By Mike Schnieders
Summary: Regardless of industry or product proclamations, there’s no universal cure to well rehabilitation. With so much safety and investment at stake, though, it’s paramount that each solution is uniquely designed for the present situation. Here are a few tips to keep the well from going dry.
While it’s not hard to imagine customer complaints of a rotten egg odor, the other effects of well fouling may be less conspicuous. A gradual loss in production can occur over time resulting in a well that’s both ineffective at meeting supply demands, but also too expensive to operate. Production loss is often associated with mineral scale, but can also be a result of a bacterial presence within the well. Screen bridging—the closing off of screen openings by biological slime—as well as heavy growth within the gravel pack can affect production rapidly. A corrosion condition could develop chemically or microbiologically within the lower portion of the well, rapidly degrading screen quality and shortening the life of the well considerably. Water quality is an aspect with an ever-increasing demand to address, both in consumer awareness and federal and state regulations. High turbidity, excessive iron, discoloration and odors are just a few of the lengthy list of parameters required for monitoring of potable water supplies.
Why does fouling occur? This is an often-asked question with many answers. Even though a pump house is clean and the pump runs efficiently with steady production, trouble could be brewing just beneath the surface. Water wells act as a great concentrator, taking on characteristics of multiple aquifer waters, lithologies (or rock types) and even the soil present. Ions and various influences, including biology, from throughout the supporting aquifers converge to a central point—the well. Here in this great gathering location, subsurface conditions are intermixed with air from the surface as well as the gravel pack material, screen and any other aquifer or rock material disrupted by the well. The concentration effect occurs at this point—the convergence of ions, temperatures and bacteria, often with diverse results.
No. 1 culprit
The most common occurrence is precipitation of a mineral scale. Minerals develop within the well such as alkalinity, pH, ion concentrations and temperature are altered to a level when the saturation point is reached and precipitation occurs. Carbonates, such as calcite, dolomite and magnesite, are common to the well environment. Just as frequent are metallic oxides such as iron oxide and manganese oxide. In some cases, sulfates can develop and include calcium sulfate.
Mineral scale development is often enhanced by the presence of a biofilm, slimy polysaccharide exopolymers extruded by sessile-type (or attached) bacteria. Bacteria exude this slime to attach themselves to a smooth surface. Biofilm acts as a suburban community within a well system, developing in numerous locations, sustaining life and rapidly expanding throughout the well environment.
Biofilm can coat gravel pack material and take up pore spaces between the granules, thereby decreasing porosity and water flow. Matter trapped in the biofilm matrix won’t readily release, requiring special treatment procedures. Present throughout nature, biofilm is an excellent source for adhesion of minerals within a well system. Biofilm can thus promote mineral buildup by providing an excellent surface for adhesion.
Biofilm and all it brings
Furthermore, biofilm can harbor troublesome bacteria. Sulfate reducing bacteria—anaerobes that reduce sulfate and produce hydrogen sulfide—can cause taste and odor problems with well water while corrosive environments “down hole” tend to inhabit biofilm. Coliforms, used as an indication of contamination, can often mask themselves by residing within biofilms and require additional efforts for disinfection. Biofilm isn’t exclusively one type of bacteria but a mixture of anaerobic and aerobic bacteria that can exist throughout the well system and nature.
Sessile, or attached, bacteria are more abundant, but a variety of bacteria exist throughout the well. Planktonic, or free-swimming, bacteria abound within the open casing environment of the well. Aerobic bacteria, which require oxygen, exist in the upper portions as well as in any aerated zones. Anaerobes, bacteria that exist in anoxic (or oxygen-depleted) conditions, can be found in the deepest regions of the well in addition to areas around clay lenses or other aquitards where flow is restricted. Sumps—static areas in the bottom of the well which are commonly added to well designs—can be excellent locations for anaerobic bacteria buildup as conditions become static and little oxygen reaches the area. Iron-oxidizing bacteria are stalked bacteria that utilize iron as an energy source and secrete an iron oxy-hydroxide mass that can be very problematic in wells. Red water, metallic taste and slimey, stringy masses are commonly associated with iron bacteria problems. Corrosion is also an unwelcome problem associated with these bacteria. The iron oxy-hydroxide stalks can rapidly bridge screen openings and reduce flow in the system as well as cover the borehole wall.
Affecting new and old
Although highly dependent on the aquifer, wells can be susceptible to the infiltration of fine-grained sediments such as mud and silt. Often this occurs in older wells, but new wells placed in adjoining silt beds can be just as likely to develop this problem. These fines plug pore throats and open spaces in the gravel pack as well as screen openings, reducing flow into the well and increasing the energy spent in pumping. Residual drilling mud remaining within a well after development can aid in this type of fouling by providing a starting point for fines accumulation.
Often a well rehabilitation project follows a predetermined method or recipe based on contractor or operator experience. Commonly, little regard is given to the actual conditions occurring down hole, or that the possibility that differences do occur. When a patient visits the doctor, the doctor has a large selection of medicines available to treat the variety of conditions that the patient may have. The same is now true of the well professional. With proper diagnosis, almost any well can be saved and production restored, even increased beyond original pumping rates.
The first step for any well rehabilitation should be identifying the problem. This step includes an investigation of pumping and use records and a laboratory analysis. The analysis should include inorganic chemistry and a microbiological analysis. The analysis will determine the actual problem occurring within the well and then design the correct treatment. Treatment may require mechanical work or be more involved and could include chemical rehabilitation as well as mechanical methods. The use of laboratory analysis is often disregarded as wasted funds, but with municipal well rehabilitation projects ranging in cost from $50,000 to $150,000 or more; a small investment could ensure that the correct procedure is followed to address the actual problem.
Consult the contractor
With the investigation complete, the well rehabilitation should be continued with use of an experienced contractor. An experienced contractor shouldn’t only understand well construction but also chemical cleaning and the correct and safe use of chemicals and their disposal requirements. The “dump and run” method of applying well treatment chemicals won’t address a stubborn biofilm and likely will be ineffective against mineral deposits. Proper chemical use, paired with correct mechanical application, is essential to success of a rehabilitation project.
Use of biodispersants, relatively new to the well market, has increased the success of chemical rehabilitation projects. Biodispersants are utilized in conjunction with an acid or caustic wash to specifically address exopolymers involved in biofilm accumulation as well as improve prevention of the re-precipitation of minerals. Although numerous products have begun to appear on the market, the USFilter Johnson Screen product NW-310 and Layne Christensen’s QC-21 have proven most effective at enhancing well rehabilitation. The proper use and application of these chemicals are essential to their success. It should be noted that these two companies are currently the only ones in the market that provide technical assistance and support in the use of their chemistry, an important consideration for rehabilitation success.
Following rehabilitation, care should be taken in the disposal of waste removed from the well. Regulatory stipulations should be followed, with attention paid to neutralizing the chemical effluent as well as returning the well to normal operating conditions.
Conclusion
Well fouling occurs for a variety of reasons. Traditional efforts at a universal solution to curing problem wells are no longer acceptable. Investigation and the design of a proper treatment process are essential for the success of a well rehabilitation project. Just as important is the choice of chemistry utilized and use of an experienced contractor. With careful attention paid to both the problem and solution, wells can be rehabilitated and restored to acceptable operating levels.
About the author
Mike Schnieders is a hydrogeologist with Water Systems Engineering Inc., of Ottawa, Kan. His work involves troubleshooting, problem resolution and technical support of water well and surface water systems utilizing the firm’s specialized laboratory capabilities. The company specializes in the chemistry and microbiology, as well as the distribution and usage, of water production. Schnieders can be reached at (785) 242-6166 or email: [email protected].