Comparison of stormwater biofiltration systems in ...water-pire.uci.edu/wp-content/uploads/2015/02/... · Focus Article Comparison of stormwater bioﬁltration systems in Southeast

Focus Article

Comparison of stormwaterbiofiltration systems in SoutheastAustralia and Southern CaliforniaRichard F. Ambrose1,2∗ and Brandon K. Winfrey1

Stormwater biofilters (also called rain gardens, bioretention systems, andbioswales) are used to manage stormwater runoff in urbanized environments.Some benefits of biofilters include flood prevention, stormwater runoff water qual-ity improvement, and wildlife habitat. This technology has been implemented ona larger scale in southeast Australia, but cities and counties in southern Californiajust beginning to construct biofilter systems to manage stormwater runoff. Biofil-ters tend to be larger in southern California than in southeast Australia. Differencesin rainfall patterns likely affect biofilter function. Southern California has muchlonger periods between rain events than southeast Australia, providing challengesto establishing and maintaining vegetation in biofilters. The use of biofilters forrestoring predevelopment flow regimes has been studied in a peri-urban water-shed in southeast Australia, but flow regime restoration is not likely in highlyurbanized locations in both Australia and southern California. However, stormwa-ter runoff treatment and harvesting in decentralized biofilters could substantiallyreduce storm flows and improve water quality in receiving waters while improv-ing urban water supply and extending the life of existing stormwater managementinfrastructure. © 2015 Wiley Periodicals, Inc.

How to cite this article:WIREs Water 2015, 2:131–146. doi: 10.1002/wat2.1064

INTRODUCTION

Urban stormwater management is important forcontrolling pollution and flooding associated

with runoff from impervious surfaces following rainevents but has often been at the cost of importantecosystem services and functioning in urban streams(i.e., the urban stream syndrome).1 Low ImpactDevelopment or Green Infrastructure stormwater sys-tems that infiltrate water through a vegetated fil-ter media can be used to capture and treat urbanstormwater runoff and re-establish predevelopmentflow patterns.2 In this article, we refer to these sys-tems as stormwater biofilters, but recognize that they

∗Correspondence to: [email protected] of Environmental Health Sciences, University ofCalifornia, Los Angeles, Los Angeles, CA, USA2Institute of the Environment and Sustainability, University ofCalifornia, Los Angeles, Los Angeles, CA, USA

Conflict of interest: The authors have declared no conflicts of interestfor this article.

also capture and treat dry-weather runoff not asso-ciated with storms. There are many similarities indesign criteria of bioswales, vegetated strips, rain gar-dens, and bioretention or biofiltration systems; weare considering the term ‘biofilter’ to encompass allsystems that filter stormwater runoff through a vege-tated filter media and convey treated stormwater intoperforated pipes leading to a discharge pipe and/orpercolate into the underlying soil. Biofilters removepollutants such as metals, solids, oils and grease,nutrients, and pathogens through a myriad of phys-ical, physicochemical, and biological processes. Allof these processes occur as a result of gravity-fedhydraulics, filter media characteristics, and captur-ing sunlight through photosynthesis, making thesesystems low-energy options for stormwater manage-ment. Biofilters are used in stormwater managementin a variety of regimes. Currently, many cities inthe United States and Australia offer rebate pro-grams and guidance documents to design and con-struct biofilters at residents’ homes. Government and

Volume 2, March/Apr i l 2015 © 2015 Wiley Per iodica ls, Inc. 131

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nonprofit organizations are also building biofilters atthe single project and neighborhood scale. Typically,single projects capture runoff from parking lots ora large commercial or industrial development; theseprojects are commonly constructed by private com-panies who own the lots from which stormwater iscollected. Neighborhood-scale projects aim to managelarger catchments and often include several types ofstormwater management technologies; these projectsare commonly constructed by government agencies,often in collaboration with other organizations, andare more extensive and expensive.

Comparing the U.S. and Australian stormwatermanagement infrastructure is useful because bothdeveloped countries are mitigating the effects ofurban stormwater runoff on aquatic ecosystems usinglow impact development.3 In this article, we comparethe implementation of stormwater biofilters in south-east Australia and southern California. Althoughstormwater biofilters have been constructed in bothof these regions, they differ in the motivations behindtheir construction. We discuss these motivations aswell as differences in the climates of the two regions,design and maintenance of biofilters, and the ecologyof biofiltration systems. We show how the differ-ent environmental settings could influence optimalimplementation in each region, although little workhas been performed so far to identify the optimalimplementations.

