This type of drilling requires a fluid, which is pumped down the hole as the cuttings are circulated to the surface. The drilling fluid, or "mud", also serves to cool the drill bit, to exert hydrostatic pressure on the formation and to form a thin coating on the borehole walls, which prevents them from collapsing (see Figure 5). Wet drilling has the disadvantage of impeding the determination of the groundwater depth. In addition to this, the well will be contaminated with the introduced fluids, and extensive well development will be necessary to overcome this.
The following link shows a film of how a wet rotary drill is used.
Air Rotary Drilling
In porous rock formations, drilling mud is usually lost. In these cases, air rotary drilling is used. Compressed air is circulated down the drill string, raising the cuttings to the surface as the drilling progresses.
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After a borehole has been drilled for the well, casing and well screens are placed, filling material is packed between the borehole and the screen, and the annular space between the borehole and the casing is filled with an appropriate sealant. An example of a typical well, completed above grade, is shown in Figure 6.
Hence the well screen, riser, filter pack and sealant are some of the essential elements required for the completion of the monitoring well. Several factors must be considered to determine the best material to be selected for these elements based on the environmental conditions expected at the site. The interaction potential of the groundwater with material being selected must be reviewed. Plastics are acceptable for sampling inorganic constituents and are resistant to most naturally occurring compounds and to corrosion. However they are susceptible to attack by organic compounds. Metals can also contaminate the water by leaching heavy metals to the water, release by-products of corrosion, also allow adsorption of organic and inorganic compounds from the groundwater onto the metal surface.
A well screen is usually a slotted pipe that allows water to flow from the formation into the well, without allowing the entrance of soil particles into the well. The well screens are placed in the well opposite the interval of the water-bearing zone or aquifer that is to be sampled.
Prior to the selection of the type of well screen or the screen material, certain criteria should be met. The criteria chosen should prohibit the entry of aquifer material, allow water to enter the well freely and provide structural support for both the column load of the well and the surrounding loose formation material. In order to obtain optimum well design, the diameter of the well, the slot size of the screen, screen material type, and the screen design should all be evaluated with respect to the objective of sampling, such as well yield and contaminants of interest.
The diameter of the well can range in size from 1.5 to 60 inches. Well screens of smaller diameter are not very expensive, but are less effective.The slot or the screen opening is another factor to be considered. Geologic and aquifer information acquired during the design of the monitoring program provide general characteristics of the aquifer, which should enable adequate preselection of screen slot size and also the gravel pack. Aquifers that have fine formation grains are compatible with a 10-slot screen (0.010 in. slot); in fine-grained aquifers, a 20-slot screen would be appropriate, and in coarse-grained aquifers, such as gravel with sand, a 30-slot screen would be appropriate. The well efficiency (maximum water production with minimum side effect) is found to be quite high when the percentage of open area in the screen is the same as, or greater than, the average porosity of the aquifer material.The screen length should also be considered when selecting a screen type. The screen lengths for monitoring wells are selected based on the area of the aquifer that is of interest. The typical total screen length of monitoring wells usually ranges between 5 and 20 ft.
There are five different types of well screen used for monitoring wells: field slotted pipe, factory slotted perforated pipe, wire-wound perforated pipe, manufactured louvre-type or bridge-slot screen, and wire-wound continuous-slot screens. Some of their advantages and disadvantages are listed in the following table.
|Field slotted pipe
|Factory slotted pipe
|Manufactured louvre-type or bridge-slot screen
|Wire-wound perforated pipe (pipe-based screen)
|Wire-wound continuous slot screen (not pipe based)
The riser (or casing) is the solid walled pipe that connects the well screen with the surface. A variety of materials including mild steel, stainless steel, polyvinyl chloride (PVC), Teflon, polypropylene and kynar can be used to construct monitoring well casings. These factors include well depth and diameter, construction techniques, material strength, groundwater corrosiveness, microbiological activity, sorptive/desorptive properties of the chemical species under consideration, and material cost. It is also important to determine that the casing does not act as a catalyst for chemical reactions or leach constituents into or adsorb contaminants. The amount of stress and temperature the casing is exposed to is determined by the depth and diameter of the monitoring well. Deeper wells with wide diameters would require casings to be resistant to stress exerted by the weight of the pipe string, unconsolidated sediment, and an increase in the temperature with increasing depth.
