Disperse the disinfection solution evenly throughout the well, including the bottom of the well.
By Michael J. Schnieders, PG, PH-GW
Well disinfection is a part of every planned maintenance effort.
However, there are many times when we are faced with a potential compromise or failure of the well and an emergency response is necessary. This is most common in flooding situations but can occur for a variety of reasons—vandalism, accidents, and other forms of natural disasters.
Depending on the circumstances surrounding the compromise or failure, other hazards may be lurking at the well site, such as damaged electrical wiring. Before addressing disinfection, you should address these challenges. Once access and power to the well site has been restored, you can safely focus on the issue at hand—well disinfection.
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One issue most commonly faced when performing an emergency well disinfection is many wells have not been maintained adequately prior to the emergency. This puts the contractor in a precarious position and may require much more effort than initially considered.
We’ll focus on the disinfection aspect, but you may need to look at more invasive cleaning or structural repair as part of the emergency response.
The main challenge with a compromise is the introduction of water and sediment that has drastically different water chemistry and microbiology as well as the potential for a variety of contaminants into the well system.
Purging Introduced Material
The first step is to purge much of the introduced material from the well. As needed, restore pump function and pump impacted wells clear for several hours to remove introduced water, sediment, and possible contaminants. Do not pump the evacuated water through treatment and distribution systems, but discharge from the first flushing tap away from the well.
The time required depends on the size of the well, aquifer hydraulic conductivity, and the degree of impact. As few as three hours and as many as 24 may be needed—a reasonable number of hours should be determined by the well size. As a rule, purge the well until visual turbidity is gone and the conductivity has reached a normal level (typically between 500 and 1000 mV).
Following effective purging of the well, a pH balanced chlorination treatment is recommended as a means of disinfection prior to returning the well to active use. Chlorination should use a pH adjusted chlorination treatment of a 250 ppm chlorine level in a pH range of 6.5 to 7.0. Using 10% strength sodium hypochlorite, this is equal to 1.25 gallons of chlorine per 500 gallons of treatment volume. The volume of the disinfection solution should be equivalent to three times the standing static volume of the impacted well.
The preferred concentration of chlorine to use varies based on the well structure, cleanliness of the well prior to disinfection, as well as the background water chemistry. Based on research conducted on fouled well systems, it is not recommended to use more than a 500 ppm chlorine concentration.
Chlorine in its gas form, as used in water treatment facilities, is the most effective form of chlorine, but its safe use and application are challenging at a well site. Calcium and sodium hypochlorite solutions are far more common for use in the field and, in particular, water well disinfection. Sodium hypochlorite in its liquid form is the more preferred chlorine solution—it goes into solution better and doesn’t contribute additional calcium downhole. Calcium and sodium hypochlorite products degrade over time and at a faster rate when exposed to direct sunlight, air, organic material, or temperature fluctuations.
You may note in the treatment recommendations pH adjustment was specified. Calcium and sodium hypochlorite solutions are buffered for safety in use and transport to a pH range between 9 and 10.5. The biocidal activity of chlorine is severely controlled by the pH of the hypochlorite solution.
A slight change in pH can result in a large reduction in the availability of the most effective chlorine form, hypochlorous acid. At a pH of 6 (of the blended solution), 98% of the chlorine product is available as hypochlorous acid. At a pH of 7.5, the percentage reduces to 40%, and at a pH of 8.5, to less than 4%.
Laboratory research shows the ideal pH for the maximum production of hypochlorous acid is between a pH of 6 and 7. In this range you can maintain 85% to 98% effective activity without forcing the reaction toward the production of dangerous chlorine gas (<4.5) or the less effective hypochlorite ion (>8.5).
There are a variety of methods used to adjust the pH. We typically recommend products that incorporate secondary cleaning and are based on the water chemistry (alkalinity), providing better disinfection results.
Using Disinfection Solution
The disinfection solution should be blended aboveground and introduced into the well by a tremie pipe or similar treatment line. If possible, disperse the disinfection solution evenly throughout the well, including the bottom of the well. Once the solution is placed into the well, it should be agitated to disperse the solution throughout the entire column. Cycling of the pump or pump-to-tank surging is recommended. Following agitation, check the chlorine residual within the well to ensure sufficient strength is present. If the residual has diminished below 150 ppm, add additional sodium hypochlorite to raise it to that level.
Allow the chlorine solution to remain downhole overnight. The following morning, check the chlorine residual within the well to make sure sufficient strength has remained. If the solution has fallen below a 25 ppm residual, additional disinfection or cleaning may be necessary. Once sufficient disinfection has been achieved, pump the well until the chlorine has been evacuated (total chlorine is <0.2 ppm) and all debris has been removed from the well (as identified by visible turbidity).
Following removal of the chlorine, collect a water sample and test for required water quality parameters, including total coliform bacteria presence and nitrates. If major structural damage has occurred to the well, the well is substandard at inspection, or does not respond to chlorine treatment, more invasive cleaning, repair, or replacement may be needed.
Once final disinfection efforts are completed, the well should be returned to an active operating schedule as soon as possible and not allowed to sit idle for any extended period of time.
“Disinfecting New and Existing Wells” Water Well Journal, August 2015
“Responding to Flooded Wells” Water Well Journal, March 2014
Michael Schnieders, PG, PH-GW, is the president and principal hydrogeologist at Water Systems Engineering Inc. in Ottawa, Kansas. Schnieders was the 2017 NGWA McEllhiney Lecturer in Water Well Technology. He has an extensive background in groundwater geochemistry, geomicrobiology, and water resource investigation and management. He can be reached at email@example.com.