How Orthophosphate Used in Drinking-Water Treatment Is Quietly Changing Urban Streams
The relationship between city water systems and nearby streams is usually treated as simple: one delivers drinking water to homes, the other carries rain and runoff away. But new research from the University of Pittsburgh shows these systems are more connected than we think. When cities add orthophosphate to drinking water to keep lead pipes from corroding, some of that treatment chemical is leaking underground and ending up in urban streams. And the effects are far from minor.
This finding comes from a detailed study published in PLOS Water, where a team of biogeochemists, hydrologists, and environmental engineers examined how drinking-water corrosion control measures influence stream chemistry. Their results point to a surprising and previously overlooked environmental impact linked to an otherwise essential public-health practice.
What the Researchers Found
The study team monitored five urban streams in Pittsburgh before, during, and after the city implemented orthophosphate-based corrosion control between February 2019 and June 2020. Orthophosphate is widely used across the United States and parts of Europe because it forms a protective coating inside old lead pipes, reducing the amount of lead that can enter drinking water.
But the new data showed that this chemical doesn’t stay confined within the underground infrastructure.
After the orthophosphate dosing began, phosphorus levels in the streams jumped by over 600%. Even more surprising, concentrations of copper, iron, and manganese — metals that corrode from old pipes — increased by almost 3,500%. These spikes provide clear evidence that treated drinking water and corrosion byproducts are escaping into soil and groundwater, eventually reaching surface streams.
The researchers note that most people assume water mains and distribution pipes form a sealed system. But in many older cities, aging, cracked, and leaking infrastructure creates pathways where drinking water slowly seeps outward. Instead of staying contained, water carrying phosphate and trace metals enters the surrounding environment.
To understand the biological effects, the team also performed nutrient-addition bioassays, testing how algae respond when small amounts of phosphorus are added to streamwater. The results were consistent with what scientists know about aquatic ecology: phosphorus is often a nutrient-limiting factor. When phosphates enter a water system that normally has very low levels of dissolved nutrients, algae grow faster and thicker. In the study, phosphorus additions notably stimulated algal biomass in the tested samples.
This enrichment effect is concerning because it can accelerate eutrophication, the process in which nutrient overload leads to excessive algal growth. When algal populations explode, they consume oxygen as they decompose, reducing water quality and harming fish and other aquatic organisms.
Why These Findings Matter Beyond Pittsburgh
Lead pipes are concentrated in the Northeast, the Great Lakes region, and the Midwest — meaning tens of millions of Americans rely on systems that use orthophosphate to control corrosion. If leakage-induced phosphorus pollution is happening in Pittsburgh, it is likely happening in other cities with similar aging pipe networks.
Phosphate leaching has historically been overlooked as an environmental issue because pollution discussions usually focus on:
- Wastewater discharges
- Agricultural runoff
- Stormwater systems
- Industrial sources
But this study identifies a new pathway: direct loss from buried drinking-water infrastructure.
Importantly, the researchers emphasize that public health must remain the top priority. Orthophosphate is one of the most effective tools for reducing lead exposure. The challenge is figuring out how to protect human health while also protecting nearby ecosystems.
How Orthophosphate Works in Drinking Water
Orthophosphate is a form of phosphorus added at low concentrations to drinking water. It interacts with the inside of lead pipes, forming a thin insulating layer of lead-phosphate scale. This layer prevents water from contacting the metal directly, greatly reducing lead dissolution.
Without corrosion control, cities can face emergencies like:
- Flint, Michigan
- Washington, D.C.
- Recent issues in Pittsburgh
These events show that corrosion control is not optional — it is essential. But the discovery that some of the chemical escapes the system means communities now have to manage both sides of the trade-off.
The Ecological Impact: From Chemistry to Algal Blooms
Streams in urban areas often have relatively low nutrient levels compared to agricultural regions. A sudden 600% rise in phosphorus shifts the chemistry fundamentally. When combined with increases in metals like copper and iron — which can influence microbial processes — the stream ecosystem begins to behave differently.
Potential consequences include:
- Higher rates of algal growth
- Shifts in microbial community composition
- Mobilization of metals in sediment
- Long-term changes in water clarity and oxygen levels
In the study, the researchers used streams that were still above ground — many Pittsburgh streams are buried in culverts — which allowed them to measure how real ecosystems respond. Their sampling monthly across two years provided enough data to clearly distinguish the effect of orthophosphate from natural seasonal variation.
Four Solutions Proposed by the Scientists
To limit phosphate leakage from drinking-water systems, the researchers suggest four main actions:
1. Repair and Replace Aging Infrastructure
Leaky pipes are common in older cities. Upgrading water mains and service lines helps prevent treated drinking water from escaping into the environment.
2. Upgrade Wastewater Treatment Facilities
Better nutrient removal at treatment plants can counterbalance unexpected phosphorus entering waterways through infrastructure leaks.
3. Optimize Orthophosphate Dosing
Utilities can examine whether lower concentrations still provide adequate corrosion protection. Not every region requires the same dosage.
4. Develop Better Monitoring Tools
Cities often lack systems that track how buried infrastructure interacts with groundwater and surface streams. Improved sensors and analytics would help detect leaks earlier.
The researchers stress that these steps should be part of a national conversation. With millions of Americans depending on old water systems, communities must balance the urgent need to reduce lead exposure with the responsibility to protect the aquatic environments next door.
Additional Context: Nutrients, Streams, and Urban Infrastructure
How Nutrients Drive Stream Health
In many freshwater systems, phosphorus is the primary limiting nutrient. Small increases can dramatically influence:
- Algal growth
- Biofilm development
- Stream oxygen levels
- Fish habitat quality
This is why phosphorus is heavily regulated in wastewater discharges. Even minor leaks from drinking water systems can have ecological consequences when sustained over time.
Why Metal Spikes Matter
Copper, iron, and manganese aren’t simply harmless bystanders. They influence:
- Aquatic toxicity thresholds
- Solubility of nutrients
- Microbial metabolism
- Sediment chemistry
A 3,500% increase in dissolved metals suggests that corrosion scales from pipes are being transported into natural streams — a sign of significant system leakage.
Drinking-Water Infrastructure as an Environmental Actor
Traditionally, environmental science treats drinking-water systems as closed, engineered structures. But this study shows that they behave more like dynamic participants in the environment. Water doesn’t always stay in the pipes, and chemicals meant exclusively for human health can escape into ecosystems.
Research Paper Link
From Pipes to Streams: The Hidden Influence of Orthophosphate Additions on Urban Waterways
