A Twenty-Year Analysis of The Ballona Freshwater Marsh’s Removal of Contaminants from Municipal Runoff, A Model for Future Restoration of the Ballona Wetlands

A Twenty-Year Analysis of The Ballona Freshwater Marsh’s Removal of Contaminants from Municipal Runoff, A Model for Future Restoration of the Ballona Wetlands | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A Twenty-Year Analysis of The Ballona Freshwater Marsh’s Removal of Contaminants from Municipal Runoff, A Model for Future Restoration of the Ballona Wetlands Annalia Henderson This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7673850/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 29 Mar, 2026 Read the published version in Wetlands Ecology and Management → Version 1 posted 6 You are reading this latest preprint version Abstract Over 95% of California’s wetlands has been lost to urbanization, prompting efforts to restore Los Angeles County’s Ballona Wetlands Ecological Reserve. Adjacent to the reserve, the Ballona Freshwater Marsh was constructed in 2003 on former agricultural land to mitigate urban and stormwater runoff. This study presents a two-decade analysis of the marsh’s effectiveness in reducing enterococci concentrations, a leading indicator of pathogenic water pollution. Enterococcus spp. levels from the marsh’s main inlets and outlet, recorded between 2005–2025, were compared to determine filtration efficacy. Results indicate the marsh often removes enterococci and can lower bacterial counts to safe levels. The geometric mean of the outlet-to-inlet ratio was 0.322 (95% CI: 0.259–0.399, p < 0.001). However, removal is not uniform, and increasingly the marsh behaves as a source rather than a sink. The California Department of Fish and Wildlife (CDFW) plans to restore 566 acres of the Ecological Reserve by removing cement levees and rerouting the adjoining Ballona Creek through a meandering path to recreate an ecologically active estuary. Separate analysis of water from the Ballona Creek over the last five years shows levels of pollution regularly above the Environmental Protection Agency (EPA) safety thresholds for fecal bacteria. We anticipate water quality improvements if the restoration proceeds. In the context of ongoing litigation over restoration of the Ecological Reserve, this study offers an assessment of engineered wetlands’ pollutant-filtering capacity, establishing a case for the research-driven management and preservation of wetlands. Enterococcus water quality Ballona Wetlands constructed wetlands urban runoff Figures Figure 1 Figure 2 Figure 3 Introduction The Ballona Wetlands in Southern California historically stretched over 2,100 acres and drained the Ballona Creek watershed into the Santa Monica Bay (Grossinger 2011). Today, only the 577-acre Ballona Wetlands Ecological Reserve remains. The Ballona Creek’s flow into this reserve was channelized to a concrete channel in the 1920s, and much of the remaining wetlands were destroyed in the 1960s, when approximately 3.5 million cubic yards of dredged material were dumped during the construction of the Marina del Rey Marina (CEQA Findings 2020 ). The Environmental Protection Agency (EPA) classified the area as impaired in 2012. In response, the California Department of Fish and Wildlife (CDFW) was tasked with restoring the ecological function of the reserve while mitigating flood risk. In 2017, the CDFW with the U.S. Army Corps of Engineering released a draft environmental impact report (EIR), proposing a “full tidal restoration” of 566 acres (Environmental Impact Report 2017 ), followed by a final report in 2019. Since then, the restoration effort has been delayed by ongoing litigation. In this context, it is an ideal time to assess lessons from a smaller, 26.1-acre restoration of the Ballona Wetlands completed 22 years ago: the Ballona Wetlands Freshwater Marsh. In 2003, the Ballona Wetlands Freshwater Marsh was created from abandoned celery fields as part of a private development of residential and commercial properties. This engineered marsh with free water surface flow has two major inlets and a single outlet (Fig. 1 ), creating an isolated environment ideal for studying water quality dynamics. Runoff from storm drains enters the marsh through the Jefferson Corridor Inlet, while runoff from a creek adjacent to Loyola Marymount University enters through the Riparian Inlet. The Ballona Outlet subsequently drains into the Ballona creek and then to the ocean. The Freshwater Marsh is home to hundreds of species of native wildlife and vegetation and serves as a resting ground for migrating birds and butterflies (Cooper 2008 , Johnston 2015). Constructed and restored wetlands are widely promoted as nature-based solutions for treating municipal wastewater and urban stormwater, often achieving substantial reductions in nutrients, suspended solids, and pathogens while providing co-benefits for wildlife and recreation (Wu 2016; Biswal 2022 ). Studies have shown that a healthy wetland ecosystem reduces fecal indicator bacteria (FIB), including E. coli and enterococci, via a combination of sedimentation, filtration, solar radiation, predation, and plant–microbe interactions (Wu 2016; Byappanahalli 2012). However, performance is highly variable across sites and seasons, and wetlands can shift from pollutant “sinks” to “sources” when internal loading, wildlife use, storm-driven resuspension of sediments, or management disturbances mobilize stored contaminants (Scott 2023, Ferguson 2005; Stein 2004 ; Dorsey 2010). Recent syntheses highlight that long-term treatment efficiency depends on hydrologic regime, catchment land use, vegetation structure, and maintenance practices, yet most studies span only months to a few years, leaving uncertainties about decadal-scale dynamics and aging of treatment wetlands (Wu 2016). This research presents the longest study to date spanning twenty years of the effectiveness of the Ballona Wetlands Freshwater Marsh as an ecological sink. Measurements of Enterococcus spp. have been recorded using consistent methodology in the Ballona Marsh since 2005 by volunteers from the Blue Water Task Force, an international program that samples water quality along the coastline. This study aims to compare the relative concentrations of enterococci at the inlets and outlet of the marsh to determine filtering capacity and consistency. By understanding the ongoing impacts of the Ballona Marsh on water quality, this research expects to provide insight into how these patterns might be reflected with further restoration of the Ballona Wetlands Ecological Reserve. Data Acquisition and Processing The Idexx Enterolert test kits (Budnick 1996) were utilized to quantify Enterococcus spp. bacteria concentrations in water samples collected by the Surfrider Foundation for the Blue Water Task Force from three predetermined locations in the Ballona Freshwater Marsh: The Jefferson Creek Inlet, Riparian Corridor Inlet, and Ballona Outlet (Fig. 1 ). Enterococcus species identified by this method are faecalis , faecium , avium , gallinarum , casseliflavus , and durans . Water quality sampling was conducted approximately twice per month, but not during the summer. During the COVID epidemic, water samples were only collected for the first couple of months of 2020. Water quality samples measured from October 2005 until May 2025 were sourced from the Blue Water Task Force public webpage database ( https://bwtf.surfrider.org/ ). Cumulative data was compiled and analyzed using Python Pandas data programming tools in Jupyter Notebook, and statistical analyses were performed with the SciPy library. Lower and upper detection limits levels were 10 most probable number (MPN)/100ml and 24,196 MPN/100ml respectively. The Ballona Creek’s water quality is monitored by the Los Angeles County Public Works. Enterococci bacteria concentrations spanning 2019–2024 were analyzed using Python Pandas software. For E. coli the lower and upper detection limits were 100 MPN/100ml and 24,000 MPN/100ml and for enterococci the respective limits were 10 MPN/100ml and 24,000 MPN/100ml. Results This study analyzed 705 enterococci bacteria concentration measurements in the Ballona Freshwater Marsh. Data from the Riparian Creek Inlet spanned 2007–2025 and data from the Jefferson Inlet and Ballona Outlet spanned 2005–2025. During this time, the Jefferson Inlet had a mean concentration of 2,047 ± 4,715 MPN/100 ml and the Riparian Creek Inlet of 3,149 ± 6,573 MPN/100 ml. The Ballona Outlet had a smaller mean concentration of 1,455 ± 4,651 MPN/100 ml. To account for the log-normal distribution of bacteria concentrations, geometric means were used to summarize results. The calculated geometric means were 409 MPN/100ml for the Jefferson Inlet, 497 MPN/100ml for the Riparian Corridor Inlet, and 186 MPN/100ml for the Ballona Outlet, each far above the EPA recommended 33 MPN/100ml or above the California threshold of 35 MPN/100 ml. The geometric mean of the outlet-to-inlet ratio was 0.322 (95% CI: 0.259–0.399), indicating that outlet enterococci concentrations were typically 