By Andrew Liao

Summary: Treating a water well is dependent on many factors. First, consider the site and its unique surroundings. Then, rehabilitation can commence, but only if certain factors such as acids and blockage types are also take into account. The following serves as a primer for water treatment dealers looking to improve well efficiency.


A contractor from Columbia City, Ind., discussed a potential well rehabilitation project for a water well that had been in service for more than 30 years. Both yield and specific capacity of this well had declined steadily in the past two years. The water quality also showed signs of deterioration such as water that was tinted light brown and sometimes smelled like rotten eggs. These are typical indicators of a well infected by either metal encrustation or bacteria or both.

This article focuses on well rehabilitation procedures, well-cleaning chemicals and effective chlorination.

Blockage types
Water flow path blockage should be one of the major factors attributing to yield decline of a well. Generally, there are at least three types of blockage—chemical, biological and physical.

Chemical blockage is due to formation of mineral scales and encrustation. Groundwater often contains high concentrations of dissolved minerals such as calcium, magnesium, iron and manganese. Some aquifers contain high concentrations of carbonic acid, produced from dissolved natural carbon dioxide. Carbonic acid helps keep minerals as ionic species in solution. During pumping, a portion of dissolved carbon dioxide is lost through increased velocity and decreased pressure, causing increase in water pH. When groundwater pH rises, it increases the tendency for a portion of the dissolved species to precipitate out of water as carbonate scales such as calcium and magnesium carbonates. Sulfate ions are also present in groundwater. When sulfate ions exceed 70 parts per million, they can precipitate as calcium or magnesium sulfates in the same manner as carbonate.

Weighing the scales
Carbonate scales are easier to dissolve by acid treatment than sulfate scales. Iron and manganese can also precipitate as iron or manganese sulfates, depending on their oxidation states. Scales with a combination of iron and manganese are harder to dissolve and might require a prolonged acid treatment.

Biological blockage is due to formation of at least three major microorganisms: slime-forming bacteria, iron-reducing bacteria and sulfate-reducing bacteria. They’re naturally occurring, common soil bacteria found in every aquifer. Many of these are highly mobile and can be either aerobic or anaerobic. Aerobic bacteria thrive in oxygen-rich areas in a well; for example, the high-velocity sections of a screen during pumping, at the static water level or in cascading water. On the contrary, aerobic bacteria-like areas of low oxygen may include non-pumping wells, low permeable zones, sumps (or static areas) beneath screens in wells, or beneath large amounts of scale and slime. Oftentimes, a submersible pump is pulled out of a well covered with slime and iron-reducing, bacteria-induced encrustation. It happens to a well screen in the same manner.

Detecting bacteria
Formation of biofilm can also clog water flow paths. It has been documented that iron bacteria produce a stalk-like structure—often filled with slime-producing bacteria—which increases the severity of the clogging problem. Sulfate reducing bacteria are anaerobic in nature. These bacteria may be found under growth and scale because of the low-oxygen environment. Because their locations are somewhat protected, they’re harder to reach. Sulfate-reducing bacteria consume sulfate by releasing not only a corrosive organic acid but also by releasing hydrogen sulfide gas, which smells like rotten eggs and produces ferrous sulfate or oxide scale.

Physical blockage can be caused by accumulation of clay and silt resulting from the migration of fines from numerous sources such as insufficient grave envelope, wrong type of screen or poor screen displacement, poor initial well development, and overpumping. An effective dispersant is needed to disperse clay/silt aggregates.

In addition to flow path blockage, other factors such as well hydraulic change, water chemistry and microbiology change, depletion of aquifer, and well configuration change can also cause decline of well yields.

Four steps to a better well
An effective well rehabilitation is to mitigate flow path blockage by mechanical and chemical means to restore the well to its most efficient condition. A four-step well rehab procedure is recommended.

Use of mechanical tools
First, pretreatment using a mechanical tool can dislodge mineral encrustation and bacteria deposits in easy-to-reach areas. Pretreatment using a mechanical tool also helps open more area in the well for chemical contact. The major benefit of mechanical pretreatment prior to chemical treatment is to reduce chemical consumption and allow chemicals to penetrate better into areas that need to be cleaned. Several common mechanical tools used for well cleaning include wire brushes, dual disk swab with or without brush, surge block, and airlift pumps with such features as dual disk swabs to sonar jetting. All debris should be bailed or airlifted out of the well prior to chemical treatment.

Chemical treatment
Second, chemical treatment in well rehabilitation commonly refers to acid treatment. Acid treatment should be site specific. Before the actual treatment, one must consider the following factors—type of acid or blend of acids to treat type of encrustation, volume of acid solution, method of displacement, surface equipment for mixing the acid(s), tanks for containing debris after bailing, and tanks for neutralizing waste acid solution before disposal. Personal safety equipment for handling acid and working space with adequate ventilation must also be part of an acidization plan (see Extra).

One can use the following two simple equations to roughly estimate the amount of solution needed for one treatment to clean a well screen or casing.

