Constructing Monitoring Wells
While a borehole is being drilled, soil and rock samples should be collected at frequent depth intervals. Careful observations of drill cuttings can guide the collection of undisturbed samples. Soil and rock samples are used to define the stratigraphy of a study area. In environmental investigations, these samples can be chemically analyzed to determine the extent of subsurface contamination. All cuttings and samples obtained during drilling should be carefully described in a field logbook.
Soil and rock samples are described according to several physical characteristics, including color, texture, degree of compaction, density and moisture content. The predominant grain size is used to name an unconsolidated sample. Other observable grain sizes may be used as adjectives. For example, a clay containing a minor fraction of sand may be described as a sandy clay.
Drilling MethodsThe method chosen for drilling a monitoring well depends on several factors, including subsurface conditions, equipment availability, versatility of the drilling method, drilling cost, site accessibility, installation time, ability to preserve natural conditions and ability to obtain reliable samples. The most common drilling methods include hollow-stem augers, solid-stem augers, cable tool, air rotary, water rotary, mud rotary, dual-wall reverse circulation, sleeve- or hammer-driving, and jet percussion. In environmental investigations involving contaminated soil and rock, the drilling equipment should be steam-cleaned between holes.
Well ConstructionOnce a hole is drilled, casing and annular material are added to complete the well. The casing consists of segments of solid pipe, whereas the screen is a slotted section of pipe. Segments of casing and screen usually are threaded, glued or welded together. Threaded joints require a rubber O-ring to prevent leakage. Glue is less preferable than other coupling alternatives because it can leach into the ground water.
The well screen should be designed to allow water but not sediment to enter the well. Screen lengths vary depending on the purpose of a well, but generally should not exceed 10 feet. Long well screens provide vertically composite rather than discrete samples. However, the screened portion of a well must allow sufficient water to enter for sampling or piezometer tests.
Casing material is chosen on the basis of cost, durability and reactivity with water. Teflon is most costly, least durable and most inert. Stainless steel is most durable, moderate in cost and essentially inert. PVC, which is available in most hardware stores, is least costly and most frequently used.
The annular space between the borehole wall and casing should be filled to prevent passage of formation materials into the well. Many bedrock wells do not require screens and thus do not require filter packs. However, most wells do require filter packs and a screened length of casing. Filter pack material should be chemically inert. Industrial grade silica sand or glass beads are appropriate. The Environmental Protection Agency recommends that filter pack constituents be well rounded and of uniform size. The filter pack should extend at least 2 feet above the top of the well screen. A tremie pipe can be used to install the filter pack and other annulus materials to prevent bridging between the casing and borehole wall.
Above the filter pack, proper sealing of the annular space between the well casing and the borehole wall is required to prevent a hydraulic connection between surface water and ground water near the well. The sealants should be chemically inert and essentially impermeable. A 2- to 3-foot plug of bentonite is usually placed directly over the filter pack. The bentonite can be emplaced by dropping pellets down the annulus of the borehole. When properly emplaced and hydrated, the pellets form a clay plug. A mixture of cement and bentonite often is used to fill the annulus above the bentonite seal.
Monitoring wells commonly are completed above ground or flush with the ground. In either case, measures should be taken to prevent infiltration of surface runoff into the well annulus and prevent damage or vandalism of the well. Concrete should be installed at the ground surface to hold the well in place. Anchored in concrete, a locking protective casing can be placed around the well casing to prevent damage or unauthorized entry.
After a well is installed, it should be developed to remove any sediment blocking the well screen. This procedure improves yield and creates a well capable of producing low-turbidity samples. Turbid samples may interfere with the chemical analysis of ground water. Well development usually is performed with a cylindrical plunging device or swab that is repeatedly lowered and raised through the water column. The plunging action forces water into and out of the well screen, thereby removing lodged silt and clay particles. Loose particles are transported into the well and removed with a bailer or pump. Other methods for developing a well include jetting with air or water and over-pumping.
Typically, a borehole contains a single monitoring well. However, it also is possible to place multilevel sampling devices in a single borehole. These devices may consist of a series of flexible tubes tapping the sidewall of a pipe at different depths. Alternatively, multiple pipes can be placed in a single borehole, but screened at different depths.