Water Quality & Stormwater
Highways, buildings, parking lots and other impervious development produces stormwater runoff that produces flooding, carries pollutants into streams and warms water. These impacts can be mitigated by the use of designed plantings to reduce runoff and filter water before it enters streams.
Combined Sanitary and Storm Sewers
Compounding this problem is that Pennsylvania’s two largest cities (as well as many others), have combined sanitary/storm sewer systems that create enormous quantities of polluted water during storm events.
A 2007 paper by the University of Pittsburgh Center for Healthy Environments and Communities reported:
The "Three Rivers” area of Pittsburgh has approximately 317 combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs)-- more than any other city in the United States, which release untreated municipal waste directly into receiving water during wet weather events (National Research Council, 2005). An estimated 16 billion gallons of sewage and stormwater are discharged yearly into receiving streams and main stem rivers in the Allegheny County Sanitary Authority (ALCOSAN) sewershed.
Plant-based green infrastructure systems can help to alleviate this problem by reducing the amount of stormwater discharged into sewage treatment systems. The Philadelphia Water Department estimates that it can afford to spend up to $260,000 per acre on green infrastructure projects (green roofs, bioswales, rain gardens, etc.) rather than continue to treat the stormwater that otherwise would flow from these project areas into their combined sanitary/stormwater system.
Conservation Reserve Easement Program Model
We have an excellent model that proves the effectiveness of using plants for this purpose, the Conservation Reserve Enhancement Program (CREP), but have not deployed it to mitigate runoff from parking lots, streets, highways and other development. Pennsylvania now leads the country with 165,000 acres enrolled in CREP. CREP uses plants to protect water resources from agricultural runoff. Without so-called "riparian buffers,” (the area along streams and rivers with native plants and trees) to slow stormwater runoff, hold the soil and filter surface water flowing to the stream, the water quality of a stream can degrade quickly.
Plants as Water Purifiers
Plants, in fact, are small water treatment and purification machines. First, a dense colony of plants in the path of stormwater can slow surface runoff, allowing soil, sediment and other impurities to simply drop out or be caught in the plant stems and roots before entering the stream. Second, plants encourage the infiltration of water into the soil by slowing stormwater’s progress to the stream, plus the roots of the plants loosen the soil and create a multitude of small fissures through which the water can infiltrate. Third, the plants themselves take up nitrogen and phosphorus, which are good for the plants, but can be bad for the stream in excess. Fourth, the plants host an entire ecosystem of micro and macro invertebrates, bacteria, fungi and other organisms that operate in the soil and water to convert pollutants to harmless and even useful simpler compounds.
Getting stormwater to infiltrate into the ground, rather than run directly into a stream, provides enormous water quality benefits. The soil is a marvelous water purification system that we take for granted. When water percolates slowly thought the soil to the water table, it is filtered physically as small particles are trapped in soil layers and biologically as natural bacteria go to work on organic pollutants. Plus, since ground water provides 25% of the drinking water supply in Pennsylvania, recharging the aquifer is a good thing too. Plants are the key to facilitating this infiltration process.
The Role of Trees in Water Quality
Trees play a significant role in slowing stormwater runoff and increasing infiltration. A recent study by the US Forest Service (USFS) determined that about two-thirds of the rain falling on a tree in a half inch rain event was held by the leaves and branches of tree where it slowly dripped to the ground after the event or evaporated back into the air. Another study by the USFS found that the tree canopy in Salt lake City, Utah in a one inch rainstorm over twelve hours, reduced surface runoff by about 11.3 million gallons, or 17%.
The lessons we have learned from the CREP program can be applied to any development to protect streams and ground water. Instead of planning to direct stormwater off the highway and into a stream as quickly as possible, we should be designing appropriate placement of meadows, bioswales, beds, trees and other plantings to slow and absorb rain water. If the highway adjoins a stream, a riparian tree, shrub and plant buffer along that stream will slow, filter and reduce the temperature of stormwater and that may flow to the stream from the highway.