IMPETUS FOR IMPLEMENTATION

In southeast Australia, a major impetus for theconstruction of stormwater biofilters was water con-servation in response to the so-called Millenniumdrought.4 Additionally, stormwater biofiltration wasrecognized as an effective method to prevent nitrogenfrom reaching Port Phillip Bay because these systemscan handle the variable concentrations and flows ofrunoff events.5,6 The major water purveyor, Victorianstate statutory authority Melbourne Water, adoptedpolicies and incentives to encourage large-scale imple-mentation of stormwater biofilters. Melbourne Waterprograms include a ‘Ten Thousand Rain Garden’program. In addition, studies by Melbourne scientistshad demonstrated an adverse effect of stormwaterrunoff from urbanized watersheds.1 Besides the activesupport of Melbourne Water, regulatory require-ments encourage the construction of stormwaterbiofilters. New developments and redevelopmentsare required to manage stormwater according toClause 56.07-4 of the Victoria Planning Provisions(http://planningschemes.dpcd.vic.gov.au/schemes/vpps/56_07.pdf). The involvement of academic

scientists in Melbourne is also noteworthy. TheWater for Liveability Centre at Monash Universityhas played a central role developing the scientificfoundations for biofilter construction and the imple-mentation of actual projects. The Little StringybarkCreek Project is a collaborative research programwhere researchers from Monash University and Uni-versity of Melbourne have investigated the potentialof watershed-scale impact of managing stormwaterrunoff with biofilters and rainwater harvesting. Thisproject is the largest of its kind in the world and has sofar resulted in the construction of dozens of biofilters.

In southern California, the impetus for stormwa-ter biofilter implementation has been control of waterpollution under the federal Clean Water Act. Initially,the State Water Resources Control Board (SWRCB)promulgated regulations requiring stormwater beretained on new construction sites. Under the2009-0009-DWQ Construction General Permit,stormwater runoff originating in new developmentsin California was recognized as point source pollutionand regulated under the Clean Water Act. The CleanBeach Initiative, a fund under California’s SWRCB,provided funding to projects aimed at reducing fecalindicator bacteria (FIB) loads to California’s beachesfrom urban runoff.7 Stormwater biofilters weresuggested as a best management practice (BMP) topreserve drainage in urban areas and prevent FIB fromreaching beaches. These stormwater biofilters (amongother techniques for retaining stormwater) werelargely constructed by permittees on private property.More recently, Municipal Separate Storm Sewer Sys-tem (MS4) regulations have led to broader adoptionof stormwater biofilters. Stormwater biofilters con-structed by government agencies are typically larger,more expensive, and may include other stormwatermanagement techniques such as infiltration galleriesand pervious pavement. Most neighborhood-scalesystems have been built by local government agencies.The Elmer Avenue Green Street project, coordinatedby a consortium including the City of Los Angeles,the Council for Watershed Health and TreePeople(nonprofit organizations), mitigated flooding in aSun Valley neighborhood by capturing runoff ina large infiltration gallery installed beneath ElmerAvenue.8 This project also included the constructionof 24 street-side biofilters and rainwater-harvestingtanks. The California Department of Transportation(CalTrans) constructed a biofilter in 2006 as part ofa pilot project to investigate the suitability of usingthis technology as a BMP for stormwater manage-ment. Monitoring of the treatment performance andmaintenance requirements of the CalTrans biofil-ter is ongoing. There has been limited engagement

132 © 2015 Wiley Per iodica ls, Inc. Volume 2, March/Apr i l 2015

http://planningschemes.dpcd.vic.gov.au/schemes/vpps/56&uscore;07.pdf

WIREs Water Comparison of stormwater biofiltration systems

Average temperature in Melbourne, VIC30

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FIGURE 1 | Mean monthly temperatures for Melbourne and Los Angeles. Data for 1994–2013: Melbourne9 and Los Angeles.10

with the academic community in the role of biofilterimplementation in southern California.

DIFFERENCE IN CLIMATESoutheast Australia and southern California also dif-fer in their climate that may have a substantial impacton biofilter performance. Southeast Australia is con-sidered a temperate climate while southern Californiahas a Mediterranean climate. Temperatures are sim-ilar, with mean monthly high temperatures in Mel-bourne and Los Angeles varying from 15 to 27∘C and

mean monthly low temperatures varying from 7 to17∘C (Figure 1). Los Angeles is slightly warmer, withboth average high and low temperatures about 4∘Chigher in the winter and average low temperaturesabout 2∘C higher in the summer. Los Angeles alsohas slightly more consistent monthly mean high tem-peratures, with a 6∘C difference between winter andsummer compared to a 12∘C difference in Melbourne.

