Causes of Borehole Deterioration and Failure: Water Level Drawdown, Mechanical Failure, Incrustation, and Corrosion

The life expectancy of a production borehole will be limited if it was incorrectly designed or not constructed for maximum efficiency, or if it has been over-pumped. Many production wells are seldom monitored or maintained; they are neglected until a problem arises. But if a borehole is properly designed, constructed with the correct materials, and given regular attention, it can produce water for 50 years or more. Common causes of borehole deterioration or failure include the following:

a. Water level drawdown
Production from a borehole or a well field can decline because of a drop in the water table, which might be due to natural causes such as drought, but also to well deterioration and over-pumping (excessive drawdown). A drop in the water level can result in submersible pumps shutting off automatically.

b. Mechanical failure
Pumps eventually lose their effectiveness as parts become worn, corroded, or clogged, and borehole screens become partly blocked by damaging organic and inorganic accumulations and scale deposits. If pumps are not turned off before they begin sucking in air, they will be irreparably damaged. Decline in or loss of production can be at least partly (if not mostly) remedied by a program of well maintenance and rehabilitation.

c. Incrustation
Most ground waters are only mildly corrosive, if at all, so corrosion is not usually a problem if good quality plastic and steel (such as stainless steel) casings and screens have been installed. The main cause of deterioration is the build-up of incrustations around screen openings, which reduce borehole efficiency.

As a borehole is pumped, pressure is reduced by the local drawdown, and water velocity and turbulence around the borehole increase. In this agitated zone, carbon dioxide gas is released from the water, which reduces the solubility of certain compounds in the water, such as calcium carbonate. Incrustation is mainly the result of the precipitation of insoluble carbonates, bicarbonates, hydroxides, or sulphates of calcium, magnesium, sodium, manganese, or iron. However, these deposits are rarely composed of a single mineral.

Normally, the level of dissolved iron in groundwater is low, but slight changes in water chemistry, such as acidification due to dissolved carbon dioxide or organic matter (humic acids) can result in higher iron concentrations (up to tens of milligrams per litre). Iron will remain in its soluble (ferrous) state unless there is a rise in the pH (alkalinity, equivalent to reduction of acidity) or Eh (redox potential) of the water. Increased oxygenation of the turbulent zone can initiate iron precipitation by oxidation from the ferrous (soluble Fe2+) to the ferric (insoluble Fe3+) form in the screen area. Serious mineral deposition can occur at the top of screens, which become exposed to air owing to excessive drawdown. Inorganic silts and clays often add to the problem, but organic deposits can also be
involved. Oxidation of ferrous to ferric iron at the borehole boundary can encourage the growth of 86 certain bacteria. Organic slime formation by species of iron bacteria is a result of the life cycle of such organisms. They inhabit groundwater by metabolizing ammonia, methane, or carbon dioxide, again changing iron into deposits of insoluble salts (mainly hydroxide), which worsens incrustation.

Iron biofouling is a complex process influenced by interactions between the aquifer environment and the borehole structures. Microbial matter consists of filamentous cell colonies, mats, and slime sheaths (which cells secrete for protection), often of a sludgy consistency, but able to harden with age. Such incrustations impair hydraulic efficiency and specific capacity, clogging pipes, filter packs, screens, and pumps. They can cement a gravel pack into something akin to concrete. They encourage corrosion and reduce water quality, but remedial measures are likely to be less effective once hardening has occurred. If an incrustation has aged and recrystallized it will be extremely difficult to loosen and remove.

d. Corrosion
The most common corrosion process is electrochemical, in which iron (or another metal) is dissolved and re-precipitated as a hydroxide deposit.
Corrosion in a water well most often occurs at localized physical imperfections on metal pipes and screens; the process can be encouraged by high salinity, high temperatures, oxygen, carbon dioxide, hydrogen sulphide, and organic acids (from peat or pollution).

Corrosion can perforate metal screens and casings, weakening the structure and allowing pollutants (or even gravel pack material) to enter the borehole. As has been mentioned before, incrustation or corrosion can be slowed down by installing screens with the greatest possible slot area, to reduce pumping rates and inlet velocities, and by periodic cleaning or redevelopment of the borehole.