Water Quality and Permeable Concrete 3

[msquared48]

Currently, as I understand it, the EPA leaves it up to the design engineer how to treat water that is infiltrated through permeable concrete, given that the concrete and gravel matrix does not do so, which is an issue in itself.

Has anyone come up with any ways to physically collect and treat water that has been infiltrated through such a mechanism, other than mechanical treatment options?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[bimr]

Permeable pavement is an infiltration practice. Generally, permeable pavement should not be used for stormwater infiltration where there is a potential for groundwater contamination. All contaminated runoff should be prevented from entering municipal storm drain systems by using best management practices (BMPs) for the specific industry or activity.

"It is believed that pervious concrete pavements will carry the soluble “first flush” pollutants into the pores of the concrete and additional rain will carry the pollutants further into the system, where they will be held until infiltrated, rather than becoming a part of the runoff stream. Compounds contributing to biological oxygen demand (BOD) and chemical oxygen demand (COD) should then undergo natural filtering and purification such that the water reaching the ground water table will be of roughly the same quality as that moving through similar in-situ soils. Greases and low-volatile content oils, such as drips from vehicles, will be typically adsorbed onto the surface of the pervious concrete or, at worst, in the pores of the pervious concrete. This is expected to result in negligible effects on porosity and permeability of the pervious concrete, although this is an area in which additional research is needed.

The effect of the total suspended solids (TSS), including the grit and fines in the runoff, carried into the pervious concrete pavement system have not been fully established and additional research is warranted."

http://www.perviouspavement.org/PCA_hydro-design/hydrologic_design_part3.htm

 

[msquared48]

bimr:

You said "Generally, permeable pavement should not be used for stormwater infiltration where there is a potential for groundwater contamination."

When would this not be the case? And that is exactly the point I have to address. How would you treat it if it were the desired alternative - as in low gravity situations to avoid hauling fill onsite?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[bimr]

The Center for Watershed Protection defines a stormwater hotspot as “as an urban land use or activity that generates higher concentrations of hydrocarbons, trace metals, toxicants than are found in typical stormwater runoff.” Examples of these areas, which would not be appropriate for porous pavement include:
• Commercial nurseries
• Auto recycle facilities
• Vehicle service and maintenance areas
• Vehicle and equipment washing/steam cleaning facilities
• Fueling stations
• Commercial/industrial parking lots
• Industrial rooftops
• Marinas (service and maintenance)
• Hazardous material generators (if the containers are exposed to rainfall)
• Outdoor loading and unloading facilities
• Public works storage area

http://www.uri.edu/ce/wq/NEMO/Publications/PDFs/PP.GWQuality.pdf

http://www.stormwaterauthority.org/assets/Porous Pavement.pdf

 

[bimr]

http://www.stormwatercenter.net/Ass...ces/Infiltration Practice/Porous Pavement.htm

 

[Ron]

Mike,
Generally we look at the separation between the bottom of the pervious pavement and the seasonal high groundwater level, and the soil types. For clean sands, filtering of the first flush can occur in about 48 to 60 inches of soil (compare that with a septic tank drainfield). The BOD and COD of pavement runoff is generally not sufficient to treat with mechanical means.

In some areas, there is a requirement for two separate ponds for runoff...the first flush is retained on site; while the remainder is allowed to run offsite provided pre-development runoff peaks are not exceeded.

In your case, you are retaining the first flush onsite through the soil void volume below the pervious pavement, and allowing percolation to the groundwater. Provided you have reasonable separation between the pavement bottom and the groundwater, it should mitigate the contamination issues.

I have no references for these items...just rules of thumb that have made it through review in the past.

 

[msquared48]

bimr:

Your last link is very interesting, especially with the infiltration limits and Hotspot/Retrofit sections.

A few questions though...

What is this "stormwater center"? Never heard of it.

And...

The retrofit mentoions the use of this porus pavement as a retrofit where a parking lot is being resurfaqced... This seems funny - do they mean "Replaced" and not "resurfaced"?
Otherwise, how would you infiltrate through the existing AC or concrete paving?

And...

Under the porus pavement section, it mentions that "porous pavement can only be applied to relatively small sites". How small is small?

