By Paul Feldman, FairWarning

Scientists have known for decades that drinking water contaminated by fertilizer nitrates can pose a threat to infants by undermining the ability of their blood to carry oxygen. The condition, known as blue baby syndrome, led federal regulators to impose an environmental standard of no more than 10 parts per million, or ppm, for nitrates in public water supplies.

Recently, though, some evidence suggests that long-term ingestion of drinking water with nitrates at just half that federal limit can prove dangerous to children and adults alike, potentially raising the risk of bladder, thyroid, kidney, ovarian and colon cancers.

A new report from the Environmental Working Group, a research and advocacy organization, indicates that more than 1,700 water districts across the U.S. recorded nitrate levels that averaged 5 ppm or more in 2014-2015. The vast majority 1,683 of the water districts were rural systems serving no more than 25,000 people and generally located in farming areas where fertilizer and manure in cropland runoff can seep into the public water supply. Included in those rural districts were 118 systems that matched or exceed the U.S. Environmental Protection Agencys limit of 10 ppm.

The report by EWG, which operates a Tap Water Database enabling consumers to check for hundreds of contaminants that might be in their water supplies, said that in recent years much attention has been given to urban drinking water problems. But Craig Cox, EWGs senior vice president for agriculture and natural resources, said when it comes to long-term ingestion of nitrates, The people who are most at risk are the people who are living in farming communities.

For example, nearly three-quarters of the communities where nitrate contamination in drinking water is at or above the legal limit are in heavily farmed counties of Arizona, California, Kansas, Oklahoma and Texas.

Many rural communities have been addressing the contamination through stopgap efforts, while looking at building sophisticated water treatment plants with ion exchange or reverse osmosis systems. In Pretty Prairie, Kansas, as a short-term fix, pregnant women and those with babies six months or younger have been provided free bottled water so that they can avoid nitrates. For a long-term solution, Pretty Prairie has been told by federal officials to build a $2 million-plus treatment system which would amount to more than $3,000 per resident, EWG said.

Nitrates also plaguing city systems

Among the 60 larger and sometimes urban water districts with nitrates matching or exceeding the 5 ppm level, farm runoff in certain cases still appeared to be a factor. One example is Des Moines, Iowa , which is surrounded by farmland and uses river water for drinking.

But that group of larger water districts also includes the systems serving the Southern California cities of Pasadena and Burbank , where officials have said nitrates leach into well and groundwater from natural erosion or sewage as well as from fertilizer use.

The Fertilizer Institute , which represents major farming and chemical manufacturing firms, has said evidence indicates that blue baby syndrome may not be solely related to nitrate exposure, and it has disputed any notion of exponential increases in nitrates in North American groundwater. But the industry group also promotes efficient farming practices, such as applying fertilizer at the best time and rate, as a way to reduce the problem.

Several conservation practices already exist to help prevent erosion of soil which may contain excess nutrients from ending up somewhere they shouldnt be, the institute says in an environmental paper on its website.

Last year, a study of post-menopausal women in Iowa by researchers from the National Cancer Institute, the National Institutes of Health and several universities concluded that long-term ingestion of drinking water with nitrates exceeding 5 ppm was associated with an increased risk of bladder cancer.

Jury is still out

Another 2016 analysis , by researchers at Texas A&M and the University of Iowa, declared that several studies published since 2000 showed an association between mothers exposed to high concentrations of nitrates in drinking water during their pregnancies with neural tube defects, oral clefts and limb deficiencies.

But Peter Weyer, a University of Iowa researcher who participated in both of those 2016 reports, cautioned that the jury is still out on the effects of nitrates at levels from 5 ppm to 10 ppm because several other studies have not shown an association with health risks at those levels. He said one possibility is that long-term health problems result from a combination of nitrates and other contaminants such as herbicides.

Still, Weyer said, consumers concerned about the contamination can buy reverse osmosis or other systems that reduce the nitrates at the tap. Its expensive but it is an option, he said. Weyer said people who own unregulated private wells may be at particular risk and would have to test their own water to know of potential contamination.

EWG urges farmers and other landowners to reduce runoff, and cut contamination from nitrates and other hazardous chemicals, by taking such steps as maintaining buffers between tilled cropland and adjacent streams.

This story was reported by FairWarning (www.fairwarning.org), a nonprofit news organization based in Pasadena, Calif., that focuses on public health, consumer and environmental issues.