68% lower than inlet levels; this reduction was statistically significant (p < 0.001, Wilcoxon signed-rank test). For the first 18 years of the data collection, 80% of the time enterococci concentrations at the Ballona Outlet indicated reduced bacterial concentrations relative to inlet levels (sink events), while 20% showed equal or higher concentrations (source events). For the last two years, the proportions shifted to less than half, 48% sink events and 52% source events, indicating a significant decline in filtration efficacy in recent years (Fig. 3 ). Log-transformed concentrations at the two inlets were moderately correlated (r = 0.51, p < 0.001). No significant difference in mean levels was detected (p = 0.34), indicating that neither inlet consistently exhibited higher concentrations. Approximately 26% of the variation in inlet concentrations (r 2 = 0.26) can be explained by the concentration of the other inlet, while the remaining 74% reflects factors not shared. For example, on September 25, 2022 the Riparian Creek inlet measured 24,196 MPN/100 ml, while the Jefferson Storm Drain had a reading of only 132 MPN/100 ml, below its geometric mean. Such discrepancies suggest that local factors may strongly influence individual inlets over larger environmental factors such as weather. ANOVA showed no significant seasonal effects on log-transformed enterococci concentrations (Spring, Winter, Fall; Summer data unavailable), F(2,244) = 1.05, p = 0.35. Using temperature and precipitation records from the National Weather Service’s KLAX station, no significant correlations were found with temperature, same day precipitation, or precipitation lagged by 1–3 days. To evaluate the water quality of Ballona Creek which the EIR proposes to redirect into the future restored wetland, this study analyzed 1,874 water samples collected by the L.A. County Public Works between 2019 − 2014 for Enterococcus spp., E. coli , and total coliform. Just upstream of the planned restoration, the Ballona Creek Estuary sampling site along McConnell Ave recorded some of the highest average levels of water pollution in the region. Mean concentrations were elevated, with enterococci at 2,143 MPN/100ml and E. coli at 8,637 MPN/100ml. These averages were skewed by days when values exceeded the assay’s upper detection limit of 24,000 MPN/100ml. The geometric mean for enterococci was at 218 MPN/100ml and E. coli at 1,033 MPN/100ml, each exceeding the U.S. EPA’s recreation water quality criteria of 33 MPN/100ml and 126 MPN/100ml, respectively. Total coliform levels were also high, with a mean of 92,919 MPN/100ml and a geometric mean of 45,060 MPN/100ml. The final 4.7 km of the Ballona Creek functions as an estuary, with concrete banks but a sediment bottom. At its distal end near Pacific Avenue, where tidal mixing occurs, the bacteria concentrations improved but remained elevated with a geometric mean of 39 MPN/100 ml for enterococci, 208 MPN/100 ml for E. coli , and 2,116 MPN/100 mL for total coliform. Discussion This two-decade record shows that the Ballona Freshwater Marsh has generally functioned as an effective, though not consistently reliable, sink for fecal indicator bacteria (Fig. 2 ). On average, enterococci concentrations at the outlet were 68% lower than at the inlets (geometric mean outlet–to–inlet ratio 0.322), equivalent to roughly a 0.5-log 10 reduction. Yet this treatment capacity has declined recently: sink events dominated from 2005–2023, whereas in the last two years the marsh was more likely to export enterococci than to remove it. Figure 3 reveals water quality improvement events have declined since 2023, indicating that the marsh is functioning less as a sink and increasingly as a source of pollutants. Prior short-term studies spanning only a few months would be unable to detect this variation in the wetland’s pollutant removal capacity. These findings fit within a broader body of work showing that wetlands, whether natural or engineered, can alternate between acting as sinks and sources of fecal indicator bacteria depending on design, loading, and management (Wu 2016). Constructed wetland systems are widely used for pathogen and FIB attenuation, but reported performance is highly variable. Free water surface wetlands typically report reductions of 0.1–2.2 log 10 units for enterococci under standard operating conditions, yet the effluent concentrations frequently remain above reuse or contact-recreation standards when influent loads are high (Kadlec 2009 ). In this context, the Ballona Freshwater Marsh’s 68% reduction (≈ 0.5 log 10 ) is modest but within the reported ranges. For example, flow-through treatment wetlands in California agricultural landscapes achieved 80–95% reductions in E. coli load and 86–94% reductions in enterococci, but only ~ 30% of effluent samples met the 33 MPN/100 mL standard (O’Geen 2015 ). Similarly, Lamori et al. ( 2019 ) reported ~ 84% removal of culturable enterococci in a tertiary treatment wetland, again with many samples exceeding guideline values. Our findings for Ballona mirror this pattern: the marsh substantially lowers enterococci concentrations but rarely brings them below regulatory thresholds, because influent levels are often orders of magnitude above those criteria. The magnitude of removal at Ballona is also plausible given its configuration. Reviews emphasize that pathogen removal in constructed wetlands is controlled by hydraulic retention time (HRT), hydraulic regime, plant community, and loading rate (Wu 2016). In the Central Valley case study, E. coli removal increased from ~ 69% to ~ 95% as HRT increased from 11 days (O’Geen 2015 ). The Ballona Freshwater Marsh is relatively small compared to its urban catchment, receives highly variable dry-weather runoff, and likely operates with shorter effective HRTs than dedicated treatment wetlands. Under those constraints, consistent ~ 0.5-log enterococcus reduction is comparable to other multi-use systems in which water-quality treatment is only one of several design goals. A key contribution of this study is the demonstration that the Ballona Freshwater Marsh has shifted from predominantly sink behavior (≈ 80% of sampling events prior to 2023) to roughly equal sink/source behavior in the last two years. Short-term studies often report only average removal efficiencies and can miss such temporal transitions. Alternation between sink and source behavior is found on other coastal and urban wetlands. In the adjacent Ballona salt marsh, Dorsey et al. ( 2010 ) showed that the system acted as a sink for FIB during daylight hours, when solar radiation and quiescent conditions promoted inactivation and sedimentation, but shifted to a source during late-day and nocturnal ebb tides when contaminated sediments were resuspended and exported. Beyond Southern California, a stormwater treatment wetland in Europe was recently reported to be a consistent net exporter of E. coli , with outlet concentrations 1.5–26 times higher than inlets (median ~ 10-fold increase) (Scott 2023). Agricultural treatment wetlands show similar patterns. In the Central Valley case study, flow-through wetlands receiving relatively contaminated irrigation tailwater reduced E. coli loads by 80–95%, whereas large flood-pulse wetlands supplied with relatively clean water often exported FIB, with negative “removal efficiencies” ranging from − 206% to − 554% (O’Geen 2015 ). These systems illustrate a key point: wetlands contain internal FIB sources—including sediments, biofilms, decaying vegetation, and wildlife—that can dominate when influent concentrations are low or hydraulic and light conditions favor regrowth and resuspension (Wu 2016). The Ballona Freshwater Marsh fits squarely within this source–sink framework. For much of its life, the marsh has behaved like the flow-through systems described above, receiving heavily contaminated urban runoff and exporting water with substantially lower enterococci concentrations. During some months, even when inlet concentrations saturated the Quanti-Tray assay (> 24,196 MPN/100 mL), outlet water was reduced to near or below recreational criteria (Fig. 2 ), demonstrating that the marsh is capable of strong sink behavior under favorable conditions. At other times, especially following large-scale vegetation disturbance (e.g., reed cutting for mosquito abatement) (Fig. 2 ) and in the most recent years, outlet concentrations exceeded inlet averages, consistent with a growing contribution from internal reservoirs and altered hydraulics. They also support the conclusion of Wu et al. ( 2016 ) that wetlands are best understood not as permanent “filters” but as dynamic reactors in which die-off, retention, and release processes continually compete. Our statistical analyses did not detect strong relationships between enterococci concentrations and temperature or short-term precipitation. Studies have been mixed about reporting changes in FIB with seasons with some reporting higher removal in warm seasons (Elhamouri 1994; Karathanasis 2003) and others reporting no seasonal change (Garcia 2008, Tunçsiper 