Thanks for any input here. Sounds like we will probably not be using it from this article, but I am still interested in the concept.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beej67]

Yeah, they mean 'replaced' and not 'resurfaced.'

I've got a hardcopy of a study laying around here someplace that some guys in France did, that shows porous pavements and the base course of stone beneath actually start to function as aerobic bioreactors after a year or two of the pavement being in place. Oils and hydrocarbons leaked into the base course of rock are frequently inundated with water, and biological components take to the rock surfaces and eat up the oil. Not necessarily suitable for hotspot treatment, but still pretty dang cool.

If you guys want I can dig through my filing cabinet and see if I can find the study.


I'm a huge fan of porous pavement designs, and wish I had the opportunity to put in more of them, but people are scared off by them quite a bit down in Georgia. The first big issue you must must must address is soils. Don't build them on clays unless you've got a way to underdrain them, because the pavement will fail when the underlying soil turns to mud. This is not nearly as big an issue in Florida, because Florida is a lot sandier, but also because in Florida your developers pretty much always start with a geotech investigation. In Georgia developers rarely bother getting the geotech work done until after the civil design is complete, so you never know whether your soils can support a porous pavement system or not until after you've pulled your permit.

The second issue is clogging. Never have any runoff from pervious surfaces (landscaping) get into your parking lot and drain into your porous area, because it will advect silt and whatnot into the pavement and clog it up. They're also best installed in low traffic areas, for the same reason - mud on tires might clog it up. The Japanese use the heck out of porous pavements, but they also vacuum their streets. Crazy Japanese.

Third issue is redundancy. You want to plan for the thing clogging even though you try to avoid it with your design. The slickest way I've seen for this is don't pave all the way to the curb line - stop a foot short and fill that last foot with rounded river stone. That way, if the pavement section clogs then water will run to the curb, and infiltrate through the river stone into the sub base, which is where you want it anyway. Then put in wheel stops to prevent folks from screwing up the stone when they park.

One good place to put these things is at the downhill side of a parking lot, in the parking spaces themselves, but not the travel lane. That way you direct all the surface runoff from the lot into the porous pavement infiltration system without having to worry as much about its capacity to bear heavy loads.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[beej67]

"How small is small?"

Eye of the beholder. What they really mean is don't drain a huge amount of runoff into a small (by ratio) porous pavement area because of the clogging issue.

Georgia Stormwater Management Manual has a pretty good treatment of porous pavements, including some design guidelines:

http://www.georgiastormwater.com/vol2/3-3-7.pdf

Their regulatory criteria don't give you a whole lot of incentives to use them in designs though. The new "Coastal Stormwater Supplement" to the GSMM does, but I haven't had the opportunity to do a design with their guidelines yet.

http://www.georgiastormwater.com/Georgia-CSS-Final-Apr-09.pdf

(this second link is a large document)

One other thing, you really need a contractor with experience to put these things in. Because of the air voids, the pavement dries out before it can cure unless the contractor seals the whole thing under plastic during the curing process. There may be other constructability issues I'm not sure about too.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[TerryScan]

The client on one of our projects wished to use a permeable paver system throughout a small marina/townhouse site. At sumps, we placed underdrains in the aggregate base which directed flow to sand filters.

 

[bimr]

The Stormwater Center seems to be funded by the EPA:

The Stormwater Manager's Resource Center (SMRC) web site is made possible through a grant from the Environmental Protection Agency, Office of Water, Office of Wastewater Management, Assistance Agreement #828077-01. The SMRC site is managed and published by the Center for Watershed Protection, Inc., a 501(c)3 organization located in Ellicott City, Maryland.

The SMRC web site was funded to provide Phase II communities with the tools and techniques necessary to protect their watersheds and to enhance and restore their local water resources. Click here for more on NPDES Phase 2 regulations, or go to

http://www.epa.gov/owm/sw/phase2

to find out more about EPA Stormwater NPDES Phase II regulations directly from the EPA.

http://www.stormwatercenter.net/

I agre with you. I think that it means replaced, not resurfaced.

Permeable pavers are more expensive than asphalt. I suspect that the cost of the permeable paveers will limit the size of project.

 

[msquared48]

Thanks guys. I really appreciate all the experience and input. We all learn something new every day here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask