Why a present/absent test doesn’t always tell the complete story.
By Jennifer Strawn
A well owner calls you out with a common complaint. Their water smells or maybe appears discolored on startup.
Both can be a sign that something in their well has changed.
Begin with a physical inspection. Drain the pressure tank, open the union joint, and inspect the buried piping to the well for debris.
“Whatever deposits that may be growing in the well will likely show up in the piping as well,” says David Hanson, owner of DavidTHansonAssociates in Excelsior, Minnesota.
If deposits are in the piping, also inspect the drop pipe, inside check valves, or water meters below the gaskets for debris. If debris is not found, the buried piping might be the issue.
“You can pressure test the line, assuming the check valve in a submersible pump is holding, or shut off the line at a check valve for vertical turbine pumps,” says Hanson, who was the 2003 NGWA Foundation McEllhiney Lecturer in Water Well Technology. “If the line leaks, you’ve found the problem. But, if there is debris at the well, the well is the likely problem. Cleaning the well, pump, and the buried
pipelines will fix the issue.”
Along with a physical inspection, you may also test for bacterial contamination—especially if the well owner is concerned with coliform or E. coli.
While test kits for coliform and E. coli are affordable and easy to use, a simple present or absent result won’t tell you the whole story, Hanson says.
“When the industry moved to presence/absence for coliform, a lot of the laboratories in the United States found that was a very easy and a cheap way to go,” he says. “Many decided they didn’t need a microbiologist to understand identity of bacteria or any other issues. Apparently, nobody thought about other bacteria that could be a problem.”
Types of bacteria
There are three categories of bacteria you’ll need to worry about. First, non-pathogens. These are generally not a health threat and are naturally occurring soil organisms present in all wells—as heterotrophic bacteria—usually at less than 60 colonies per milliliter in a natural aquifer.
“It’s important to understand numbers in a normal aquifer,”Hanson says. “Excessively higher numbers can indicate a surface water source or even a continuing source due to a failure in the well such as corrosion or failed fittings, a failure of grout in the annulus, or a failure of a casing seat into hard rock.”
Heterotrophic bacteria are all aerobic in nature and are found in low numbers in all aquifers, naturally. When slime formation in the well occurs, the numbers of heterotrophic bacteria can often rise into the hundreds/thousand colonies permilliliter. Slime-forming bacteria constitute 88% of the slime that most people refer to as iron bacteria. These slime-forming bacteria can cause plugging in the pump, well, or piping when the numbers rise above 200 to 300 cfu/mL. Commonly found families of these bacteria include: Pseudomonas, Aerobacter,
Flavobacter, Acinetobacter, and Bacillus.
The second category—pathogens—are a health threat to people. This category includes E. coli and fecal coliform, among others, which must be identified by a microbiologist. These bacteria come from a source, whether it’s a single source deposited into a well—such as a failed well cap, a flood, or a continuing source.
The third category—opportunistic pathogens—are also a health threat for people who have compromised immune systems such as the very young, very old, or other individuals with immune deficiencies such as someone undergoing chemotherapy. So, suspect this if only an elderly person, baby or another individual is sick when no one else is. Opportunistic pathogens are heterotrophic bacteria, so they also require anidentity of bacteria.
Opportunistic pathogens are not naturally occurring in groundwater and are generally tied to surface water or shallow groundwater. Like pathogens, they require either a single or continuous source into deeper aquifers. Some of the more common families of these bacteria include Pseudomonas aeruginosa, Aeromonas hydrophilia, all Citrobacteria families, and all Enterobacter families.
Out of the three categories, opportunistic pathogens are least likely to be heard of by some labs or by some health departments, Hanson says.
What to test for
Present/absent tests can only tell you whether bacteria are present, not the identity of the bacteria or the number of bacteria. If you get an “absent,” you’re done. But if you’ve completed an enzyme test kit and received several “present” results for bacteria—especially following chlorination—it could mean that something else is going on.
Opportunistic pathogens often cause a “present” on these enzyme test kits, so you should also do a test for coliform counts.
“If then you see ‘present,’ but the count is zero, you should suspect something is drastically wrong,” Hanson says. “This may be a health issue.”
In this case, you should also include a heterotrophic plate count to help determine a potential for a continuing source.
Look for a lab that can identify bacteria and request a heterotrophic plate count, an identity of the bacteria on the plate count, and a coliform count. These three tests will give you a better look at the problem.
Hanson often uses the example of a job he worked on with Hartman Well Co. in Lakeville, Minnesota, as an example of how to understand well problems.