2012). The absence of such signals at Ballona may reflect (1) limited sampling during storm events, (2) the relatively narrow seasonal temperature range in coastal Southern California, and (3) the dominance of local episodic factors—such as illegal dumping, maintenance actions, or wildlife usage—over climatic drivers at the time scale of our sampling. Enterococcus spp. and E. coli levels measured in Ballona Creek upstream of the Ecological Reserve were consistently elevated, with geometric means (218 MPN/100 mL for Enterococcus and 1,033 MPN/100 mL for E. coli ) well above U.S. EPA recreational criteria, but they are within the range reported for heavily urbanized catchments with substantial impervious cover and chronic dry-weather flows (Wu 2016, Kadlec 2009 ). Downstream, where the concrete-lined channel transitions to a tidally influenced estuary with a sediment bottom, bacterial concentrations decline but remain elevated relative to regulatory thresholds. This longitudinal pattern mirrors other coastal systems in which natural or restored marshes reduce FIB loads from upland sources but do not fully eliminate contamination. Dorsey ( 2006 , 2010) and Myers & Ambrose ( 2015 ) all found that Southern California salt marshes reduce FIB fluxes from upstream creeks, yet can episodically export bacteria back to the estuary during specific tidal or wind conditions (Dorsey 2015 ). The Ballona Freshwater Marsh thus provides a relevant analog for the proposed re-routing of Ballona Creek through a restored tidal wetland. Diverting polluted creek water through vegetated floodplains is likely to produce measurable improvements in downstream water quality, even if regulatory standards are not consistently met. The marsh has repeatedly demonstrated the capacity to transform extremely contaminated inflows, sometimes at or above assay detection limits, into much cleaner outflows. At the same time, the decline in sink performance and increasing frequency of source events highlight the importance of design and adaptive management. Equally important is reducing loads before they reach the wetland. Recent work on the Ballona Creek rain garden shows that small-scale bioretention systems can significantly attenuate FIB and other pollutants in upland urban stormwater (Buckhard 2024). Coupling such distributed green infrastructure with a large restored estuarine wetland increases the likelihood that the system as a whole will function as a net sink rather than a source. This retrospective, observational analysis has important limitations and does not establish causality or isolate specific drivers of sink-versus-source behavior. The nonuniform temporal coverage (notably no summer data and a collection gap during early 2020), may reduce sensitivity to seasonal or episodic dynamics and may under-capture short-lived storm pulses. Also enterococci are a proxy for fecal contamination risk rather than a direct measurement of pathogens; accordingly, an increase in enterococci does not necessarily imply a proportional rise in clinically relevant pathogens. Finally, wetlands provide additional water-quality services beyond FIB attenuation, including removal or transformation of metals and organic contaminants. Our Ballona Creek measurements showed metals and pesticides were usually below regulatory thresholds (data not shown). From a management perspective, the Ballona Freshwater Marsh demonstrates both the promise and the limits of wetlands as nature-based water-treatment infrastructure. Over two decades, the marsh has provided substantial pollutant removal under real-world loading and maintenance regimes, similar in magnitude to other multi-purpose constructed wetlands. Yet its recent shift toward source behavior underscores that these systems are not static. Their water-quality functions depend on sustained hydrologic, vegetation, and sediment management, as well as on the broader watershed context. For the Ballona Wetlands Ecological Reserve, our results add to the literature supporting proceeding with restoration that reconnects Ballona Creek to a more natural floodplain and tidal network, while explicitly incorporating hydrologic and ecological design criteria known to enhance FIB removal. They also justify long-term monitoring beyond the typical 1–3-year project horizon. Without sustained datasets like the one presented here, managers risk misinterpreting short-term sink behavior as guaranteed and may overlook gradual declines in treatment performance. Finally, by documenting both strong sink events and periods of net export, this study reinforces a central message from the broader literature: wetlands can substantially improve water quality, but they do so as dynamic ecosystems whose performance is contingent on design, operation, and the quality of water they receive. Recognizing and planning for that dynamism is essential to realizing the full ecosystem service benefits of wetland restoration in urban coastal watersheds. Declarations Statements & Declarations: The author declares that no funds, grants, or other support were received during the preparation of this manuscript. The author has no relevant financial or non-financial interests to disclose. Author Contribution Annalia J Henderson is solely responsible for the study conception and design, data analysis, and manuscript. Acknowledgement Craig Cadwallader (Surfrider Foundation) collected twenty years of water quality samples at Ballona Freshwater Marsh. Guidance and advice were gratefully received from Mary Simun (Surfrider Foundation) and Michelle Pena-Ortiz (Blue Water Task Force). Los Angeles County Public Works water quality data was provided by Josafat Flores. I am grateful to John Dorsey, Gregory Henderson, David Kay, and Raymond Wells for review and feedback on the manuscript. Data Availability Ballona Fresh Water Marsh water quality samples collected between October 2005 and May 2025 were sourced from the Blue Water Task Force database (https://bwtf.surfrider.org/). Additional water quality data from the Ballona Creek from 2019 to 2024 were provided by the Los Angeles County Public Works. References Biswal BK, Balasubramanian R (2022) Constructed Wetlands for Reclamation and Reuse of Wastewater and Urban Stormwater: A Review Front Environ Sci, 10. https://doi.org/10.3389/fenvs.2022.836289 Buckhard JL, Johnston KK, Saez JA Dorsey JH (2024) Water Quality and Hydrological Benefits of a Natural Treatment System in an Urban Watershed in Southern California. Bull Southern California Acad Sci 123 (3): 105–134. https://doi.org/10.3160/0038-3872-123.3.105 Budnick GE, Howard RT, Mayo DR (1996) Evaluation of Enterolert for enumeration of enterococci in recreational waters. Appl Environ Microbiol 62:3881. https://doi.org/10.1128/aem.62.10.3881-3884.1996 Byappanahalli MN, Nevers MB, Korajkic A, Staley ZR, Harwood VJ (2012) Enterococci in the environment. Microbiol Mol Biol Rev. Dec;76(4):685–706. https://doi.org/10.1128/mmbr.00023-12 CEQA Findings (2020) CEQA Findings of Fact of the California Department of Fish and Wildlife as Lead Agency under the California Environmental Quality Act (Pub. Resources Code, § 21000 et seq.) for the Restoration of the Ballona Wetlands Ecological Reserve as analyzed in the Ballona Wetlands Restoration Project Final Environmental Impact Report. State Clearinghouse No. 2012071090 Cooper DS (2008). The Use of Historical Data in the Restoration of the Avifauna of the Ballona Wetlands, Los Angeles County, California. Natural Areas Journal, 28(1), 83–90. https://www.jstor.org/stable/43914034 Dorsey JH (2006). Densities of Fecal Indicator Bacteria in Tidal Waters of the Ballona Wetlands, Los Angeles County, California. Southern California Academy of Sciences, 105 (2), 59 – 7. https://doi.org/10.3160/0038-3872(2006 )105[59:DOFIBI]2.0.CO;2 Dorsey JH, Carter PM, Bergquist S, Sagarin R (2010) Reduction of fecal indicator bacteria (FIB) in the Ballona Wetlands saltwater marsh (Los Angeles County, California, USA) with implications for restoration actions. Water Res, 44(15):4630–42. https://doi.org/10.1016/j.watres.2010.06.012 Dorsey JH (2015) The Ballona Wetlands: Nature’s water treatment system. CURes Blog, Loyola Marymount University, Los Angeles. https://curesblog.lmu.edu/the-ballona-wetlands-natures-water-treatment-system/ Accessed 28 Nov 2025. Elhamouri B, Khallayoune K, Bouzoubaa K, Rhallabi N, Chalabi M (1994) High-rate algal pond performances in faecal-coliforms and helminth egg removals. Water Res 28 (1), 171–174. https://doi.org/10.1016/0043-1354(94)90131-7 Environmental Impact Report (2017) Draft Ballona Wetlands Restoration Project Environmental Impact Statement/Environmental Impact Report. State Clearinghouse No. 2012071090 Ferguson DM, Moore DF, Getrich MA, Zhowandai MH (2005). Enumeration and speciation of enterococci found in marine and intertidal sediments and coastal water in southern California. Journal of applied microbiology, 99(3), 598–608. https://doi.org/10.1111/j.1365-2672.2005.02660.x García M, Soto F, González JM, Bécares E (2008) A comparison of bacterial removal efficiencies in