A Marsh for All Reasons
A controversial development uses a freshwater wetlands system to treat stormwater and urban runoff from a 1,000-ac. watershed.
In Spring 2003, Los Angeles residents witnessed the grand opening of the freshwater marsh at Playa Vista, a profusion of colorful native trees and shrubs, where ducks float past elegant egrets and great blue herons. But what on the surface appears to be a welcome natural oasis in the midst of urban Los Angeles is actually much more.
On land originally designated for a concrete flood control channel and several hundred residences, stormwater from a 1,000-ac. watershed will be collected and treated as it makes its way along what will eventually be a 51-ac. freshwater wetlands system. The marsh was designed to achieve a balance of beneficial uses between flood control, improving water quality, and protecting and enhancing habitat values.
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The project owner, Playa Capital, is keeping its (and the previous owner’s) promise to the community that after the freshwater wetlands system is finished, stormwater entering Ballona Channel and flowing out to the Pacific Ocean will be cleaner than it is today - even with development of the entire Playa Vista master-planned community. Best management practices (BMPs) that treat just the runoff from new development typically only reduce the net increase in pollution. The difference at Playa Vista is that the project owner is taking on treatment of runoff from significant offsite areas as well. The wetlands will treat stormwater runoff from 572 ac. of the Playa Vista project area and runoff from 538 ac. of the surrounding urbanized community of homes, office buildings, and commercial/industrial facilities. The project demonstrates the value of regional treatment-system approaches in watersheds that are already partially developed.
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| Wetlands draining into Playa Vista property |
Crucial in Settling Lawsuit
The opening of the freshwater marsh, the initial and largest portion of the freshwater wetlands system, is a hallmark in the history of Playa Vista; the freshwater wetland concept was crucial in settling a major environmental lawsuit that finally allowed the project to move forward. Construction is nearing completion on portions of this new Los Angeles community less than a mile from the Pacific. The first residents and office tenants are now moving in, and upon completion, Playa Vista will include some 5,800 residences; 3.2 million ft.2 of office space, most of which is within a business campus; and a mixed-use village center of approximately 150,000 ft.2 of retail uses with upper-story residences and offices.
The development’s lengthy and controversial planning history dates back to the late 1970s, when the heirs of Howard Hughes first decided to turn the land surrounding the Hughes Aircraft plant into a mixed-use community. By the late 1980s, developer Maguire Thomas Partners was under fire to preserve more marshland. Since the project needed a flood control component, Psomas, the project engineer, and Woodward-Clyde Consultants, the project stormwater expert, led a team of engineers, biologists, and water-quality professionals to design - with input from environmental groups - one of the first and largest stormwater treatment wetlands in southern California.
Pollutants of Concern
To predict potential water-quality impacts the project might have, a comprehensive, statistically based stormwater pollutant loadings model was developed. The model considers event-mean concentrations for land-use types, percent imperviousness, relationships between imperviousness and runoff, and an analysis of hourly rainfall data to develop a long-term characterization of storm events that produce predicted runoff and pollutant loads.
Pollutant loading estimates were initially based on information from the Santa Monica Bay Restoration Project, the Environmental Protection Agency, and the Federal Highway Administration. More recently, there have been efforts to characterize land-use concentrations of stormwater in the Los Angeles Basin as a part of National Pollutant Discharge Elimination System permit efforts. The Los Angeles County stormwater program monitored runoff from various land-use stations, and these data have been used in the modeling work.
Results from these monitoring efforts resulted in identification of seven primary pollutants of concern that could be modeled with enough data available: suspended solids, Kjeldahl nitrogen, phosphorus, oil and grease, copper, lead, and zinc. Based on these findings, the wetland system planning considered heavy metals and total suspended solids (TSS) as design parameters for water-quality features but included the consideration of source control measures to limit activities and vegetation in order to reduce pollutants, such as nutrients and pesticides (see sidebar).
Wetlands and Water Cleansing
Water cleansing is a natural function of wetlands, offering a wider range of treatment mechanisms than almost all standard BMPs. Wetland treatment systems have the best effluent quality in the National BMP Database. Sedimentation of particulates is the major removal mechanism. However, performance is enhanced as plant materials decelerate the flow of stormwater and allow pollutants to come into contact with vegetation, organic matter, and soils that together act as a natural filtration system. Plants also uptake some pollutants as nutrients, and the sun, as well as bacteria around the roots of plants, kills fecal coliform bacteria. Copper, lead, and zinc are removed primarily through attachment to particulate matter, which settles relatively quickly in wetland systems.