A farm site was just purchased with a test done by the seller at a lab in Rochester, Minnesota. Zero coliform and no E. coli were present in the well. As a precaution, the new well owner sent several samples to a nearby lab in Bloomington, Minnesota, and received multiple “present” results. Each time, the health department advised her to chlorinate her well. First, it was a gallon of bleach. When that didn’t work, it was two gallons. Then, it was three gallons.
“The homeowner said she was about to come unglued when the same recommendation didn’t solve the problem,” Hanson says.
Hanson talked to the Bloomington lab to understand their capabilities and requested the lab do a heterotrophic plate count, present/absent for coliform, a coliform count, and identify the bacteria.
Sampling a well
How, when, and where you take a sample is as important as what you test for.
First, talk with the lab ahead of time, especially if you’ll be using a new lab you’re not familiar with. Not only do you want to be sure they can run all the tests you need, you’ll need to know the volume of samples required, if samples will need chilled, how you will keep them cold, and any specific instructions the lab has for testing.
Make sure the homeowner knows when you plan to take the samples and you allow enough time to collect the samples and/or drop them off or ship them to the lab.
When you’re ready to take a sample, you generally want to take it as close to the wellhead as possible to remove the possibility of a failed buried piping. If you are taking samples some distance from the well, make sure you add in the volume of water in the piping. Because many sample taps or ports are put in after the fact, they can get dirty. Flush away any debris; disinfect the spigot with a flame, alcohol swab, or a chlorine solution (about 5 to 10 parts per billion); and rinse the spigot again with fresh well water.
Handle the sample bottles with clean hands and be careful not to accidentally contaminate the sample. Bacteria are everywhere. Hold the sample cap in one hand during the process. Do not set the cap on any surface. The sample bottles should be lab approved, and you should wait to remove the cap and break the seal until just before collecting the sample.
If the sampling port is near the ground, you may want to consider laying plastic down on the ground to reduce the chances of dirt splashing back into the sample.
Calculating timed samples
To determine whether the bacteria is coming from a single source or a continuing source, you’ll want to take “timed” samples so you can get a
sample from the casing and from the aquifer.
A casing sample is a sample that is captured outside of the pump column but still inside the borehole.
To get the casing sample for the Lakeville site, Hanson calculated the amount of water in the 1-inch buried piping and the 1-inch drop pipe to the pump. Then, he divided the total amount of water by the gallons-per-minute rate.
In this case, it would take 45 seconds for the water to vacate the lines. Hanson suggests adding 30 seconds to that time to capture the sample outside the pump.
Before taking the casing sample, let the well sit for six hours to allow any slime-forming bacteria to grow. When you start the pump back up, the sudden movement of water to the pump intake can create enough force to pull some of the more loosely bound bacteria away,
causing a discoloration of water upon startup. Take the sample, even if it’s discolored.
To collect an aquifer sample, calculate the amount of water in the well (total depth minus static level) and multiply that amount of water times the amount of water per diameter of well. Multiply that total volume of water by 20 or 30 times. Then, divide by the GPM.
Pump the well continuously for that minimum amount time before taking the sample. This could be anywhere from one to three hours for a domestic well and even longer for a municipal well. The sample should be clear.
When the Lakeville “timed” tests came back from the lab, the casing sample was “present” with a coliform count of zero and a heterotrophic count of 782 cfu. The identity of the bacteria was Aeromonas hydrophila, an opportunistic pathogen.
The aquifer sample came back “absent” with a coliform count of zero and a heterotrophic count of 2 cfu. The identity of the bacteria was Bacillus, which are normal bacteria in a well.
“The heterotrophic plate count was enormous in the casing and low to normal in the aquifer, which indicated slime formation in the casing and not from a continuing source,” Hanson says. “A physical inspection by the new owner also found black, sludgy debris in the piping at the union joint.”
After contacting the previous owner, the new well owner found out the well had been flooded about three years prior, which led to the growth of opportunistic pathogens.
“Six months later the new well owner (in Lakeville) called and said, ‘My husband is getting better,’” Hanson says. “When they had moved to the home, her husband was getting sicker and they assumed it was due to his chemotherapy, but they now think it was the
The Lakeville project also proves why it’s critically important to look further than a present/absent test to help you develop an appropriate treatment plan.
“If you do a test and it comes up absent, then everybody’s happy,”Hanson says. “But if it comes up repeatedly present, we have to start looking for more and better answers. Chemistry won’t fix a physical issue in the well, and as an industry, we have to provide better answers to solve our customers’ problems.”
Jennifer Strawn was the associate editor of Water Well Journal from 2004 to 2007. She is currently in the internal communications department at Nationwide in Columbus, Ohio. She can be reached at firstname.lastname@example.org.