Water Test Results: Why They Vary and Why it Matters

Our environment is constantly changing, and results for water quality tests can vary as well. Whether you’re a repeat customer comparing results of several water tests, or just want to know more about factors that can cause variability in water chemistry, this quick guide will highlight the basics of why water quality can vary over time, and what that means for you.

Table of Contents:

• What Factors Contribute to Water Quality Variability?
• How Sample Collection Contributes to Variability
• What Does this Mean for Me?
• What’s the Takeaway?

What Factors Contribute to Water Quality Variability?

The precise answer depends on how you source your water—from a utility or a private well.

Private Wells

Private wells provide water to buildings and homes in rural areas not connected to a public utility. If you are a homeowner reliant on a private well, regular water quality testing is your responsibility in order to ensure your water supply is safe. If you’re considering testing, check out how to test your well water in a lab to learn more about Tap Score’s recommended approach for sampling your well, and how to interpret water quality for key contaminants.

Key factors that can change your well’s water quality include:

• Local groundwater pumping activity: Changing pumping behavior may impact levels of common contaminants. For example, in a study in Wisconsin, different pumping rates changed the conditions in a specific well and caused large differences in the concentrations of arsenic measured from the well.[1]
  
  
• Nearby septic systems: A high density of septic systems in the area can impact contaminant levels, especially if they are improperly maintained or leaking. Septic systems can be a source of nitrates and fecal bacteria for nearby well waters.
  
  
• Nearby land use activity: Certain land uses can lead to seasonal or long-term variability of certain contaminants.[2] These land use patterns may include heavy industry, dumps or landfills, hydraulic fracturing activity, mining, and agricultural practices. Some common contaminants from these sources include:
  
  
• Nitrates
• Coliform bacteria
• Heavy metals—like lead, arsenic, chromium, mercury, cadmium
• Pesticides—including glyphosate (RoundUp)
• VOCs—including those associated with petroleum products like BTEX (benzene, toluene, ethylbenzene, xylene) and MTBE
• Emerging contaminants—like PFAS and pharmaceuticals, which are often found in fertilizer recycled from sewage
  
  
• Changes to land cover or foliage: When vegetation and topsoil are removed, soil is less able to filter contaminants from surface water before it reaches the groundwater. In addition, converting natural land into agricultural fields can increase nitrate contamination of groundwater.[3]
  
  
• Seasonal variation: Rocks and minerals in contact with the water source may contain certain radioactive elements and harmful metals, which can enter the groundwater in varying amounts. Seasonal changes to groundwater levels can impact how much of these contaminants get into the water. For example, uranium concentrations can change seasonally as groundwater levels fluctuate and cause weathering of the aquifer rock.[4]
  
  
• Weather events: Extreme weather events like hurricanes and severe flooding can disrupt the water quality in private wells and even damage equipment. Wells typically need to be disinfected after flood waters recede.

Public Water Systems

The majority of the US population relies on a public water system for supplying their drinking water. Unlike private wells, all public water systems are required to meet primary drinking water standards. There are a variety of actions regularly taken by water systems in the interest of meeting these standards that can cause variability in water chemistry.

Non-routine actions taken by utilities that could lead to water quality variability include:

• Changes in source water: Utilities may change the source(s) of water they use for a variety of reasons. The Flint water crisis is an example of a source water change (from Lake Huron to the Flint River) gone awry. A change in source water may lead to variability in water quality throughout the distribution system and at the household level. If you suspect or have been notified that your utility’s source water has changed, you may notice changes in the taste, odor, or appearance of your water. In some cases, as with Flint, harmful contaminants like lead can be released if a change is not well-executed.
  
  
• Changes to water treatment methods: Water systems adjust their treatment processes for various reasons, including changing source water characteristics, changing regulations, technological advancements, cost considerations, etc. These adjustments can sometimes lead to changes in the water’s taste, odor or color, and other changes to the water’s chemistry. For example, in order to reduce the production of harmful disinfection byproducts, many utilities switched from using chlorine to chloramines for disinfection after the EPA changed regulations in 1998.
  
