This spring in the well water clinic we run each year we found 25 wells out of 114 that had coliform "PRESENT." On a state level, the occurrence of coliform is higher. Of the approximately 7,000 households that participated in the Virginia Household Water Quality Program clinics from 2007 to 2015 they found that 41% of the wells had coliform bacteria, and 9% had E. coli bacteria. Though the 7,000 households may not be representative of all private drinking water wells in Virginia, it is the largest database on private drinking water wells available. It is safe to say that coliform contamination is widespread.
If your water is contaminated with coliform but not fecal coliform or E. coli, don't panic. You have a nuisance bacteria problem and the source may be infiltration from the surface from rain or snow melt. Typical causes are improperly sealed well cap, well repairs performed without disinfecting the well, failed grouting or surface drainage to the well. If your well had coliform bacteria present you should shock chlorinate the well, repack the soil around the well pipe to flow away from the well and replace the well cap. Then after at least two weeks and the next big rainstorm retest the well for coliform. If coliform bacteria is still present then a long-term treatment should be implemented: using UV light, ozonation, or chlorine for continuous disinfection. These systems can cost up to $2,000 installed.
If your well test PRESENT for coliform standard protocol is:
- Carefully check the well and water system for points of contamination. Make sure you have a sound and secured sanitary well cap and that the soil around the well is packed to drain water away from the well.
- Then treat the well and plumbing system with chlorine for 12-24 hours to disinfect system (the 12-24 hours is essential). Then flush the chlorine from the system- not to your septic system. Make sure that this is done correctly.
- Retest the water after the chlorine has left the system in about 10 days to two weeks. If coliform bacteria is “ABSENT” you’re done. If not, then it is time to install a long term disinfection system. (UV light or continuous chlorination)
- A missing or defective well cap and check seals around wires, pipes, and where the cap meets the casing may be cracked, letting in contaminants.
- Contaminant seepage through the well casing - cracks or holes in the well casing allow water that has not been filtered through the soil to enter the well. This seepage is common in the wells made of concrete, clay tile, or brick. This can also happen to a steel pipe well that was hit by a piece of equipment such as a car, snow blower, lawn tractor or mower or that has rusted.
- Contaminant seeping along the outside of the well casing - many older wells were not sealed with grout when they were constructed or the grouting has failed. Check the grouting carefully especially if water seems different after severe rains.
- Well flooding - a common problem for wellheads located below the ground in frost pits that frequently flood during wet weather.
Bacteria washed into the ground by rainfall or snowmelt are usually filtered out as water seeps through the soil, so properly constructed water wells do not typically harbor Coliform bacteria. However, coliform bacteria can persist within slime formed by naturally occurring ground water microorganisms. The slime (or biofilm) clings to the well screen, casing, drop pipe, and pump and may even invade filter systems. Disturbances during pumping or well maintenance can cause the slime to dislodge, releasing the coliform bacteria.
Keep in mind that coliform bacteria do not always show up in every sample. They can be sporadic and sometimes seasonal when they occur in a water supply. You should not continue drinking water contaminated with coliform, either boil the water drink bottled water until you disinfect your well. Bring the water to a rolling boil for one to five minutes (the higher the elevation the more time is necessary) to kill the bacteria. You may also want to consider using bottled water as a temporary drinking and cooking water source.
You may have received a total coliform count. This gives you a general indication of the sanitary condition of a water supply and extent of the problem. Bacteria can be introduced into a new well during construction and can remain if the water system is not thoroughly disinfected and flushed. Well construction defects such as insufficient well casing depth, improper sealing of the space between the well casing and the borehole, corroded or cracked well casings, and poor well seals or caps can allow surface water or insects to carry coliform bacteria into the well. These problems are common and the most likely source of the coliform bacteria contamination. Unplugged abandoned wells can also carry coliform bacteria into deeper aquifers.
Since bacterial contamination cannot be detected by taste, smell, or sight, all drinking water wells should be tested at least annually for Coliform bacteria.
|a sanitary well cap|
|typical drilled well|