By Marvin F. Glotfelty, RG
There are many things to consider on the topic of water well resilience. The recent hurricanes and associated flooding in the southeastern United States and Puerto Rico underline the need for these considerations.
The basic premise of water well preparedness is the well infrastructure should be located and constructed in accordance with an anticipated worst-case scenario, which may someday become a reality. Here are some of the water well concepts to consider.
Water well site selection: The first level of protection of the well is to locate it out of harm’s way. In the case of storm damage and flooding, this means seeking to locate the well on higher ground, and further inland when possible, away from storm surge. Such locations may sometimes be in indirect conflict with the need for water sources close to points of water use, or near water transmission pipelines. Land ownership and, of course, hydrogeological conditions are also primary considerations for well siting.
From a practical standpoint, the consideration of extreme weather and flooding should be added to the existing list of well siting considerations, but some of the other well siting variables (that will occur with more certainty and on a regular basis) may take priority over well resiliency as criteria for well site selection.
Wellhead armoring and sealing: In those cases where the well must be located on lower ground where flooding is possible, the annular seal of the well is critically important. Some well designs include annular sounding tubes or gravel feed pipes extending down the outside of the well casing, from the land surface through the annular seal, to the area adjacent to the filter pack and well screen.
These annular tubing strings are not a good idea in areas of potential flooding, as they can become an avenue for severe cross-contamination of the well. Even with a good annular seal to prevent the flow of floodwater down the outside of the well casing, the well will still be susceptible to contamination from floodwater if it doesn’t have a durable sanitary well seal. The well seal provides the critical boundary between the floodwater and the aquifer during a flood.
Even the most structurally sound wellhead cannot withstand abrasion and collisions from aggregate and cobbles that are moved along with the floodwater. Therefore, it is important to armor the wellhead at vulnerable locations. This can be done with thicker and heavier concrete surface pads, protective gabions or bollards around the wellhead, or, in some cases, elevated wellheads that extend above grade by several feet (this makes pump installation/removal difficult, but can be effective in some cases).
Well design: In addition to the obvious need for a good annular seal and armored wellhead, water wells located in potential flood areas should include some other characteristics that will facilitate effective rehabilitation (cleaning, disinfection, and structural repair) in the event there is flooding of the well.
One attribute that provides resiliency to wells is strong and stable well casing and screen. For smaller wells, polyvinyl chloride (PVC) is often used due to its lower cost and inert characteristics, but in areas susceptible to flooding, steel casing and screen may be a better choice. The higher strength of steel will withstand the force of water and alluvium that will flow against and around it during a flood event, whereas PVC or very thin-walled steel casings are more likely to be severed by floodwaters.
Less reactive materials, such as stainless steel, should also be considered in flood-prone areas, because in cases where the well becomes fouled by floodwater, these materials are less sensitive to the biological and chemical impacts of the flood. Chemical cleaning of stainless steel will be more successful in most cases compared to cleaning of mild steel casing and screen.
An additional well design characteristic that should be considered for flood-prone areas is accessibility for well redevelopment. This generally involves designing the well with an adequate diameter, screened interval, and screen type that will accommodate thoroughly cleaning out any and all debris or contaminants that may be introduced during flooding.
Marvin F. Glotfelty, RG, principal hydrogeologist at Clear Creek Associates LLC in Scottsdale, Arizona. Glotfelty was the 2012 NGWA Foundation McEllhiney Lecturer who presented “Life-Cycle Economic Analysis of Water Wells—Considerations for Design and Construction.” He can be reached at firstname.lastname@example.org.