What Are the Most Common Contaminants Found in Well Water?
If you get your water from a private well, you are the water utility. There is no municipal treatment plant testing samples every day and adjusting chemistry behind the scenes. That responsibility sits with the homeowner, and most people who rely on well water do not think much about what is actually flowing through their taps until something changes: a strange smell, a rusty stain in the sink, or a lab result that raises an eyebrow.
Well water quality depends entirely on the geology and land use around a specific property, which is why two homes a mile apart can have completely different water. Understanding the contaminants that show up most often in wells gives you a much better starting point for deciding what to test for and what kind of treatment might eventually make sense.
Why Private Well Water Needs Regular Attention
Groundwater moves slowly through soil and rock, picking up minerals, gases, and sometimes chemical residue along the way. That slow movement is part of what makes wells useful, since the ground itself acts as a natural filter for many surface contaminants. But it also means that whatever is naturally present in the bedrock, or whatever has leached into the soil nearby, can end up in the water supply over time.
Unlike a city water system, a private well is not routinely monitored by a health department. Homeowners are on their own for scheduling tests, interpreting results, and deciding how to respond. That is not meant to sound alarming. Most well water is perfectly safe with basic awareness and the occasional lab test. The goal is simply to know what to look for.
Bacteria and Microbial Contamination
Bacteria are one of the most common reasons a well water test comes back flagged, and coliform bacteria are usually the first thing a lab checks for. Coliform itself is not always dangerous, but its presence signals that surface water or organic material may be finding its way into the well, which raises the possibility that more harmful organisms could follow the same path.
Wells near septic systems, livestock, or areas with heavy seasonal flooding are more prone to this kind of contamination. A cracked well cap, a poorly sealed casing, or a well that sits too close to a drainage area can all be entry points. Regular testing, especially after heavy rain or nearby construction, helps catch this early.
When bacteria or viruses are a recurring concern, disinfection at the point of entry is often the most reliable fix. Ultraviolet light treatment neutralizes microorganisms without adding chemicals to the water, which is why so many well owners turn to UV system experts for safe water once a test confirms ongoing microbial activity rather than a one-time fluke.
Arsenic in Groundwater
Arsenic is naturally present in certain types of bedrock, and it dissolves into groundwater gradually over long periods of time. It has no taste, no smell, and no color, so there is no way to notice it without a laboratory test. That makes it one of the more concerning contaminants precisely because it gives no warning signs.
Areas with granite or certain metamorphic rock formations tend to see higher arsenic levels in well water, and this is highly localized. Two wells drilled a short distance apart can show very different arsenic readings depending on which rock layer they draw from. This is one of the strongest reasons every new well should be tested before it becomes a primary water source, and existing wells should be retested periodically since levels can shift as groundwater conditions change.
Radon Gas in Well Water
Radon is a radioactive gas that forms naturally as uranium in rock and soil breaks down. It is best known as an air quality concern, but it also dissolves into groundwater and can be released into indoor air every time water runs from a faucet, shower, or washing machine. Because it is invisible and odorless, radon in water is almost always discovered through testing rather than observation.
Homes with granite bedrock, which is common across large parts of New Hampshire, are more likely to have elevated radon readings in their well water. The concentration can vary significantly even between neighboring properties, so a radon test from a neighbor’s well does not tell you much about your own.
Because radon behaves differently than most other contaminants, treating it typically requires a dedicated approach rather than a general filter. Aeration systems that vent the gas outdoors before it reaches household plumbing are one of the more established methods, and homeowners dealing with confirmed radon levels usually look into radon mitigation for drinking water as a distinct step separate from any other filtration already in place.
Volatile Organic Compounds
Volatile organic compounds, often shortened to VOCs, are a broad category of chemical contaminants that can come from a range of sources: nearby fuel storage, industrial activity, agricultural chemicals, or even certain older plumbing materials. Because VOCs cover so many different compounds, from solvents to pesticide residue, the health effects and detection methods vary depending on exactly what is present.
Rural and suburban wells located near old gas stations, dry cleaners, farmland, or industrial sites have a higher likelihood of VOC exposure, though contamination can occur in less obvious settings too. Since these compounds usually have no smell or taste at low concentrations, a targeted lab test is the only reliable way to know if they are present and at what level.
Treating VOCs generally calls for activated carbon filtration, aeration, or a combination of methods depending on which specific compounds show up in the test results. Homeowners who discover VOCs in their water often look for thorough VOC water cleaning in New Hampshire so the treatment approach actually matches the chemicals identified rather than relying on a generic filter that may not address the issue.
