Arguments Against Infiltration Often Don’t Hold Water
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!
