Chemicals Used in Water Well Rehabilitation -- Part 1
Chemicals are used in water well rehabilitation to remove products that are either contaminating the well water or blocking or plugging the well environment.
Contamination can be the result of chemical infiltration into the well, such as the leakage of turbine oil into the well environment or the appearance of coliform bacteria from some contaminating source. Well blockage or slowdown can be caused by the mineral buildup on screens or in flow spaces of the gravel pack or formation, and/or the biological buildup on these same surfaces.
Most often a combination of chemicals is used for the rehabilitation of the well. Usually these are chosen because of availability and not because of their aptitude to the specific well problem. Following are some of the general classes of chemicals with a few examples of each. I have added a few comments to this list to help guide you in your selection.
The Mineral Acids
Acids, particularly the mineral acids, are used to dissolve mineral precipitates like calcium carbonate and sulfate, magnesium hydroxide, iron and manganese oxide, phosphates, silicates, and, of course, mixtures of all of these. Of those listed, the carbonates are the most easily dissolved with the resulting release of carbon dioxide. A simple example would be the action of hydrochloric acid on calcite (calcium carbonate).
In this reaction, the acid HCl breaks apart the calcite, (CaCO3) into the salt (CaCl2) and carbonic acid (H2CO3) which further breaks down into water (HOH) and carbon dioxide (CO2), which bubbles off. This reaction is carried out by most mineral acids against carbonate deposits and accounts for the rapid evolution of gas during cleaning of wells with this type of deposit. While the CO2 gases pushing from the well can cause dangerous expulsion of cleaning chemicals, the evolution of gas at the deposit level often results in breakup of the hard scale, and leads to better dissolution of combined deposits not as easily attacked by the acid.
In the above reaction, part of the carbonate molecule is removed from the reaction by the CO2 evolution. In the dissolving of gypsum, calcium sulfate and most other mineral deposits, we are not as fortunate. In these reactions, we see the accumulation of byproducts in the cleaning solution.
The ions or byproducts of the dissolving reactions remain in the cleaning mixture and result in the accumulation of high concentrations of dissolved products. This congested or crowded environment can prevent the complete dissolution of the mineral deposits. It is because of this crowding principal that proper choice of both the acid and the concentration used is necessary for a good rehabilitation job.
The principal of crowding further explains why some acids are not good choices if certain types of mineral deposits are suspected. In general, two factors are important - one, the total concentration of dissolved solids, and the other, the concentration of any specific dissolved solids.
Excessive concentration of dissolved solids can be caused by choosing too high a concentration of acid, or by having too much deposit to dissolve - a factor we cannot control. An excessive level of acid adds to the total dissolved solids, limiting the amount of mineral deposit that can be dissolved. We control this by using a more dilute concentration, thus providing more dilution of the resulting total dissolved solids (TDS) concentration. We can control the deposits to be dissolved by staging two cleaning procedures - divide the cleaning chemistry using the first batch to dissolve part of the mineralization, pump it from the well and use the second batch to dissolve the remaining blockage. In each situation, the TDS concentration will be lower than it would be if all the cleaner had been used in one application.
The most common and useful mineral acids used in water well rehabilitation are hydrochloric, muriatic, phosphoric and sulfamic. Muriatic acid is, of course, an industrial name for hydrochloric and usually designates a concentration of approximately 18 percent.
Hydrochloric Acid (HCl) - Hydrochloric is available commercially in concentrations of 31 percent and the highest at approximately 38 percent. While there are some food grades and NSF-certified hydrochloric solutions available, they are difficult to find. This acid, however, is very effective against many mineral deposits, is very inexpensive and readily available. The purity, however, should always be questioned and a certificate of analysis secured before the acid is used in a potable water well or any well where contamination of an aquifer is a possibility.
Hydrochloric acid is very corrosive to most metals and particularly corrosive to stainless steel because of its chloride content. Since the vapor given off from the acid is hydrogen chloride, it too is very corrosive. When the gas comes into contact with water, such as condensation or water vapor in the lungs, it produces a very corrosive reaction. Because hydrogen chloride gas is heavy, it will settle into lower areas. Therefore, work with hydrochloric should only be done in very open spaces.
Phosphoric Acid (H3PO4) - This acid usually is available in food grade and NSF-certified quality. The two most common concentrations are 75 percent and 85 percent. It does not give off harmful vapors, however, sprays or mists of the acid would be acidic and dangerous. It is far less corrosive to metal than hydrochloric and can lead to some passivation of the metal with the proper chemistry. It is a slower reacting acid than hydrochloric, and, while it has less ability to dissolve phosphate (because of the similar ion concentration), it is very effective against iron and manganese compounds because of its ability to sequester these metals. Its sequestering ability also leads to a greater ability against large concentrations of calcium and magnesium minerals.
Sulfamic Acid (H2NSO3H) - Another chemical name for this acid is amidosulfonic. Sulfamic acid is moderately soluble in water, being less soluble in warmer water. It is a strong acid and reacts very quickly against carbonates. However, the sulfamate ion produced by the acid hydrolyzes over time to a sulfate, rendering this acid almost ineffective against gypsum or other sulfate deposits. The acid should be dissolved in water prior to addition to the well as it requires good mechanical mixing to be put into solution. Powdered acid added directly to the well can remain in the bottom of a well for long periods of time before it is fully dissolved. Usually, the acid is reserved for smaller systems since large wells would require considerable acid and handling large amounts of dry acid could become quite a problem.
