The regulatory gap at the heart of the study is enormous. The Safe Drinking Water Act of 1974 empowers the EPA to set enforceable standards for roughly 90 microbial and chemical contaminants in public water systems, but wells serving fewer than 25 people fall entirely outside that framework. More than 43 million Americans rely on private wells for drinking water, including approximately 350,000 Maryland households, and most of the state’s 12,600 farm operations almost certainly draw their household water from private wells because of their rural locations. On farms, the risk profile is amplified: domestic and wild animals, manure storage, and routine agricultural activity all create potential pathways for fecal pathogens to reach groundwater that no agency is monitoring.

To capture that risk, the researchers recruited 77 farmers or farm family members through University of Maryland Extension agricultural agents, ultimately collecting 84 water samples from wells in 20 of Maryland’s 23 counties. Participants answered a 28-question survey covering well type, age, depth, condition, maintenance history, treatment devices, and testing practices, while trained participants or Extension agents collected samples from kitchen or bathroom cold water faucets after removing screens and aerators and flushing the lines for at least five minutes. Samples were packed on ice and driven to the university’s Water Quality, Outreach, and Wellness Laboratory, where they were processed within 24 hours using U.S. EPA Method 1604, a membrane filtration technique that filters 10, 100, and 500 milliliter volumes of each sample through 0.45-micrometer filters placed on highly selective MI agar.

The laboratory approach matters for the precision of the results. Unlike the IDEXX enzyme-based assays many health departments use, membrane filtration on MI agar allows researchers to count the exact number of target bacteria rather than estimate concentrations statistically, and it preserves live isolates for downstream confirmation. Presumptive E. coli colonies were purified on MacConkey and tryptic soy agar and then verified by polymerase chain reaction targeting the uidA gene, which encodes β-glucuronidase, an enzyme characteristic of E. coli. Each PCR run included a phosphate-buffered saline negative control and a known E. coli strain as a positive control, adding a layer of molecular certainty on top of the culture results.

The geographic pattern in the data was striking. Total coliform presence differed significantly by county, region, and the owner’s description of well condition, while E. coli presence was significantly associated with region, geological province type, county, and wellhead cover material. The Western region of Maryland, in the Appalachian portion of the state, fared worst by a wide margin: every one of the eight wells sampled there was positive for total coliforms, and 62.5 percent carried E. coli. In the province west of the Fall Line, the geological boundary dividing Maryland’s crystalline and sedimentary uplands from its coastal sediments, 29 percent of wells were E. coli-positive, compared with just 2 percent in the Atlantic Coastal Plain Province. Wells shallower than 9.1 meters and those with concrete or other nonstandard wellhead covers also showed significantly higher odds of E. coli contamination.

The geology offers a plausible explanation. West of the Fall Line, groundwater moves through fractures and bedding-plane partings in consolidated igneous, metamorphic, or sedimentary rock, and fractured media can transport contaminants rapidly and over long distances. Some provinces in this region also contain limestone aquifers, which previous research has linked to elevated contamination risk, and the Piedmont hosts more unconfined aquifers that lack the protective confining layers of deeper systems. By contrast, the Atlantic Coastal Plain east of the Fall Line consists of unconsolidated sand and gravel aquifers typically overlain by layers of low-permeability clay that act as a natural barrier, blocking surface contaminants from percolating downward into the water supply.

Perhaps the most counterintuitive finding concerns the weather. While earlier studies in Pennsylvania and New Jersey found that heavy rainfall increases the likelihood of coliform and E. coli detection in private wells, this study found the opposite: the odds of E. coli presence dropped significantly as precipitation levels in the 14 days before sampling increased, a result that held up in the multivariable logistic regression model with an odds ratio of 0.11. The authors suggest the inverse relationship may reflect the danger of antecedent dry conditions. Recent research on drought and childhood diarrheal disease has hypothesized that bacteria concentrate in the environment during dry spells and are then washed into water sources when rain finally returns, meaning the contamination signal may depend on what happened weeks before the rain, not the rain itself. Precipitation levels in the state also varied significantly by county, region, and province, complicating any simple rainfall-to-risk narrative.

Even more unsettling than the bacteria counts was what the survey revealed about owner perception. Eighty-five percent of participants described their wells as being in good or excellent condition, yet among wells with a known condition rating, 51 percent of those rated good or excellent were positive for total coliforms and 11 percent carried E. coli. In the multivariable model, wells described as merely okay or fair actually had significantly lower odds of total coliform contamination than wells rated good or excellent, an odds ratio of 0.10. The researchers interpret this paradox as evidence that many well owners fundamentally misunderstand the actual condition of their systems, and that self-assessed maintenance and condition are poor proxies for water safety. Notably, maintenance factors such as recent repairs, treatment devices, and testing history did not significantly predict bacterial presence, whereas geography and well construction did.