The Playa Vista wetland system goes beyond simply meeting federal- and state-mandated stormwater management requirements in reducing both the amount and concentrations of pollutants. The system is significantly oversized, treating 1 in. of rainfall for the entire watershed rather than the 0.75 in. of rainfall that would be required for the development area alone.
System Design
The freshwater wetlands system ultimately will comprise a 25-ac. riparian corridor that flows into a 26-ac. freshwater marsh. Both the freshwater marsh and the riparian corridor will receive runoff from the Playa Vista development, as will the hundreds of acres of offsite tributary watersheds to the Playa Vista site. It is projected that daily flows from these sources will range from about 0.5 cfs to 640 cfs (peak flows during a 50-year storm event). Most of the main freshwater marsh has been constructed. The riparian corridor and the rest of the marsh will be completed in the future.
Flows to the wetlands and the riparian corridor will come from stormwater runoff (estimated at an average 35.2 million gal. per year), dry-weather urban flows, natural groundwater inflows, and - if needed - pumped and treated groundwater from the former Hughes Aircraft plant. Other groundwater might also be a source of water for the marsh. The riparian corridor, which due to the flat slope is essentially a long linear wetland, is designed to slow the flow of water so contaminants can be absorbed by herbaceous freshwater marsh species or become sediments. Contours of the 2-mi. channel at bottom range in elevation from -1 ft. mean sea level (msl) at the outlet of the riparian corridor to the freshwater marsh, to +16 ft. msl at the eastern end of the project site, a slope of about 0.25%.
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Under dry weather, the contribution from the riparian corridor to the freshwater marsh will be primarily a function of the pumped groundwater, along with the dry-weather flows arising from the existing development areas (reduced by losses due to percolation and evapotranspiration). The average daily dry-weather flow is estimated to be about 0.4 cfs on average and 0.6 cfs at peak. Playa Vista is employing BMPs to minimize dry-weather flows from the project area (see sidebar); however, one or two wells will be built to supplement flows so that the riparian corridor remains at an acceptable water level. Upon build-out of the project, about 50% or more of the stormwater discharged to the freshwater marsh will enter via the riparian corridor.
The L-shaped freshwater marsh runs along Lincoln Blvd., a major north-south thoroughfare at the western end of Los Angeles. It is designed to store as much as 100 ac.-ft. of water during larger storms - or approximately the runoff from a one-year storm event. During the winter, there will be a 20-ac.-ft. wet pool and another 40 ac.-ft. of extended detention volume, enough to treat a more-than-1-in. storm. For the 1-in. volume, 60 ac.-ft. would be stored and released slowly.
Connecting the riparian corridor to the freshwater marsh are two culverts that slope into the main body of water. The two culverts will also contain runoff from the Lincoln Blvd. storm drain, which drains runoff from the Playa del Rey bluffs to the south. The water entering the 26-ac. freshwater marsh from the riparian corridor combines with water entering from two other storm drains, the central drain (all Playa Vista runoff) and the Jefferson drain (mostly offsite runoff). Trash racks and other devices to remove debris are located at the outlet of each drain and the riparian corridor. The runoff water from these three main drains flows slowly over shallow, thickly vegetated areas that form three pretreatment (or primary treatment) areas where the majority of pollutants will attach to plants and soils. These areas can be easily cleaned as needed - about every five years or more as monitoring of the system dictates.
The main body of the marsh will serve as "water polishing" areas and habitat functions. It consists of permanently flooded or open-water areas with partially submerged vegetation, surrounded by freshwater emergent marsh and willow scrub and mixed riparian woodlands. Five strategically located habitat islands, totaling about 2.2 ac., further reduce the speed of flow and disperse the waters.