constructed wetlands and algae-based systems. Ecol Eng, 32(3), 238–243. https://doi.org/10.1016/j.ecoleng.2007.11.012 Grossinger R, Stein ER, Cayce K, Askevold R, Dark S, Whipple A (2011) Historical wetlands of the Southern California Coast: An Atlas of US Coast Survey T-sheets, 1851–1889. Oakland: San Francisco Estuary Institute. Johnston KK, Medel ID, Abbott RC, Grubbs MW, Del Giudice-Tuttle E, Piechowski C, Wong Yau M, Dorsey J (2015) Ballona Wetlands Ecological Reserve: Comprehensive 5-Year Monitoring Report. Report prepared by The Bay Foundation for the California State Coastal Conservancy. Karathanasis AD, Potter CL, Coyne MS (2003) Vegetation effects on faecal bacteria, BOD, and suspended solid removal in constructed wetlands treating domestic waste-water. Ecol Eng, 20(2), 157–169. Kadlec RH, Wallace SD (2009) Treatment Wetlands Second Edition. CRC Press, New York Lamori, JG, Xue J, Rachmadi AT, Lopez GU, Kitajima M, Gerba CP, Pepper IL, Brooks JP, Sherchan S (2019) Removal of fecal indicator bacteria and antibiotic resistant genes in constructed wetlands. Environ Sci Pollut Res, 26, 10188–10197. https://doi.org/10.1007/s11356-019-04468-9 Myers MR, Ambrose RF (2015) Salt marsh reduces fecal indicator bacteria input to coastal waters in Southern California. Bull South Calif Acad Sci, 114(2), 76–88. https://doi.org/10.3160/0038-3872-114.2.76 O’Geen AT, Bianchi ML (2015) Using Wetlands to Remove Microbial Pollutant from Farm Discharge Water. ANR Publication 8512. Stott R, Sukias JPS, McKergow LA, Davies-Colley RJ, Tanner CC (2023) Net export of E. coli from a constructed wetland treating agricultural drainage. Ecol Eng, 194:107023. https://doi.org/10.1016/j.ecoleng.2023.107023 Sharley DJ, Sharp SM, Marshall S, Jeppe K, Pettigrove VJ (2017) Linking urban land use to pollutants in constructed wetlands: Implications for stormwater and urban planning. Landscape and Urban Planning, 162, 80–91. https://doi.org/10.1016/j.landurbplan.2016.12.016 Stein E (2004) Characterization of Dry Weather Metals and Bacteria in Ballona Creek. Southern California Coastal Water Research Project. Tunçsiper B, Ayaz SÇ, Akça L (2012) Coliform bacteria removal from septic wastewater in a pilot-scale combined constructed wetland system. Environ Eng Manag J (EEMJ), 11(10), 1873–1879. Wu S, Carvalho PN, Müller JA, Manoj VR, Dong R (2016) Sanitation in constructed wetlands: A review on the removal of human pathogens and fecal indicators. Science of The Total Environment, 541, 8–22. https://doi.org/10.1016/j.scitotenv.2015.09.047 . Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 29 Mar, 2026 Read the published version in Wetlands Ecology and Management → Version 1 posted Editorial decision: Revision requested 22 Jan, 2026 Reviews received at journal 20 Jan, 2026 Reviewers agreed at journal 28 Dec, 2025 Reviewers invited by journal 28 Dec, 2025 Submission checks completed at journal 15 Dec, 2025 First submitted to journal 14 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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09:22:02","extension":"html","order_by":10,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":73540,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7673850/v1/7246c39cdff4f90b1ec03ce0.html"},{"id":99792594,"identity":"9a9b917d-6846-4fbe-a699-71a148e90db4","added_by":"auto","created_at":"2026-01-08 13:22:48","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":5210853,"visible":true,"origin":"","legend":"\u003cp\u003eBallona Wetlands Freshwater Marsh. Inlet and Outlet locations. \u003cem\u003eSource: Google Maps.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7673850/v1/95de2f9dafe9cb93161d0d05.png"},{"id":99792533,"identity":"2363532d-0924-4003-8ae6-13f7abfb0f4b","added_by":"auto","created_at":"2026-01-08 13:21:54","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":286525,"visible":true,"origin":"","legend":"\u003cp\u003eRemoval of \u003cem\u003eEnterococcus\u003c/em\u003e by Ballona Freshwater Marsh. As inlet concentrations increase (x-axis), there is a steady positive and increasing removal of bacteria (y-axis). 76% of outlet samples had lower bacterial counts than the inlets (a sink), while 21% of samples had higher counts (a source). Over the last two years (points shown in green) the marsh has more often served as a source. On October 9, 2016, vegetation and reeds in the Ballona Marsh had been drastically cleared for mosquito abatement (personal observation and photo by Craig Cadwallader) and the Ballona Marsh turned into a source (before: panel A; after: panel B). By contrast, on January 25, 2015, despite very high inlet concentrations, the outlet effectively lowered the \u003cem\u003eEnterococcus\u003c/em\u003e to safe levels.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7673850/v1/fd9b53286e1c37371334565c.png"},{"id":99594901,"identity":"716e5149-503a-432a-a1fe-f2df599974fa","added_by":"auto","created_at":"2026-01-06 09:21:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":198036,"visible":true,"origin":"","legend":"\u003cp\u003eAnnual geometric-mean percent change in \u003cem\u003eEnterococcus\u003c/em\u003e. Each point shows the yearly geometric mean of event-level outlet:inlet ratios. Values less than 0% indicate net removal (sink; green band), while values greater than 0% indicate net increase (source; red band). Error bars indicate the 95% confidence intervals for each year.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7673850/v1/dfe834b4b5dfe9f6ad4e177d.png"},{"id":105755926,"identity":"8eddcbbe-bb91-4ba0-bffc-cf60dd82bb2c","added_by":"auto","created_at":"2026-03-30 16:33:08","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6804038,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7673850/v1/09018244-a4c6-420f-ab3c-ec2a8b69b66c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"A Twenty-Year Analysis of The Ballona Freshwater Marsh’s Removal of Contaminants from Municipal Runoff, A Model for Future Restoration of the Ballona Wetlands","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Ballona Wetlands in Southern California historically stretched over 2,100 acres and drained the Ballona Creek watershed into the Santa Monica Bay (Grossinger 2011). Today, only the 577-acre Ballona Wetlands Ecological Reserve remains. The Ballona Creek\u0026rsquo;s flow into this reserve was channelized to a concrete channel in the 1920s, and much of the remaining wetlands were destroyed in the 1960s, when approximately 3.5\u0026nbsp;million cubic yards of dredged material were dumped during the construction of the Marina del Rey Marina (CEQA Findings \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The Environmental Protection Agency (EPA) classified the area as impaired in 2012.\u003c/p\u003e \u003cp\u003eIn response, the California Department of Fish and Wildlife (CDFW) was tasked with restoring the ecological function of the reserve while mitigating flood risk. In 2017, the CDFW with the U.S. Army Corps of Engineering released a draft environmental impact report (EIR), proposing a \u0026ldquo;full tidal restoration\u0026rdquo; of 566 acres (Environmental Impact Report \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), followed by a final report in 2019. Since then, the restoration effort has been delayed by ongoing litigation. In this context, it is an ideal time to assess lessons from a smaller, 26.1-acre restoration of the Ballona Wetlands completed 22 years ago: the Ballona Wetlands Freshwater Marsh.\u003c/p\u003e \u003cp\u003eIn 2003, the Ballona Wetlands Freshwater Marsh was created from abandoned celery fields as part of a private development of residential and commercial properties. This engineered marsh with free water surface flow has two major inlets and a single outlet (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), creating an isolated environment ideal for studying water quality dynamics. Runoff from storm drains enters the marsh through the Jefferson Corridor Inlet, while runoff from a creek adjacent to Loyola Marymount University enters through the Riparian Inlet. The Ballona Outlet subsequently drains into the Ballona creek and then to the ocean. The Freshwater Marsh is home to hundreds of species of native wildlife and vegetation and serves as a resting ground for migrating birds and butterflies (Cooper \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2008\u003c/span\u003e, Johnston 2015).