The Martha Gardens Green Alleys Pilot Project: A Successful Collaboration of City and State to Improve Water Quality
The City of San Jose (City) retrofitted three blocks of existing alleyways with Low Impact Development (LID) features in the Martha Gardens Green Alley Pilot Project. The $1.4 million retrofit, funded substantially through State Water Resources Control Board (SWRCB) Proposition 84 Stormwater Grant funds, is helping the City raise public awareness of stormwater issues and the relationship between local runoff and local waterways, and test LID in the field. The retrofit project eliminated sediment sources and substantially reduced stormwater runoff from the treated area by infiltrating stormwater through porous pavers into infiltration trenches. Performance monitoring showed significant reductions of Total Suspended Solids (TSS), and associated contaminant loads, most notably mercury, lead, and nickel, to adjacent storm drains and streets. Overall, this successful collaboration of City and State resources produced many lessons to share.
The Martha Gardens Project is located south of downtown San Jose, Santa Clara County, draining to the Guadalupe River. The Guadalupe River is listed as impaired by Mercury, Pesticide Toxicity, and Trash. Prior to the project, the alley surfaces were poorly paved to completely eroded, creating hazards for residents and generating dust and sediment. In rainstorms, they regularly flooded, allowing sediment to run off and track onto adjoining streets where it was then carried to nearby catch basins. Approximately 35,000 square feet of surface was replaced with durable high albedo "green concrete.” Stormwater from 2.3 acres of urban development drains through approximately 5,000 square feet of porous pavers into infiltration trenches and then infiltration galleries located at either end of each alley, thereby reducing the volume and rate of runoff discharged to the storm drain system.
Performance evaluation used SWRCB-approved methods to determine the retrofits’ sediment, TSS, and pollutant load reduction effectiveness. US Geological Survey methods were adapted to estimate street dirt and sediment volume yield. Pre- and post- construction sediment samples were collected from the alleyways during dry and wet seasons, and analyzed for total mercury, total PCBs, pyrethroid pesticides, total metals, gasoline and diesel range organics, grain-size distribution, and dry weight. In addition, water samples were collected from the adjacent catch basins during four separate storm events both pre- and post- construction, for TSS analysis.
Offsite sediment loads were reduced by nearly 70% while TSS concentrations were reduced approximately 70%. Contaminant load reductions for Mercury, Nickel, and Lead were calculated at 88%, 79%, and 74% respectively. Final construction costs were 18% below budget with a combined design, engineering, and construction cost of $32.40 square foot. Post-construction inspections have verified that the pavers are draining properly, keeping polluted stormwater runoff from flowing directly into the stormwater system. This successful project addressed long-time concerns of residents, and can be used to estimate the cost effectiveness of similar projects in the region. Lessons learned can help municipalities plan similar projects in the future.
Carol Boland is an Environmental Services Specialist with the City of San Jose Stormwater Management team, specializing in water quality monitoring, LID effectiveness assessment, and environmental outreach. She holds a BS in Ecology and Systematic Biology (2005), and MS in Biology (2008), both from California Polytechnic State University, San Luis Obispo. Carol happily lives among the redwoods in Santa Cruz County, where she is an avid surfer and mountain biker.