Data from the National BMP Database (www.bmpdatabase.org) were used to predict the expected water quality of the marsh. Via this database, and based on the sizing of the wetland system, the freshwater marsh is expected to result in water quality that meets the California Toxics Rule for heavy metals. It is also expected to result in significant reductions in TSS, nutrients, and other pollutants. The modeling showed that with the project there would be no increase in the pollutants of concern for all pollutants modeled in flows to the Ballona Creek Estuary (a tributary to Santa Monica Bay). Pollutant loadings to the Ballona Wetlands salt marsh located directly to the west would be reduced significantly.
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Managing Water Balance
After flowing through the freshwater wetland system, about 90% or more (on an annual average basis) of the cleansed water will be released into Ballona Channel where it flows into Santa Monica Bay. The other 10% will drain into the Ballona Wetlands. Three 60-in. pipes with flap gates located at the northwest corner of the marsh system allow for a one-way flow of water from the freshwater marsh to Ballona Channel.
The freshwater marsh will be allowed to fill with water to +4 ft. msl during the spring, summer, and early fall months. Freshwater in excess of this amount (rare during these seasons) will be allowed to spill over a control weir and flow into Ballona Channel. In mid- to late fall, the water level will be dropped to +2 ft. msl by lowering the control weir. This decrease in surface elevation and permanent pool volume will provide greater capacity to accommodate water during storm events. With an approximately one-year (or greater) storm event, fresh water will travel over a spillway into the Ballona Wetlands. As noted above, on an average annual basis it is predicted that 10% or less of all runoff will spill to the Ballona Wetlands.
Meeting Freshwater Needs of Ballona Wetlands
Another attribute of the freshwater marsh is its ability to divert urban waters from the adjacent Ballona Wetlands, which has been degraded by urban runoff. Pollutant loads to the Ballona Wetlands will be greatly reduced, both by the redirection of stormwater away from the salt marsh and by the improved water quality of those flows that do reach the salt marsh from the freshwater marsh.
Salt marshes certainly benefit from periodic "freshening," and the freshwater wetlands system has been designed to meet the freshwater needs of any future salt-marsh restoration. This will offer the opportunity to create a brackish area in the marsh to stimulate seed germination.
Specific features designed into the freshwater wetland system will allow flexibility in managing water flowing into the salt marsh. A berm with slopes varying from 5:1 to 10:1 separates the freshwater marsh from the salt marsh. Three structures located on the outer margin of the freshwater marsh provide the means to control the amount of freshwater flow into the proposed salt marsh: a sluice gate at the south end of the freshwater wetlands berm, a spillway to the salt marsh located in the north-central portion of the berm, and a control weir that outlets to Ballona Channel at the northwest corner of the freshwater wetlands.
Monitoring and Maintenance
Maintenance will be required in perpetuity to ensure that three overriding goals - habitat protection and enhancement, stormwater runoff containment, and water-quality improvement - are achieved. An adaptive management approach set forth in an operations, maintenance, and monitoring manual allows the marsh manager flexibility to ensure the success of this important restoration project. The manual describes operational procedures for the freshwater wetland system during dry-summer and winter-storm seasons, as well as procedures for monitoring vegetation, birds in nesting season, and water-quality parameters. Monitoring will be conducted primarily over the first five years of the operation, with more limited activities to be performed in perpetuity. Water monitoring includes continuous flow monitoring and grab sampling for water-quality analyses.
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Stormwater as a Resource
Playa Vista’s freshwater wetlands system demonstrates how important goals can be achieved through viewing stormwater as a resource. In addition to improving water quality and flood control, the freshwater marsh is providing a vast representation of plant life that is already attracting an impressive diversity of insects, birds, and wildlife. More than 3,000 native trees and 10,000 native shrubs and grasses were planted in the freshwater marsh. Many other native species are "volunteering" on their own. The riparian corridor, banked by a willow woodland, will provide added natural habitat.
An example of sustainable design, the Playa Vista freshwater wetlands system has provided the project and the community with an important amenity; hundreds of people per day visit the marsh, and there is a premium on Playa Vista properties that overlook the wetlands. Moreover, the developer has saved money by building a wetlands system instead of a traditional concrete flood control channel and/or basin. The environment has gained valuable plant and animal habitats, and the region has gained improved water quality for Santa Monica Bay, a vital marine-life habitat and major recreational resource for southern California.
Author's Bio: Eric Strecker, P.E., principal, is with Geosyntec Consultants in Portland, OR, and Acton, MA.