  
• Replacement of lead service lines (LSLs): Removal of lead service lines by utilities is now a major priority because of the dangers of lead ingestion at any level. But removing the pipes can cause lead levels to spike temporarily because lead deposits in the pipes may be disturbed during the process and flow to your tap. Concentrations of other heavy metals could also vary.[5,6] Once new service lines are in place, the water’s taste, odor, and bacterial population could change.[7]
  
  
• Intermittent supplies (rare): If water supplies are intermittent, or water is not delivered 24 hours per day, the shutdown and restart of flow may lead to the growth of bacteria such as E. coli.[8] In the United States, intermittent supply events are typically short and isolated, but water quality may still be impacted. When the supply is restored, water quality should improve after 1-2 days.
  
  
• Weather events: Similar to wells, extreme weather events can impact water coming from a public utility. Impacted utilities must issue advisories to customers in the event of any deviations from regulatory requirements. It is a good idea to consider your water unsafe for consumption until told otherwise in the event of extreme weather.
  
  

How Sample Collection Can Contribute to Variability

Believe it or not, the act of collecting samples can cause variation in your results, so follow instructions carefully!

There are two main types of samples in drinking water analysis:

• First draw sample: Sample is collected after water has been stagnant in the pipes for at least 6 hours. This water has had time to interact with the pipes and will be more reflective of potential issues in the plumbing system.
  
  
• Fully-flushed sample: Sample is collected after running the tap continuously for about 5 minutes. This water comes from further back in the distribution system and is more reflective of the water quality of the water entering your home (rather than any potential issues caused by your home plumbing system).
  

Contaminant concentrations in a first draw sample may be completely different from those in a fully-flushed sample from the same tap. The test you choose should specify what type of sample is most appropriate.

Contamination
Analysis for some contaminants can be particularly vulnerable to contamination—bacteria and PFAS are both good examples. It can be easy to contaminate a sample container with bacteria if you happen to have the target species on your hands (which is not uncommon). 

PFAS are common culprits as well due to the fact that they are present (or suspected to be present) in so many everyday items, like certain rain jackets, fast food wrappers, the ink from some permanent markers (like Sharpies), etc.[9] Writing on a sample bottle with a permanent marker suspected to contain PFAS or wearing clothing treated with PFAS while sampling can change your results.

Headspace
Another common mistake made during sample collection is leaving headspace in sample containers when testing for volatile compounds, typically volatile organic compounds (VOCs). Headspace refers to space in the sample container above the water itself, meaning you didn’t fill the container all the way to the top. This is a problem for VOCs because they can leave the water sample for the air in the headspace, which results in the concentrations measured in the water being artificially low. 

Follow sampling instructions closely; if you’re filling a sample container that specifies that you should NOT leave headspace, make sure you fill the container all the way to the top.

What Does this Mean for My Sample Results?

It is important that your samples are “representative,” or reflective of everyday scenarios—which can be impacted by where, when, and how the sample was obtained. When you compare sample results over time and see differences, it can be hard to know if there may be issues with your water quality.

Testing more often is helpful to get a more complete picture of your water quality.

There are a few things you can do to make sure that your samples are as representative as possible of your actual water quality:

1. Choose the right test for your situation. Make sure the test you choose measures everything you are concerned about in your water.
2. Collect your sample properly. Representative sampling requires knowing if your sample is to be first draw or fully-flushed. Make sure you don’t introduce contamination, and fill the sample container appropriately (i.e., without headspace, if specified).
3. Only compare samples taken from the same location in the same manner. For example, you can directly compare two first draw samples taken from a kitchen sink in the morning a couple months apart; you may still see a difference, but it would likely be coming from the water source itself and not variability introduced during the sample collection step.

If you’re following these guidelines, then any variation you might see in your test results is likely due to a change in water quality at the source.

Summing Up

As discussed above, the factors that impact private wells can be different from those that impact water from public utilities, so it’s important to think through your particular situation to better understand what may be going on.

If you’re in doubt, it’s always a good idea to take more samples if possible—the more data the better when you’re trying to understand variability in results. All in all, water chemistry is complicated and small changes in water quality are to be expected.

Tap Score streamlines the testing process by offering easy-to-use water testing kits that are linked to a nationwide network of certified labs. Additionally, we offer first-in-class customer service to help guide you every step of the way. If you have further questions, reach out to our science team-we’re here to help!