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| Percolator picture courtesy of The Sun |
Problem: Impervious Surfaces
An outstanding feature in the Lower Allegheny Watershed is the city of Pittsburgh. As such, a large human impact in that portion of the watershed is actually impervious surfaces. Impervious surfaces are caused by pavements used for roads, highways, parking lots, and other surfaces like building rooftops (Pennsylvania Environmental Council, 2011). With the pavement, precipitation such as rain does not have the capability of penetrating the soil surface and percolating down to the groundwater. The water simply collects and flows directly on the pavement, carrying with it pollutants from motor vehicles, the pavement materials, and litter. Such pollutants carried in stormwater include benzene, hydrocarbons, oil, grease, and salt (Pennsylvania Environmental Council, 2011).
Besides limited water quality and groundwater infiltration, impervious surface also creates high peak runoff events during storms. Much of this runoff is diverted into pipes and has the potential to overload sewage pipes to the point where treatment plants cannot process the water fast enough, leading to the polluted and sewage water being diverted and discharged into local creeks and waterways. High peak runoff events also mean increased water velocity as it flows, increasing erosion and undercutting vegetation (Pennsylvania Environmental Council, 2011). Table from the Pennsylvania Environmental Council shows the percentage of impervious surface cover at a building site and relative stream health in regards to runoff.
Table : Stream Health Relative to Impervious Cover
Impervious Cover | Stream Health |
Less than 10% | Protected |
10-29% | Impacted if not mitigated |
30% and more | Degraded if not mitigate |
Solutions
Mitigating impervious effects due to runoff can be as simple as increasing permeable surfaces through rain gardens and planted medians along roads and sidewalks as plants and soil have the ability to store, convert, and transform many contaminants before they reach groundwater and waterways. Also, discharge points could be retrofitted and redesigned to reduce peak runoff and enhance infiltration (Pennsylvania Environmental Council, 2011).
Recommendation 1:
Create permeable areas such as rain gardens and planted medians.
Recommendation 2:
Retrofit old spillways and discharge points to reduce peak runoff and erosion