| Permeable pavers, porous asphalt, and bioretention cells at the Silver Lake beach parking lot, Wilmington (GeoSyntec Consultants)
Demonstration 3
Permeable Paving Materials in a Parking Lot
Location: Silver Lake Beach Parking Lot, Wilmington,
MA
Purpose:
- Reduce the quantity of stormwater runoff and nonpoint
source pollution to Silver Lake and maximize infiltration
to groundwater.
- Demonstrate the use and performance of different types of permeable
paving materials to infiltrate stormwater.
- Demonstrate the use of bioretention
cells to reduce runoff and pollutants from impermeable areas.
- Assess and characterize any potential impacts to groundwater
quality that might result from the use of permeable pavement.
Description: Silver Lake is an important recreational
resource that supports swimming, fishing, wildlife viewing, and
boating. However, the lake is degraded from nutrients, sediment,
and bacteria from the surrounding conventional stormwater system
(which includes several direct discharge outlets to the lake) and
nonpoint source runoff. Beach closures due to high bacteria counts have occurred repeatedly.
This project incorporates three LID practices designed to reduce
runoff volume, improve water quality, and enhance groundwater recharge:
- Permeable paving materials
- Bioretention cells
- Vegetated water quality swales
Four types of permeable paving materials were used to repave about half of the parking lot. The parking spaces themselves, consisting of about 8,000 square feet, were paved with porous pavers, which allow stormwater to soak into the ground between the paving units. The parking aisles, consisting of 16,600 square feet, were paved with porous asphalt, a special mix that allows water to pass through the pavement surface. Underneath the porous pavers and asphalt is an infiltration bed of sand and gravel that filters contaminants and allows water to slowly percolate into the underlying soil. Additionally, a porous paving system called Gravelpave, consisting of interlocking plastic cells filled with gravel, was installed in a portion of the overflow parking area. Finally, a porous material called Flexi-Pave, made from recycled tires, was installed in another small section of the overflow parking lot. For comparison purposes, one half of the existing main parking lot was resurfaced with standard asphalt.
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Bioretention cell
detail
(GeoSyntec Consultants) |
Surface runoff from the standard pavement and any overflow from
the permeable paving surfaces will drain to bioretention cells,
which will provide treatment as water percolates through the soil,
and nutrients and contaminants are processed by the plants. The
parking lot design includes two center bioretention islands and
seven bioretention cells around the perimeter.
Two ten-foot-wide vegetated water quality swales were installed
to replace existing piped stormwater outfalls at either end of the
beach. As with the bioretention cells, these swales will improve
water quality through the physical, chemical, and biological processes
that occur as runoff percolates through soil and is taken up by
plants. The northwestern swale replaced a grassed area that
formerly attracted geese and other waterfowl, leading to the buildup
of droppings that contribute to bacteria loading and closure of
the public swimming beach area. The planting plan for this swale
includes native shrubs that are not attractive to grazing waterfowl.
Data Collection and Analysis: The U.S. Geological
Survey (USGS) installed seven wells in the parking lot. Four wells
provide data on groundwater levels, and three provide data on groundwater
quality. USGS monitored preconstruction conditions quarterly
and after a few large storms. Following construction, USGS monitored groundwater levels and collected samples monthly. Samples
were analyzed for a range of parameters, including conductivity,
temperature, pH, nitrogen, phosphorous, metals, and total petroleum
hydrocarbons. Sampling was designed to detect any changes in groundwater
quality associated with recharge from the parking lot.
This project is being closely coordinated with additional LID
work occurring across the lake from the town beach. For details,
see Demonstration 4: Lake Water
Quality Improvement Using LID Retrofits to Replace Conventional
Stormwater Discharge
Partners:
- Town of Wilmington
- U.S. Geological Survey
Design/Engineering:
Construction:
Click here to see educational poster Status: See Progress Report
Acknowledgements: DCR would like
to acknowledge contributions to this project from Invisible Structures,
Inc. (porous asphalt), Analyze Property Company (Flexi-Pave) and Oldcastle/Foster-Southeastern (permeable pavers)
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