Bioretention on Monterey Bay
The bioretention concept is part of a greater storm water management model known as Low Impact Development (LID) and was originally developed by the Prince Georges County, Maryland, Department of Environmental Resources in the early 1990s. LID began as an alternative to traditional best management practices (BMPs) installed at construction projects and residential communities. Eventually, officials found that the traditional practices associated with BMPs, such as detention ponds and retention basins, were not cost-effective and the results did not meet water quality goals.
LID is supported by the U.S. Environmental Protection Agency (EPA) as a medium to meet the goals of the Clean Water Act and is an important part of Smart Growth and Sustainable Infrastructure land use planning. Along with bioretention, other LID techniques include dry wells, filter strips, vegetative buffers, level spreaders, grass and bio-swales, rain barrels, cisterns, and infiltration trenches.
The bioretention method combines physical filtering and adsorption with biological processes. These facilities may range from simple shallow depressions to more complex designs, but all are structurally engineered to provide the following roles with respect to storm water quantity control: interception or capture, infiltration, filtration, storage, and water uptake by vegetation. The nine major components of the bioretention facility are: pretreatment, flow entrance, ponding area, plant material, organic layer or mulch,planting soil and filter media, pea gravel diaphragm, under-drain and outlet, and surface overflow.
Bioretention works by reproducing the physical, chemical and biological processes of the natural environment to create a more efficient, on site, water treatment facility. The incorporation of biomass (plants, mulch, soil) introduces biological processes and cycles to naturally deal with the hydrologic cycle. A key design aspect of a bioretention facility is its depressed bowl-shaped topography, creating a "ponding area". This ponding area allows for surface storage of runoff when the soil storage is at capacity, promotes evaporation, and allows sedimentation of particulate matter prior to infiltration. They are commonly located in parking lot islands or within small pockets in residential land uses.
Bioretention areas have a few limitations. They cannot be used to treat a large drainage area, limiting their usefulness for some sites. In addition, although the practice does not consume a large amount of space, incorporating bioretention into a parking lot design may reduce the number of parking spaces available if islands were not previously included in the design.
Bioretention requires landscaping maintenance, including measures to ensure that the area is functioning properly, as well as maintenance of the landscaping on the practice. In many cases, bioretention areas initially require intense maintenance, but less maintenance is needed over time. In many cases, maintenance tasks can be completed by a landscaping contractor, who may already be hired at the site. Landscaping maintenance requirements can be less resource intensive than with traditional landscaping practices such as elevated landscaped islands in parking areas.
First, the majority of storm water discharge for Marina Station will be handled by a percolation pond located in the western portion of the Plan area1. The Specific Plan states the system will consist primarily of storm water inlets with underground piping systems in residential streets, alleys and parking areas and that the percolation pond is an existing feature of the Plan area's topography, which currently accepts runoff from its natural tributary drainage basin.
The use of this kind of storm water retention facility is considered a BMP and is consistent with The City of Marina code which requires the runoff from a 10-year 24-hour storm be retained onsite and disposed of on-site via percolation. Development of the Plan area will increase the amount of storm water runoff pollutants such as sediment, nutrients, bacteria, oil and grease, heavy metals and debris. In addition to City standards for retaining and disposing of storm water, storm water discharges are regulated under the Federal Clean Water Act through the National Pollutant Discharge Elimination System. The percolation pond is expected to successfully remove high levels of particulates and dissolved pollutants as well as provide the added benefit of ground water recharge in an area where recharge is important to protect against seawater intrusion.
Second, the Specific Plan will utilize its landscape design to incorporate the site drainage and infiltration as an integral part of the site development. As stated in the Plan, "The intent is to improve the water quality as storm water is filtered through planted areas wherever possible." To aid in achieving this goal the project will use bioswales and water retention areas to reduce the need for conventional storm drainage piping systems. Bio-swales may be integrated into the parking areas to improve storm water quality and improve infiltration of storm water into the subgrade. Furthermore, permeable concrete and paving stone areas may be used to reduce storm water runoff and reduce the scale of the larger parking areas.
Finally, in accordance with the Specific Plan, "Marina Station will be a leading example of Sustainable Community Design with many special features of a green community." To achieve this goal, the Specific Plan is expected to meet the American Planning Association's (APA) four objectives for sustainability. Of these four, Objectives Two and Three relate specifically to the use of storm water runoff. Stated here are those objectives and their relationship to storm water management as stated by the Specific Plan.
Objective Two: Reduce residents' and businesses' dependence on chemicals and other manufactured substances that can accumulate in nature.
• Disposal of all storm water runoff into a retention pond, which will clean the water and recharge the aquifer.
Objective Three: Reduce residents', businesses' and development's encroachment upon nature.
• Management and retention of storm water in a way that restores the quality of on-site run-off and percolates 100% of the run off to the underground aquifers.
Also of some concern, the Specific Plan does not adequately address how it may include those techniques closest to bioretention facilities such as bioswales. It is assumed that bioswales will be incorporated wherever appropriate, but there is no assurance these sites will be utilized under all general circumstances despite the statement that "the intent is to improve the water quality as storm water is filtered through planted areas wherever possible." No specific examples are given and the use of permeable concrete and paving stone areas is vague.
Finally, the use of such BMP's is crucial to remain consistent with Marina City code, however LID practices such as bioretention can dramatically alleviate stress on the planned percolation pond. These facilities will encourage infiltration, filtration, and uptake by vegetation on an evenly distributional scale. Further, laboratory and field studies performed by the University of Maryland have shown that bioretention is between sixty to eighty percent efficient at removing heavy metals such as copper, lead and zinc, and organic compounds such as ammonia and phosphorus2. Also there was a marked decrease in thermal pollution, a form of pollution often forgotten when dealing with run-off.
2 Winogradoff, D. A. (2001). The Bioretention Manual. Upper Marlboro: Programs & Planning Division Department of Environmental Resources Prince George's County, Maryland.