January- February 2004
A Marsh for All Reasons
A controversial development uses a freshwater wetlands system to treat stormwater and urban runoff from a 1,000-ac. watershed.
In Spring 2003, Los Angeles residents witnessed the grand opening of the freshwater marsh at Playa Vista, a profusion of colorful native trees and shrubs, where ducks float past elegant egrets and great blue herons. But what on the surface appears to be a welcome natural oasis in the midst of urban Los Angeles is actually much more.On land originally designated for a concrete flood control channel and several hundred residences, stormwater from a 1,000-ac. watershed will be collected and treated as it makes its way along what will eventually be a 51-ac. freshwater wetlands system. The marsh was designed to achieve a balance of beneficial uses between flood control, improving water quality, and protecting and enhancing habitat values.
|
The project owner, Playa Capital, is keeping its (and the previous owner’s) promise to the community that after the freshwater wetlands system is finished, stormwater entering Ballona Channel and flowing out to the Pacific Ocean will be cleaner than it is today - even with development of the entire Playa Vista master-planned community. Best management practices (BMPs) that treat just the runoff from new development typically only reduce the net increase in pollution. The difference at Playa Vista is that the project owner is taking on treatment of runoff from significant offsite areas as well. The wetlands will treat stormwater runoff from 572 ac. of the Playa Vista project area and runoff from 538 ac. of the surrounding urbanized community of homes, office buildings, and commercial/industrial facilities. The project demonstrates the value of regional treatment-system approaches in watersheds that are already partially developed.
|
| Wetlands draining into Playa Vista property |
Crucial in Settling Lawsuit
The opening of the freshwater marsh, the initial and largest portion of the freshwater wetlands system, is a hallmark in the history of Playa Vista; the freshwater wetland concept was crucial in settling a major environmental lawsuit that finally allowed the project to move forward. Construction is nearing completion on portions of this new Los Angeles community less than a mile from the Pacific. The first residents and office tenants are now moving in, and upon completion, Playa Vista will include some 5,800 residences; 3.2 million ft.2 of office space, most of which is within a business campus; and a mixed-use village center of approximately 150,000 ft.2 of retail uses with upper-story residences and offices.
The development’s lengthy and controversial planning history dates back to the late 1970s, when the heirs of Howard Hughes first decided to turn the land surrounding the Hughes Aircraft plant into a mixed-use community. By the late 1980s, developer Maguire Thomas Partners was under fire to preserve more marshland. Since the project needed a flood control component, Psomas, the project engineer, and Woodward-Clyde Consultants, the project stormwater expert, led a team of engineers, biologists, and water-quality professionals to design - with input from environmental groups - one of the first and largest stormwater treatment wetlands in southern California.
Pollutants of Concern
To predict potential water-quality impacts the project might have, a comprehensive, statistically based stormwater pollutant loadings model was developed. The model considers event-mean concentrations for land-use types, percent imperviousness, relationships between imperviousness and runoff, and an analysis of hourly rainfall data to develop a long-term characterization of storm events that produce predicted runoff and pollutant loads.
Pollutant loading estimates were initially based on information from the Santa Monica Bay Restoration Project, the Environmental Protection Agency, and the Federal Highway Administration. More recently, there have been efforts to characterize land-use concentrations of stormwater in the Los Angeles Basin as a part of National Pollutant Discharge Elimination System permit efforts. The Los Angeles County stormwater program monitored runoff from various land-use stations, and these data have been used in the modeling work.
Results from these monitoring efforts resulted in identification of seven primary pollutants of concern that could be modeled with enough data available: suspended solids, Kjeldahl nitrogen, phosphorus, oil and grease, copper, lead, and zinc. Based on these findings, the wetland system planning considered heavy metals and total suspended solids (TSS) as design parameters for water-quality features but included the consideration of source control measures to limit activities and vegetation in order to reduce pollutants, such as nutrients and pesticides (see sidebar).
