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Field Evaluation of Emergency Well Disinfection for Contamination Events
Stuart Smith, MS, CGWP

This article summarizes recently published research on emergency well disinfection practices following large scale flooding caused by Hurricane Floyd. The work includes recommendations for state and local emergency management agencies to plan for, and respond to, such events. The author is a partner with Smith-Comeskey Ground Water Science in Upper Sandusky, Ohio.

In September 1999, Hurricane Floyd caused extensive flooding in North Carolina and adjacent Atlantic coastal areas. In the flooding, thousands of homes were made uninhabitable and 48 people died. Potential fecal and pathogenic bacterial contamination of wells due to their immersion by floodwaters was identified by state authorities as a significant health risk. To restore many of the 12,000 affected wells in North Carolina (over 2000 of which showed total coliform positive (potentially unsafe) results), wells were disinfected in the affected areas. However, a significant fraction of wells were still not providing coliform-free water after multiple treatments. Treatments seemed to be ineffective, probably due to:

  • Floodwaters contain very high loads of sediment, debris, and chemical and biological contaminants.
  • Immersion can force contaminants into the aquifer formation.
  • Also, a significant number of wells have construction deficiencies.

To improve response to such large-scale flooding events in the future, the U.S. Federal Emergency Management Agency (FEMA) asked the National Ground Water Association (NGWA) to investigate and recommend procedures and protocols for emergency well disinfection that it can recommend to state and local emergency management agencies. Smith-Comeskey Ground Water Science, and well service specialists EGIS P.A. of Chapel Hill, NC conducted this work on behalf of the NGWA. The NGWA formed a Peer Review Team who advised on the project as needed. The North Carolina Department of Environment and Natural Resources also assisted in this work.

To develop these plans and recommendations, the project team conducted a two-step investigation:

1. Background phase: This phase included a literature review and a survey of disinfection providers nationally and in NC.

2. Field phase: This was a field evaluation of well disinfection methods, involving identification and testing of potential candidate wells in NC, and disinfecting a representative sample and evaluating results. This task was undertaken in 2002 in Edgecombe and Pender counties, NC. Those disinfected were bored and drilled 2-in. wells in Edgecombe County and 2-in. drilled and driven wells in Pender County.

Literature Review and Survey

There is abundant advice published on the practice of well disinfection. On balance, published advice tends to be based on the incidental experience of case histories. Officially published well disinfection procedures seem to be simply copied from source to source since the 1950s with little evaluation of efficacy. Recently, work in Illinois and Michigan has resulted in the first published systematic studies of well disinfection.

The survey helped to narrow the search for wells to the counties that reported actual well effects. Most NC counties affected reported that multiple treatments were needed to achieve coliform-negative results. Among the contractor, expert, and health personnel respondents, a wide range of method chemistry and application was reported.

Field Testing Phase

Wells tested in the study areas were analyzed for physical-chemical parameters in the field and sampled for analysis of total coliform bacteria and heterotrophic plate count. Wells were also sampled for indicators of microbial ecology that may affect chlorination (culturing by BART Method, Droycon Bioconcepts Inc.). Profiles of area water quality were established, which were also relevant to disinfection treatment chemistry.

Because of the potential for harm to functioning potable water wells, and the availability of abandoned but unplugged wells on "FEMA buy back" properties in the study areas (in close proximity to tested wells), the field team made the decision to seek to use FEMA wells for treatment experimentation. Wells available included shallow bored wells and a deep two-inch well in close proximity to one another in Edgecombe County and two-inch wells in Pender County, also in close proximity to tested wells both with and without reported disinfection problems post-flooding in 1999.

Disinfection methods were designed to 1) incorporate recent recommendations on solution chemistry (maximizing disinfecting hypochlorous acid (HOCl) in solution) but trying both solid and liquid hypochlorite products, 2) test the effects of improved application (brushing dug wells, development and mixing in others) and 3) be achieved with components available off-the-shelf from hardware or home-improvement stores.

Conclusions and Observations

Water quality

The well water quality could be profiled readily with field instruments. Such profiles are useful in identifying the aquifers and zones tapped by wells, especially when little other information is available. For example, wells in vulnerable aquifers, or wells that will have higher chlorine demand during treatment, can be identified.

Total coliform (TC) and other biological results were site-specific. TC results from the reconnaissance phase were generally negative, but positives were common for the shallow bored wells. The only E. coli results were from a shallow bored well in Edgecombe County. After almost three years since the 1999 inundations, surface-derived coliforms may have declined below detection by standard methods in other wells. However, biological activity reaction test (BART) results indicated the presence of bacteria known as "environmental" coliforms (known to be native to aquifers but part of the TC bacteria group) and other heterotrophic bacteria in high numbers in all the tested wells. The BART profiles suggest that a residual effect of inundation on microbial ecology still persists.

Such ecological profiling by BART methods could potentially be an easy-to-use and cost-effective method that is applicable to widespread study of the long-term effects of events such as aquifer inundation, and in designating vulnerable areas.

Disinfection procedure test results

Disinfection methods selected and tested on both shallow bored wells and 2-inch wells were generally successful in producing disinfecting conditions despite well faults:

1. Solutions of chlorinated chemicals achieved desired oxidation reduction potential (ORP) and chlorine residual levels. Maintaining target total chlorine values in two-inch wells required repeated treatment.

2. Acidification aided in forming optimal solutions favoring HOCl. This was accomplished with small amounts of acid, and can be done safely by trained personnel. Dilute acetic acid (e.g., distilled white vinegar) is safe and effective.

3. Mixing was required to distribute disinfecting solutions through water columns.

4. Disinfecting solutions and residual water quality effects can be persistent.

5. Researchers evaluated two forms of chlorine used for disinfection, dry calcium hypochlorite (Ca(OCl)2) granuals and solutions of sodium hypochlorite (NaOCl). Ca(OCl)2 is more easily stored for long periods and drops to well bottoms better, but is harder to regulate in solution. NaOCl is easier to use in mixing solutions.

6. The treatment program was conducted successfully using off-the-shelf equipment and solutions. However, expertise and time are required to assemble the proper equipment and solutions, and to apply them to make these procedures work.

7. Disinfection can be temporary if impaired water can return to the well. Eliminating well construction and structural faults and aquifer-scale contamination is critical.

Recommendations for a well recovery Emergency Response Plan

1. In implementing an Emergency Response Plan (ERP), prepare to respond rapidly with equipment and training, and have people available to respond effectively locally.

2. Emphasize restoring pump function and pumping wells clear for several hours (or more if severely affected by dirty water) as a first step, then go to disinfection, as needed.

3. The involvement of experienced, trained people and effective well cleaning equipment is also crucial for success.

4. In preparation for a future large-scale inundation event such as a large hurricane, implement preventative measures, such as improved well code enforcement, starting now.

5. Baseline data collection is recommended as part of the prevention and response process.

6. Specify a standard disinfection procedure that includes:

  • Clearing the well,
  • Injecting a 100-200 mg/L NaOCl solution (acidified to < pH 6) to the bottom of the well, (note: Ca(OCl)2 may be used during the initial emergency response),
  • Mixing solution throughout the water column.

While not specifically based on the findings of this project work, the following are recommended:

1. Local environmental health personnel would benefit from being trained and equipped to conduct the recommended well reconnaissance, supervise emergency treatment, and provide training oversight.

2. Greater cooperation with and involvement of water well contractors (equipped to work on wells) is encouraged.

3. Additional research and improved publications on treatment under a broader range of conditions, as well as plans to apply results in developing countries are recommended.


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