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• Bacteria and Beaches

Urban-runoff retrofit units being installed at the edge of sidewalks

On the western edge of Los Angeles, CA, roughly halfway between Santa Monica and the Los Angeles International Airport, Marina del Rey is an increasingly popular Southern California entertainment destination. Built around the West Coast’s largest small-boat harbor, it draws tourists who come to boat, hike on the myriad of recreational paths, shop, and eat at California-cool restaurants. One thing marring visitors’ enjoyment is the increased bacteria levels from urban runoff during the area’s May to October dry season.

Urban Density Means High Pollutant Discharge
Densely populated, Marina del Rey suffers from high levels of various pollutant discharges originating from the area’s residential, commercial, and retail activities. Bacteria and other pathogenic discharges in stormwater and dry-weather runoff can significantly harm the health of the harbor and potentially hurt the local economy.

To control bacteria, pathogens, and other pollutants, and to meet federal and local regulations, the Watershed Management Division of the County of Los Angeles Department of Public Works is working on several best management practices (BMPs) to attack the problem.

We manage the watershed holistically using a systems-based approach,” says Angela George, a senior civil engineer and Santa Monica Bay watershed manager for the LA Department of Public Works.

Holistic Watershed Management
According to George, the Watershed Management Division is responsible for a number of flood-protection projects within the watershed, including elements of water quality, habitat restoration, aesthetic improvements, and recreational benefits. Her division engaged the Department of Public Works’ Design Division to develop a retrofit stormwater solution for the Marina del Rey watershed.

A Mountain Flax plant installed over a bioretention system

Installation of a Yedda Hawthorn

Managing Dry-Season Runoff
Throughout the dry-weather months, pollutants that collect on the streets of cities and unincorporated areas within the Marina del Rey watershed are carried by runoff through the flood-control system’s network of storm drains and into the harbor. Due to high levels of E. coli, fecal coliform, enterococcus bacteria, and other pollutants found in several basins within the harbor, communities within the watershed must minimize their contaminated runoff in the dry season.

During dry weather, most of the communities’ storm drain outlets use low-flow diversions to route urban runoff into sanitary sewers for treatment. Ideally, this reduces polluted dry-weather runoff entering the Santa Monica Bay harbor. “As one alternative, we proposed a low-flow diversion to direct flows from the pipeline to the sanitary sewers for treatment,” George says. “However, during design, we found that we could not address all the flows carried through the system to the harbor because of seawater intrusion and a high groundwater level. So we had to come up with another plan.”

“The existing local storm drain system is submerged and subject to tidal influence all year,” explains Atashzay. “So it was not feasible to divert the low-flow runoff effectively from the drain system to the sanitary sewers for treatment. Our new plan was to capture and treat the surface low-flow runoff before it entered the drain system.”

Using Several BMPs
To decrease both bacteria and pollutants within the communities’ watershed, LA County uses a suite of social marketing and structural BMPs to improve water quality in the harbor incrementally. Designed to change public behavior, its social BMPs have resulted in dog owners picking up their pets’ feces and restaurant owners handling trash and food waste in ways that reduce bacteria contamination in the harbor.

One of the structural BMPs addressing the county’s bacteria problem is the Filterra Bioretention System. Because this was a retrofit project using existing catch basins, the county needed a small-footprint solution. It also preferred natural or bioretention processes to filter dry-weather runoff. A natural approach also complemented other BMPs used to address high levels of fecal coliform and enterococcus bacteria, as well as other pollutants found in several basins within the Marina del Rey watershed.

Favoring Natural Processes
“The county prefers using a natural process whenever possible, because it adds intrinsic aesthetic values to the community,” says George. “The Filterra data showed that its units had the capacity to treat the pollutants we wanted to target, like bacteria and heavy metals. So it appeared to be a good fit.”

For the retrofit, the county selected Clarke Contracting, which installed the Filterra systems. “The urban density of Marina del Rey offers limited space for controlling urban runoff and reducing TMDL loads for collection and treatment, making a biologic process very attractive,” notes Bob Clarke of Clarke Contracting. “The Filterra units we installed are also easy to maintain and clean out. They’ve only had to be maintained once since they were installed.”

The county had the contractor install and blend the urban-runoff retrofit units in at the edge of sidewalks along the roadway. By design, the retrofit needed to preserve and connect to existing, shallow catch basins.

Clarke Contracting installed five bioretention systems along several roadways. Preparing the sites and installing the units took about a month, according to Clarke. “We had to tear up the sidewalk and install the Filterra systems a little shallower than we’d expected so we could use existing catch basins,” he says. “This meant we ran more 4-inch PVC pipe. But we were able to tie it to the deeper end of the catch basins to get the right depth and the correct flow.”

Biotreatment Fits Naturally
Each of the Filterra Bioretention Systems includes a below-grade concrete container, a 3-inch layer of mulch, 1.5 to 3.5 feet of filter media, an observation and cleanout pipe, an underdrain system, and an environmentally suitable plant. Plants selected for the five Marina del Rey installations are suitable for the area and include Western Redbud, Mountain Flax, and Yedda Hawthorn.

“When we considered the Filterra device, it had a strong resemblance to biotreatment solutions that we are already using in our wetland project, for example,” says George. “Filterra also added trees to the streets, something we try to incorporate as often as possible in our watershed.”

As a natural solution for treating dry-weather runoff within the watershed, Filterra systems use Bacterra, a bioretention medium that optimizes the removal of bacteria and other pathogens as well as total suspended solids (TSS) and heavy metals. Physical, chemical, and biological processes all aid in the removal of pollutants.

Now, dry-weather runoff drains directly from the unincorporated communities’ sidewalks, streets, and other
impervious surfaces through a curb inlet into a concrete box and passes through the mulch, plant, and Bacterra medium. Treated water then flows out through an internal underdrain pipe connected to the storm drainpipe.

Improved Aesthetics, Decreased Bacteria
“While we had to tear up the sidewalks, the installation was easy, and we certainly improved the aesthetics of the neighborhood,” Clarke says. “People passing by were happy to see us planting trees.”

Removing nutrients and organics from the dry-weather runoff by using plants and biomass to capture, transform, and hold bacteria and pollutants, the Bacterra medium helps to prevent bacteria from growing downstream after processing. Although laboratory tests showed bacteria removal efficiencies between 77 and 99%, continuous onsite testing by CRG Marine Laboratories of Torrance, CA, has shown higher removal rates ranging from 94 to 99%. In the long term, reducing bacteria rates to these low levels not only improves the harbor’s water quality, but also makes the community a healthier place to visit and live.

“For Marina del Rey’s highly urbanized environment, and with its very limited space for treatment, Filterra was the right choice based on the existing conditions in the watershed,” says George.