In contrast to relatively consistent temperaturepatterns, the rainfall patterns of southeast Australiaand southern California differ dramatically. Meanmonthly rainfall in Melbourne varies only between 4



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FIGURE 2 | Precipitation patterns for Melbourne and Los Angeles. Data for 1994–2013: Melbourne9 and Los Angeles.10

and 6 cm (Figure 2). In contrast, mean monthly rain-fall in Los Angeles varies between 0 and 12 cm, with80% of the rain falling in December to March. Asa result, there are much longer periods without rainin Los Angeles. This difference is apparent in the fre-quency of antecedent dry days (ADD), defined as thenumber of days preceding a rain event over 1 mm/day.Between 1994 and 2013, periods of ADD over 30 daysoccurred 51 times in Los Angeles but only 3 timesin Melbourne (Figure 3). In Melbourne, the pattern

of ADD was similar in the driest and wettest yearsover the 20-year period examined, even though rain-fall differed by a factor of two. In contrast, the driestand wettest years in Los Angeles had distinctly dif-ferent ADD distributions, with widely spaced rainfallduring the driest year. The more consistent rainfallin southeast Australia means shorter periods of dryweather, which can influence biofilters performance.Besides longer periods of dry weather in Los Angeles,there are larger differences from year to year, meaning



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FIGURE 3 | Distribution of antecedent dry days for Melbourne and Los Angeles. Inset: Distributions of antecedent dry days for Melbourne and LosAngeles during Driest and Wettest Years. Data for 1994–2013: Melbourne9 and Los Angeles.10

that there are more years when biofilters would receiveonly sporadic rainfall. Nitrogen and metals removalefficiencies are reduced by biofilter drying.11,12 In addi-tion, a saturated or submerged zone that can improvenitrogen and metals removal12 would be easier tomaintain consistently with regular rainfall. Because ofthe extended dry periods, a saturated zone likely couldonly be maintained year-round in southern Californiaif there was considerable dry-weather runoff or if itwas replenished with potable water. Although biofilterdrying generally reduces pollutant removal efficiencyfor many pollutants and can challenge the mainte-nance of a biofilter, prolonged dry periods enhancedthe removal of micropollutants.13

Stormwater pollutant loads differ somewhatbetween southeast Australia and southern Califor-nia, and this could influence biofilter performanceor which design would be optimal in each location.In both regions, pollutant loads range widely forevery constituent (Table 1), although there are somegeneral patterns. Nutrients tended to be higher insouthern California stormwater. Although there was

broad overlap, total suspended solids ranged higherin southeast Australia stormwater. Lead and E. colialso ranged higher in southeast Australia stormwa-ter. These data reflect stormwater runoff that reflectsinflow after rain events but may not represent dryweather runoff pollutant loads.

DESIGN SPECIFICATIONSAND MAINTENANCE

There are differences in key design characteristics ofstormwater biofilters in southeast Australia and south-ern California. We compared 13 Victoria and 13southern California stormwater biofilters (Table 2).These biofilters represent various ages, sizes, anddesigns of the major neighborhood-scale biofiltersfound in Victoria and southern California. Mostbiofilters in southern California were constructed inthe past 4 years, while Victoria biofilters have beenbuilt steadily over the past 10 years. The biofilters inboth regions cover a wide size range, but southern



TABLE 1 Typical Concentrations of Constituents in Stormwater Runoff in Southern California and Southeast Australia

Constituent Unit Southern California* Southeast Australia**

Total suspended solids mg/L 30–70 40–150

Total nitrogen mg/L 2–10 1–3

Total phosphorus mg/L 0.2–0.9 0.1–0.4

Cadmium μg/L 2–5 4–5

Copper μg/L 8–100 10–60

Lead μg/L 2–30 10–140

Zinc μg/L 80–500 100–300

E. coli MPN/100 mL 360–1800 600–31,000

*Data from Refs 14–17.**Data from Refs 5, 11, 18, and 19.

California biofilters were not as small as many of theVictoria biofilters, and three of the southern Californiabiofilters were larger than any of the Victoria biofil-ters. The mean biofilter size in southern California was646 m2 while the mean size in Victoria was 160 m2.Catchment areas also differed somewhat between thetwo regions. Eight of the 13 Victoria biofilters drainedcatchments of less than 1 ha compared to only onesouthern California biofilter. Both regions includedone project greater than 50 ha. The mean catchmentareas of southern California and Victoria biofilterswere 10 and 7 ha, respectively, while the median areaswere 5.0 and 0.43 ha, respectively. Catchment ratio(biofilter area/impervious catchment area) has beenused as an indicator of biofilter effectiveness on acatchment scale, with Australian researchers suggest-ing that biofilters should cover 2% of the imperviouscatchment area for optimal performance.20 SouthernCalifornia BMP guidelines suggest designing biofiltersto infiltrate the runoff from a 2-cm (3∕4-inch) stormevent within the drainage area within 48 h.21 Sizingcalculations include percolation rates of filter mediaand underlying soils, impervious surface area, and therunoff coefficients for drainage areas.21 Four Victo-ria and four southern California biofilters reach theAustralian catchment ratio target. All but one of theVictoria biofilters were located in residential areas (theexception is located in an industrial and commercialarea, while seven southern California biofilters werein commercial districts (two combined with industrialand one combined with residential, Table 2).

