8.3 Drilling Methods

A variety of drilling methods have been developed over time to account for the many geologic conditions. Some formations are very hard, such as granite, while others are soft or unconsolidated such as the sands and gravels found in an alluvial setting. Drilling and subsequent well completion should be considered together when deciding which drilling method is most appropriate. Some methods may be faster than others but may result in disturbing the aquifer to the point of reduced yield. For example, if the production zone is in unconsolidated sediments, high-pressure drilling fluids may actually cause damage to the aquifer. In this case, production wells need to be drilled using a slower, less-disruptive method so that production-zone disturbance is minimized. This chapter will explore the most common truck-mounted drilling methods. For a more complete discussion of drilling applications in the water-well industry, the reader is referred to Driscoll (1986).

Monitoring wells provide a point of access to the aquifer. They need to be constructed so that the hydraulic head and water-quality information collected is representative of the aquifer under investigation. Again, the drilling method will depend on the geologic conditions. The well-completion materials need to be appropriate for the water quality of the site, which may include such factors as pH, presence of organics, water temperature, or depth.