While the expected service life of a well depends upon the design, construction, development and operation of the well, proper maintenance helps to improve the performance and increase the life of the well.
Proper records of power consumption, well discharge, drawdown, operating hours, periodical chemical analysis of water and other such observations will help in devising proper maintenance procedures.
The sudden pressure drop and increase in the entrance velocity near the screen due to high pumping rates releases carbon dioxide and causes precipitation of calcium carbonate and iron deposits near the screen.
The change in entrance velocities results in precipitation of iron and mang anese hydroxides.
The presence of oxygen in the well can change soluble ferrous iron to insoluble ferric hydroxide.
The perforations can be cleaned by adding hydrochloric (muriatic) acid or calgon followed by agitation and surging which removes the incrusting deposits.
Normally, the volume of acid required for a single treatment will be about 1.5 to 2 times the volume of water in the screen.
Sulphuric acid can also be used instead of hydrochloric acid but its action is a little slower and requires a longer contact time in the well.
Fig. Acid Treatment
The yield of the well may decrease due to the deposition of incrustation of fine particles of silt and clay near the screen.
This can be removed by the use of a dispersing agent such as polyphosphates. For effective treatment, 15 to 30 kg of polyphosphate is added to every 1000 litres of water in the well.
1 kg of calcium hypochlorite should be added for every 1000 litres of water in the well to facilitate the removal of iron bacteria and their slimes, and also for disinfection purposes.
The solution of polyphosphate and hypochlorite is poured into the well and a surge plunger or the jetting technique is used to agitate the water.
The well may be treated 2-3 times for better results.
The perforations may become plugged with algae or bacterial growths.
Chlorine treatment of wells has been found more effective than acid treatment in loosening bacterial growths and slime deposits which often accompany the deposition of iron oxide.
Since a very high concentration of 100 to 200 ppm of available chlorine is required, the process is known as shock treatment with chlorine.
Calcium or sodium hypochlorite may be used and the chlorine solution in the well must be agitated by using the high velocity jetting technique or by a surge plunger.
Faulty well construction such as poor casing connections, improper perforations or screens, defective gravel packs and poorly seated valves should be located and set right immediately.
Sudden failure of a casing pipe or strainer, resulting in the entry of sand, will require replacement of the well as a whole.
An additional precaution against corrosion of the screen is provided by cathodic protection by suspending in the well a rod of a metal low on the electrochemical scale, such as magnesium.
This rod will corrode instead of the metal perforations or the screen, and can be replaced when necessary from time to time.
Development methods by compressed air or dry ice are sometimes effective in cleaning up corrosion deposits.
Depletion of ground water supply can sometimes be remedied by decreasing pumping drafts, resetting the pump or deepening the well.