Scientists Don't Know Best Way to Disinfect a Well

A couple of weeks ago I replaced my well pump, the wiring and some fittings. After opening a well or doing any work it is recommended that the well be disinfected by shock chlorination. Shock chlorination is also recommended when a well is flooded or tests positive for bacteria.  The problem is there is no standard rigorously tested method of disinfection though there are many ways that well water can become contaminated with bacteria. For example, improper construction or completion of the well. The well might not have a sanitary well cap, the well might not be grouted. The well might have become damaged over time, for example the well casing may develop holes from rust or cracks or the grouting might be damaged by time or accident. These deficiencies can provide direct pathways for surface water to infiltrate the well and increase the likelihood of microbial contamination. Another way a well can become contaminated is if the groundwater itself is contaminated by a nearby failed septic system or in aquifer in Karst terrain that is under influence of surface water.

During well disinfection, free chlorine is introduced into the well water; however, there is no one standard for how to accomplish this disinfection. Based on a survey of emergency disinfection protocols performed by Dr. Kelsey J. Pieper et. al and published earlier this year “Improving state-level emergency well disinfection strategies in the United States”, there is no single method.

The researchers searched for Emergency well disinfection protocols to be used in the case of flooding. They found well disinfection protocols for 43 of the 50 states. For the seven states without an emergency protocol, five of the states had routine disinfection protocols and two states had disinfection protocols in their well construction regulations. Emergency well disinfection protocols from 34 states were reviewed based on instructions for creating chlorine solutions; circulating chlorine solutions throughout the distribution system; maintaining adequate contact time and post-disinfection guidance.

The scientists found that many protocols were missing key information about fundamentals of disinfection. Only two protocols instructed well users to verify chlorine residuals and three protocols instructed users to measure water pH. Most protocols recommended that high chlorine doses be introduced into the well, circulated throughout the system, and stagnated for several hours. It is important that residual chlorine be measured because if too much of the chlorine solution reacts with iron or organic substances the effectiveness for disinfection is reduced. Likewise if the pH of the well water falls outside of the ideal range of 6.5-7 the formation of HOCl will be impaired and the disinfection will be less effective.

Although there is widespread use and data on the efficacy of chlorine-based disinfectants, little research has been done to evaluate the efficacy of well disinfection strategies. Of the 8 steps identified by the scientists, the reviewed protocols contained between 2 and 7 steps, and no protocol included all 8 steps.

1. Determine  chlorine volume based on well characteristics to add to the well
2. Measuring water pH before adding chlorine solution and after
3. Pump contaminated water out of the system
4. Circulate the chlorine solution through both the well and home plumbing systems and
5. Measure the chlorine residual to ensure there is adequate chlorine to disinfect the well
6. Have adequate contact time to disinfect the well and that inactivates Cryptosporidium (for flooding)
7. Chlorine solution should be removed from the well and home plumbing
8. Water should be tested for confirmation of microbial reduction

As the scientists point out “ several studies have highlighted that emergency and routine chlorination methods do not always reduce total coliform and E. coli bacteria in well systems (Branz et al., 2017; Cavallaro et al., 2011; Garandeau et al., 2006; Luby et al., 2006; Rowe et al., 1998; Swistock and Sharpe, 2005). Since the concentration of chlorine ...is assumed to be that of the added disinfection solution, any disappearance of chlorine from the water via chemical reactions would cause disinfection efficacy to be overestimated. For example, reactions with high levels of organic matter, ferrous iron, and manganese in water can cause chlorine to disappear quickly (Cavallaro et al., 2011; Garandeau et al., 2006; Luby et al., 2006; Oliphant et al., 2002). Moreover, there are concerns about well users performing disinfection steps correctly (Eykelbosh, 2013). Lastly, researchers have highlighted the high variability and uncertainty associated with drinking water grab samples. For instance, microbial detection rates are higher when wells are sampled more frequently (Atherholt et al., 2015). Despite these challenges, officials continue to promote well disinfection protocols during both routine and emergency conditions because there are no other practical alternatives.”

The bottom line is that the protocols for well disinfection needs to be studied and improved. Most of the protocols recommended some version of high chlorine concentrations should be introduced into the well, circulated throughout the system, and stagnated for several hours. The optimal conditions were not identified. Dr. Pieper et. al. identified two research gaps; determining whether chlorine doses should differ based on well water chemistries and evaluating the appropriate chlorine dose that should be recommended for inactivating pathogens.

Read the full article:

Pieper, Kelsey & Rhoads, William & Saucier, Leslie & Katner, Adrienne & Barrett, Jason & Edwards, Marc. (2020). Improving state-level emergency well disinfection strategies in the United States. Science of The Total Environment. 720. 137451. 10.1016/j.scitotenv.2020.137451.