Wetlands and Water Cleansing
Water cleansing is a natural function of wetlands, offering a wider range of treatment mechanisms than almost all standard BMPs. Wetland treatment systems have the best effluent quality in the National BMP Database. Sedimentation of particulates is the major removal mechanism. However, performance is enhanced as plant materials decelerate the flow of stormwater and allow pollutants to come into contact with vegetation, organic matter, and soils that together act as a natural filtration system. Plants also uptake some pollutants as nutrients, and the sun, as well as bacteria around the roots of plants, kills fecal coliform bacteria. Copper, lead, and zinc are removed primarily through attachment to particulate matter, which settles relatively quickly in wetland systems.
The Playa Vista wetland system goes beyond simply meeting federal- and state-mandated stormwater management requirements in reducing both the amount and concentrations of pollutants. The system is significantly oversized, treating 1 in. of rainfall for the entire watershed rather than the 0.75 in. of rainfall that would be required for the development area alone.
System Design
The freshwater wetlands system ultimately will comprise a 25-ac. riparian corridor that flows into a 26-ac. freshwater marsh. Both the freshwater marsh and the riparian corridor will receive runoff from the Playa Vista development, as will the hundreds of acres of offsite tributary watersheds to the Playa Vista site. It is projected that daily flows from these sources will range from about 0.5 cfs to 640 cfs (peak flows during a 50-year storm event). Most of the main freshwater marsh has been constructed. The riparian corridor and the rest of the marsh will be completed in the future.
Flows to the wetlands and the riparian corridor will come from stormwater runoff (estimated at an average 35.2 million gal. per year), dry-weather urban flows, natural groundwater inflows, and - if needed - pumped and treated groundwater from the former Hughes Aircraft plant. Other groundwater might also be a source of water for the marsh. The riparian corridor, which due to the flat slope is essentially a long linear wetland, is designed to slow the flow of water so contaminants can be absorbed by herbaceous freshwater marsh species or become sediments. Contours of the 2-mi. channel at bottom range in elevation from -1 ft. mean sea level (msl) at the outlet of the riparian corridor to the freshwater marsh, to +16 ft. msl at the eastern end of the project site, a slope of about 0.25%.
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Under dry weather, the contribution from the riparian corridor to the freshwater marsh will be primarily a function of the pumped groundwater, along with the dry-weather flows arising from the existing development areas (reduced by losses due to percolation and evapotranspiration). The average daily dry-weather flow is estimated to be about 0.4 cfs on average and 0.6 cfs at peak. Playa Vista is employing BMPs to minimize dry-weather flows from the project area (see sidebar); however, one or two wells will be built to supplement flows so that the riparian corridor remains at an acceptable water level. Upon build-out of the project, about 50% or more of the stormwater discharged to the freshwater marsh will enter via the riparian corridor.
The L-shaped freshwater marsh runs along Lincoln Blvd., a major north-south thoroughfare at the western end of Los Angeles. It is designed to store as much as 100 ac.-ft. of water during larger storms - or approximately the runoff from a one-year storm event. During the winter, there will be a 20-ac.-ft. wet pool and another 40 ac.-ft. of extended detention volume, enough to treat a more-than-1-in. storm. For the 1-in. volume, 60 ac.-ft. would be stored and released slowly.
Connecting the riparian corridor to the freshwater marsh are two culverts that slope into the main body of water. The two culverts will also contain runoff from the Lincoln Blvd. storm drain, which drains runoff from the Playa del Rey bluffs to the south. The water entering the 26-ac. freshwater marsh from the riparian corridor combines with water entering from two other storm drains, the central drain (all Playa Vista runoff) and the Jefferson drain (mostly offsite runoff). Trash racks and other devices to remove debris are located at the outlet of each drain and the riparian corridor. The runoff water from these three main drains flows slowly over shallow, thickly vegetated areas that form three pretreatment (or primary treatment) areas where the majority of pollutants will attach to plants and soils. These areas can be easily cleaned as needed - about every five years or more as monitoring of the system dictates.
The main body of the marsh will serve as "water polishing" areas and habitat functions. It consists of permanently flooded or open-water areas with partially submerged vegetation, surrounded by freshwater emergent marsh and willow scrub and mixed riparian woodlands. Five strategically located habitat islands, totaling about 2.2 ac., further reduce the speed of flow and disperse the waters.
