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Technology

A borehole is the term for any narrow shaft bored in the ground, either vertically or horizontally.  A borehole may be constructed for many different purposes, including the extraction of water.

The following is a description of the basics of building a borehole ...

1.  Water Well Basics

  • A water well is a hole or shaft for the purpose of extracting water.
  • Mostly vertical, but some can be horizontal (river beds)
  • Oldest known wells are hand dug horizontal shafts extending into the mountains of the Old Persian Empire in present day Iran.

2.  Determining a Well Location

  • For drinking water, groundwater quality and long-term supply are the most important considerations.
  • This requires a knowledgeable driller or professional consultant.
  • Sufficient water must be available to meet the demand.
  • To determine whether there is sufficient water available in a particular location, drillers and hydrogeologists rely on:
  •  Their prior knowledge of the local groundwater system, Experience in similar areas,
  • Land surface topography, Groundwater chemistry, Thickness, Depth, Rock fracturing, Local geology
  • Permeability of local aquifers from existing wells, Groundwater levels
  • Satellite or aerial photographs, Geophysical measurements

    Location is further limited to:

  • Property ownership
  •  Minimising transportation of pumped water
  • Access restrictions for drilling equipment
  • Potential sources of contamination – loos, farmyard

3.  Water Well Design and Installation

  • When location is determined – a preliminary well design is completed
  • Objective 
    • Structurally stable, Long lasting, Efficient well, Space to house pumps,  Allows free movement of groundwater and sediment free, From the aquifer into the well, At the desired volume and quantity
    • Prevents bacterial growth and material decay
  • A well consists of a well screen, well casing (pipe) and surrounding gravel pack, & surface seals
  • Water enters the well through perforations in the well screen
  • There are different types of well screens (slotted, louvered, bridge slotted and continuous wire wrap)
  • Wire wrap screens give best performance (well development and efficiency)
  • A blank well casing is used to prevent fine particles from entering the well and to provide open pathway from the aquifer to the surface and to protect the pumped groundwater from interacting with shallower ground water that may be of a lower quality.
  • Annular space between well screen, well casing and borehole is filled with gravel and coarse sand (gravel pack) (protects the well)
  • Uppermost section is sealed with bentonite (clay formed by breakdown of volcanic ash) and cement
  • A surface casing is installed to protect the well.

4.  Well Drilling

  • Common techniques:
  • Rotary (mud flows down under gravity, drilling mud pumped up the center of the drilling tube) (need lots of surface water)
  • Reverse rotary
  • Air rotary
  • Cable tool
  • Auger (helical type - Shallow)
  • Measurements take during drilling provide information about location & thickness of the aquifer layers, important for the determination of the depth of the well screens, size of the screens and size of the gravel pack material.
  • Often worthwhile to drill pilot hole to allow prior determination and ordering of the well materials.

5.  Well Development

  • Cleans the borehole and casing and settles the gravel pack
  • Typical method is to surge or jet water or air in and out of the well screen. (may take several days)
  • Properly developed gravel pack keeps fine sediments out of the well and provides clean unrestricted flow path for water.
  • Results in lower pumping costs, longer pump life, fewer biological problems (iron bacteria and slime build up)
  • Poorly developed wells subject to more frequent pump failures
  • Poor well efficiency (where water cannot enter the well screen because of a lack of open area in the screen
  • Well efficiency is different from pump efficiency; the former relates to the water yield from the borehole, whilst the latter relates to the amount of water displaced by the pump.
  • Aquifer test – continuous or stepwise increased rates for up to 7 days whilst checking the water levels
  • This determines the efficiency and capacity of the well and provides information about the permeability of the aquifer
  • Critical for the correct pump size

6.  Wellhead Protection

  • Final well seal is intended to provide protection from leakage and to keep runoff from entering the well.
  • Often used with backflow devices to stop contaminated water flowing back into the well.

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