Monitoring Wells

Taking environmental samples to monitor the concentration of a pollutant in river, lake, and atmospheric systems is relatively easy, although there are strict procedural guidelines for sampling and analysis. In these systems, you can see and feel the sample matrix. However, the sampling of groundwater is very different. For simplification purposes, we will limit our discussions primarily to saturated groundwa-ter systems, but remember that contamination also can occur between the ground surface and the flowing aquifer in the unsaturated zone.

First, one has to determine the depth of the water table (boundary of the saturated zone; Figure 8.1) and then determine which way the water is flowing. Although today there are nonintrusive techniques that do this, we usually resort to the very intrusive and potentially destructive method of installing a monitoring well, since this also enables us to obtain water samples to analyze for pollutants. A monitoring well consists of a metal or PVC casing (tube), installed from the land surface to a given depth in the aquifer. There is no good way to install such a well without significantly disrupting the nature of the aquifer. In environmental sampling, we always want to take a "representative and unbiased" sample, so that the water we obtain is characteristic of any water in the system. Obviously, if you violently drill a hole into a system, grinding and mixing dirt and rock fragments with the water, it is difficult to obtain a representative sample. But for now, this is the only technically feasible and economical method we have of obtaining groundwater samples.

We will next present a few of the well-installation techniques used today, since installing groundwater monitoring wells is often an expensive part of a characterization process and since our modeling results depend on obtaining samples to validate our modeling efforts. Also, as noted earlier, the sampling of groundwater requires special considerations. The types of drilling techniques we will cover include the cable tool, direct rotary drills, and auger and coring systems.

8.3.1 Cable Tool Percussion Method

The cable tool percussion method of drilling wells was probably used in the first attempts at installing a deep well. The technique goes back at least 4000 years and was developed by the Chinese, who successfully drilled wells to approximately 915m (3000 feet). A photograph of a modern system is shown in Figure 8.2. The drilling process works by repeated lifting and dropping of the drill bit (shown in Figure 8.3). Water is usually used to suspend the broken rocks and media, but this is not always necessary. Periodically, when sufficient debris has accumulated at the bottom of the well being drilled, a drill bit is replaced with a bailer and the solids are removed. The cable tool technique can be used in almost any geologic media, but works especially well in coarse glacial sediments (till), boulder deposits, or rock strata that are highly disturbed, broken, fractured, or cavernous. Well depths typically run from 90m (~300ft) to 1520m (5000 ft). Major advantages of the cable tool technique include the following (Johnson Filtration Systems, 1986):

• Drill rigs are relatively inexpensive.

• Drill rigs are simple in design and use.

• Machines have low energy requirements.

• Recovery of samples of geologic media is possible.

• Wells can be drilled in areas with little water supply.

• Wells are not contaminated by drilling equipment (muds, discussed later).

• Drill rigs can be operated in all temperature regimes.

• Wells can be drilled in media where loss of circulation is a problem (discussed in the next section, rotary drill rigs).

• Water yield from the formation can be determined at any point in the drilling operation.

Figure 8.2. A cable tool drill rig (U.S. DOE-INEL photograph).

Disadvantages include relatively slow drilling rates and high casing costs resulting from heavier diameter wall casing needed.

8.3.2 Direct Rotary Drill Method

A much faster drilling technique is the direct rotary drill method. In this approach, the drill bit is directly spun (motor driven) to break the geologic media. The drill

Figure 8.3. A cable tool drill bit and bailer (U.S. DOE-INEL photograph).

shaft is hollow, and some form of fluid is forced down the inside of the shaft, removing the cuttings from the well by forcing them up the well between the drill shaft and the borehole. You may have seen this type of drilling technique in use, or in movies on television, since it is used in wells for water, gas, and petroleum exploration. A typical rotary drill rig is shown in Figure 8.4. Drilling fluids must be used to remove cutting, to prevent the drill bit from becoming lodged in the borehole.

Figure 8.4. A direct rotary drill rig (U.S. DOE-INEL photograph).

Air, water, and synthetic "mud" (a clay mineral, bentonite) have been used for this purpose. Synthetic mud contains clay in suspension and, in some cases, surfactants. Air and clean water are preferable for installing monitoring wells for environmental sampling, since synthetic mud is transported into the geologic medium during the installation process. Once there, the mud cannot be completely removed upon well development (discussed later) and affects the movement of pollutants. Water and synthetic mud are necessary, however, in traditional direct rotary drilling in unconsolidated media, since they also serve to hold the formation open (prevent collapsing) around the drill bit and borehole. With this in mind, it follows that air can only be used in semiconsolidated or consolidated media, which will not be subject to collapse, as would unconsolidated material. One added feature of the rotary drill technique is that it can drill monitoring wells under permanent facilities by drilling at steep angles, as illustrated in Figure 8.5.

The major advantages of the direct rotary drill method include the following (Johnson Filtration Systems, 1986): relatively high drill rates in all types of media, minimal casing required during the actual drilling process, and rapid rig mobilization and demobilization.

Figure 8.5. A direct rotary drill rig being used to drill a diagonal well (U.S. DOE-INEL photograph).

Major disadvantages of this technique include high cost of drilling rigs, high level of maintenance required by drilling rigs, special procedures required for collection of media samples, and possible plugging of the drill bit and borehole caused by loss of pressure of the drilling fluid in fractured formations.