Nitrates and Nitrites
Nitrate contamination usually points back to agricultural runoff, fertilizer use, or septic system discharge making its way into groundwater. It is a particular concern for households with infants, since high nitrate levels have long been linked to health risks in very young children, but it is worth monitoring in any home that relies on a well near farmland or dense septic use.
Nitrate levels can rise seasonally, often following spring fertilizer application or periods of heavy rain that push runoff toward the water table faster than usual. A single test result is a snapshot in time, so wells in agricultural areas benefit from being tested more than once a year if nitrate has ever shown up as elevated.
Iron, Manganese, and Staining Minerals
Iron and manganese are two of the more visible well water problems, and homeowners often notice them before ever getting a test done. Reddish-brown stains on sinks, tubs, and laundry point to iron, while manganese tends to leave behind dark brown or black staining and can give water a slightly metallic taste.
Neither mineral is typically a serious health concern at the levels usually found in residential wells, but they cause real headaches around the house: stained fixtures, discolored laundry, and buildup inside pipes and appliances over time. Because they are naturally occurring minerals dissolved from rock and soil, their presence says more about the property’s geology than about any contamination event.
Sulfur and the Rotten Egg Smell
Few well water issues are as immediately noticeable as sulfur. Even in small amounts, hydrogen sulfide gas produces a distinct rotten egg odor that is hard to miss, especially when running hot water. It is unpleasant more than dangerous in most residential cases, but it can also indicate bacterial activity within the well or plumbing system, which is worth ruling out with a test.
Sulfur odor can come and go depending on groundwater levels, temperature, and how long water sits in the pipes, which is why some homeowners notice it more in certain seasons. A water test can confirm whether the smell is coming from dissolved sulfur compounds or from something else in the plumbing, since the source affects which treatment approach actually works.
Water Hardness from Calcium and Magnesium
Hard water is one of the most widespread well water characteristics, caused by dissolved calcium and magnesium picked up as groundwater moves through limestone and similar rock. It is not a health hazard, but it is a persistent nuisance: scale buildup in pipes and appliances, soap that will not lather properly, spots on dishes and glassware, and dry skin and hair after washing.
Over years, hardness minerals can shorten the lifespan of water heaters, dishwashers, and washing machines by leaving mineral deposits inside components that were designed to handle plain water. Because hardness is so common in groundwater generally, it is one of the first things worth checking when a household water test is performed, even if there are no obvious symptoms yet.
Lead and Corrosion Byproducts
Lead is rarely present in the groundwater itself, but it can enter well water through old plumbing, fixtures, or solder that contains lead, especially in homes built before more recent plumbing standards. Corrosive water, meaning water with low pH or low mineral content, can accelerate the breakdown of these materials and pull more lead into the water supply over time.
Because lead exposure is cumulative and especially harmful to children, testing for it is worth doing even in newer wells if the home’s plumbing is older or if water has a metallic taste. A simple pH test can also indicate whether the water is corrosive enough to be a contributing factor, which helps narrow down where the lead might be coming from.
pH Imbalance and Acidic Water
Well water pH varies quite a bit depending on local geology, and water that sits outside the normal range can cause problems even when no specific contaminant is present in dangerous amounts. Acidic water, with a low pH, tends to be corrosive to pipes and fixtures, which can introduce metals like copper or lead into the water supply as a secondary effect.
Signs of acidic water include blue-green staining around drains, a metallic taste, or a shortened lifespan for plumbing fixtures. Since pH imbalance is more of a root cause than a standalone contaminant, addressing it often prevents other issues from developing further down the line.
How Often Private Wells Should Be Tested
Most water quality organizations recommend testing well water at least once a year for bacteria and nitrates, since these can change relatively quickly depending on seasonal conditions and land use nearby. Contaminants like arsenic, radon, and VOCs tend to be more stable over time, but they still warrant periodic retesting, particularly if there is any change in water taste, smell, appearance, or if nearby land use changes.
It is also worth testing any time something changes physically, such as after well repairs, nearby construction, flooding, or a new water heater or plumbing installation. A fresh baseline test helps confirm that recent changes have not introduced a new issue, and it gives homeowners a clear record to compare against in future years.
Matching Treatment to What Is Actually in the Water
The biggest mistake well owners make is guessing at treatment before knowing what is actually in their water. A whole-house softener will not touch bacteria, a carbon filter alone will not remove arsenic, and no single generic filter addresses every contaminant on this list. Effective treatment starts with a real water test, followed by a system designed around those specific results rather than a one-size-fits-all setup.
Because well water chemistry is so specific to each property, even neighboring homes on the same street can need different combinations of treatment. Taking the time to test first, then treat based on what is found, is the difference between a system that actually solves the problem and one that just adds cost without addressing the real issue.