Data from the National BMP Database (www.bmpdatabase.org) were used to predict the expected water quality of the marsh. Via this database, and based on the sizing of the wetland system, the freshwater marsh is expected to result in water quality that meets the California Toxics Rule for heavy metals. It is also expected to result in significant reductions in TSS, nutrients, and other pollutants. The modeling showed that with the project there would be no increase in the pollutants of concern for all pollutants modeled in flows to the Ballona Creek Estuary (a tributary to Santa Monica Bay). Pollutant loadings to the Ballona Wetlands salt marsh located directly to the west would be reduced significantly.
|
|
Managing Water Balance
After flowing through the freshwater wetland system, about 90% or more (on an annual average basis) of the cleansed water will be released into Ballona Channel where it flows into Santa Monica Bay. The other 10% will drain into the Ballona Wetlands. Three 60-in. pipes with flap gates located at the northwest corner of the marsh system allow for a one-way flow of water from the freshwater marsh to Ballona Channel.
The freshwater marsh will be allowed to fill with water to +4 ft. msl during the spring, summer, and early fall months. Freshwater in excess of this amount (rare during these seasons) will be allowed to spill over a control weir and flow into Ballona Channel. In mid- to late fall, the water level will be dropped to +2 ft. msl by lowering the control weir. This decrease in surface elevation and permanent pool volume will provide greater capacity to accommodate water during storm events. With an approximately one-year (or greater) storm event, fresh water will travel over a spillway into the Ballona Wetlands. As noted above, on an average annual basis it is predicted that 10% or less of all runoff will spill to the Ballona Wetlands.
Meeting Freshwater Needs of Ballona Wetlands
Another attribute of the freshwater marsh is its ability to divert urban waters from the adjacent Ballona Wetlands, which has been degraded by urban runoff. Pollutant loads to the Ballona Wetlands will be greatly reduced, both by the redirection of stormwater away from the salt marsh and by the improved water quality of those flows that do reach the salt marsh from the freshwater marsh.
Salt marshes certainly benefit from periodic "freshening," and the freshwater wetlands system has been designed to meet the freshwater needs of any future salt-marsh restoration. This will offer the opportunity to create a brackish area in the marsh to stimulate seed germination.
Specific features designed into the freshwater wetland system will allow flexibility in managing water flowing into the salt marsh. A berm with slopes varying from 5:1 to 10:1 separates the freshwater marsh from the salt marsh. Three structures located on the outer margin of the freshwater marsh provide the means to control the amount of freshwater flow into the proposed salt marsh: a sluice gate at the south end of the freshwater wetlands berm, a spillway to the salt marsh located in the north-central portion of the berm, and a control weir that outlets to Ballona Channel at the northwest corner of the freshwater wetlands.
Monitoring and Maintenance
Maintenance will be required in perpetuity to ensure that three overriding goals - habitat protection and enhancement, stormwater runoff containment, and water-quality improvement - are achieved. An adaptive management approach set forth in an operations, maintenance, and monitoring manual allows the marsh manager flexibility to ensure the success of this important restoration project. The manual describes operational procedures for the freshwater wetland system during dry-summer and winter-storm seasons, as well as procedures for monitoring vegetation, birds in nesting season, and water-quality parameters. Monitoring will be conducted primarily over the first five years of the operation, with more limited activities to be performed in perpetuity. Water monitoring includes continuous flow monitoring and grab sampling for water-quality analyses.
Stormwater as a Resource
Playa Vista’s freshwater wetlands system demonstrates how important goals can be achieved through viewing stormwater as a resource. In addition to improving water quality and flood control, the freshwater marsh is providing a vast representation of plant life that is already attracting an impressive diversity of insects, birds, and wildlife. More than 3,000 native trees and 10,000 native shrubs and grasses were planted in the freshwater marsh. Many other native species are "volunteering" on their own. The riparian corridor, banked by a willow woodland, will provide added natural habitat.
An example of sustainable design, the Playa Vista freshwater wetlands system has provided the project and the community with an important amenity; hundreds of people per day visit the marsh, and there is a premium on Playa Vista properties that overlook the wetlands. Moreover, the developer has saved money by building a wetlands system instead of a traditional concrete flood control channel and/or basin. The environment has gained valuable plant and animal habitats, and the region has gained improved water quality for Santa Monica Bay, a vital marine-life habitat and major recreational resource for southern California.