8.3.3 Earth Augers

The most common type of drilling method for installation of shallow monitoring wells is the hollow stem auger. A small, hand-operated system is shown in Figure 8.6, but larger truck-mounted systems are common. These systems are only used in unconsolidated media. The hollow stem allows soil samples to be collected and analyzed for pollutant contamination. This technique is used both for soil sampling and for installation of monitoring wells. These systems allow relatively rapid installation, but are limited to shallow depths.

8.3.4 Well Casing, Grouting, and Sealing the Well Casing

In most cases, once a borehole is drilled it must be lined with casing to prevent collapse of the geologic media. Commonly available casing materials include

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Figure 8.6. A handheld auger drilling system. (Courtesy of Forestry Suppliers, Inc. http://www.forestry-suppliers.com).

aluminum, carbon steel, stainless steel, and PVC. For monitoring wells, the type of casing used depends on the type of contamination. For example, PVC casing would not be used for monitoring the subsurface around a leaking underground organic chemical tank, since the liquid chemical might dissolve the PVC casing. PVC casing would also sorb any hydrophobic pollutant from the water. Similarly, metal casing would unlikely be used in wells for monitoring metal contamination.

Grouting is a term used to describe the filling of the space between the well casing and the borehole (referred to as the annular space). This filling is needed to prevent water from entering the well from the surface and in order to isolate different regions of the well. Some monitoring operations, especially in the unsaturated zone of a formation, use lysimeters for obtaining water samples. A lysimeter is a porous cylinder (usually ceramic) that contains two tubes that lead back to the land surface. A vacuum can be pulled on the lysimeters, allowing the collection of water over long periods of time. When sufficient time has passed, one tube of the lysime-ter is pressurized, which lifts the waters in the lysimeter to the surface through the second tube for sampling. Lysimeters are placed at areas in the unsaturated subsurface where water is expected to collect during infiltration from rain or flooding. Collection areas can be located at different depths in unconsolidated media or at fractures in consolidated media. In any event, water must be prevented from entering at the land surface or flowing between different lysimeters. To prevent the inflow of water from another region of the subsurface, a clay medium, usually bentonite (one form of grout), is placed above and below the lysimeter, and sand or silica is placed immediately around the lysimeter to allow water to freely pass to the collection point. The addition of grouting materials to a borehole is illustrated in Figure 8.7. Above the lysimeter, more bentonite is placed to seal the lower lysime-ter from water flowing above. Another lysimeter is placed at the next collection point, and so on, until each fracture or sampling locale has a lysimeter and the well is full. Many wells/boreholes are sealed at the surface with cement, but given the high pH associated with the Ca(OH)2, cement would not be used near any sampling location.

Wells at monitoring stations are drilled to the desired depth and then typically a porous section of pipe (referred to as a screen) is placed in the borehole to allow collection of aquifer water. Screen depths, where water is expected to be collected, can range from a few centimeters to several meters, depending on the section of the aquifer that you desire to sample. Sand or silica is placed between the screened casing and the borehole, and grout, usually bentonite, is placed above the screened area. Again, cement is avoided except at the land surface.

8.3.5 Well Development

As noted at the beginning of this section, the installation of monitoring wells is an intrusive and destructive process, and the area immediately adjacent to the borehole/well needs to be returned to its original condition, or as close to it as possible. Thus, well development refers to (1) the repair of damage to the subsurface (geological) formation and (2) the alteration of the aquifer to allow water to flow freely to the well for collection. First and foremost, it includes removal of any drilling fluid

Figure 8.7. The addition of grouting materials to a borehole (U.S. DOE-INEL photograph).

(other than air) that was used in the drilling process. As noted earlier, the installation of most environmental monitoring wells does not allow the use of synthetic muds or surfactants. Yet it is necessary to remove any drill cutting (rock chips and disrupted material) that may be present in the formation and contaminate subsequent samples. In order to restore the aquifer to near-normal conditions, the well can be "overpumped" (pumped at a high water removal rate for hours or days). Another technique is to reverse the flow several times to dislodge particles in the formation. In general, states, federal agencies, or concerned clients have an established procedure for developing the well. It should be noted that you do not simply walk up to a monitoring well and take a sample. Usually, three to five well casing volumes of water must be removed before a water sample is taken for chemical analysis. The purpose of this is to ensure that you have a representative sample from the aquifer and not stagnant water from the well casing.

8.3.6 But How Good Is Our Well?

Groundwater monitoring wells have been used for decades, but as amazing as it may seem, the effectiveness of well grouting was only recently evaluated. Dunnivant et al. (1997) found that standard well installation procedures for consolidated media were highly effective in isolating different productive zones within a borehole. An example of this effectiveness is shown in Figure 8.8. The dark areas of the borehole represent the location of bentonite used to seal the borehole and prevent water from flowing from above or below. This lysimeter installation was later flooded with at least 1 m of water for 50 days, yet no water passed directly down the borehole. Water and tracer (75Se) did migrate through the subsurface via fractures in the basalt media and arrived at some lysimeter locations but not at others. The first lysimeter placement at 6.4 m shows a typical tracer profile, with time, for a pulse input. The next two lysimeters in the borehole at 10.4 m and 18.9m did not receive water or tracer over the entire experiment (50 days of water application). The next lysimeter at 37.8 m did receive water, apparently from the beginning of the flooding when no tracer was present. The next two lysimeters at 42.7 m and 52.7 m received water and tracer. Again, these and other experimental measurements confirm that water did not flow in the borehole between lysimeter placements, which validates the grouting technique used. These and other borehole studies during the experiment by Dunnivant et al. (1997) found that the standard grouting technique was effective in sealing the borehole in consolidated media. Another study has found standard grouting techniques to also be effective in unconsolidated media (Christman et al., 2002).

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