Biofilters designed for infiltration are not linedwith an impermeable layer, allowing treated waterto flow vertically to the underlying soil, potentiallyrecharging groundwater. Biofilters with underdrainscan be lined or unlined. These biofilters containslotted or perforated pipes plumbed to flow to theadjacent stormwater conveyance system or receivingstream (or potentially to be captured for use in

irrigation or other purposes). Underdrains can bedesigned with outflows at an elevation higher thanthe bottom of the biofilter to retain water betweeninflow events, which results in a submerged zone.Almost all biofilters in both regions were designed forinfiltration (Table 2). In Los Angeles, three biofiltershad underdrains as well as infiltration, and again twowere underdrain alone. In Melbourne, seven biofiltershad underdrains as well as infiltration, and two wereunderdrain alone. Hereford Road Raingarden hasboth types of flow regimes, providing groundwaterrecharge via infiltration during most rain events andflood protection with underdrains during larger rainevents by preventing high ponding (Box 1).

We classified biofilters as curb cutouts or stan-dalone systems to make comparisons between theirimmediate surroundings. Curb cutout biofilters aresystems located adjacent to the street and sidewalk(Figure 4(a)). These biofilters may be required toadhere to certain regulations set by transportationagencies regarding visibility, safety, and connectivityto the stormwater conveyance system. The City of LosAngeles provides standard plans that identify planttype, ponding depth, and dimensions required to com-ply with local regulations.21,22 Similarly, MelbourneWater provides engineering plans for biofilters andother green infrastructure that provides guidance tocomply with local regulations.23 Standalone biofiltersrefer to systems not located between a street and side-walk. These systems are typically in public parks oreasements (Figure 4(b)). Typically, standalone biofil-ters have more flexibility in design, but could haverestrictions based on safety if they are located in a pub-lic area, particularly regarding plant type and pond-ing depth. One of the Ballona Creek Rain Gardensis located between a bike path and residential area(Box 2). This biofilter infiltrates runoff that wouldotherwise flow directly into Ballona Creek. This long,narrow design would not likely be possible with other



TABLE 2 Characteristics of Biofilters in Victoria and Southern California

Site Name

Age

(years)

Size

(m2)

Catchment

(ha)

Catchment

Ratio (%)

Catchment

Land Use

Drainage

Type Setting

Southern California

Elmer Avenue (Multiple) 4 12–24 16 — res Infiltration Curb cutout

Riverdale Avenue 4 12–20 5.7 — res Infiltration/Underdrain Curb cutout

Oros Street 7 12–20 2.0 — res Infiltration/Underdrain Curb cutout

Bicknell Avenue (Multiple) 5 12–20 not avail. — res Infiltration Curb cutout

Baldwin Avenue (Multiple) 2 120 2.2 0.5 res Infiltration Curb cutout

Ballona Creek East 3 1900 5 3.8 ind/comm Infiltration Standalone

Ballona Creek West 3 810 5 1.6 res Infiltration Standalone

Hope Street 4 10–15 not avail. — comm Infiltration Curb cutout

Woodman Ave (multiple) 0.5 2500 51 0.5 res/comm Infiltration Curb cutout

Chatsworth Station (multiple) 2 Varies 15 — comm Underdrain Parking Lot

LA Zoo Parking Lot (multiple) 3 1400 5.5 2.5 ind/comm Infiltration Parking Lot

Irvine- CalTrans 9 810 1.6 5.0 comm Underdrain Standalone

Scripps Institution of Oceanography 4 150 0.7 2.3 comm Infiltration/Underdrain Standalone

Average 4 646 10 2.3

Victoria

Hereford Rd 4 100 0.9 1.1 res Infiltration/Underdrain Standalone

Spring Street 2 14 0.1 1.3 res Infiltration/Underdrain Standalone

Stringybark Blvd South 3 70 0.4 1.5 res Infiltration/Underdrain Standalone

Fernhill Rd (multiple) 1 5–15 Varies — res Infiltration Curb cutout

Morrison Reserve 1 500 16 0.3 res Infiltration Standalone

Otter St (multiple) 6 105 1 0.9 res Infiltration/Underdrain Curb cutout

Napier and Kerr (multiple) 7 24 0.4 0.6 res Infiltration Curb cutout

Cremorne St (multiple) 9 85 0.1 6.4 ind/comm Infiltration/Underdrain Curb cutout

Parker St (multiple) 9 33 0.1 4.7 res Infiltration/Underdrain Curb cutout

Avoca Crescent (multiple) 9 13 0.1 2.5 res Infiltration/Underdrain Curb cutout

Clifton Hill (multiple) 6 200 3 0.7 res Underdrain Standalone

Alleyne Ave (multiple) 8 76 0.1 15 res Infiltration Curb cutout

Edinburgh Gardens 3 700 60 0.1 res Underdrain Standalone

Average 5 160 7 2.9

Where biofilter size varied within a site, averages were calculated based on the smallest size. Biofilter systems were identified and described using informationgathered from Internet keyword searches for terms ‘rain garden’, ‘biofilter’, ‘stormwater biofilter’, ‘stormwater biofiltration’, and ‘stormwater LID’ preceded bylocations of interest (Los Angeles, Culver City, Irvine, San Diego, Orange County, or Melbourne). Additionally, websites of local watershed protection agenciesand personal communication with agency personnel were used to determine site locations. The selected sites do not represent a random sample of all existingbiofilters in each region, but biofilters were not selected to represent any particular characteristic(s), except that Victoria biofilters were selected to represent arange of ages. “res”= residential, “ind”= industrial, “com”= commercial

types of stormwater management systems. Both Mel-bourne and Los Angeles had seven curb cutout biofil-ters. Only Los Angeles had parking lot biofilters (2),with the remainder in both regions being standalone.

A submerged (saturated) zone can lead tomore consistent biofilters performance. Maintaininga submerged zone led to stable hydraulic perfor-mance during prolonged wet and dry periods, whilethe outflow rate of biofilters with no submerged

zone was reduced during prolonged wet periods.13

Submerged zones have been shown to enhanceremoval of nitrogen24–27 and heavy metals.28 The-oretically, a submerged zone could increase plantsurvivorship by allowing plant roots to have access towater for an extended period of time. However, we arenot aware of any studies documenting this potential,nor evaluating its importance for different species.The presence of a submerged zone could influence



BOX 1

THE HEREFORD ROAD RAINGARDEN

The Hereford Road Raingarden was constructed in 2010 to replace a stormwater retention basinin an area adjacent to Hereford Rd. and a petrol station. This 100-m2 stormwater biofilter treats runofffrom a 9300-m2 drainage area in residential Mt. Evelyn, VIC, Australia. A sand transition layer andgravel drainage layer allow water to infiltrate into underlying soils or are collected in perforated pipesflowing first to an overflow pit, then to Little Stringybark Creek. The loamy sand layer is the filtermedia where most plant roots occupy. Hereford Road Raingarden is planted with species that have beenreported to promote high treatment performance: Carex appressa, Juncus flavidus, and Melaleuca sp.This biofilter was constructed as part of a catchment-scale effort to restore the Little Stringybark Creek(LSC) stream ecosystem through flow-regime management. Infiltration and collection pipes preventstormwater runoff from entering Little Stringybark Creek directly during low-flow events and provideflood protection during high flow events.

Littlestringybarkcreek Loamy sand

Sand and fine gravelGravel

Inflow pitOverflow

pit

Runoffcollection

Hereford road raingarden

the plant species used in a biofilter, both in terms ofpollutant removal and long-term maintenance. Again,we know of no studies evaluating these aspects; themajor study evaluating the influence of plant traits onbiofilter performance29 did not evaluate submergedzones. Because of the difference in the frequency of

ADD, submerged zones would be easier to maintain insoutheast Australia than in southern California. Insouthern California, submerged zones would likelyrequire supplemental water to maintain them throughthe dry season unless dry-weather runoff was sub-stantial. In some settings, such as golf courses, it



(a)

(b)

FIGURE 4 | Examples of typical stormwater biofilter settings. (a) Curb cutout biofilter located in Culver City, CA, U.S.A. on Baldwin Avenue; and(b) standalone biofilter located in Mt. Evelyn, VIC, Australia in Morrison Reserve.

might be possible to direct runoff to biofilters tomaintain submerged zones. Similarly, greywater fromcommercial or residential buildings could be treatedusing biofilters while providing water for maintainingsubmerged zones. Alternatively, submerged zonescould be allowed to dry out and then be re-establishedfollowing storm events. Submerged zones are begin-ning to be incorporated into stormwater biofiltersin Melbourne, but are not currently being used insouthern California.

Plants represent critical features of stormwaterbiofilters, but there are relatively few design guide-lines available. In both regions, native plants arerecommended.21,23 In Los Angeles, standard plans forcurb cutout biofilters include only plants with matureheights below 91 cm (3 feet) in order to maintain sightlines, but offer no guidance on planting densities.22 Insouthern California, plants are often (but not always)selected by landscape architects, apparently with

little knowledge of their performance in stormwaterbiofilters. In southeast Australia, research on thebiofilter performance of native plants29 has providedsome information for plant selection, although thishas not necessarily been followed. For example, thesingle species with the best pollutant removal perfor-mance, Carex appressa, is rarely used in Melbournestormwater biofilters, possibly because of its sharpleaves. There is a need for more information aboutthe performance of native species, especially in Cal-ifornia, more complete consideration of all relevantplant traits, including aesthetics and maintenanceconsiderations as well as pollutant removal, andbetter incorporation of these considerations into theselection of plants for a particular biofilter system.

