A great deal of recent media attention has been paid to phosphorus as a pollutant;
this article is intended as a primer on that topic. Specifically, this article will explore
what phosphorus is, why it is used and how it enters the water system. Beyond that,
the problems created by excess phosphorus will be examined along with the constructive
ways that states and communities have dealt with the issues.
To be academically correct, phosphorus is actually an element; as it is commonly
used in the water management industry and in the media, however, it refers to
compounds of phosphorus, or phosphates (PO4). In its elemental form, phosphorus
is highly toxic, yet its compounds are critical for life on Earth. In fact, phosphorus is
the ‘glue’ that binds the sugars that form the backbone of DNA. Additionally, in plants, phosphates are key in nutrient uptake, flower formation and photosynthesis. Three
types of phosphates exist – orthophosphates, polyphosphates (also called metaphosphates) and organophosphates. Orthophosphates are associated with natural processes,
human-made fertilizers and sewage treatment. Polyphosphates are commonly used in detergents but when introduced to water, polyphosphates will change to the
ortho-form. Organophosphates are the phosphates that are actually being used by and actively bound in the plant material, so they are rarely found ‘free’ in water systems.
In addition to detergents (mentioned above) that can enter the water system via semi-treated household and industrial sewage and phosphates that naturally leech
out of rocks and soil, a major introduction of phosphates into watersheds comes from agricultural and urban runoff; more specifically, agricultural fertilizers and lawn-care
products breakdown to release phosphates into the soil. Because plant growth can be
limited by the amount of phosphorus available in the soil, it has become common practice
to apply fertilizers which enhance the amount of phosphorus available and thus increase
the crop yield. Because they are soluble in water, phosphates that have not been
taken up by plants will tend to “runoff†with the rainwater after a heavy rain. It is
often by this means that phosphates enter creeks, rivers, streams and, in urban areas,
storm sewers. While the use of phosphate-enriching products is aimed at noble end-goals, most notably increasing food availability, problems arise when excess phosphates enter the water system.
In terrestrial plant systems, phosphates are necessary, but nitrogen is the primary
nutrient that limits plant growth; in aquatic systems, however, phosphorus is the
primary limiter of growth. Essentially, when aquatic plants have consumed all of the
available phosphorus, all growth will stop; as a result, excess phosphorus will lead to
rapid and prolific aquatic plant growth. Most commonly, this growth comes in the
form of algae blooms and it is often these algae blooms that are the first obvious
indication that there is a problem. While the growth of algae can be a problem by itself,
when the algae begins to die, the microorganisms that consume the dead algae also
remove dissolved oxygen from the water in a process called eutrophication. If left
unchecked, the water will become highly eutrophic, essentially devoid of oxygen;
in this state, higher aquatic life such as fish, worms, frogs or turtles cannot survive. Additionally, the algae itself can pose a threat; blue-green algae produces natural
toxins that can be lethal when consumed by livestock or wildlife who drink from a contaminated source.4
The problems that result when excess phosphates enter the water system are
obvious; what may not be as obvious is that fairly simple practices can mitigate
the problem. Many states are encouraging the use of ‘buffer strips’ where agricultural
land abuts creeks, streams and rivers. These strips of land are not tilled or planted
and are essentially left to ‘go wild’; the resulting effect is typically a zone of high
grasses and wildflowers. These tall grasses and plants are barriers to the rainfall
runoff from the agricultural field and serve to slow the velocity of the water. By
slowing the runoff velocity, the phosphate-rich water has more of an opportunity
to leech into the soil where it will be bound by the roots of plants in the buffer strip.
This practice has been shown to be effective in numerous studies by the Army
Corp of Engineers, the US EPA and several state EPAs. Additional reductions in
amount of phosphates that can enter a body of water can be achieved by
encouraging tree and shrub growth in these strips. These simple techniques
can reduce overall phosphate runoff by well over 50%. Farmers seem willing to
adopt these techniques as the zones near creeks and streams are often naturally
difficult to cultivate. In addition state governments have encouraged this practice
by offering credits or outright paying farmers to adopt the practice; Maryland,
Pennsylvania, New York, Ohio and several other states have specific programs
aimed at encouraging this practice.
Wetlands, whether natural or man-made, also offer a means to reduce phosphorus
entry into a watershed. Typically shallow and rich in plant life, wetlands are an effective means of slowing the runoff water while simultaneously offering abundant plant life
that can assist in the rapid uptake of the phosphates. Wetlands are often viewed
as wildlife sanctuaries but, in fact, they are a vital player in maintaining the health
of freshwater systems. Wetlands, often referred to as ‘the kidneys’ of the watersheds,
serve to filter and prevent entry of many pollutants into the water systems of the world.
Beyond states encouraging the creation of filter zones and buffers, some cities have developed plans of their own. New York City’s water system, managed by the nonprofit Watershed Agricultural Council (WAC), serves nearly 10 million people. To combat the
problem of phosphates and other runoff-related pollutants, WAC partnered with the U.S. Department of Agriculture Natural Resources Conservation Service, and soil and water conservation districts to develop “whole-farm” plans to help farmers reduce harmful farm runoff and protect the watershed. Also, in the state of Vermont, the state government directs the municipal governments to develop local runoff abatement plans.
Aside from taking active steps to reduce the amount of phosphates entering the water system, there is significant effort focused on monitoring phosphate levels and tracking phosphate entry into watersheds. The Phosphate Index (P Index) has been widely
adopted as a means of quantifying the potential of agricultural phosphate to enter
surface water. In states such as Minnesota and Iowa, the P Index is used to identify areas where the state needs to take quick action to prevent phosphate migration into a watershed. In New York and other states, active monitoring of the phosphate
levels in the water itself helps to avoid excess phosphates making it into a drinking water reservoir.
The use of phosphates is widespread in both industry and agriculture and it is
likely that phosphates will remain in use for many years to come. While problems
that excess phosphates create in fresh water systems are serious, solutions exist
to manage and mitigate these problems. Through effective education and outreach
programs, monitoring and active remediation, the states and the federal government
can begin to deal with the issue of phosphates.