Responding to Flooded Wells

The revised article includes new chlorinated isocyanurates that are gaining popularity for disinfection.

By Michael Schnieders, PG, PH-GW

This set of samples show the progress made with pumping of a flooded well that sat out of service following impact. The sample on the far left shows heavy turbidity with a lot of iron oxide and sediment present. Pumping of the well over the course of several hours yielded the improvement visible in the samples, moving left to right.

Flooding is the covering or submerging of normally dry land with a large amount of water. Floods can happen during heavy rains, storm surges (coastal or inland), or when dams or levees fail.

Impacts from flooding can occur because of only a few inches of water in a short expanse of time or complete walls of water that will submerge an area for weeks. Worldwide, floods are the most common and widespread of all weather-related natural disasters.

Aside from direct damage to infrastructure or the limiting of access to well systems, one of the challenges our industry faces is dealing with the after effects of wells and watersheds that have been flooded. The occurrence of high-water events can impact a well beyond the immediate submergence of the well head and can alter conditions within the watershed and influence an aquifer for an extended number of years.

Wells that are susceptible to flooding or potential contamination include:

• Older wells completed in areas now designated as floodplains
• Wells in which the casing is not finished above the high-water level
• Wells not adequately capped or sealed, or older wells with shallow or insufficient grout
• Wells in areas geologically unstable, or where erosion or subsidence has compromised the structure and surrounding land.

Any well that has been flooded or is suspected of having been influenced by floodwaters should be viewed as unsafe for potable use until it has been evaluated and tested to ensure it is safe for consumption. Floodwaters carrying debris from the flood can structurally impact the well by damaging or destroying caps, vents, piping, and the well casing and pump components.

Additionally, by destabilizing the ground around a well, flooding can weaken the structural integrity of a well, jeopardizing the sanitary seal.

Floodwaters entering the well introduce not only surface water with drastically different water chemistry and microbiology, but also sediments and a variety of foreign contaminants into the well system. Increased levels of bacteria and microorganisms and other “natural” contaminants like sewage and animal waste can be found in floodwaters. This contaminated water can enter the well casing through the top of the well or through defects and damage in the well’s casing or seal.

Additionally, floodwaters can mobilize environmental contaminants beyond controlled areas, increasing risks associated with contact. One of the most common examples of this is the mobilization of hydrocarbon spills and the appearance of rainbows and oil films on the surface of the water.

The general health concern with regard to floodwaters is the higher risk of infection and diarrheal disease associated with exposure to contaminated water. Floods may also potentially increase the transmission of communicable diseases including both waterborne illnesses like typhoid fever, cholera, and hepatitis, as well as vector-borne diseases like malaria and dengue fever.

In addition to these health risks, floodwaters typically contain an elevated sediment load. Sediments found in the floodwaters can enter the well through vents and wiring conduits and cause mechanical fouling, increased wear of the pump components, and plugging of the aquifer.

Addressing a Flooded Well

It is advisable to document the conditions from the start, and to record all work, noting any damage to the well and surrounding area. Also, document any trash or debris that is present as well as any visible discoloration of the water surface or vegetation.

The first step in responding to a flood-impacted well is to evaluate the electrical supply. Ensure that the power is safely turned off and confirm that none of the components were jeopardized which could cause added problems to the site during treatment. There exists a significant danger of electrical shock, personal injury or death, and damage to the well, the pump, and the water system.

During this step, you can begin to clear access to the well. Depending on the extent of the flooding, this may be a minor or a more involved task. Remove all debris from the well site, including any standing water.

Once you’ve determined it is safe to proceed, begin to pump the well with the existing pump or a back-up. The initial objective is to purge as much of the introduced material from the well. You should pump an impacted well for several hours to clear sediment and floodwater from the borehole. Do not pump this flushed water through existing treatment and distribution systems but discharge it away from the well. The time required is dependent on well size, aquifer hydraulic conductivity, and floodwater depth and quality. As few as three hours or as many as 24 hours may be needed.

It is recommended to purge the well until visual turbidity is gone and the conductivity has reached a normal level (typically less than 1000 mV). Visual turbidity is a means of evaluating a sample visually against an opaque background for signs of contamination or sediment. Conductivity is the measure of the ability of water to pass an electrical current. Conductivity in water is affected by the presence of inorganic dissolved solids such as chlorides, nitrates, sulfate, and phosphate anions (negatively charged ions) or sodium, magnesium, calcium, iron, and aluminum cations (positively charged ions).

As such, conductivity is an easy means of evaluating water quality. Typically, groundwater has a lower conductivity than surface water. Groundwater conductivity values change considerably, so it is advised to secure a general background level from historical test records prior to using it as a means of monitoring progress.

Once the well is flushed of the immediate threat, it may be necessary to conduct a video survey to assess any potential damage and document the well’s condition. This would be an important step if the well was exposed to prolonged influence from the floodwaters or was suspected of having damage and fouling prior to the flooding event. During the video survey, evaluate the upper casing for signs of damage or instability, as well as the lower extension of the well for accumulations of sediment and biological debris.