  • (diameter of well screen or casing in inches)² ÷ by 24.5 = gallons per foot (gal/ft)
  • (gal/ft) × (length of screen or casing in feet) = total volume of solution

For a screen with gravel envelope, it’s advised to multiply the total volume of solution from the second equation with a factor of 1.5 to 2 to obtain a sufficient solution volume.

Monitoring acid solution pH every four to six hours is one method to determine effectiveness of an acid treatment. As acid dissolves scales and encrustation, the acid becomes neutralized and pH will rise. Once the acid solution pH rises above 3.5, the strength of the acid(s) is diminished and the time required to dissolve a given amount of encrustation becomes much longer. Once pH rises about 3.5 during acid treatment, it’s advisable to replenish the acidizing solution with fresh acid(s) to lower the solution pH to 1.2 or less.

Repeat the process of checking the pH and replenish with fresh acid throughout the acid treatment, if necessary. The treatment is complete when the solution of pH hasn’t risen above 3.0 for four to six hours or if it has risen much more slowly than during prior acid replenishment.

Pump the acid solution from the bottom of the hole until the pH is within 0.5 of the original value of acid put into the well. Neutralize the waste acid solution with either soda ash (preferred) or lime. Check with local regulations on the pH window for disposal. Most allow a pH of 6 to 9 for a non-hazardous disposal classification.

Redevelopment
Next, after acid treatment, a well has to be redeveloped. Redevelopment involves removing all the debris and dislodged deposits from the well and transmitting sufficient hydraulic energy to the screen and gravel envelope or the formation by using mechanical means such as jetting, airlifting or bailing.

An effective dispersing agent, either phosphate based or nonphosphate based (preferred), is recommended as part of an “expert level” redevelopment process.

Chlorination
Finally, using either sodium hypochlorite (liquid) or calcium hypochlorite (solids) for chlorination and disinfection, respectively, one should maintain the chlorine concentration high enough so the chlorine remains several hours after treatment, usually four to 12 hours. The pH of the chlorine solution should be kept at 6 to 7 in order to produce sufficient amounts of hypochlorous acid or “bactericidal” chlorine. Don’t use any strong acid such as muriatic acid for this purpose. The use of muriatic acid might generate toxic chlorine gas when pH drops too low (below 5) and too fast (uncontrollable).

Conclusion
As mentioned, well rehabilitation is site specific. Only a guideline has been given here. Before taking any well cleaning project, one should ask some pertinent questions. For example:

  1. What types of scales/encrustation and/or bacteria deposits are plugging the well?
  2. Where are the encrustation and/or biofouling?
  3. What type of acid or blend of acids will remove the crud and bugs efficiently?
  4. What type of screen has been used and what is its condition?
  5. What types of mechanical tools are available to facilitate the cleaning process?
  6. How was the well constructed (well history), and when?
  7. Where can I obtain information on the groundwater chemistry for the well?
  8. What are the local regulations on fluid disposal?
  9. What is the estimated rehab cost?
  10. Can I do the project safely?

There are still other questions one may ask. The list given here is a good start.

About the author
Andrew Liao is product champion for Baroid Industrial Drilling Products, of Houston, a product and service line of Halliburton Energy Services Inc. He has been with the company for 16 years. This article first appeared in Water Well Journal in July 2001.


EXTRA—Types of acids
The pros and cons of four acids commonly applied for well cleaning are briefly described here.

Muriatic acid
Also known as hydrochloric acid. This very strong acid provides fast and violent chemical reaction to dissolve carbonate scales and metal encrustation, particularly oxides of iron and manganese. Muriatic acid without NSF certification often contains trace heavy metals. The acid doesn’t remove biofilm efficiently.

Sulfamic acid
A granular acid (not a liquid) and also known as amidosulfuric acid. It provides moderate chemical reactions and takes a longer time to dissolve scales and encrustation. It’s safer to use than muriatic acid, as it’s not very corrosive and produces no hazardous fumes. As with muriatic acid, sulfamic acid doesn’t dissolve biofouling very effectively.

Phosphoric acid
A mild acid that contains phosphate ions, which might cause some environmental concerns while discharging to a storm sewer after the acid is neutralized. It’s somewhat effective in dissolving iron and manganese oxides. Phosphoric acid doesn’t remove biofilm efficiently.

Glycolic acid
Also known as hydroxyacetic acid. This acid can effectively disperse and remove biofilm. The chemical reaction is slow but creates no hazardous fumes.

Acid combination and delivery
To effectively remove both biological deposits (bugs) and metal encrustation (crud), treatment involving a combination of two acids is a common practice. For example, one recommendation involves the following recipe for making a blend of acids for well cleaning.

  • 100 gallons of make-up water
  • One unit (5-gallon pail) of granular acid (50 pounds per pail; about 6 percent by weight)
  • One unit (5-gallon pail) of liquid acid (about 5.6 percent by volume)

The above combination of acid solution has been successfully applied to several well-cleaning operations. The acid solution can be delivered through a tremie line just above the screen, which is about the area that needs to be cleaned.

Reprinted by permission of Water Well Journal. © Copyright 2001.

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