Ground water remediation practitioners typically rehabilitate wells in response to the occurrence of significant biofouling rather than using biofouling controls in a preventative manner. Rehabilitation most commonly involves physical swabbing and pumping procedures, coupled with the use of aggressive and potentially hazardous chemicals. Heat may also be used to augment the performance of biocidal chemicals. Rehabilitation is recommended if the well yield, efficiency or specific capacity declines by more than 25 percent, but due to the cost of these procedures, well rehabilitation often is initiated only when well yields decline by 50 percent to 75 percent.
Physical Displacement
Surging with overpumping is a common well rehabilitation procedure. Surging can be performed by using surge blocks or by injecting air in the casing above the well screen. It is labor-intensive, and often requires specialized equipment (e.g., service rigs). Manual brushing also is effective in dislodging material from the well screen and casing. Over-pumping involves removing water from the well, either by bailing or pumping, and allows water from the aquifer to flow into the well, removing any fines or biofilm fragments that were dislodged through surging or brushing.Jetting approaches also may be used to dislodge fines and biofilms from well screens. Jetting is carried out using a perforated jetting tool and a high-pressure water source. Because jetting has the potential to pack debris against the borehole wall, it is coupled with an airlift pump to promptly remove the debris.
Chemical Treatment
Historically, shock chlorination has been used to prevent biofouling. Chlorine is added at concentrations in the 500 mg/L to 2,000 mg/L range, and generally precedes acid treatment. After treating for 24 hours, the chlorinated water is surged within the well, and then pumped out. Purge water with any chlorine residue is pumped to open retention ponds or tanks to allow the chlorine to dissipate prior to discharge to a wastewater treatment facility. Use of chlorine can result in the formation of disinfection byproducts (e.g., trihalomethanes) through reaction with natural organic carbon or other organic compounds present in the ground water.Muriatic acid (industrial grade hydrochloric acid), sulfamic acid and glycolic acid also are commonly used for well rehabilitation. Acids are used to dissolve iron and manganese oxides and carbonate encrustation, and exert an antibacterial effect by providing a pH shock to bacteria typically adapted to neutral pH.
Muriatic acid is a powerful acid, and is most effective for the removal of mineral scale. It is hazardous to handle, requiring field personnel to wear full-body splash protection and respirators, as it can generate toxic fumes. Muriatic acid also can be contaminated with trace levels of arsenic and other metals (undesirable for introduction to ground water environments), and poses purge water-handling problems due to its low pH. In contrast to muriatic acid, sulfamic acid comes as a solid, which is stable and relatively safe to handle and mix; however, it can form ammonia upon dissolution. Glycolic acid, also known as hydroxyacetic acid, is a liquid organic acid, commercially available in 70 percent concentrations as LBA. It is safer to use than sulfamic and muriatic acids, being noncorrosive and producing little or no toxic fumes. Glycolic acid has antibacterial and metal chelating properties, and is particularly suited to attacking iron bacteria biofilms. Being weaker than sulfamic acid, longer contact times are required, which can translate into longer enhanced in-situ bioremediation system (EISB) shutdowns and higher operations and maintenance (O&M) costs.
After an acid is added to a well, water is added to the well to push the acid solution through the screen and into the filter pack and formation immediately surrounding the well. The acid solution is mechanically agitated, left in the well to react with encrustations and biofilms, agitated again, and then pumped to waste. The treatment time varies from a few hours to more than 15 hours, depending of the severity of the fouling and the type of acid used. Acidic purge water requires neutralization prior to being pumped to a wastewater treatment facility, or containerized and disposed of in an environmentally safe manner.
Other Rehabilitation Methods
Hot water (130°F) has been used to augment or replace chemical treatment to kill and disperse iron bacteria in wells. However, heat may enhance bacterial growth away from the thermal shock area, resulting in fouling within the aquifer itself. Heat also can cause shrinking of bentonite grout, adversely affecting well integrity. Other well rehabilitation technologies include Aqua Freed, which involves the injection of cryogenic carbon dioxide, hydrogen peroxide and blended method treatments, such as the blended chemical heat treatment process that incorporates physical, chemical and heat treatments.Advantages and Disadvantages
The advantages of conventional well rehabilitation procedures include restoration of injection well performance (although this usually is temporary) using methods that are relatively straightforward and widely available. For shallow sites (e.g., less than 20 ft. deep), it may be possible to maintain delivery well capacity through simple and cost-effective brushing, surging, jetting and/or overpumping techniques (with or without added chemicals) as part of a prescribed O&M program.The disadvantages of well rehabilitation include the cost, requirement for process shutdown and, in some cases, transient improvement in well performance. For example, in a U.S. Army Corps of Engineers repair, evaluation, maintenance and rehabilitation study evaluating potentially cost-effective rehabilitation techniques for relief wells at the Leesville Dam in Ohio, the rehabilitation procedure employed polyphosphate addition, surging (2-4 hrs.), and shock-chlorination (1,000 mg/L for 12 hrs.), followed by well redevelopment using surging and over-pumping. While this was highly effective in restoring well performance, the application of such intensive rehabilitation measures on a frequent basis (e.g., monthly or quarterly) would increase the operating cost of an EISB treatment system to the point that the technology might not be cost-effective relative to conventional remediation technologies (e.g., pump-and-treat).
At many EISB sites, well rehabilitation can be the most significant operating cost. Well rehabilitation has been estimated to cost in the vicinity of $3,000 to $12,000 per well (depending on well diameter, depth and degree of fouling), when subcontractor, contractor oversight, decontamination and purge water treatment costs are included. One of the greatest indirect costs of well fouling is the loss of well and process efficiency. For example, plugging increases the energy burden of the pump to move the same volume of water, and biofilms immediately surrounding or fouling injection wells can increase nutrient consumption.
Rehabilitation often is only partially successful. In ideal cases, the well may remain unclogged for years. However, it is much more common that performance is maintained only for weeks to months.
ND
This article is provided through the courtesy of the Environmental Security Technology Certification Program. Learn more about innovative, cost-effective environmental technologies at www.estcp.org.
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