Rehabilitation of Relief Wells
Rapidly developing technology in the fields of chemistry and microbiology, as they are related to wells and aquifers, could negate portions of the following rehabilitation techniques, but the items noted are at least broadly covered and represent normal practices. Environmental concerns (past and present chemical usage) also require that certain federal, state and local laws be followed, and rehabilitation techniques may have to be modified to comply with these laws.
Mechanical ContaminationPlugging of relief wells by silts, clays or other particulate media entering the filter pack - either from the formation or through the top of the well - usually is difficult to determine except as indicated by periodic pumping tests. If significant reductions in specific yield are noted, rehabilitation of the well is in order. Mechanical redevelopment of the well similar to that used to develop a new well should be the first step. Overpumping or pumping the well at the highest rate attainable generally is advantageous. Surging and the use of horizontal jetting devices also may produce beneficial results.
Chemical Treatment with PolyphosphatesMechanical plugging of relief wells often is corrected by chemical treatment with polyphosphates. These chemicals act as dispersing agents that cause silt and clay particles to repel one another and calcium, magnesium and iron ions adhering to the particles to remain in a soluble state. The most widely used chemicals for this purpose are the glassy sodium phosphates, which are inexpensive and readily available. The chemicals usually are applied in concentrations of 15 pounds to 25 pound per 100 gallons of water, in combination with at least 50 ppm of chlorine (about 0.5 gallons of 3% household bleach in 100 gallons of water). Phosphate solutions are mixed in a barrel or tank adjacent to the well. The material is best dissolved in small amounts in a wire basket or perforated container in agitated or swirling water. If the material is dropped directly into the tank or well, it will sink to the bottom and form a large gelatinous mass that could remain undissolved for some time. One of the most effective means of introducing the phosphate and chlorine solution into the well is by means of a horizontal jetting device. The well should then be surged vigorously prior to pumping. Three or more repetitions of injecting, surging and pumping over a two-hour to four-hour cycle will be much more effective than a single treatment with a longer detention time.
Chemical IncrustationsIf the cause of reduced well efficiency is determined to be chemical incrustation, more frequent cleaning and maintenance should be initiated. If the efficiency remains low, consideration should be given to treating the well with a strong acid solution, which can chemically dissolve the incrusting materials so that they can be pumped from the well. Acids most commonly used in well rehabilitation is hydrochloric acid, sulfamic acid and hydroxyacetic (glycolic) acid. Acid treatment should be used with caution on wooden screen wells, as the acid may tend to attack the lignin in the wood and cause severe damage. Acid treatment methods require great care and only experienced personnel with specialized equipment should be employed.
Bacterial IncrustationIncrustation of wells by iron bacteria is best controlled by a combination of chemical and physical treatments. Many chemical treatments have been suggested and applied in practice but their success has been variable as evidenced in many cases by recolonization or regrowth in the treated wells. A strong oxidizing agent such as chlorine is widely used to limit the growth of iron bacteria. Chlorine, in the form of a gas, is used in the restoration of commercial wells; however, safety and experience requirements limit its general application. A more convenient alternative is the use of hyperchlorite or other chlorine products.
Recommended TreatmentAs clogging of well screens and filter materials is caused not only by the organic material produced by the bacteria but also by oxides and hydroxides of iron and manganese, better results usually are obtained by treating the well alternately with a chlorine compound to attack the organic material and a strong acid to dissolve the mineral deposits. Between each treatment the well is pumped to waste to ensure that chlorine and acid are not in the well at the same time. A recommended procedure using the two procedures is:
1. Inject a mixture of acid, inhibitor and wetting agent. The addition of a chelating agent such as hydroxyactic acid may sometimes be beneficial. An inhibitor is needed only if the well screen is metal. The amount of acid typically should be one-and-a-half to two times the volume of the well screen. If a chelating agent is not used, iron will precipitate out if the pH rises above 3. The precipitate can result in clogging; therefore, the pH should be monitored throughout the acid treatment and not be allowed to rise above 3, regardless of whether or not a chelating agent is used.
2. Gently agitate the solution with a jetting tool at 10-minute intervals for a period of one to two hours.
3. Pump out a volume of solution equal to the volume of the well.
4. Determine the pH of solution removed from the well. If the pH is higher than 3, repeat steps 1-3.
5. Allow the acid to remain in the well for a minimum of 12 hours and then pump to waste.
6. Inject a mixture of chlorine and one or more chloric-stable surfactants (e.g., detergents and wetting agents). The concentration of the chlorine should exceed 1,000 ppm.
7. Gently agitate the solution with a jetting tool at 10-minute intervals every two hours for the first eight hours, and then at eight-hour intervals for at least 24 hours.
8. Pump out a volume of solution equal to the volume of the well.
9. Determine the chlorine concentration. If the concentration is less than 10 percent of the original concentration, repeat steps 6-8.
10. Perform a pumping test on the well. If the specific capacity has improved by more than 5 percent, repeat the entire procedure until the specific capacity does not improve by 5 percent.
Specialized TreatmentThe USAE Waterways Experiment Station developed a field procedure for cleaning water wells that provides initial kill of the active bacteria in the well, dissolves the biomass in the screen, in the gravel pack, and some distance into the aquifer, and provides some inhibition of future growth. The procedure was developed using a patented process known as the Alford Rodgers Cullimore Concept (ARCC). The procedures in general include an initial well diagnosis performed with a prepackaged, field microbiological test kit that is designed to give a qualitative indication of the types of bacterial and chemical agents at work in the wells, and a very general indication of the bacterial concentrations. The initial water chemistry also is measured prior to treatment. A treatment then is designed with the information from the tests, targeting the problematic agents with an appropriate set of chemicals. Redevelopment of the wells using the ARCC method is based on the use of blended chemicals and high temperature (BCHT) and is divided into three principle elements of treatment:
Shock - This phase is achieved by adding high-temperature chlorinated water to the well and surrounding aquifer to "shock" kill or reduce the impact of deleterious algae and bacteria. The water is chlorinated to >700 ppm with gaseous chlorine to avoid binders found in powdered chlorine and is applied to the well as steam until the well temperature is brought above 120 degrees F for massive bacteria kill. The chlorine treatment remains in the well for a specified period of time; mechanical surging is used; and pumping follows for the removal of the initial loosened biomass.
Disrupt - This phase is achieved by the addition of chemical agents, acids, surfactants and steam to the well and surrounding aquifer while the well is pressurized. Mechanical surging to break up organic and mineral clogging in the system also is used. The mechanical surging and chemical set time are important during this phase to achieve dissolution of the remaining biomass.
Disperse - This phase of treatment consists of removal of the material that has been clogging the well and aquifer. Acceptance criteria for the well are checked and further cycles are considered or a final cold chlorination treatment is applied for inhibition of any remaining bacterial colonies.