Phosphate in drinking water can contribute to nutrient pollution, study says
April 3, 2023
A water main break in Richmond, Virginia, in July 2016, shown in "Water Main Break on 900 block of W Fanklin" by Eli Christman, licensed under CC By 2.0.
Nutrient pollution remains a critical problem in many U.S. waterways, leading to such concerns as toxic algal blooms and eutrophication. Addressing this problem requires a clear understanding of the sources of nutrients, including phosphorus. Effluent from septic systems and wastewater treatment facilities and runoff from agricultural lands are known to be common sources of phosphorus. But what about treated drinking water?
A recently published study by researchers at the British Geological Survey, the U.K.’s Lancaster University, and Union College in Schenectady, New York, estimates that outdoor water use and leakage from water mains constitute a significant source of phosphate entering the environment in areas where the compound is used to control lead and copper corrosion in distribution systems.
Developing estimates
In 2015, as much as 14.9 kt of phosphate was added into U.S. drinking water distribution networks to control corrosion, according to the article by Flint et al. titled “Watermains Leakage and Outdoor Water Use Are Responsible for Significant Phosphorus Fluxes to the Environment Across the United States,” which was published online by the journal Global Biogeochemical Cycles on March 11.
To obtain this estimate, the authors of the article relied on data from the U.S. Environmental Protection Agency indicating the number of public water systems having active phosphate-dosing systems for corrosion control in 2015. This data then was used to determine the fraction of a county’s population served by public water systems that add phosphate.
Based on the total volume of freshwater withdrawn for drinking water per county and target dosing concentrations for phosphate, the researchers estimated the total amount of phosphate added to drinking water in each county. County-level estimates then were aggregated to obtain a total national estimate.
In most cases, state or national leakage factor data was used to estimate the volume of water leaked from mains and the amount of phosphate within the lost water. Similarly, state or national leakage factor data for outdoor water use was used to estimate the amount of phosphate within the water applied outside.
Following a water main break in New York City in September 2011, workers from the Metropolitan Transportation Authority pumped out up to 10 feet of floodwater from tracks of the city's subway system.
Photo Credit: Metropolitan Transportation Authority / Patrick Cashin.
Regional fluctuations found
Of the 14.9 kt/yr of phosphate estimated to have been added to U.S. water supplies, most of this phosphate load was returned to wastewater treatment facilities, according to the article. However, between 0.7 to 2.6 kt/yr, or 5 to 17%, was estimated to have been released into the environment by water main leaks, according to the article. Another 0.8 to 3.1 kt/yr, or 5 to 21%, was released into the environment by outdoor water usage.
Combined, the upper bound estimates of phosphate fluxes are “equivalent to around 12% of [phosphorus] inputs to the environment from urban fertilizer, 2.6% of the [phosphorus] load to surface waterbodies from point sources, and 0.3% of [phosphorus] inputs to the environment from farm fertilizers and manure application,” according to the article.
However, these fluxes were highly localized, in keeping with the localized nature of phosphate dosing. For example, the researchers found that 4,572 of the 152,104 active U.S. public water systems had at least one facility for dosing drinking water with phosphate to control corrosion. These systems were located within 1,402 of the 3,109 counties included in the study. All told, only 3% of U.S. public water systems used phosphate dosing in 2015. However, many of those systems serve large populations, such that 25% of the U.S. population receives water from such systems.
Midwestern, Northeastern, and Mid-Atlantic states, as well as California, were found to have the highest numbers of drinking water systems using phosphate and the largest populations served by such systems. Within these regions, the highest estimated volumes of phosphate entering the environment by means of drinking water were found in heavily urban counties.
Ultimately, the lower and upper county-level estimated phosphate fluxes exceed phosphorus inputs to the environment from urban and farm fertilizer usage and manure application across 16–56, 13–21, and 17–32 counties, respectively, and exceed phosphorus inputs from point sources to freshwaters across 461–541 counties, according to the article. Meanwhile, “when upper bound county-level estimates are considered, 39 counties have combined [water main leakage and outdoor water use] fluxes that exceed the sum of all major [phosphorus] inputs to the environment,” the article states.
Improving ongoing efforts
The study results could be used to boost ongoing efforts to improve water quality by reducing nutrient pollution, says Elizabeth Flint, a Ph.D. student hosted by the British Geological Survey and the lead author of the study. “With the U.S. having one of the highest water use volumes per capita anywhere in the world and persistent nutrient pollution of the country’s freshwaters, we have been working to understand how these two major environmental pressures may be linked,” Flint says. “We hope that the outputs of this research will inform more sustainable management of freshwater quality and quantity across the United States.”
“The localized significance of phosphorus inputs from leakage and outdoor water use should help to promote targeted lead service line replacements and support the need to reduce the use of treated tap water for domestic irrigation,” Flint says. “Although leakage and outdoor water use are relatively smaller inputs of phosphorus on a national-level, their quantification on this spatial scale will facilitate a more integrated understanding of nutrient cycling, and will allow for more accurate quantification of other phosphorus inputs across the country.”
In fact, reductions in phosphate fluxes could help lead to improvements in some locations where nutrient pollution has long persisted. “Many of the areas that were found to have the highest release of phosphorus from leakage and outdoor water use also have persistent nutrient pollution,” Flint says. “For example, around half of the land of Pennsylvania drains into the Chesapeake Bay watershed, which ultimately contributes to the eutrophication of the Chesapeake Bay. We estimated Pennsylvania to have one of the largest inputs of phosphorus. Reducing the release of phosphorus via leaking water mains and limiting the use of dosed water for domestic irrigation purposes may help the state reach its phosphorus reduction goals and contribute to the improvement of water quality within the bay.”
Given the importance of phosphate in helping to prevent lead and copper corrosion, utilities that rely on the compound for this purpose cannot discontinue its use unless or until they have removed those components from their systems. That said, water providers can take steps to reduce phosphate fluxes. “Although phosphate dosing will be a necessity until lead service lines are fully replaced, utilities should be striving to optimize their dosing of water, as well as to minimize leakages,” Flint says.