The survey also documented risky baseline practices. Fifty-six percent of the wells were 26 years or older, 68 percent had received no repairs or well management in the previous decade, and although 62 percent of owners had tested their water at some point, only a minority had tested for the full suite of common contaminants. Fifty-eight percent of participants reported having observed water quality problems such as odors, off tastes, or scale on fixtures, and nearly half had installed some form of treatment device, from carbon filters to ultraviolet systems, without necessarily knowing what they were treating for.

The public health stakes extend well beyond Maryland’s farmsteads. Previous studies estimate that 40 to 58 percent of private wells nationwide contain at least one contaminant exceeding a Safe Drinking Water Act standard, and reliance on private wells has been repeatedly linked to acute gastrointestinal illness, with improper treatment and maintenance compounding the risk. E. coli infection can cause fever, vomiting, diarrhea, nausea, and abdominal cramps, and while many strains are harmless, their presence signals that fecal matter, and potentially far more dangerous pathogens, has entered the water supply. The study team, funded in part by a Northeast Sustainable Agriculture Research and Education grant, recommends that well-owner education incorporate geographic risk information, including region, geology, wellhead type, and well depth, alongside traditional maintenance guidance. Because samples were collected only between May and August, the researchers caution that seasonal variation and the exploratory nature of their models warrant confirmation with larger, multi-season studies. Still, the message for the millions of Americans who drink from unregulated wells is unambiguous: clean-tasting, well-kept water is not necessarily safe water, and the only reliable defense is regular testing, particularly for the farmers whose livelihoods and households depend on what lies beneath their fields.

Subject of Research: Microbial contamination of private drinking water wells on Maryland farms and the influence of well properties, geography, and precipitation

Article Title: E. coli and total coliform presence in private drinking water wells on Maryland farms: Determination of the impact of well properties, geographic, and precipitation factors

Article References: Smith, C., Woerner, E. M. H., Leslie, A., Beale, B., Nichols, K., Dill, S., Hirsh, S., Semler, J., Kness, A., Keane, J., Costa, M., Yang, J., Cruz-Cano, R., Lazur, A., & Rosenberg Goldstein, R. (2026). E. coli and total coliform presence in private drinking water wells on Maryland farms: Determination of the impact of well properties, geographic, and precipitation factors. PLOS Water, 5(9), e0000636. https://doi.org/10.1371/journal.pwat.0000636

Image Credits: AI Generated

DOI: 10.1371/journal.pwat.0000636

Keywords: E. coli, total coliforms, private wells, drinking water, Maryland farms, groundwater contamination, well maintenance, geology, precipitation, Safe Drinking Water Act, public health, water testing

The findings arrive amid a widening scientific debate over fluoride, a substance added to community water supplies at 0.7 milligrams per litre to prevent tooth decay, yet one that occurs naturally in groundwater at levels that can vary dramatically with local geology. In rural New Hampshire, where many families rely on private, unregulated wells drilled into fluoride-bearing rock, concentrations can be far higher than in fluoridated municipal systems; roughly thirteen percent of wells in regions underlain by felsic igneous rock exceed the U.S. Environmental Protection Agency’s secondary maximum contaminant level of 2.0 milligrams per litre. Against this backdrop, the research team set out to test whether prenatal fluoride exposure is linked not to intelligence, which has dominated the literature, but to the social-emotional and behavioural dimensions of child development that have received far less scrutiny.

Methodologically, the study distinguishes itself through its choice of exposure biomarker. Most previous prospective studies relied on maternal urinary fluoride, which reflects intake over only about six hours—a snapshot that can fluctuate with tea drinking, meal composition, and water consumption on a given day. Toenails, by contrast, grow slowly and integrate fluoride exposure over approximately three to six months, capturing cumulative intake from all sources: drinking water, beverages prepared with fluoridated water, seafood, tea, and the inadvertent swallowing of fluoridated dental products. Because fluoride readily crosses the placenta and penetrates the blood-brain barrier, the developing fetal brain is directly exposed, making the timing and magnitude of maternal exposure biologically consequential.

The researchers analyzed toenail clippings collected from women at roughly 28 weeks of gestation and again about five weeks after delivery, yielding estimates of fluoride exposure during early pregnancy and mid-to-late pregnancy respectively. Samples were analyzed at the Indiana University School of Dentistry using a hexamethyldisiloxane microdiffusion procedure coupled with fluoride ion-selective electrode detection, and specimens weighing less than 2.5 milligrams were excluded after validation work showed that smaller samples produce unreliable results. Household tap water collected at enrollment was analyzed with a fluoride ion-selective electrode as well. The final analytic samples comprised 358 mother-child pairs with toenail fluoride data and 381 pairs with water fluoride data, drawn from a cohort recruited between 2009 and 2013 from prenatal clinics serving women using private water systems.