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eConstructed and restored wetlands are widely promoted as nature-based solutions for treating municipal wastewater and urban stormwater, often achieving substantial reductions in nutrients, suspended solids, and pathogens while providing co-benefits for wildlife and recreation (Wu 2016; Biswal \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Studies have shown that a healthy wetland ecosystem reduces fecal indicator bacteria (FIB), including \u003cem\u003eE. coli\u003c/em\u003e and enterococci, via a combination of sedimentation, filtration, solar radiation, predation, and plant\u0026ndash;microbe interactions (Wu 2016; Byappanahalli 2012). However, performance is highly variable across sites and seasons, and wetlands can shift from pollutant \u0026ldquo;sinks\u0026rdquo; to \u0026ldquo;sources\u0026rdquo; when internal loading, wildlife use, storm-driven resuspension of sediments, or management disturbances mobilize stored contaminants (Scott 2023, Ferguson 2005; Stein \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Dorsey 2010). Recent syntheses highlight that long-term treatment efficiency depends on hydrologic regime, catchment land use, vegetation structure, and maintenance practices, yet most studies span only months to a few years, leaving uncertainties about decadal-scale dynamics and aging of treatment wetlands (Wu 2016).\u003c/p\u003e \u003cp\u003eThis research presents the longest study to date spanning twenty years of the effectiveness of the Ballona Wetlands Freshwater Marsh as an ecological sink. Measurements of \u003cem\u003eEnterococcus\u003c/em\u003e spp. have been recorded using consistent methodology in the Ballona Marsh since 2005 by volunteers from the Blue Water Task Force, an international program that samples water quality along the coastline. This study aims to compare the relative concentrations of enterococci at the inlets and outlet of the marsh to determine filtering capacity and consistency. By understanding the ongoing impacts of the Ballona Marsh on water quality, this research expects to provide insight into how these patterns might be reflected with further restoration of the Ballona Wetlands Ecological Reserve.\u003c/p\u003e"},{"header":"Data Acquisition and Processing","content":"\u003cp\u003eThe Idexx Enterolert test kits (Budnick 1996) were utilized to quantify \u003cem\u003eEnterococcus\u003c/em\u003e spp. bacteria concentrations in water samples collected by the Surfrider Foundation for the Blue Water Task Force from three predetermined locations in the Ballona Freshwater Marsh: The Jefferson Creek Inlet, Riparian Corridor Inlet, and Ballona Outlet (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). \u003cem\u003eEnterococcus\u003c/em\u003e species identified by this method are \u003cem\u003efaecalis\u003c/em\u003e, \u003cem\u003efaecium\u003c/em\u003e, \u003cem\u003eavium\u003c/em\u003e, \u003cem\u003egallinarum\u003c/em\u003e, \u003cem\u003ecasseliflavus\u003c/em\u003e, and \u003cem\u003edurans\u003c/em\u003e. Water quality sampling was conducted approximately twice per month, but not during the summer. During the COVID epidemic, water samples were only collected for the first couple of months of 2020. Water quality samples measured from October 2005 until May 2025 were sourced from the Blue Water Task Force public webpage database (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://bwtf.surfrider.org/\u003c/span\u003e\u003cspan address=\"https://bwtf.surfrider.org/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). Cumulative data was compiled and analyzed using Python Pandas data programming tools in Jupyter Notebook, and statistical analyses were performed with the SciPy library. Lower and upper detection limits levels were 10 most probable number (MPN)/100ml and 24,196 MPN/100ml respectively.\u003c/p\u003e \u003cp\u003eThe Ballona Creek\u0026rsquo;s water quality is monitored by the Los Angeles County Public Works. Enterococci bacteria concentrations spanning 2019\u0026ndash;2024 were analyzed using Python Pandas software. For \u003cem\u003eE. coli\u003c/em\u003e the lower and upper detection limits were 100 MPN/100ml and 24,000 MPN/100ml and for enterococci the respective limits were 10 MPN/100ml and 24,000 MPN/100ml.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eThis study analyzed 705 enterococci bacteria concentration measurements in the Ballona Freshwater Marsh. Data from the Riparian Creek Inlet spanned 2007\u0026ndash;2025 and data from the Jefferson Inlet and Ballona Outlet spanned 2005\u0026ndash;2025. During this time, the Jefferson Inlet had a mean concentration of 2,047\u0026thinsp;\u0026plusmn;\u0026thinsp;4,715 MPN/100 ml and the Riparian Creek Inlet of 3,149\u0026thinsp;\u0026plusmn;\u0026thinsp;6,573 MPN/100 ml. The Ballona Outlet had a smaller mean concentration of 1,455\u0026thinsp;\u0026plusmn;\u0026thinsp;4,651 MPN/100 ml. To account for the log-normal distribution of bacteria concentrations, geometric means were used to summarize results. The calculated geometric means were 409 MPN/100ml for the Jefferson Inlet, 497 MPN/100ml for the Riparian Corridor Inlet, and 186 MPN/100ml for the Ballona Outlet, each far above the EPA recommended 33 MPN/100ml or above the California threshold of 35 MPN/100 ml.\u003c/p\u003e \u003cp\u003eThe geometric mean of the outlet-to-inlet ratio was 0.322 (95% CI: 0.259\u0026ndash;0.399), indicating that outlet enterococci concentrations were typically 68% lower than inlet levels; this reduction was statistically significant (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, Wilcoxon signed-rank test). For the first 18 years of the data collection, 80% of the time enterococci concentrations at the Ballona Outlet indicated reduced bacterial concentrations relative to inlet levels (sink events), while 20% showed equal or higher concentrations (source events). For the last two years, the proportions shifted to less than half, 48% sink events and 52% source events, indicating a significant decline in filtration efficacy in recent years (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eLog-transformed concentrations at the two inlets were moderately correlated (r\u0026thinsp;=\u0026thinsp;0.51, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). No significant difference in mean levels was detected (p\u0026thinsp;=\u0026thinsp;0.34), indicating that neither inlet consistently exhibited higher concentrations. Approximately 26% of the variation in inlet concentrations (r\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.26) can be explained by the concentration of the other inlet, while the remaining 74% reflects factors not shared. For example, on September 25, 2022 the Riparian Creek inlet measured 24,196 MPN/100 ml, while the Jefferson Storm Drain had a reading of only 132 MPN/100 ml, below its geometric mean. Such discrepancies suggest that local factors may strongly influence individual inlets over larger environmental factors such as weather. ANOVA showed no significant seasonal effects on log-transformed enterococci concentrations (Spring, Winter, Fall; Summer data unavailable), F(2,244)\u0026thinsp;=\u0026thinsp;1.05, p\u0026thinsp;=\u0026thinsp;0.35. Using temperature and precipitation records from the National Weather Service\u0026rsquo;s KLAX station, no significant correlations were found with temperature, same day precipitation, or precipitation lagged by 1\u0026ndash;3 days.\u003c/p\u003e \u003cp\u003eTo evaluate the water quality of Ballona Creek which the EIR proposes to redirect into the future restored wetland, this study analyzed 1,874 water samples collected by the L.A. County Public Works between 2019\u0026thinsp;\u0026minus;\u0026thinsp;2014 for \u003cem\u003eEnterococcus\u003c/em\u003e spp., \u003cem\u003eE. coli\u003c/em\u003e, and total coliform. Just upstream of the planned restoration, the Ballona Creek Estuary sampling site along McConnell Ave recorded some of the highest average levels of water pollution in the region. Mean concentrations were elevated, with enterococci at 2,143 MPN/100ml and \u003cem\u003eE. coli\u003c/em\u003e at 8,637 MPN/100ml. These averages were skewed by days when values exceeded the assay\u0026rsquo;s upper detection limit of 24,000 MPN/100ml.\u003c/p\u003e \u003cp\u003eThe geometric mean for enterococci was at 218 MPN/100ml and \u003cem\u003eE. coli\u003c/em\u003e at 1,033 MPN/100ml, each exceeding the U.S. EPA\u0026rsquo;s recreation water quality criteria of 33 MPN/100ml and 126 MPN/100ml, respectively. Total coliform levels were also high, with a mean of 92,919 MPN/100ml and a geometric mean of 45,060 MPN/100ml.\u003c/p\u003e \u003cp\u003eThe final 4.7 km of the Ballona Creek functions as an estuary, with concrete banks but a sediment bottom. At its distal end near Pacific Avenue, where tidal mixing occurs, the bacteria concentrations improved but remained elevated with a geometric mean of 39 MPN/100 ml for enterococci, 208 MPN/100 ml for \u003cem\u003eE. coli\u003c/em\u003e, and 2,116 MPN/100 mL for total coliform.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis two-decade record shows that the Ballona Freshwater Marsh has generally functioned as an effective, though not consistently reliable, sink for fecal indicator bacteria (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e2\u003c/span\u003e). On average, enterococci concentrations at the outlet were 68% lower than at the inlets (geometric mean outlet\u0026ndash;to\u0026ndash;inlet ratio 0.322), equivalent to roughly a 0.5-log\u003csub\u003e10\u003c/sub\u003e reduction.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eYet this treatment capacity has declined recently: sink events dominated from 2005\u0026ndash;2023, whereas in the last two years the marsh was more likely to export enterococci than to remove it. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e reveals water quality improvement events have declined since 2023, indicating that the marsh is functioning less as a sink and increasingly as a source of pollutants. Prior short-term studies spanning only a few months would be unable to detect this variation in the wetland\u0026rsquo;s pollutant removal capacity. These findings fit within a broader body of work showing that wetlands, whether natural or engineered, can alternate between acting as sinks and sources of fecal indicator bacteria depending on design, loading, and management (Wu 2016).\u003c/p\u003e \u003cp\u003eConstructed wetland systems are widely used for pathogen and FIB attenuation, but reported performance is highly variable. Free water surface wetlands typically report reductions of 0.1\u0026ndash;2.2 log\u003csub\u003e10\u003c/sub\u003e units for enterococci under standard operating conditions, yet the effluent concentrations frequently remain above reuse or contact-recreation standards when influent loads are high (Kadlec \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). In this context, the Ballona Freshwater Marsh\u0026rsquo;s 68% reduction (\u0026asymp;\u0026thinsp;0.5 log\u003csub\u003e10\u003c/sub\u003e) is modest but within the reported ranges.\u003c/p\u003e \u003cp\u003eFor example, flow-through treatment wetlands in California agricultural landscapes achieved 80\u0026ndash;95% reductions in \u003cem\u003eE. coli\u003c/em\u003e load and 86\u0026ndash;94% reductions in enterococci, but only\u0026thinsp;~\u0026thinsp;30% of effluent samples met the 33 MPN/100 mL standard (O\u0026rsquo;Geen \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Similarly, Lamori et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) reported\u0026thinsp;~\u0026thinsp;84% removal of culturable enterococci in a tertiary treatment wetland, again with many samples exceeding guideline values. Our findings for Ballona mirror this pattern: the marsh substantially lowers enterococci concentrations but rarely brings them below regulatory thresholds, because influent levels are often orders of magnitude above those criteria.\u003c/p\u003e \u003cp\u003eThe magnitude of removal at Ballona is also plausible given its configuration. Reviews emphasize that pathogen removal in constructed wetlands is controlled by hydraulic retention time (HRT), hydraulic regime, plant community, and loading rate (Wu 2016). In the Central Valley case study, \u003cem\u003eE. coli\u003c/em\u003e removal increased from ~\u0026thinsp;69% to ~\u0026thinsp;95% as HRT increased from \u0026lt;\u0026thinsp;1 day to \u0026gt;\u0026thinsp;11 days (O\u0026rsquo;Geen \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The Ballona Freshwater Marsh is relatively small compared to its urban catchment, receives highly variable dry-weather runoff, and likely operates with shorter effective HRTs than dedicated treatment wetlands. Under those constraints, consistent\u0026thinsp;~\u0026thinsp;0.5-log enterococcus reduction is comparable to other multi-use systems in which water-quality treatment is only one of several design goals.\u003c/p\u003e \u003cp\u003eA key contribution of this study is the demonstration that the Ballona Freshwater Marsh has shifted from predominantly sink behavior (\u0026asymp;\u0026thinsp;80% of sampling events prior to 2023) to roughly equal sink/source behavior in the last two years. Short-term studies often report only average removal efficiencies and can miss such temporal transitions. Alternation between sink and source behavior is found on other coastal and urban wetlands. In the adjacent Ballona salt marsh, Dorsey et al. (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) showed that the system acted as a sink for FIB during daylight hours, when solar radiation and quiescent conditions promoted inactivation and sedimentation, but shifted to a source during late-day and nocturnal ebb tides when contaminated sediments were resuspended and exported. Beyond Southern California, a stormwater treatment wetland in Europe was recently reported to be a consistent net exporter of \u003cem\u003eE. coli\u003c/em\u003e, with outlet concentrations 1.5\u0026ndash;26 times higher than inlets (median\u0026thinsp;~\u0026thinsp;10-fold increase) (Scott 2023).\u003c/p\u003e \u003cp\u003eAgricultural treatment wetlands show similar patterns. In the Central Valley case study, flow-through wetlands receiving relatively contaminated irrigation tailwater reduced \u003cem\u003eE. coli\u003c/em\u003e loads by 80\u0026ndash;95%, whereas large flood-pulse wetlands supplied with relatively clean water often exported FIB, with negative \u0026ldquo;removal efficiencies\u0026rdquo; ranging from \u0026minus;\u0026thinsp;206% to \u0026minus;\u0026thinsp;554% (O\u0026rsquo;Geen \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). These systems illustrate a key point: wetlands contain internal FIB sources\u0026mdash;including sediments, biofilms, decaying vegetation, and wildlife\u0026mdash;that can dominate when influent concentrations are low or hydraulic and light conditions favor regrowth and resuspension (Wu 2016).\u003c/p\u003e \u003cp\u003eThe Ballona Freshwater Marsh fits squarely within this source\u0026ndash;sink framework. For much of its life, the marsh has behaved like the flow-through systems described above, receiving heavily contaminated urban runoff and exporting water with substantially lower enterococci concentrations. During some months, even when inlet concentrations saturated the Quanti-Tray assay (\u0026gt;\u0026thinsp;24,196 MPN/100 mL), outlet water was reduced to near or below recreational criteria (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e2\u003c/span\u003e), demonstrating that the marsh is capable of strong sink behavior under favorable conditions. At other times, especially following large-scale vegetation disturbance (e.g., reed cutting for mosquito abatement) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and in the most recent years, outlet concentrations exceeded inlet averages, consistent with a growing contribution from internal reservoirs and altered hydraulics. They also support the conclusion of Wu et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) that wetlands are best understood not as permanent \u0026ldquo;filters\u0026rdquo; but as dynamic reactors in which die-off, retention, and release processes continually compete.\u003c/p\u003e \u003cp\u003eOur statistical analyses did not detect strong relationships between enterococci concentrations and temperature or short-term precipitation. Studies have been mixed about reporting changes in FIB with seasons with some reporting higher removal in warm seasons (Elhamouri 1994; Karathanasis 2003) and others reporting no seasonal change (Garcia 2008, Tun\u0026ccedil;siper 2012). The absence of such signals at Ballona may reflect (1) limited sampling during storm events, (2) the relatively narrow seasonal temperature range in coastal Southern California, and (3) the dominance of local episodic factors\u0026mdash;such as illegal dumping, maintenance actions, or wildlife usage\u0026mdash;over climatic drivers at the time scale of our sampling.\u003c/p\u003e \u003cp\u003e \u003cem\u003eEnterococcus\u003c/em\u003e spp. and \u003cem\u003eE. coli\u003c/em\u003e levels measured in Ballona Creek upstream of the Ecological Reserve were consistently elevated, with geometric means (218 MPN/100 mL for Enterococcus and 1,033 MPN/100 mL for \u003cem\u003eE. coli\u003c/em\u003e) well above U.S. EPA recreational criteria, but they are within the range reported for heavily urbanized catchments with substantial impervious cover and chronic dry-weather flows (Wu 2016, Kadlec \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2009\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDownstream, where the concrete-lined channel transitions to a tidally influenced estuary with a sediment bottom, bacterial concentrations decline but remain elevated relative to regulatory thresholds. This longitudinal pattern mirrors other coastal systems in which natural or restored marshes reduce FIB loads from upland sources but do not fully eliminate contamination. Dorsey (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2006\u003c/span\u003e, 2010) and Myers \u0026amp; Ambrose (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) all found that Southern California salt marshes reduce FIB fluxes from upstream creeks, yet can episodically export bacteria back to the estuary during specific tidal or wind conditions (Dorsey \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe Ballona Freshwater Marsh thus provides a relevant analog for the proposed re-routing of Ballona Creek through a restored tidal wetland. Diverting polluted creek water through vegetated floodplains is likely to produce measurable improvements in downstream water quality, even if regulatory standards are not consistently met. The marsh has repeatedly demonstrated the capacity to transform extremely contaminated inflows, sometimes at or above assay detection limits, into much cleaner outflows. At the same time, the decline in sink performance and increasing frequency of source events highlight the importance of design and adaptive management.