If prolonged exposure to floodwaters has occurred or if sediment infiltration is suspected, the pump and associated equipment should be removed from the well. At a minimum, place the pump and column pipe on an elevated surface or on plastic sheeting, powerwash, and disinfect using a 50-ppm chlorine solution prior to returning them to the well. Disassembly of the pump is recommended for effective cleaning and removal of biomass and sediment accumulations. After cleaning, the pump and column pipe should be evaluated for corrosion and material degradation that may have occurred as a result of the congested water and sediment present.

Disinfection

Disinfection of a flooded well is recommended regardless of the degree of impaction.

A combined chemical and mechanical cleaning of the well may be required for effective disinfection. Well system owners often postpone maintenance or have a “run-to-failure” mentality, so it is fairly common to have a flooded well needing to be chemically and mechanically cleaned before returning to service.

If cleaning of the well is needed, balance the mechanical efforts with the structural integrity of the well. The chemicals used for cleaning should be selected based on the suspected fouling mechanisms present and the materials used in well construction. As with all well rehabilitation projects, the chemicals selected should be NSF Standard 60 approved for their use in potable well systems.

After the well has been purged of floodwaters and any necessary cleaning efforts have been completed, a multi-volume, pH-adjusted chlorination treatment is recommended as a means of disinfection prior to returning the well to active service.

Chlorination as a means of disinfection can take on several forms depending on which products are available. Regardless of the products available, the volume of the disinfection solution should be equivalent to three times the standing volume of the affected well to flood the well, borehole, and adjacent aquifer interface.

If using traditional sodium hypochlorite (NaClO), confirm that the chlorine solution is of recent manufacture and free of additives (such as detergents or fragrances). Due to the buffering of liquid sodium hypochlorite, pH adjustment is needed to maximize the biocidal effectiveness needed during disinfection. Chlorination should utilize a target pH of 6.5 to 7. Chlorination should use a target of 200 ppm chlorine concentration. Using 10% strength sodium hypochlorite is equivalent to using 1 gallon of chlorine per 500 gallons of treatment volume.

There are several commercially available products on the market that aid in buffering of the pH to an effective level during chlorination. Various products have additional benefits such as increased penetration of biomass and surfactant qualities to increase the zone of treatment. When choosing the best product, we recommend selection of an NSF-approved chemical that accounts for the water chemistry (pH and total alkalinity) and the strength and type of chlorine used to allow for best results.

An alternative to traditional chlorine is the use of chlorinated isocyanurates. Chlorinated isocyanurates are chlorine-containing compounds used as sanitizers, disinfectants, and bleaching agents. The white, crystalline powder is used in a variety of applications, readily goes into solution, and does not require pH adjustment like traditional bleach.

Similarly, chlorination should use a target of 200 ppm chlorine concentration. Using powdered chlorinated isocyanurate, at a rate of 0.3 pounds (5 ounces) of product per 100 gallons of water. As noted above, utilize only those products that are NSF Standard 60 certified for use in potable well systems.

Regardless of the chlorine source, it is not recommended to use more than a 500-ppm chlorine concentration.

The disinfection solution should be blended above ground and introduced into the well by a tremie pipe or similar treatment method. Efforts should be made to evenly 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 all throughout the entire column. Swabbing of the well, cycling of the pump, or a 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 50 ppm, add additional material to raise it to that level.

It is important to note that if a chlorine residual during disinfection is unable to be maintained, this may indicate that additional flood contamination remains, and more invasive cleaning is needed. High levels of organic material, dissolved solids, and variable pH levels (all associated with floodwater) can degrade chlorine concentrations during treatment.

Allow the chlorine solution to remain downhole overnight. The following morning, check the chlorine residual within the well to ensure sufficient strength has remained. If the solution has fallen below a 50-ppm residual, additional disinfection or cleaning may be necessary.

Post-Disinfection

Once sufficient disinfection has been achieved, begin evacuation of the well. Purge 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. Conduct pipeline and water system disinfection of associated components per state rules or recommendations. 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.

If major structural damage has occurred to the well, or if the well is deemed substandard during inspection, or does not otherwise respond to chlorine treatment, more invasive cleaning, repair, or replacement may be needed.

Although all wells should be periodically monitored and evaluated, it is especially important that follow-up evaluations be conducted on flooded wells. The evaluations should include periodic water testing to make sure disinfection (or cleaning) was sufficient, and no contaminants have impacted the aquifer and watershed or have migrated into the well.

Additionally, within six months of the receding of floodwaters and again after one year, the well site should be evaluated for subsidence or other changes to the land within the vicinity of the wellhead.

Flooded wells are often just one in a host of problems during a deluge that can have far-reaching impacts. Understanding the well and the steps necessary to get it back online is important.

As always, being prepared is important! The risks that flooded water systems pose to well structures and population health can be minimized if these risks are recognized beforehand and response plans are set in place before a flooding event occurs.

 Michael Schnieders, PG, PH-GW, is the president and principal hydrogeologist at Water Systems Engineering Inc. He has an extensive background in groundwater geochemistry, geomicrobiology, and water resource investigation and management. He specializes in the diagnosis and treatment of fouled wells and potable water systems. Schnieders is the 2017 McEllhiney Distinguished Lecturer in Water Well Technology. He can be reached at mschnieders@h2osystems.com.