Children’s behaviour was assessed at age five with the Behavior Assessment System for Children, Second Edition, Parent Rating Scale for preschoolers, a validated instrument that generates age-standardized T-scores across clinical scales such as hyperactivity, aggression, attention problems, anxiety, depression, and withdrawal, along with composite indices of externalizing problems, internalizing problems, and overall behavioural symptoms. Scores of 60 or above fall into the at-risk range. In this sample, most children scored near the normative mean, with between six and twenty-eight percent in the at-risk range depending on the scale. The analysis was preregistered, and the modeling strategy used generalized estimating equations to estimate the odds of at-risk outcomes per one microgram per gram increase in toenail fluoride, with interaction terms testing for differences by child sex and exposure window.

The headline result was statistically robust: each one microgram per gram increase in toenail fluoride reflecting early pregnancy was associated with thirty-nine percent higher odds of scoring in the at-risk range for externalizing problems, with a confidence interval running from fifteen to sixty-seven percent above baseline. The late-pregnancy estimate was similar in magnitude, at thirty percent higher odds, though its confidence interval narrowly included the null. On the individual clinical scales, early-pregnancy fluoride was linked to twenty percent higher odds of at-risk hyperactivity, and the late-pregnancy estimate of twenty-four percent approached significance. The pattern suggests that no single trimester stands out as uniquely vulnerable; rather, fluoride exposure across pregnancy appears relevant to externalizing behaviour.

The sex-specific findings added a further layer of complexity. Among girls, each one microgram per gram increase in early-pregnancy toenail fluoride was associated with thirty-seven percent higher odds of at-risk depressive symptoms, while no association appeared in boys—a statistically significant interaction. Suggestive but imprecise signals also emerged for anxiety and withdrawal in girls in relation to late-pregnancy exposure. This pattern is intriguing because prior fluoride studies, particularly those examining intelligence outcomes, have often reported stronger effects in boys. The authors note that internalizing and externalizing problems may share transdiagnostic roots, including emotion dysregulation, and that sex-specific biological responses to prenatal exposures could shape which behavioural domain is affected.

Plausible biological mechanisms are beginning to take shape in the literature. Fluoride has been implicated in disruption of the thyroid axis, a concern in pregnancy because the fetus depends entirely on maternal thyroid hormones during the first trimester, and even mild maternal thyroid dysfunction has been linked to both internalizing and externalizing problems in children. Fluoride exposure has also been shown to induce oxidative stress in animal and human studies, a pathway independently associated with behavioural problems in offspring. Low iodine status, which compromises thyroid hormone synthesis, may exacerbate fluoride’s effects, and emerging evidence suggests thyroid regulation and oxidative stress responses differ by fetal sex. None of these mechanisms is confirmed as the driver of the observed associations, but together they offer a coherent framework for future investigation.

Why did water fluoride fail to predict outcomes while toenail fluoride succeeded? The authors argue that toenails capture total intake from all dietary and product sources, whereas a single tap-water measurement reflects only one exposure pathway and ignores individual differences in consumption volume. This mismatch can produce exposure misclassification that biases associations toward the null. Median water fluoride in the sample was about 0.2 milligrams per litre—comparable to other communities with naturally occurring fluoride—but values ranged from 0.02 to 13.8 milligrams per litre, and correlations between water and toenail measures, while positive, were moderate. The authors also caution that some participants may have reduced tap-water consumption after learning their wells exceeded arsenic guidelines during the study.

The study’s limitations temper its implications. The cohort was predominantly white, college-educated, married, and relatively affluent, and relied on unregulated private wells, so the findings cannot be assumed to extend to populations drinking optimally fluoridated municipal water. Postnatal fluoride exposure was not measured, toenail growth rates vary between individuals, and only a small fraction of children scored in the at-risk range, limiting statistical power. Yet extensive sensitivity analyses—adjusting for co-occurring metals, imputing missing data, removing influential observations, and modeling outcomes continuously—left the primary conclusions intact. The authors call for longitudinal follow-up, validation of toenail fluoride reference ranges in pregnant populations, and exposome-wide approaches that jointly model fluoride alongside the many other prenatal environmental exposures that shape the developing brain.

Subject of Research: The association between prenatal fluoride exposure measured in maternal toenails and social-emotional and behavioural outcomes in preschool children.

Article Title: Long-term exposure to prenatal fluoride and social-emotional and behavioural outcomes in children in the New Hampshire Birth Cohort Study

Article References: Goodman, C. V., Karagas, M. R., Peacock, J. L., Korrick, S., Flora, D., Lanphear, B., Martinez-Mier, E. A., Tamayo-Cabeza, G., Lippert, F., & Till, C. (2026). Long-term exposure to prenatal fluoride and social-emotional and behavioural outcomes in children in the New Hampshire Birth Cohort Study. Environmental Advances, 25, Article 100753. https://doi.org/10.1016/j.envadv.2026.100753

Image Credits: AI Generated

DOI: 10.1016/j.envadv.2026.100753

Keywords: fluoride, prenatal exposure, child behaviour, hyperactivity, New Hampshire Birth Cohort Study, toenail biomarker, drinking water, neurodevelopment, thyroid, externalizing problems, internalizing problems, private wells