New water supplies, whether drawn from freshwater sources, groundwater wells, or recycled wastewater, routinely contain a mix of contaminants that range from pesticide residues and industrial chemicals to bacteria and microplastics. The specific findings depend on the water source, surrounding land use, and treatment methods, but testing across the United States has revealed a consistent pattern of substances that show up again and again.
Pesticides and Their Breakdown Products
Pesticides are the most frequently detected contaminants in new water sources. A statewide study across Minnesota’s drinking water systems found that atrazine, a widely used herbicide, appeared in 86% of surface water samples. Metolachlor, another common agricultural herbicide, was also a frequent detection. What makes pesticides especially persistent is that even after the original chemical breaks down, its transformation products linger in the water. These degraded forms actually represented the majority of contaminants found in 25% or more of all samples tested.
Groundwater supplies near agricultural land showed a similar profile. The most common detections were atrazine, a breakdown product called deethylatrazine, and a metolachlor byproduct. These chemicals seep into aquifers slowly, meaning a brand-new well drilled in farming country can tap into contamination that accumulated over years or decades.
PFAS: The “Forever Chemicals”
Per- and polyfluoroalkyl substances, commonly called PFAS, are synthetic chemicals used in nonstick coatings, food packaging, and firefighting foam. They earned the nickname “forever chemicals” because they don’t break down naturally in the environment. In Minnesota’s water testing, PFBA (a short-chain PFAS compound) was the most frequently detected, showing up in 67% of surface water samples.
The type of PFAS found depends heavily on what’s happening on the land above. Groundwater near areas influenced by wastewater (from septic systems, treatment plant discharge, or industrial sites) showed four different PFAS compounds in more than 25% of samples, along with an antibiotic (sulfamethoxazole) and atrazine. Groundwater near farmland, by contrast, showed mostly pesticides with only one PFAS type appearing regularly.
Microplastics in Tap Water
Tiny plastic fragments now appear in virtually every water source tested worldwide. Tap water concentrations vary enormously, from nearly undetectable levels to roughly 100 particles per liter across studies. Freshwater sources span an even wider range. Bottled water consistently contains more microplastics than tap water, likely due to particles shed from bottle walls and caps during manufacturing, plus airborne contamination in bottling factories.
The challenge with microplastics is measurement. Concentrations reported in published studies span ten orders of magnitude across different water types and sampling methods, making it difficult to pin down a single “typical” number. What’s clear is that they’re present in new and old water systems alike, and no standard treatment process was originally designed to remove them.
Bacteria and Biofilm in New Pipes
Even in brand-new distribution systems, bacteria begin colonizing pipe surfaces almost immediately. Within the first week, a selective group of pioneer bacteria attaches to pipe walls and starts forming biofilm, a thin, sticky layer of microorganisms. Research tracking biofilm development found that initial colonization was highly selective: water flowing into a new system contained roughly 1,677 distinct microbial types, but only about 189 managed to establish themselves on pipe surfaces in the first seven days.
The more concerning finding involves Legionella pneumophila, the bacterium responsible for Legionnaires’ disease. It rapidly colonized new biofilms, becoming the dominant species in its group, with concentrations peaking after just four weeks. The bacterium thrives alongside tiny single-celled organisms called amoebae that also live in the biofilm, which provide a protective environment for Legionella to multiply. Biofilm diversity continued shifting over the first several weeks, with significant changes observed between weeks two and three, and again between weeks six and seven.
Lead and Copper From New Plumbing
New plumbing fixtures that meet the current U.S. “lead free” standard can still release lead into drinking water at levels above 1 part per billion. The legal definition of “lead free” allows trace amounts of lead in fixtures and solder, and those trace amounts leach into water, especially in the first months of use. After a lead service line is replaced, water utilities are required to provide residents with a certified pitcher filter and three months of replacement cartridges, followed by a water test between three and six months later.
Copper pipes similarly leach metal into water when they’re new. The protective mineral layer that eventually forms inside copper pipes takes time to develop, and until it does, copper levels can be elevated, particularly in water that sits in pipes overnight.
Naturally Occurring Radioactive Elements
When new wells are drilled into deep rock formations, they can tap into naturally occurring radioactive elements. Uranium is present in many types of rock and soil, and as it decays over time, it produces radium. Radium then breaks down further into radon, a radioactive gas. All three dissolve in water and accumulate in wells. The concentrations depend on local geology, so two wells drilled a few miles apart can have dramatically different readings. Private wells are especially vulnerable because they aren’t subject to the same monitoring requirements as public water systems.
What Testing Looks Like for New Supplies
The CDC recommends that private well owners test their water at least once a year for total coliform bacteria, pH, total dissolved solids, and nitrates. Fecal coliforms and E. coli should also be tested. Beyond that baseline, your local health or environmental department can tell you whether your area warrants testing for volatile organic compounds, lead, arsenic, mercury, radium, or pesticides.
Public water systems face more extensive requirements, but the Minnesota study illustrates an important gap: many of the contaminants detected, including pharmaceuticals, hormones, illicit drugs, and wastewater indicators, aren’t covered by current federal drinking water standards. Treatment plants may reduce their concentrations, but they aren’t specifically designed or required to eliminate them.
How Recycled Water Gets Cleaned
A growing number of communities are turning to direct potable reuse, which means treating wastewater to drinking water standards. California’s regulations for this process are the most detailed in the country and give a sense of how intensive the purification must be. The treatment train must reduce viruses by a factor of 10 raised to the 20th power, Giardia parasites by 10 to the 14th, and Cryptosporidium by 10 to the 15th. In practical terms, that means if you started with a trillion pathogen particles, you’d need to reduce them to a number so small it’s essentially zero.
To achieve this, California requires at least four separate treatment processes using three different mechanisms: physical separation through membranes like reverse osmosis, chemical disinfection with ozone, and UV light treatment. The mandated sequence is ozonation paired with biologically activated carbon, followed by reverse osmosis, then advanced oxidation. No single step can be credited with doing more than a small fraction of the total work, which builds in redundancy so that if one process fails, the others still protect the water. These systems must maintain their target performance at least 90% of the time they’re producing water in any given month.