Bioretention, 3/7/2019

Posted on March 7, 2019

A colleague asked about the “bioretention area” in the 380 Maple West proposal (Maple and Wade Hampton, 40 condos).  I had to learn about this topic nearly ten years ago, for a construction project at the church I was attending.  On this page, I’ll try to put that learning to use, and explain what that bioretention area is, what key role it plays here, and what the potential drawbacks are.

What is a bioretention area? 

In this case, it’s a puddle. It’s a large, fairly deep, engineered puddle.  But, at root … it’s a puddle.  It’s a place to allow standing water to remain on the lot.  The “bio” part just means that, in this case, the water in the puddle filters through dirt before being absorbed into the soil or being piped to the storm drain system.

Bioretention areas come in a lot of configurations, with a lot of different names.  You’ll hear the terms rain garden, vegetated swale, dry well, infiltration trench, percolation trench, stormwater planter, and so on.  The temporary storm water retention ponds that you see around the area can be considered bioretention areas.  Anything that lets water stand and filter slowly through or into the soil is, in theory, a bioretention area.

In this case, it looks like the builder has chosen to build a stormwater planter, based on the description as an “urban bioretention”.  The illustration below is from Arlington County’s construction details for an urban bioretention area.

Water will flow from the building’s downspouts into the planter, and pond up to 6′ above the soil level.  (Beyond that, it just drains directly into the storm sewers via the overflow pipe.)  The water will, in theory, filter slowly through the soil layer, then through the gravel and out into the storm sewers.

The bottom of the box could be open to the soil beneath, so that the water would actually percolate directly back into the ground.  But around here, as any gardener will tell you, what we have is mostly dense clay soil that does not drain well.  On the 380 Maple West site, the plans show that 84% of the dirt is class “D” soil, which is described here as:

... very low infiltration rates when thoroughly wetted and consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface and shallow soils over nearly impervious material.

So, likely, there would not be much difference between a concrete bottom to the planter, and no bottom to the planter.  Once its wet, typical soil in this area drains somewhat better than concrete does, but not by much.

What does this do for the environment? 

Ever since the 1988 passage of the Chesapeake Bay Preservation Act, local governments have been tasked with doing better management of storm water runoff.

One thing that does is reduce the amount of sediment that is washed into the Bay, both by filtering it out of the storm water and by making urban streams less “flashy” (so that they don’t scour out their stream beds and deposit that in the Bay.)  In the case of roof runoff, the sediment is in the form of dirt and dust that has accumulated on the roof.

Second, it reduces the levels of two key nutrients — nitrogen and phosphorus — flowing into the Bay.  (Nitrogen here means biologically available (“fixed”) nitrogen, not nitrogen gas.)  And, oddly, for purposes of this discussion, those nutrients mostly come out of thin air.  Rain water absorbs nitrogen and phosphorus from vehicle exhaust and coal-fired power plant emissions.

Third, and not really relevant here, controlling parking-lot and street runoff can reduce heavy metals, petroleum products, and other pollutants that are largely a consequence of vehicle traffic.  There is a fourth issue — reducing pathogens — (e.g., from animal feces) that does not really apply to roof runoff.

The really odd thing about these storm water management practices is that the amount of nutrients kept out of the Bay is small.  Effectiveness is measured in “pounds of nitrogen per year” or “pounds of phosphorus per year” that are (in theory) kept out of the Bay.  Here, this stormwater planter is estimated to remove 0.59 pounds of phosphorus per year, and 4.92 pounds of nitrogen.

I don’t say that to dismiss the impact of this stormwater planter.  I say that mainly to emphasize how environmentally destructive lawn fertilizer is, in the Chesapeake Bay watershedCommon lawn fertilizer (e.g., by the bag, at Home Depot) is typically about 30% available nitrogen.  So you can easily ask yourself, how much would it cost you to buy enough available nitrogen to offset the entire annual benefit from that stormwater planter?  The answer is $12.  That buys a 15 lb bag of 29-0-4 lawn fertilizer at Home Depot.   And at the recommended use rate, I should, in theory, spread four bags of that on my lawn.   Any bit of carelessness — say, applying it the day before a big downpour — and I could easily swamp the nitrogen savings from that stormwater planter.

Around here, if you see a beautiful, lush, deep-green lawn, chances are excellent that the owner has been putting putting nitrogen fertilizer on it.  I proudly enjoy my totally mediocre lawn.

A final oddity, for the Town of Vienna, is that the Virginia requires the Town to reduce such pollutants flowing out of the Town, as a whole.  And the Town nimbly sidestepped doing anything of substance by claiming that its long-standing street-sweeping program was adequate to produce full compliance with the law (the gist of this document (.pdf).  Which, I guess, is true legally, but ludicrous from the standpoint of pollution reduction.  This is one of the many reasons why I don’t put much faith in the Town’s efforts to present itself as “green”.

What does this do for the builder?

In short, it allows him to have more impervious surface (building and pavement) than he would otherwise.  Another way of looking at it is that this allows him to cut down on green space on the lot, and so build a larger building than would otherwise be possible.

Just how little green area does this proposed building have? That’s easy enough to see, because the plans for the proposed 380 Maple West building have to show that calculation.  The figure below is from page 7,  item 2 of the materials posted for the 3/4/2019 Town Council work sessionThe MAC building will actually have less green space than the lot does now.  Currently, 16% of the lot is calculated as grassy area.  For the proposed building, 15% will be.  So much for the vast public benefits of MAC redevelopment.

And it’s the stormwater planter that allows them to do that.  That’s how they can reduce the green space, and yet manage to meet the target for reducing pollutant runoff from the lot.  In affect, the stormwater planter (aka urban bioretention area) acts like super-turf.  For minor rainfalls, it will prevent much of the water from running off the lot.  And even for major rainfalls, it will, in theory, filter a significant portion of the two main pollutants (available nitrogen and phosphorus) out of the water.

What can go wrong?

In a nutshell, water retention areas like this typically eventually clog with sediment and then require maintenance in order to function properly.

Based on my experience with my church’s parking lot, Fairfax County is going to require that the owners of this property keep that stormwater planter in good working order, in perpetuity.  Which makes sense, because the proper operation of that planter is how this property complies with the law.

Eventually, with these types of bioretention systems, the soil layer clogs with sediment washed down from the roof, or the underlying drain pipe clogs with sediment washed down through the soil layer. This is particularly true in our area, as windblown soil sediments are mostly clay (because our soil is mostly clay).  The estimated life of the system that my church installed was, I think, about seven years.  After which, they’d have to dig it up, clean it out, and rebuild it — in theory.  In practice, I don’t think Fairfax does much to make sure these systems are kept up.

For some types of bioretention systems, you can tell the system has failed because there will still be standing water 48 hours after a rainstorm.  When Fairfax looked at the earliest generation of such bioretention systems around the county, they found that the majority of them had failed.  (Unfortunately, I cannot find the reference for that).  But here’s an example of a failed rain garden in Fairfax County.  As I understand it, failure (under the standing-water-at-48-hours-after-an-inch-of-rain criterion) is more likely for a simple basin (no drain underneath) than for a system that drains through pipes embedded in the bottom.  But note that the failed system above was in fact drained by embedded pipes.

The upshot is, when these fail, if they are not redone, you end up with standing water, and a mosquito-breeding area.  Finally, I’ll just point out that this planter has kind of an odd design, in that it is surrounded by a tall decorative wall.  That looks nice now, but is going to make it hard to maintain this over the long run.  When it clogs, it looks to me like it will tricky to dig up and replace the soil.