In monitoring wells, the filter gravel pack used is composed of graded silica sand. These filter packs can be of two types, a natural filter and an artificial filter pack. In a naturally developed well, fine materials surrounding the well are removed during the process of well development and are replaced by the natural filter pack. In the artificially gravel packed well, sand or gravel which is coarser than the natural formation is placed immediately surrounding the well screen along the length of the screen. The purpose of this filter pack method is to create a more permeable zone surrounding the well screen and increase the effective diameter of the well, thereby enhancing the inflow of groundwater. The artificial gravel pack is placed a few feet above the screen because it creates a zone of protection from settling.
Grout and sealant
In order to prevent the migration of water from the surface and from overlying or adjacent formations into the monitoring well, a bentonite or cement grout is placed in the annular space of the borehole, above the filter pack. A seal of bentonite is placed above the filter pack, in most cases. This seal creates a buffer zone between the sand or gravel pack and the overlying cement grout which could otherwise seep through the sand or gravel into the well screen and hance into the well. Such a seal consists of 3 to 5 feet of bentonite clay pellets. Above this seal the annular space is usually grouted with a cement, bentonite, and water slurry.
The entire process of well installation is illustrated in Figure 7.
The following films demonstrate the proper construction of a monitoring well in a soil boring dug with a hand auger.
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Prior to collection of ground water samples, monitoring wells must be developed by the removal of some minimum quantity of water. One procedure is to pump water out of the well until the pH becomes stable. If the well has been dug using dry methods such as rotary air drilling, only three to ten casing volumes must be purged out in order to collect representative groundwater samples. However, if wet drilling has been used, fluids that were introduced during drilling will be more difficult to purge out.
Some wells are equipped with pumps. This reduces the potential for cross-contamination when using the same pump or bailer for obtaining samples at different wells. However, these wells require further development in order to remove a large fraction of the fine materials from the filter pack and the formation adjacent to the borehole, to avoid clogging the pump and the well screen.
Development of a well is taken out through a combination of pumping and surging. Surging consists of running a close fitting cylinder up and down the inside of the well to cause a flushing action in the gravel pack around the well. This loosens the fine sediment, which then requires pumping to be carried out to the surface.
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Sampling equipment for groundwater includes grab mechanisms such as bailers and syringes, suction methods such as jet pumps, and positive displacement methods such as gas pumps. Some of the available equipment, their advantages and disadvantages, are described in Table 2.
A peristaltic pump is shown in Figure 8. To see how it works, follow this link:
Another common type of pump is a submersible pump whose operation is described in the following film:
|Bailers||Portable, simple to operate and clean.||Aereation of the sample during transfer of water to sample bottle.|
|Syringes||Offer no contact with atmosferic gases. Are portable, inexpensive and easy to operate.||Inefficient for large volumes.|
|Centrifugal pumps||Allow high pumping rates.||Potential contamination from gasoline motor. Aereation and turbulence occurrs.|
|Submersible pumps||Portable, inexpensive, can be used for well evacuation.||High solids content might cause operational problems.|
|Piston pumps||Allow high pumping rates even at large depths.||Expensive. Not portable. Particles may cause damage.|
|Air lift samplers||Portable.||Cause degassing and pH changes in water.|
|Bladder pumps||Driving gas does not contact sample. Allows high pumping rates.||Inefficient for large volumes. Suspended solids may damage valves.|
|Jet pumps||Simple to operate and adaptable to different installations||Cause degassing of water. Inefficient for large volumes.|
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The data collected from the monitoring wells must be acceptable according to EPA standards. This means that sample collection and handling must follow proper protocol. All data collected during groundwater monitoring should be recorded on field data sheets and in field logbooks.
