Last week we commented on issues surrounding infiltration - a critical approach to stormwater volume control in most of Pennsylvania’s developing municipalities.  Let’s go a step further this week.   Some stormwater managers have argued against any level of infiltration in and around limestone, pointing to creation of sinkholes and threats of groundwater contamination.

Some quick background:  limestone/carbonate/karst geology exists in many areas in the state, some experiencing considerable development pressures.  These limestone formations in sum are typically distinguished by:

• subsurface channelization/interconnections that facilitate migration of pollutants, if/when introduced (i.e., quality issues)
• solution channel formation that result in subsidence problems and sinkholes (”quantity” issues)

Our position continues to be that infiltration is an important strategy for stormwater in limestone formations, though it must be accomplished with special care for both the quality and quantity reasons mentioned above.

Note, first of all, that limestone formations tend to be wonderful aquifers - water holding units and often are tapped as wonderful water supply sources.  Limestone tends to be associated with quality, permeable soils which allow for substantial infiltration and replenishment of groundwater volumes.

Secondly, under so-called normal or natural conditions, some subsidence issues do emerge and some sinkholes have been known to form.  Without attempting to describe all of the processes that are involved in sinkhole formation, we have typically argued for simply trying to maintain the pre-development hydrologic regime after development - avoid substantial concentration of stormwater flows in the stormwater plan.

Infiltrate, but infiltrate over broad evenly distributed areas so that infiltration is not concentrated and does not promote solution channel formation.

And always make sure that there is reasonable soil thickness (at least 4 feet, better if there is more) between the infiltrating surface and the subsurface rock, to buffer infiltrated precipitation both in terms of quality and its subsurface movement.

There’s a lot more to it than that - this is extremely summarized.

Let us know what you think!

admin From the Editors groundwater, hydrology, infiltration, karst, limestone, sinkholes, volume control, water quality, water quantity

Are you thinking of washing your car this weekend?  Think again!  Unless you plan to treat that sudsy water.  New data has sparked controversy and an article in the Stormwater Journal about Residential Car Washing.  “Society has been slow to recognize the link between individual behaviors and practices, and the detrimental impacts that they may have on our natural aquatic resources.”  Find out what the state of Washington is doing to combat suds.  Then check out one case of Successful Residential and Community-Based Nonpoint-Source Pollution Prevention.

PEC seeks bids for Ohiopyle Green Infrastructure Project.  The Pennsylvania Environmental Council received funding from the American Recovery and Reinvestment Act for the project.  Bids will be accepted now through October 6, 2009 for

• Installation of rain barrels with downspout and gutter repair/installation throughout the Ohiopyle Borough
• And pervious pavement, bioswales, rain gardens, landscaping, asphalt pavement repair, and concrete sidewalks on demonstration streets

A pre-bid meeting will be held September 15^th.  Click here for more information.

Upcoming Events

The Growing Greener Watershed Restoration Program will be ten years old in December and to help celebrate, 10 organizations have come together to invite watershed groups to share their success stories on a special Growing Greener website.  Thanks to the Foundation for Pennsylvania Watersheds and an anonymous benefactor, two $250 grants will be given to the watershed groups with the best article and photo or video each month through December.  The next deadline is September 20.

Come celebrate the successful work of a grassroots operation throughout the Southeast at the 13^th Annual Clean Water Fund Auction and Party.  Each year, 120 or more supporters, friends and leaders in the work of the Clean Water Fund in Southeast Pennsylvania gather along Philadelphia’s historic boathouse row, in a beautiful setting that overlooks the Schuylkill River.  Join them October 16, 2009 from 6-9pm.  Get tickets here.  Debut of a new art installation, Tsunami, created by Dave Holley for the event.

Reminders:

September 14-18 - Permit Writers’ training course in Atlanta, GA

September 22-25 - Watershed Institute in Columbia, SC

CALL FOR PAPERS - World Environmental and Water Resources Congress 2010: Challenges of Change. Due September 17. Submit here.

October 14-15 - Bailing Out Stormwater: Pennsylvania Stormwater Management Symposium at Villanova University. Registration Opens September 1.

admin Weekly News clean water, growing greener, Nonpoint source pollution, pervious pavement, rain barrels, stormwater best management practices

For your consideration:

“You can’t infiltrate here….infiltration and recharge are fine in some places, but our soils here just don’t drain….” is a frequently heard complaint across Pennsylvania municipalities.  This complaint is often used to argue against use of infiltration-oriented Best Management Practices in stormwater management programs.  Because infiltration BMPs are the cornerstone of volume-control strategies (not to mention their proven excellence in water quality performance), at least in many developing townships, rejecting infiltration often ends up meaning indirectly rejecting volume control and sacrificing water quality performance as well.

“You can’t infiltrate here….”

• First of all, several basic points need to be made.  In much if not most of Pennsylvania - with the exception of carbonate formations, water movement on the surface tends to be mirrored by water movement underground.  Unlike some areas in western states, surface basins tend to coincide with groundwater basins.
• Secondly, streams that are perennial are created by base flow which emerges as the result of infiltrated precipitation; often times this cycling of precipitation to groundwater to base flow is relatively brief, a matter of days or weeks or months (for example, all of us know local streams where after a few weeks without rain, stream flows decline significantly - proof that this rather superficial cycling occurs relatively quickly - in stark contrast to the Ogallala Aquifer developed millions of years ago).  Of course, different rock and different soils on top of the rock vary in their permeability and rates of infiltration, but virtually all soils and rock infiltrate to some extent.
• Thirdly, the vast bulk of the water that enters the ground and merges as stream base flow does so rather superficially, in contrast to some groundwater movement in western states where water may move very far down and across very large distances in the course of many years.  The bottom line here is that if you have local streams, you have local infiltration.

Another important point on this infiltration argument is that heavy clayey soils with low permeabilities function very differently when undisturbed and in well-rooted vegetated cover, offering an array of micropore/mesopore/macropore opportunities.  Conversely, a clayey soil that has been stripped and graded and compacted during the land development process undergoes a profound textural change and looses an enormous amount of its permeability.  If portions of sites with low permeability are kept undisturbed, these areas can be used productively for stormwater infiltration.

Maybe that’s enough for now, we’ll talk more about infiltration strategies next week.

Let us know what you think!

admin From the Editors BMPs, groundwater, infiltration, volume control, water quality