The Five Thousand Pound Gorilla in the room

There is a five thousand pound gorilla in the water quality design and approval room that no one seems to want to acknowledge.  While I and my colleagues in SWEMA are looked upon with distain by many in the regulatory world, we actually have met the gorilla, and have him in protective custody, if not completely under control.

While the available BMP databases, modeling programs, and regulatory protocols have characterized the average wash off concentrations at 100 to 200 mg/L, the gorilla sits on display for all to see, waiting patiently for someone to do the math.  A recent article in Stormwater Magazine cites the typical total suspended solids at about 1,100 pounds per acre per year (PAY), an oft repeated figure, and one that is accepted as “common knowledge.”  (Note: Stormwater Magazine is an excellent publication.  I read every issue cover to cover.  You should too.)  The reason most people do not question the number is that they have no idea of how a concentration of solids in the range of 100 to 200 milligrams per liter would convert to PAY.  The conversion depends on the amount of rainfall (in inches) that a region has annually.  To convert milligrams per liter to PAY, you have to convert the annual volume of rainfall depth (one acre-inch is 3,630 cubic feet, and one cubit foot is 28.317 liters, so one acre inch is 102,790.2 liters).  A concentration of 1 mg/L would then be 102,790.2 mg, or 102.79 grams.  A mass of 102.79 grams is 0.2266108 pounds.  Every acre-inch of runoff that carries a concentration of 1 mg/L will carry 0.2266108 pounds of mass.  If the concentration is 100 mg/L then the runoff will carry 22.66108 pounds.

Now we can look at an area that has 40 inches of rainfall annually and see what the wash off results might be.  At a concentration of 100 mg/L, then 40 times 22.66108 pounds would be the amount of solids that would be carried off of the site, or 906.44 pounds.  At 200 mg/L the PAY would be 1,812.88 pounds, which brackets the estimated solids loading in the magazine article quite nicely.  To be fair to the article and the magazine, the figures were not cited from specific studies, but were the assumed numbers that have been repeated over and over in the literature.  Even the gorilla would point out that not all the annual rainfall runs off, and that concentrations can vary greatly, but if less water is running off at that concentration, the PAY numbers would go down, not up.

The gorilla is the actual PAY we have found in manufactured devices over the past ten years and 5,000 maintenance and cleaning operations.  The fact is that the average solids removed from these devices has been 5,000 PAY.  Even if we assume that 50 inches of runoff occurred, this is an even 100 pounds of solids per acre-inch, or an average concentration of 441.59 mg/L.  One must remember that the devices that collected this material are the red-headed step-children of BMPs, and are considered to intercept only 50% of the solids (at the most).  This means that the full average concentration of solids must be in the 883 mg/L, if we are to believe the “common” knowledge and assumptions.

The fact is that the assumptions are wrong.  Reliance on wash-off models that predict low concentrations is wrong, and can easily be proven wrong by simply examining the solids in manufactured devices, or in public domain sumps, vaults, oil-grip separators, or fore bays.  These gross errors in estimates have been perpetuated by people with specific interests, including those who want to simplify water quality down to a manual with a few standard practices (all magically removing the targeted percentage in the local permit), and a nice chart or spreadsheet “proving” that the goals have been met.  The gorilla is unimpressed.

Why is the gorilla important?  He is important because perfectly innocent wet ponds, constructed wetlands, porous pavement, infiltration systems, bio-retention cells and many other BMPs are failing early and often because no one wants to admit that there is a gorilla in the room.  No amount of scheming for low concentrations and small particle based BMPs can protect our land based systems and eventually our streams from the onslaught of 5,000 pounds of sediments per acre per year.  If we want to actually clean up our watersheds, we ignore the gorilla at our own peril.

On February 23, 2011, posted in: Blog by
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