\u003c/p\u003e \u003cp\u003eEqually important is reducing loads before they reach the wetland. Recent work on the Ballona Creek rain garden shows that small-scale bioretention systems can significantly attenuate FIB and other pollutants in upland urban stormwater (Buckhard 2024). Coupling such distributed green infrastructure with a large restored estuarine wetland increases the likelihood that the system as a whole will function as a net sink rather than a source.\u003c/p\u003e \u003cp\u003eThis retrospective, observational analysis has important limitations and does not establish causality or isolate specific drivers of sink-versus-source behavior. The nonuniform temporal coverage (notably no summer data and a collection gap during early 2020), may reduce sensitivity to seasonal or episodic dynamics and may under-capture short-lived storm pulses. Also enterococci are a proxy for fecal contamination risk rather than a direct measurement of pathogens; accordingly, an increase in enterococci does not necessarily imply a proportional rise in clinically relevant pathogens. Finally, wetlands provide additional water-quality services beyond FIB attenuation, including removal or transformation of metals and organic contaminants. Our Ballona Creek measurements showed metals and pesticides were usually below regulatory thresholds (data not shown).\u003c/p\u003e \u003cp\u003eFrom a management perspective, the Ballona Freshwater Marsh demonstrates both the promise and the limits of wetlands as nature-based water-treatment infrastructure. Over two decades, the marsh has provided substantial pollutant removal under real-world loading and maintenance regimes, similar in magnitude to other multi-purpose constructed wetlands. Yet its recent shift toward source behavior underscores that these systems are not static. Their water-quality functions depend on sustained hydrologic, vegetation, and sediment management, as well as on the broader watershed context.\u003c/p\u003e \u003cp\u003eFor the Ballona Wetlands Ecological Reserve, our results add to the literature supporting proceeding with restoration that reconnects Ballona Creek to a more natural floodplain and tidal network, while explicitly incorporating hydrologic and ecological design criteria known to enhance FIB removal. They also justify long-term monitoring beyond the typical 1\u0026ndash;3-year project horizon. Without sustained datasets like the one presented here, managers risk misinterpreting short-term sink behavior as guaranteed and may overlook gradual declines in treatment performance.\u003c/p\u003e \u003cp\u003eFinally, by documenting both strong sink events and periods of net export, this study reinforces a central message from the broader literature: wetlands can substantially improve water quality, but they do so as dynamic ecosystems whose performance is contingent on design, operation, and the quality of water they receive. Recognizing and planning for that dynamism is essential to realizing the full ecosystem service benefits of wetland restoration in urban coastal watersheds.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eStatements \u0026amp; Declarations:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe author declares that no funds, grants, or other support were received during the preparation of this manuscript.\u003c/p\u003e\n\u003cp\u003eThe author has no relevant financial or non-financial interests to disclose.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAnnalia J Henderson is solely responsible for the study conception and design, data analysis, and manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eCraig Cadwallader (Surfrider Foundation) collected twenty years of water quality samples at Ballona Freshwater Marsh. Guidance and advice were gratefully received from Mary Simun (Surfrider Foundation) and Michelle Pena-Ortiz (Blue Water Task Force). Los Angeles County Public Works water quality data was provided by Josafat Flores. I am grateful to John Dorsey, Gregory Henderson, David Kay, and Raymond Wells for review and feedback on the manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eBallona Fresh Water Marsh water quality samples collected between October 2005 and May 2025 were sourced from the Blue Water Task Force database (https://bwtf.surfrider.org/). Additional water quality data from the Ballona Creek from 2019 to 2024 were provided by the Los Angeles County Public Works.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBiswal BK, Balasubramanian R (2022) Constructed Wetlands for Reclamation and Reuse of Wastewater and Urban Stormwater: A Review Front Environ Sci, 10. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3389/fenvs.2022.836289\u003c/span\u003e\u003cspan address=\"10.3389/fenvs.2022.836289\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBuckhard JL, Johnston KK, Saez JA Dorsey JH (2024) Water Quality and Hydrological Benefits of a Natural Treatment System in an Urban Watershed in Southern California. Bull Southern California Acad Sci 123 (3): 105\u0026ndash;134. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3160/0038-3872-123.3.105\u003c/span\u003e\u003cspan address=\"10.3160/0038-3872-123.3.105\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBudnick GE, Howard RT, Mayo DR (1996) Evaluation of Enterolert for enumeration of enterococci in recreational waters. Appl Environ Microbiol 62:3881. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1128/aem.62.10.3881-3884.1996\u003c/span\u003e\u003cspan address=\"10.1128/aem.62.10.3881-3884.1996\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eByappanahalli MN, Nevers MB, Korajkic A, Staley ZR, Harwood VJ (2012) Enterococci in the environment. Microbiol Mol Biol Rev. Dec;76(4):685\u0026ndash;706. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1128/mmbr.00023-12\u003c/span\u003e\u003cspan address=\"10.1128/mmbr.00023-12\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCEQA Findings (2020) CEQA Findings of Fact of the California Department of Fish and Wildlife as Lead Agency under the California Environmental Quality Act (Pub. Resources Code, \u0026sect; 21000 et seq.) for the Restoration of the Ballona Wetlands Ecological Reserve as analyzed in the Ballona Wetlands Restoration Project Final Environmental Impact Report. State Clearinghouse No. 2012071090\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCooper DS (2008). The Use of Historical Data in the Restoration of the Avifauna of the Ballona Wetlands, Los Angeles County, California. Natural Areas Journal, 28(1), 83\u0026ndash;90. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.jstor.org/stable/43914034\u003c/span\u003e\u003cspan address=\"https://www.jstor.org/stable/43914034\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDorsey JH (2006). Densities of Fecal Indicator Bacteria in Tidal Waters of the Ballona Wetlands, Los Angeles County, California. Southern California Academy of Sciences, 105 (2), 59\u0026thinsp;\u0026ndash;\u0026thinsp;7. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3160/0038-3872(2006\u003c/span\u003e\u003cspan address=\"10.3160/0038-3872(2006\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e)105[59:DOFIBI]2.0.CO;2\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDorsey JH, Carter PM, Bergquist S, Sagarin R (2010) Reduction of fecal indicator bacteria (FIB) in the Ballona Wetlands saltwater marsh (Los Angeles County, California, USA) with implications for restoration actions. Water Res, 44(15):4630\u0026ndash;42. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.watres.2010.06.012\u003c/span\u003e\u003cspan address=\"10.1016/j.watres.2010.06.012\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDorsey JH (2015) The Ballona Wetlands: Nature\u0026rsquo;s water treatment system. CURes Blog, Loyola Marymount University, Los Angeles. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://curesblog.lmu.edu/the-ballona-wetlands-natures-water-treatment-system/\u003c/span\u003e\u003cspan address=\"https://curesblog.lmu.edu/the-ballona-wetlands-natures-water-treatment-system/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e Accessed 28 Nov 2025.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eElhamouri B, Khallayoune K, Bouzoubaa K, Rhallabi N, Chalabi M (1994) High-rate algal pond performances in faecal-coliforms and helminth egg removals. Water Res 28 (1), 171\u0026ndash;174. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/0043-1354(94)90131-7\u003c/span\u003e\u003cspan address=\"10.1016/0043-1354(94)90131-7\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eEnvironmental Impact Report (2017) Draft Ballona Wetlands Restoration Project Environmental Impact Statement/Environmental Impact Report. State Clearinghouse No. 2012071090\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFerguson DM, Moore DF, Getrich MA, Zhowandai MH (2005). Enumeration and speciation of enterococci found in marine and intertidal sediments and coastal water in southern California. Journal of applied microbiology, 99(3), 598\u0026ndash;608. