29 February 2016

Lead, Plumbosolvency, and Phosphates in the Environment

Posted by John Freeland

What’s happened to the water supply in Flint Michigan is especially ironic considering the state is surrounded by four of the five Great Lakes, which make up 95% of the surface fresh water in the United States. If Michiganders can’t have safe public water, who can?

The problem, briefly summarized here, boils down to variable source water chemistry, plus common chemicals added at the water-works, interacting with outdated lead plumbing components in a process called plumbosolvency. The problem extends far beyond Flint .

How serious a problem is lead in the environment? Lead toxicity affects every organ system of the body. The renowned soil chemist and UC Berkeley professor Garrison Sposito, writes that lead, a potent neurotoxin, “probably represents the most important environmental hazard on a global scale” (Sposito 1989).

More information: Vox article with CDC map of US counties with Elevated Blood Lead Levels

Mitigating the Risk Using Lime and Phosphates
To inhibit related health risks from lead dissolution, lime and phosphate are commonly added to water supplies to adjust pH and form a protective low-solubility secondary mineral coating inside the water pipes. Maintaining a stable pH is absolutely critical because secondary mineral species are stable within specific, often narrow pH ranges. When pH fluctuates, stable metallic-phosphate compounds become unstable, dissolve and begin to form new compounds at the new pH level. The relationship between pH and mineral stability is illustrated using a Pourbaix diagram, familiar to soil chemists, geochemists and toxicologists. During this dissolution and re-precipitation process, lead and other metals are mobilized in solution – a dangerous situation.

Unintended Environmental Consequences – Eutrophication
Human health and safety have to trump all other concerns. If adding phosphates to drinking water is necessary to prevent lead poisoning, toxicity, then so be it. However, it’s also worth noting that these phosphates are additional anthropogenic nutrients that enter aquatic ecosystems as municipal water returns to waterbodies by way of sewage treatment plants, stormwater systems, runoff, and leakage. The phosphates we use to prevent lead poisoning contribute to another environmental problem – eutrophication.

How significant is drinking water phosphate to the overall nutrient budgets of lakes and rivers? The state of Minnesota investigated that question and summary notes are available here The Minnesota study included phosphates added to drinking water by several communities (about 400 Minnesota communities add phosphates to drinking water) and found concentrations ranging from approximately 0.5 to 9.5 ppm. Below is a list of important phosphate concentration thresholds (mg/L = ppm):

0.01 – 0.03 mg/L – the level in uncontaminated lakes
0.025 – 0.1 mg/L – level at which plant growth is stimulated
0.1 mg/L – maximum acceptable to avoid accelerated eutrophication
Greater than 0.1 mg/L – accelerated growth and consequent problems
(some information from Water, Water Everywhere. HACH Company. Second Edition. 1983.)

The phosphate concentrations in the Minnesota study data by far exceeded the maximum level to avoid eutrophication. The study also found that summer use of the public water was 2.5 times higher than the rate of winter use. This is due to such common summer activities as car washing, lawn watering, golf course irrigation, and fire hydrant flushing. This water – spiked with phosphorus – ends up in waterbodies by stormwater discharge.

Elevated water use during the summer is ecologically important because summer is also the time when algae are growing in lakes and rivers. The additional phosphorus promotes algae growth, which can lead to nuisance algal blooms.

The Minnesota study found that the total “phosphate corrosion control for drinking water treatment contributes about 9% of total “P” loading.” Because the some forms of phosphorus are more bioavailable than others, and the drinking water phosphate additives are loaded to the environment at a higher rate during the summer, the added phosphate may have a greater environmental impact than the 9% figure suggests.

Can it be fixed?
The phosphates added to drinking water to protect us from lead and other heavy metal poisoning can be significantly reduced by replacing the lead plumbing with plastic pipe. In Burton, Michigan, a more affluent suburb of Flint, the pipe replacement project to do just that has recently won an award.

Upgrading municipal water systems would help solve two public health problems: lead poisoning and nuisance algal blooms that contaminate water supplies with microcystin. The technology is in place.

Sposito, G. 1989. The Chemistry of Soils. Oxford University Press, New York.