In southern California, larger systems aretypically maintained by the agency responsible forconstruction. Procedures include removing undesiredvegetation, trash and debris, accumulated sediments,



BOX 2

THE BALLONA CREEK RAINGARDENS

The Ballona Creek Raingardens were constructed in 2012 to filter runoff from residential,commercial, and industrial areas in Culver City, CA, U.S.A. before infiltrating into the underlying soil.Two biofilters (810 and 1900 m2) were constructed to filter runoff from 5 ha of residential area (810-m2

biofilter) and 5 ha of industrial/commercial area (1900-m2 biofilter). Biofilters were sized to capture andinfiltrate 2.5 cm of rain over 24 h. Stormwater runoff enters a debris trap before flowing over a rock-linedchannel into the 8-m wide biofilters filled with a sandy loam filter media sloped at a 3:1 ratio to pondwater and channel flow. Overflow pipe conveys water to Ballona Creek. Grasses and sedges are thedominate plant community in these rain gardens. Volunteers from the community remove non-nativespecies annually. The debris filter will be cleaned every 3 years.

Ballona creek rain gardens

Rock-linked inflowchannel

Handrail

Ballonacreek

Bike path Gravel

Loamy sand

Debrisfilter



and residue from oils and grease; replanting desiredvegetation as needed; releveling eroded areas andfilling rutted areas with gravel; and observing perfor-mance under wet conditions.21 Biofilters in southeastAustralia are similarly maintained with the exceptionof irrigation following plant establishment. Addition-ally, the top 2–5 cm of filter media is scraped off everyfew years in some Australian biofilters in order tomaintain hydraulic conductivity and remove heavymetals, as suggested by Hatt et al.30 Responsibilityfor the maintenance of smaller systems in southernCalifornia, such as curb cutouts, is less clear. Initialmaintenance may be performed by the agency con-structing the biofilters, with subsequent maintenancethe responsibility of the landowner. Transferringmaintenance responsibility might be attractive to gov-ernment agencies; however, distributing responsibilityamong many parties can be problematic becausediffering levels of maintenance can occur.

Because of the large number of variables influ-encing stormwater biofilter performance, specifying asingle optimal design would be difficult. One studyfrom Melbourne specifically addressed this prob-lem, concluding that the optimally designed biofil-ter is at least 2% of its catchment area and pos-sesses a sandy loam filter media planted with Carexappressa or Melaleuca ericifolia.20 Although a goodstart, this study has limitations, including the factthat it was conducted in laboratory columns and didnot evaluate the performance of species combina-tions. No similar study evaluating optimum biofilterdesign has been conducted for southern California.Besides the need to evaluate California native plantspecies, there may be other differences between south-east Australia that need to be considered in south-ern California biofilter design. For example, Aus-tralian plants are adapted to low levels of soil nutri-ents, particularly phosphorus, so sandy loam may bemore suitable there than other regions.20 In otherareas with different design criteria, catchment cover-age of 5–10% has been estimated to be required tomeet phosphorus reduction targets based on model-ing studies.31 There have been very few field-basedevaluations of biofilter performance in southeast Aus-tralia (but see Refs 11 and 32–34) or southern Cal-ifornia. Most field studies of biofilter performancehave been undertaken in North Carolina35–40 andMaryland.41–44

Although biofilters have the potential to harveststormwater (or other runoff) to increase water supply,few biofilters in southeast Australia or southern Cal-ifornia have been constructed to take advantage ofthis potential. One exception is Edinburgh Gardensbiofilter in Fitzroy North, VIC, Australia. Stormwater

runoff is collected from the surrounding the resi-dential area, filtered through the 600-m2 biofilter,and stored in an underground tank. Harvested andtreated runoff provides 50% of the water needed forirrigating this 24-ha park (http://www.yarracity.vic.gov.au/environment/Parks-and-reserves/Edinburgh-Gardens/Proposed-Raingarden/).

BOX 3

KEY DIFFERENCES BETWEEN BIOFILTERSIN SOUTHEAST AUSTRALIA ANDSOUTHERN CALIFORNIA

• Climatic differences dictate vegetation choiceand hydraulic design. More drought tolerantplant species are expected in southern Cali-fornia biofilters. Ponding zones are generallydeeper in southern California biofilters.

• Most southern California systems are curbcutouts or located in parking lots. More biofil-ters in southeast Australia are standalone sys-tems. This could be due to higher developmentpressure and urban density in southern Cali-fornia, where most systems are retrofitted toexisting developments.