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/j.1365-2672.2005.02660.x\u003c/span\u003e\u003cspan address=\"10.1111/j.1365-2672.2005.02660.x\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGarc\u0026iacute;a M, Soto F, Gonz\u0026aacute;lez JM, B\u0026eacute;cares E (2008) A comparison of bacterial removal efficiencies in constructed wetlands and algae-based systems. Ecol Eng, 32(3), 238\u0026ndash;243. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ecoleng.2007.11.012\u003c/span\u003e\u003cspan address=\"10.1016/j.ecoleng.2007.11.012\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGrossinger R, Stein ER, Cayce K, Askevold R, Dark S, Whipple A (2011) Historical wetlands of the Southern California Coast: An Atlas of US Coast Survey T-sheets, 1851\u0026ndash;1889. Oakland: San Francisco Estuary Institute.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJohnston KK, Medel ID, Abbott RC, Grubbs MW, Del Giudice-Tuttle E, Piechowski C, Wong Yau M, Dorsey J (2015) Ballona Wetlands Ecological Reserve: Comprehensive 5-Year Monitoring Report. Report prepared by The Bay Foundation for the California State Coastal Conservancy.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKarathanasis AD, Potter CL, Coyne MS (2003) Vegetation effects on faecal bacteria, BOD, and suspended solid removal in constructed wetlands treating domestic waste-water. Ecol Eng, 20(2), 157\u0026ndash;169.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKadlec RH, Wallace SD (2009) Treatment Wetlands Second Edition. CRC Press, New York\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLamori, JG, Xue J, Rachmadi AT, Lopez GU, Kitajima M, Gerba CP, Pepper IL, Brooks JP, Sherchan S (2019) Removal of fecal indicator bacteria and antibiotic resistant genes in constructed wetlands. Environ Sci Pollut Res, 26, 10188\u0026ndash;10197. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s11356-019-04468-9\u003c/span\u003e\u003cspan address=\"10.1007/s11356-019-04468-9\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMyers MR, Ambrose RF (2015) Salt marsh reduces fecal indicator bacteria input to coastal waters in Southern California. Bull South Calif Acad Sci, 114(2), 76\u0026ndash;88. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3160/0038-3872-114.2.76\u003c/span\u003e\u003cspan address=\"10.3160/0038-3872-114.2.76\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eO\u0026rsquo;Geen AT, Bianchi ML (2015) Using Wetlands to Remove Microbial Pollutant from Farm Discharge Water. ANR Publication 8512.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStott R, Sukias JPS, McKergow LA, Davies-Colley RJ, Tanner CC (2023) Net export of \u003cem\u003eE. coli\u003c/em\u003e from a constructed wetland treating agricultural drainage. Ecol Eng, 194:107023. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ecoleng.2023.107023\u003c/span\u003e\u003cspan address=\"10.1016/j.ecoleng.2023.107023\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSharley DJ, Sharp SM, Marshall S, Jeppe K, Pettigrove VJ (2017) Linking urban land use to pollutants in constructed wetlands: Implications for stormwater and urban planning. Landscape and Urban Planning, 162, 80\u0026ndash;91. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.landurbplan.2016.12.016\u003c/span\u003e\u003cspan address=\"10.1016/j.landurbplan.2016.12.016\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStein E (2004) Characterization of Dry Weather Metals and Bacteria in Ballona Creek. Southern California Coastal Water Research Project.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTun\u0026ccedil;siper B, Ayaz S\u0026Ccedil;, Ak\u0026ccedil;a L (2012) Coliform bacteria removal from septic wastewater in a pilot-scale combined constructed wetland system. Environ Eng Manag J (EEMJ), 11(10), 1873\u0026ndash;1879.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWu S, Carvalho PN, M\u0026uuml;ller JA, Manoj VR, Dong R (2016) Sanitation in constructed wetlands: A review on the removal of human pathogens and fecal indicators. Science of The Total Environment, 541, 8\u0026ndash;22. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2015.09.047\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2015.09.047\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":true,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"wetlands-ecology-and-management","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wetl","sideBox":"Learn more about [Wetlands Ecology and Management](https://www.springer.com/journal/11273)","snPcode":"11273","submissionUrl":"https://submission.nature.com/new-submission/11273/3","title":"Wetlands Ecology and Management","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Enterococcus, water quality, Ballona Wetlands, constructed wetlands, urban runoff","lastPublishedDoi":"10.21203/rs.3.rs-7673850/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7673850/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eOver 95% of California\u0026rsquo;s wetlands has been lost to urbanization, prompting efforts to restore Los Angeles County\u0026rsquo;s Ballona Wetlands Ecological Reserve. Adjacent to the reserve, the Ballona Freshwater Marsh was constructed in 2003 on former agricultural land to mitigate urban and stormwater runoff. This study presents a two-decade analysis of the marsh\u0026rsquo;s effectiveness in reducing enterococci concentrations, a leading indicator of pathogenic water pollution.\u003c/p\u003e \u003cp\u003e \u003cem\u003eEnterococcus\u003c/em\u003e spp. levels from the marsh\u0026rsquo;s main inlets and outlet, recorded between 2005\u0026ndash;2025, were compared to determine filtration efficacy. Results indicate the marsh often removes enterococci and can lower bacterial counts to safe levels. The geometric mean of the outlet-to-inlet ratio was 0.322 (95% CI: 0.259\u0026ndash;0.399, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001). However, removal is not uniform, and increasingly the marsh behaves as a source rather than a sink.\u003c/p\u003e \u003cp\u003eThe California Department of Fish and Wildlife (CDFW) plans to restore 566 acres of the Ecological Reserve by removing cement levees and rerouting the adjoining Ballona Creek through a meandering path to recreate an ecologically active estuary. Separate analysis of water from the Ballona Creek over the last five years shows levels of pollution regularly above the Environmental Protection Agency (EPA) safety thresholds for fecal bacteria. We anticipate water quality improvements if the restoration proceeds.\u003c/p\u003e \u003cp\u003eIn the context of ongoing litigation over restoration of the Ecological Reserve, this study offers an assessment of engineered wetlands\u0026rsquo; pollutant-filtering capacity, establishing a case for the research-driven management and preservation of wetlands.\u003c/p\u003e","manuscriptTitle":"A Twenty-Year Analysis of The Ballona Freshwater Marsh’s Removal of Contaminants from Municipal Runoff, A Model for Future Restoration of the Ballona Wetlands","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-06 09:21:54","doi":"10.21203/rs.3.rs-7673850/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-01-22T14:28:18+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-20T20:39:08+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"334207395746542586414458998031954221921","date":"2025-12-28T19:57:37+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-28T16:46:52+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-15T10:40:18+00:00","index":"","fulltext":""},{"type":"submitted","content":"Wetlands Ecology and Management","date":"2025-12-14T23:56:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"wetlands-ecology-and-management","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wetl","sideBox":"Learn more about [Wetlands Ecology and Management](https://www.springer.com/journal/11273)","snPcode":"11273","submissionUrl":"https://submission.nature.com/new-submission/11273/3","title":"Wetlands Ecology and Management","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a68c87c6-3a0e-4c03-a966-7d14e7cde027","owner":[],"postedDate":"January 6th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-03-30T16:26:41+00:00","versionOfRecord":{"articleIdentity":"rs-7673850","link":"https://doi.org/10.1007/s11273-026-10125-z","journal":{"identity":"wetlands-ecology-and-management","isVorOnly":false,"title":"Wetlands Ecology and Management"},"publishedOn":"2026-03-29 16:09:47","publishedOnDateReadable":"March 29th, 2026"},"versionCreatedAt":"2026-01-06 09:21:54","video":"","vorDoi":"10.1007/s11273-026-10125-z","vorDoiUrl":"https://doi.org/10.1007/s11273-026-10125-z","workflowStages":[]},"version":"v1","identity":"rs-7673850","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7673850","identity":"rs-7673850","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}