Other Mineral Acids - Many other acids are available, but because of their chemistry - such as the sulfate portion of sulfuric or the highly oxidative nature of nitric, HNO3, with its dangerous off-gassing of oxides of nitrogen - they are unsuitable for the cleaning of well systems. Hydrofluoric (HF) has been used where heavy silica or silicate blockage has been suspected. It is a relatively weak acid in an aqueous solution. However, it is extremely toxic and corrosive to skin and eyes and is therefore used only at very low concentrations mixed with other acids such as hydrochloric. It should only be used by experts in the handling of this type of chemistry.
While the term organic acid may refer to any organic substance that imparts hydrogen ions into solution, generally those acids that are characterized by the presence in the molecule of a carboxyl group (COOH) are useful in well cleaning. The acids of this group most often used in well rehabilitation are acetic, citric, hydroxyacetic and oxalic.
Acetic Acid (CH3 COOH) - Generally supplied as glacial, acetic acid is an excellent solvent for many organics, phosphates and some sulfate compounds. It should not be used on wood-cased wells, as it delignifies the wood fiber. It may be used at low levels of 3 percent to 5 percent to improve removal of organic contaminates or to enhance phosphate removal. Glacial acetic is very corrosive to the skin and produces a pungent vapor that can cause mild to severe lung damage.
The empirical formula is C2H4O2 and it is a mono carboxylic acid. In its more mild form, it is known as vinegar, but in higher concentrations, it should be treated as a strong, dangerous acid.
Hydroxyacetic Acid (HO) CH2 (COOH) - Also known as glycolic acid, it is sold as a 70 percent solution. It is a mono carboxylic acid with one hydroxyl group. Up to a few years ago, it was registered as a biocide, proven effective against microorganisms. Hydroxyacetic acid's formula is C2H4O3. It is a weak acid, considered a mild irritant to skin or mucous membranes. It's excellent as a buffering acid for the control of pH during chlorination. Generally, it is used with mineral acid to improve iron solubility at a 2- to 5-percent concentration in the total well volume.
Citric Acid (HO) C-(CH2)2-(COOH)3 - This is a crystalline powder sold as 100 percent acid. It has three carboxyl groups that may be responsible for its effect on phosphate solubility. It is very soluble in hydrochloric acid and can be used with this acid to improve solubility of phosphates and iron deposits. Solutions of citric acid will support bacterial growth. In general, the acid is weak and should always be used with a stronger mineral acid. It is usually used at 5- to 10-percent concentrations of the well volume.
Oxalic Acid (COOH)2 - Oxalic acid is a strong, reducing acid, and as such, is excellent against oxide of iron and manganese. The acid is a granular product sold at approximately 99 percent active ingredients in 50-pound bags. It is corrosive to the skin, and salts of this acid are poisonous. Exceptional care must be taken to remove all traces of the acid from the well system. Oxalic is a carboxylated acid with two carboxyl groups. It is more soluble in warmer water. Its use rate is between 5 percent and 10 percent by weight in the well volume.
Alkalies or caustics occasionally are used in water well cleaning as the base chemistry to remove oil contamination and heavy biological fouling. Caustic will have no effect on mineral deposits, and they could create considerable precipitation if dispersants or specific polymer chemistry is not employed to prevent crystallization, agglomeration and deposition. Caustics usually used for this purpose are sodium hydroxide and potassium hydroxide.
Alkaline products also are used to neutralize acid cleaners before disposal, following well pump out of the acid cleaning solution. The products usually used for this reaction are lime, soda ash and magnesium hydroxide.
Sodium Hydroxide (NaOH) - This product is known commercially as caustic soda and is available as a 50-percent solution. It also is sold as a 100 percent active pellet, but it is considerably easier to use the solution as dissolving of the pellets cause a severe exothermic reaction which can be extremely unsafe. The solution also will produce considerable heat when being diluted. As with strong acids, always add the concentrate to the water and not water to the product being diluted. Sodium hydroxide (NaOH) will provide an extremely high pH - 12 to 14 - even at low concentrations. The usual dosage for most caustics is a 3-percent solution in the total well volume. While sodium hydroxide usually is used more as a cleaner, it could be used to neutralize used acid. However, never add the concentrated material directly to a strong acid solution. Dilute the caustic to a maximum 10 percent concentration before using for neutralization.
Potassium Hydroxide (KOH) - Known commercially as caustic potash, it is available in both a pellet and a 45-percent solution. The solution is by far the easier product to use. The pellets, as with sodium hydroxide, produce considerable heat when diluted with water. Since the 45-percent solution also produces heat, it, too, should be handled with care. Potassium hydroxide has some advantages over sodium hydroxide because of its solubility and also because it is less corrosive to the metal structure of the well. It is used at approximately 3 percent of the total well volume as a cleaning solution and can be used as an acid-neutralizing agent. However, the same precautions apply.
Calcium Oxide or Lime (CaO) - This product is available in powder or granular form and is used primarily to neutralize acid cleaning solutions following pump out of the well. Often, it is mixed directly into the acid solution if the spent solution is in a holding tank, or the lime is placed in a trough that provides for neutralization as the acid solution is pumped to disposal. The greatest danger in the use of lime is the heat produced if the product is dissolved in water, and the fine dust that is extremely corrosive to eyes and mucous membranes.
Soda Ash (Na2 CO3) - This product is 99 percent sodium carbonate granular and is used primarily to neutralize discharged acid following well cleaning. It is available in 50- and 100-pound bags. Because it is a carbonate, considerable carbon dioxide will be liberated as the acid is neutralized. Care should be taken so this release does not cause splashing or the dispersal of the cleaning solution, causing a safety concern.
Magnesium Hydroxide (Mg (OH) 2) - Magnesia, as the powder is often called, is available in a 50-percent slurry commercially and is an excellent product for the neutralization of acids used in well cleaning. The product imparts only a slight alkaline reaction to the water, so overuse will not produce a pH higher than 9.5. This is very important where discharge is being monitored for high pH release.