The purpose of the casing and screen in a groundwater monitoring well is to provide access from the surface to the groundwater in order to collect groundwater samples or measure groundwater elevations. The casing prevents geologic materials from collapsing into the borehole, while the screen allows groundwater to enter the monitoring well. The screen is generally attached to the subsurface end of the casing.
Several types of casing and screen materials are available including stainless steel, galvanized steel, carbon steel, PVC, Teflon, and aluminum. Selecting the monitoring well casing and screen material depends on the physical strength and chemical reactivity of the material under subsurface conditions. With regard to physical strength, the casing and screen should be able to withstand the forces exerted on them by the surrounding geologic materials. These forces can be significant for deep monitoring wells (greater than 30 meters). Nielson and Schalla (1991) provide data on the physical strength of different types of casing and screen materials.
With regard to chemical reactivity, the material of the casing and screen should neither adsorb nor leach chemical constituents which would bias the representativeness of the samples collected. In addition, the material must be durable enough to endure chemical attacks (corrosion or chemical degradation) from the natural chemical constituents or the contaminants in the groundwater. Teflon is probably the most chemically resistant material used in monitoring well installation, but the cost of Teflon is high (Barcelona et al. 1990). Stainless steel offers good strength and chemical resistance in most environments (except in highly acidic conditions), but it too is expensive. Galvanized steel is less expensive; however, it can impart iron, manganese, zinc, and cadmium to many waters. PVC has good chemical resistance except to low molecular weight ketones, aldehydes, and chlorinated solvents (Miller 1982).
Two types of screens are commonly used in monitoring wells: machine-slotted pipes and continuous-clot wire-wound screens. Machine-slotted pipes are readily available and inexpensive, but the low amount of open area in these screens makes development of the well difficult. Continuous-slot screens, in contrast, are more efficient, but their cost is relatively high. The design of the slot size of the screen must be based on the characteristics of the filter pack material and the grain size of the stratum. The optimum slot size should provide maximum open area for water to flow through and minimum entry of fine particles into the well during piping (Nielson and Schalla 1991; Aller et al. 1991).
The depth of placement of the screen as well as its length are usually determined based on the depth and thickness of the water-bearing zone to be monitored. When the objective of the well is to monitor a potable water supply aquifer, then a longer screen, perhaps over the entire thickness of the aquifer, might be selected. On the other hand, when the objective of the well is to vertically delineate a plume, such as with cluster wells, then shorter screens at specific intervals might be selected.
The screen should be fully submerged to prevent contact between the contaminated groundwater and the atmosphere, particularly for volatile compounds. The screen is, however, extended above the water table for wells constructed to monitor floating products. In this case, the screen length and position must accommodate variations in water table elevation.
The casings are produced in various diameters (2, 4, 6, and 8 inches) and various lengths (5, 10, and 20 feet) that are joined by various coupling methods during installation. The casing diameter depends on the future use of the well, the type of pumping equipment, and the method of drilling. Small diameters (2 and 4 inches) are used for monitoring wells, while large diameters (6 and 8 inches) are used for recovery wells.
The casing must extend from the top of the screen to the ground surface level. The casing is protected at the surface by a metal protective casing or a manhole. Multiple casings are installed for wells penetrating more than one water-bearing formation. The purpose of multiple casings is to prevent a hydraulic connection and potential cross contamination between the water-bearing formations along the annular space produced by the installation of well casings. Figure 9.15.3 shows an example of a double-cased well installation where the outer casing is anchored into the confining layer before the borehole is advanced and the well is installed through the inner casing.
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