In order to provide a quality control check, field blank samples are required. The pump or bailer should be subjected to exactly the same decontamination procedures used prior to every other sampling point. A sample of deionized water is then decanted from the bailer or run through the sampling pump and submitted to the same laboratory analysis as the rest of the samples. Sometimes a trip blank is also necessary. This sample is not affected by the sample-collection process since it is prepared in the laboratory, but it must be taken to the field trip and then through the analytical process with the rest of the samples. When volatile organic constituents are to be determined, ground water samples must be collected free of head space in order to keep volatile constituents in solution. Sediments can interfere in some analysis. For example, when heavy metals are to be determined, samples are usually preserved with acid. If the sample has not been filtered, any sediment that might be present might then leach metals into the water.
A preliminary assessment of the contaminated zone can be done through field testing. Additionally, field testing can provide information for the proper selection of samples for laboratory analysis. The most common field tests are temperature, contuctivity and pH. Other field tests include redox potential and dissolved oxygen. Some test kits ara available fror a variety of constituents, although their detection limits is not sufficiently low to provide a definitive diagnose. Confirmation of results in the laboratory is always necessary, except for some mobile laboratories that follow EPA Quality Assurance/Quality Control procedures. Samples of water that have been tested in the field should not be included in the samples collected for laboratory analysis.
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Measures should be taken to prevent the loss of contaminant mass from the sample, for instance, by volatilization or biodegradation. The method of sampling depends on the constituent of interest. There exist established guidelines for appropriate containers and preservation methods depending on the analyte being determined. It is important to prevent the introducton of contaminants into the sample from some other source (cross-contamination). Sampling equipment should be cleaned between sample locations, and locations should be sampled in order of increasing contaminant concentration. Samples should be transported to the laboratory as soon as possible, and the shipment must be tracked with and log that includes date and time of collection and transfer, as well as the person collecting, transporting and receiving them.
Water levels must be measured prior to sample collection, to ensure data consistency each time samples are collected, but also to record fluctuations. This can be measured with a water level inticator probe or a steel tape and chalk. Wells should be checked for the presence of free-phase hydrocarbon layers using an electric hydrocarbon interface probe or transparent bailer. The thickness of the hydrocarbon layer at each location should be measured to the nearest 0.01 ft. If a well contains a free-product layer, the water may not accurately reflect the dissolved-phase concentration. Such a well is frequently not analyzed.
Prior to sampling, the required purge volume is calculated based on the volume of water in the casing wells. An acceptable purge is usual from three to five well casing volumes. Removal of three to five well casing volumes is the generally accepted minimum purge. Another option is to monitor pH while purging, and sample only after pH stabilizes. Samples should be collected within 24 hours of well purging. Collection of contaminated purge water for proper treatment or disposal is frequently required by regulatory agencies.
Gloves should be worn during sample collection and changed between sample wells. Prioir to use in each well, al equipment should be cleaned with steam or detergent, rinsed first in clean water, then in a solvent such as ethanol or methanol, and finally in deionized water. The rope used to lower the bailer should be changed between different well samplings. All sampling and purging equipment must not be allowed to contact the ground.
The materials of pumps or bailers used for well purging and sampling should not react with the contaminant in the sample. Usually stainless steel or Teflon are used. Water quality samplers should also be constructed of such materials. Some air lift sampling methods such as gas-driven pumps can cause volatile-gas stripping, oxidation, degassing or pH shifts. Sample composition can also change due to depressurization during collection. If a constant flow rate is maintained these problems may be reduced.
Follow this link to appreciate some of the sampling procedures:
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We would like to thank Dr. William Reay, Dr. Michael Robinson and Dr. Daniel Gallagher for providing the video and the photographs.
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Sampling & Monitoring Primer Table of Contents