Permeable pavers and one of the many raingarden installed along the street edge of Silver Lake Avenue
Stormwater Quality Improvements Using LID Retrofits in Lake-Side Neighborhood
Location: Wilmington, MA, near Silver Lake
- Demonstrate the use of a variety of LID
retrofit techniques to reduce nonpoint source pollution to a recreational
surface water body.
- Quantify the cumulative reduction in runoff and the improvement
in water quality associated with the collective LID strategies.
- Reduce or eliminate contamination of the lake from neighborhood
stormwater system by reducing surface runoff volume and improving
- Increase groundwater recharge in neighborhood abutting
Description: Silver Lake is an important recreational
resource that supports swimming, fishing, wildlife viewing, and
boating. However, the lake was 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 had occurred repeatedly for many years prior to this demonstration.
This project incorporated several LID techniques to replace the
conventional stormwater collection system in two streets draining
to Silver Lake. Stormwater flow paths were disconnected from
the piped drainage system by directing stormwater to rain
gardens and porous
pavers. Twelve rain gardens were located
in the roadway rights-of-way. The rain gardens provide treatment
as water percolates through the soil, and nutrients and contaminants
are processed by the plants. The roadway edges in two areas along
Silver Lake Avenue were resurfaced with porous pavers with underlying
infiltration beds. The porous pavers allow stormwater to soak into
the ground between the paving units.
In addition to the LID techniques, the work included replacement
of two catch basins with deep-sump catch basins, to improve capture
and removal of sediment, and excavation of accumulated sediment
in the area around an outfall to Silver Lake, in order to restore
proper flow through the drainage system.
Click here for project photos.
Transducer and weir (USGS)
Data Collection and Analysis: The U.S. Geological
Survey collected water samples from a storm drain pipe on Silver
Lake Avenue. Two types of equipment were used. An automated sampler
collected water quality samples, which were analyzed for a range of
parameters, including bacteria. A continuous flow monitor provided
data on the quantity of stormwater discharged at the sampling point.
Preconstruction conditions were compared to post-construction
conditions to detect any changes in water quality associated with
the LID practices and to determine whether and by how much the LID
practices reduce the quantity of surface runoff from the surrounding
This project was closely coordinated with additional LID
work occurring across the lake at the town beach. For details, see
Demonstration 3: Permeable Paving
Materials and Bioretention in a Parking Lot
Click here for planting list for the raingardens:
- Construction completed May 2006
- Monitoring of stormwater quality and quantity conducted by USGS:
- Preconstruction: August - November 2005
- Postconstruction: June 2006 - December 2007
- Light maintenance of planted areas performed spring and fall, 2006 through 2008
- “Garden Party” (raingarden maintenance work day) with neighborhood residents held June 2008: residents selected additional plantings for the raingardens, learned about long-term maintenance needs, and enjoyed a day of gardening.
Design, engineering, permitting, and construction management (combined costs for Demonstration Nos. 3 and 4): $92,000
Construction, and three years of maintenance (combined costs for Demonstration Nos. 3 and 4): $355,000
Key Results and Conclusions:
- For small storms, less than or equal to 0.26 inches, the LID features reduced runoff volume relative to pre-installation conditions (although sample sizes were too small for the reductions to be statistically significant).
- The LID features fully retained the runoff generated by many storms in this small size category, whereas all storms in this size category produced measurable runoff during the pre-installation monitoring.
- For larger storms, greater than 0.26 inches, pre- and post-LID conditions performed similarly.
- Storms less than or equal to 0.26 inches represent the majority of annual rainfall events, and comprised 60% of the storms monitored during the study.
- Water quality sampling was limited to the larger storms, to ensure sufficient volume for water quality analysis, and no differences in pollutant concentrations or loads were discerned.
- Monitoring suggested that the LID features in the Silver Lake neighborhood study area reduced the effective impervious area from 10% to 4.5%.
Publications and Related Materials:
Project partner: Town of Wilmington
Design/Engineering/Construction oversight: GeoSyntec Consultants
Construction and three years of maintenance: Cali Corporation
Monitoring and Analysis: U.S. Geological Survey
Ongoing Maintenance: Wilmington Department of Public Works and neighborhood residents