Conservation Minnesota

Why Phosphorus?

When discussing runoff pollution and the various ways to mitigate its impacts, phosphorus is often the first, or at least the most prominent, pollutant mentioned.  However there is often confusion among the kinds of pollution (phosphorus, nitrates, sediments, pesticides, etc.) that are impacting our waters and the efficacy with which mitigation measures treat each type.

Water pollution issues first became part of the public dialog in the late 1960s.  At that time, Lake Erie (and many other lakes) were experiencing severe algae blooms, which tainted drinking water and caused fish kills.  Remember, “Is Lake Erie Dead?” from the cover of Time Magazine?

The proximate cause of these blooms was excess phosphorus in the lake water.  “Phosphates” (phosphate is the active form of phosphorus) were the recognized enemy of clean water.  At that time, there were hundreds of municipal sewage discharges to the Great Lakes and the treatment systems of the day did not remove phosphorus.  At the same time, many soaps and detergents had phosphates as an additive, so when rinse waters were sent to the treatment plants, the phosphates in them went right to the lake.

Eventually phosphates were banned from being added to detergents and sewage plants were upgraded to remove phosphorus.  Yet, excess phosphorus in runoff to lakes remains a problem.

To address phosphorus (and other kinds of pollution) in runoff, “best management practices” or BMPs were developed to mitigate runoff pollution.  Today, we know these BMPs as stormwater ponds, buffer strips, wetlands, rain gardens, etc.

Because of this history, we take advantage of our collective experience with water pollution involving phosphates, so messages still include mention of phosphorus in the list of benefits.

But beware.  As I have described previously, our lakes are by-and-large not improving with respect to phosphorus and algae problems.  Here’s why:

  1. Lakes that are impaired, that is, over the edge, are not responsive (with few exceptions) to phosphorus reductions from runoff.
  2. Excess phosphorus from runoff is a result of long-standing land alterations, which have caused increases in phosphorus delivery to lakes, often 10- to 20-times more than background levels.  These underlying changes are hardwired into the landscape and practically impossible to reverse.
  3. BMP performance for phosphorus removal is poor compared to the removal of other kinds of pollution.
  4. Combining these factors means we can reduce phosphorus to lakes using BMPs by, at best, 25%.  If the inputs are 10- to 20-times more than a healthy lake can handle, a 25% reduction means the phosphorus input remains 7- to 15-times in excess.

BMPs remove various types of pollutants with varying efficiencies.  For example a stormwater pond may remove 99% of the sediments (comparing the amount flowing in to the amount flowing out), but only 15% of the phosphates.  Other types of pollution, such as nitrogen, lead, pesticides, tend to fall in between.

In this sound-bite world, much of the message is often distilled and we may not have the entire story.  Saying a buffer strip removes 90% of pollution may be accurate for sediments, but it is misleading for phosphorus.  Even saying buffer strips (or any BMPs) remove a given percentage of pollution is still contingent on the buffer strip being sufficiently wide, appropriately sited, properly installed and adequately maintained.

Also, different water bodies are more or less sensitive to phosphorus pollution.  Lakes are sensitive, but rivers and streams not so much.

Another factor too often skimmed over is the fact that BMPs require maintenance to sustain their pollution removal abilities.  Reported removal rates are most often applicable for the ideal situation, but in reality the removal efficiencies are much less or unknown.  Sadly, most BMPs are not adequately maintained.

National studies of BMP performance show that only the BMPs that hold back runoff water for extended periods, can significantly mitigate phosphorus.  Of course the phosphorus doesn’t disappear, so it can show up later or in subsurface water and may still get to lakes.  Other BMPs, such as buffer strips, porous pavement, or rain gardens, do not significantly remove phosphorus.

All these BMPs have a place.  When used at the proper time and place to address specific water pollution concerns for downstream receiving waters, BMPs are beneficial.  Let’s beware for the one-size-fits all approach.

About Dick Osgood

Dick Osgood
Dick Osgood has authored numerous scientific journal papers, made hundreds of presentations at professional meetings, and recently co-authored his first book.  Now he’s working on his second book titled A Lake Manager’s Notebook.  Dick is a Certified Lake Manager and offers lake management consulting services through his business Osgood Consulting LLC.
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