• Most biofilters in southern California infiltrateto the groundwater exclusively (e.g., Box 2).In southeast Australia, many systems collectoutflow via an underdrain as well as infiltrate(e.g., Box 1).

• Varying types of filter media tends to be lay-ered and contain transition zones in southeastAustralia (e.g., Box 1). Very little informationwas available on filter media in southern Cali-fornia, but the Stormwater Best ManagementPractice Design and Maintenance Manual forPublicly Maintained Storm Drain Systems pro-vide desired particle size distribution for filtermedia.

ECOLOGY OF BIOFILTRATIONSYSTEMS

A biofilter ecosystem consists of the physical elements,plants, animals, and microbial community. Most stud-ies of stormwater biofilters have focused on physicalelements, particularly the media used and its arrange-ment within the biofilter. Much less has been publishedabout the biological elements of biofilters. Of these,by far the most work has been performed on plants.Vegetated biofilter mesocosms removed more nutri-ents than unvegetated mesocosms.45,46 Read et al.


http://www.yarracity.vic.gov.au/environment/Parks-and-reserves/Edinburgh-Gardens/Proposed-Raingarden/


have examined the role of different plant traits instormwater biofilter performance using laboratorymesocosms.29 The strongest contributors to N and Premoval were related to root extent, with the plantsassociated with the highest pollutant removal, e.g.,Carex appressa, combining these root traits withhigh growth rates. Assimilation by plants has beenshown to be the primary mechanism for remov-ing nitrate under typical stormwater conditions.47

Plants can be important in maintaining hydraulicconductivity in stormwater biofilters.11 Le Cous-tumer et al. found that species with thick roots,such as Melaleuca spp., were able to maintain highhydraulic conductivity over time; they argue thatthe choice of plant species is a key design ele-ment because of the potential maintenance of systemhydraulics.48

Studies to date have focused on plant speciesfrom southeast Australia. Although general planttraits (such as extensive root systems or thick roots)may lead to similar stormwater biofilter performancein southern California, this has yet to be tested;we are currently conducting mesocosm experimentsto evaluate these factors using native southern Cal-ifornia plants. Although the results of these experi-ments are not yet available, there are some differencesbetween southeast Australia and southern Californiathat are apparent. It is critical that plant species used instormwater biofilters be well adapted to the local con-ditions, and as noted above the precipitation regimesare markedly different between southeast Australiaand southern California. Because of frequent rain-fall throughout the year, plants in southeast Australiastormwater biofilters can more easily survive withoutsupplemental water, particularly if a submerged zoneis incorporated into the biofilter design. In contrast,plants in southern California biofilters should be ableto withstand an extended dry period; although it ispossible to provide supplemental irrigation, this is notdesirable, particularly with the current and projectedshortage of water in southern California.49 Althoughwetland plants are frequently considered for plant-ing in biofilters, in southern California biofilters willbe ‘wet’ for only a relatively short period of time,so plants will need to tolerate saturated conditionsseparated by dry conditions.50 Therefore, native ter-restrial plants (from chaparral, coastal sage scrub,or grasslands) might be more appropriate for south-ern California biofilters, although these species havenot yet been evaluated. In their review, Houdeshelet al. suggest planting deep-rooted shrubs that do notrequire irrigation following establishment in arid cli-mates but do not provide information on biofilterperformance.51

To date, very few of the plant species native tosoutheast Australia and southern California have beentested for biofilter performance, or even planted inbiofilters. Both of these regions have rich native flora,with more than 800 endemic plant species in southernCalifornia52 and more than 1800 indigenous plantspecies in the Melbourne area.53 The most extensiveinvestigation to date of plants for use in biofiltersevaluated 20 plant species,29 so clearly there are manycandidate species that have not yet been studied.Both regions also include many non-native invasiveplant species. Biofilters in both regions could beparticularly prone to colonization of invasive speciesdue to the higher moisture and nutrient content thansurrounding soils and receiving seeds from runoff.Managers of biofilters in both regions have noted thatweed suppression has a high maintenance cost.

Besides the identity of species planted, southeastAustralia and southern California may have differentplanting schemes, particularly in the mix of speciesplanted in individual biofilters. However, no studieshave systematically evaluated the plant communitiesin stormwater biofilters in either region. We arecurrently conducting these studies.

In contrast to the number of studies on the roleof plants in stormwater biofilters, very little workhas been carried out on animals and microbes. Earth-worms and other burrowing terrestrial macroinverte-brates have the potential to affect the hydrology bycreating macropores in the filter media.54 These ani-mals could also affect nutrient cycling by providingan anaerobic environment in their guts capable ofdenitrification of soil nitrate and through providing aconduit to the surface through macropores.55 Addi-tionally, the interaction between plants and animals inbiofilters and the consequences in function have notbeen examined.56

The role of the microbial community instormwater performance is acknowledged, but fewdetails have been studied. For nitrogen removal,the importance of microbially mediated denitrifi-cation is recognized,57 and conditions supportingincreased denitrification, particularly a submergedzone, identified.25,27 Although some studies of specificmicrobes, or expressed genes, have been conducted,these focus narrowly on nitrogen transformations58

rather than a broader ecological roles of the micro-bial community (e.g., mycorrhizae supporting plantgrowth and diversity, competition between denitrify-ing bacteria and plant roots for nitrate).

As biological elements in an urbanized land-scape, biofilters can provide important ecologicalvalues and ecosystem services. This benefit of biofil-ters has received little attention, but particularly as



more biofilters are constructed in a catchment, theirecological influence will become more important.Stormwater biofilters are explicitly designed to pro-vide several important ecosystem services, includingflood attenuation, groundwater recharge and waterquality improvement. However, they may also provideother ecosystem services for which they have not beenexplicitly designed (at least to date). One documentedecosystem service provided by stormwater biofiltersis the support of biodiversity. Stormwater biofilterssupport a higher diversity of aboveground terrestrialinvertebrates than surround gardens and lawns.59

Kazemi et al. have argued that transitioning from tra-ditional urban landscapes such as lawns to biofilterswould enhance urban biodiversity.12,59,60 Few studieshave evaluated stormwater biofilter characteristicsthat would increase the support of biodiversity, butKazemi et al. have suggested that greater leaf/plant lit-ter depth and higher plant species richness contributeto increased biodiversity in biofilters.61 Kazemi’sstudies were conducted in Melbourne; although theresults may apply to southern California, no similarstudies have yet been conducted there.

Other ecosystem services that may be per-formed by stormwater biofilters include carbonsequestration,62 pollinator habitat,59 aesthetics, andpotentially water supply.2 Constructing biofilterswith underdrains connected to stormwater harvest-ing tanks or infiltration-type (unlined) biofilterscould help restore predevelopment flow regimes insmaller watersheds.2 Biofilters can artificially rechargegroundwater when underlying soils have adequatelyhigh infiltration rates.63 This recharge could be ben-eficial if the biofilter area is sufficient, soil pollutionand depth to groundwater are low, and the increasedwater table does not adversely affect infrastructurebelowground.

CONCLUSIONS

Biofiltration is a promising approach to managingstormwater runoff in urban areas with temperate

and Mediterranean climates. While this technologyis implemented on a large scale in southeast Aus-tralia, southern California is still in the early stagesof constructing a system of biofilters as a BMP tocontrol urban runoff. There are notable differencesin biofilter design between southeast Australia andsouthern California, most notably the larger aver-age size of southern California biofilters and higherdiversity of drainage type in southeast Australiabiofilters. The most striking difference with regardsto stormwater management in these two locations isthe greater seasonality of rainfall in southern Califor-nia, with extended dry periods between rain events,even in the wet season. This climatological differenceundoubtedly affects the function of biofilters in theMediterranean climate of southern California. Moreresearch is needed to optimize biofilter design in thisclimate. One interesting design aspect worth investi-gation is the treatment of greywater using biofilters.Greywater can supply a continuous flow of waterto maintain submerged zones during dry periods,potentially maintaining plant life and an anaerobiczone for denitrification.

The benefits of biofilters are rarely seen at thewatershed level with the exception of projects like Lit-tle Stringybark Creek in Mt. Evelyn, VIC, where thestrategic implementation of infiltrating biofilters andrainwater harvest tanks at the watershed scale haveworked toward restoring the predevelopment flowregime.2 Due to the importation of water from north-ern California and the Colorado River to southernCalifornia, a predevelopment flow regime in southernCalifornia watersheds may not be possible, espe-cially in highly urbanized areas with high imperviouscover. Nonetheless, southern California biofilters withunderdrains and saturated zones used to harvest andtreat stormwater runoff could substantially reducestorm flows and improve water quality in receiv-ing waters. Biofilters, along with other low impactdevelopment strategies, can be used to improve andextend the life of existing stormwater managementinfrastructure.

ACKNOWLEDGMENTS

This project was supported by the U.S. National Science Foundation Partnerships in International Research andEducation (PIRE) grant OISE-1243543 to the University of California, Irvine (UCI). We appreciate discussionswe have had with PIRE team members, especially Stan Grant of UCI and Ana Deletic and Belinda Hatt of MonashUniversity. We also thank stormwater managers and others in Melbourne and California including Tim Fletcher,Chris Walsh, and Matt Burns of University of Melbourne, Wing Tam, Susie Santilena, and Deborah Deets ofthe City of Los Angeles Bureau of Sanitation, and Bhaskar Joshi and Mark Keisler of California Department ofTransportation.



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