How Wetlands Mitigate the Effects of Runoff on Coastal Water Quality


Project: Coastal Water Quality
PI: Stanley B. Grant, Chem Eng & Mat Sci, UCI; Brett F. Sanders, Civ & Envir Eng, UCI; Richard F. Ambrose, Envir Sci & Eng, UCLA; and Clinton Winant and Lisa Levin, UCSD/SIO
Funding: UC Council Coastal Environmental Quality Initiative
Division: UCI
Corporate Partners: Southern California Edison
Start Date: July 1, 2002

>Talbert Marsh

California's economy depends heavily on tourism, and large numbers of tourists of course are drawn to California because of the state's beaches. But many beaches are not safe for swimming due to runoff pollution from neighboring watersheds.

This interdisciplinary team is conducting studies to understand the role that coastal wetlands play in modulating the input of fecal indicator bacteria (FIB) into southern California's coastal waters. FIB are important because they may indicate the presence of coastal pollution (sewage, agricultural runoff, and stormwater runoff), they are correlated with whether it's safe to swim, and they are used as a water-quality index by state and local officials for decisions regarding beach closures. Coastal wetlands are located at the outlet of many urban watersheds along the southern California coastline and, because of their location between urban watersheds and the ocean, they are uniquely situated to mitigate the impacts of land runoff on coastal pollution.

The team is testing two hypotheses: (1) The degree to which a coastal wetland removes FIB from land runoff depends on the wetland's structure and ecology, the nature of water circulation within the wetland, and the tidal exchange of water between the wetland and the ocean; and (2) The input of FIB into a wetland from land runoff depends on specific features of the surrounding watershed. In addition, the team anticipates that the features that most affect FIB loadings from watersheds will be different during dry and wet weather seasons. Their research involves field work, data analysis, and numerical modeling.

"In addition to our fecal bacteria indictor studies, we monitor runoff caused by storms. The vast majority of pollutants get eroded during storms, flowing out of the watershed into the ocean. We can work with satellite experts to track these pollution plumes in the ocean, with the long-term goal of providing to the public near-real-time information on coastal water quality. These plumes show up on the satellite images, and, because the water flowing out of the watersheds during storms is so turbid, it looks like raging chocolate milk."
- Stan Grant, PI, UCI

This project leveraged a unique opportunity: In November 2001, the outlet of the San Dieguito Lagoon (Del Mar, CA), a regionally important tidal salt water marsh in San Diego County, became blocked with sand during a large wave storm. Over the next several months, water quality inside the lagoon steadily worsened due to declining dissolved oxygen concentrations, declining salinity, and increasingly frequent fish kills. This deterioration was believed to be caused by the absence of tidal flushing plus the accumulation of nutrient-laden runoff from the surrounding area, which includes the Del Mar Race Track and Fairgrounds, many horse stables, and a large residential community. Eleven months later, on October 4, 2002, the City of Del Mar excavated (breached) the outlet. This event enabled the research team to conduct before-and-after studies to learn more about the cleansing impacts of tidal flushing.

Using this opportunity, the team characterized water circulation in the lagoon and inshore region of the ocean; studied the temporal evolution of FIB in the lagoon, surf zone, and offshore; measured the spatial distribution of FIB in lagoon sediments, the relationship between FIB-laden sediment and landscape features, and the ecological response of the wetlands to the breach event; and studied linkages between patterns of FIB occurrence and within-wetland circulation via hydrodynamic modeling.

The breach occurred in conditions characterized by unusually small waves, and weak littoral and inshore currents. As a result, contaminants from the lagoon effluent were taken offshore initially, and some fraction appears to have been recycled back into the surf zone by shoreward-directed currents. The breach caused significant change in the spatial distribution of FIB in the lagoon, along the shoreline, and offshore. Before the breach, the highest concentrations of FIB were observed at the site closest to the tidal outlet (associated with low-salinity water and "nuisance" runoff from the horse stables). The FIB gradient reversed several days after the breach with concentrations increasing with increased distance inland. Within 24 hours of the breach, FIB from the lagoon were detected over a 4 km2 area offshore, stretching some 4 kilometers along shore and 1.2 km off shore. Consistent with the calm ocean conditions the day of the breach, the FIB plume was most concentrated just offshore of the tidal outlet. The concentration of FIB in tidal sediments decreased after the breach but only after the outlet had been open to tidal flushing for about one month. FIB concentrations in the lower marsh areas remained low and showed no apparent response to the breach.

Year 2 follow-up studies are being planned and are likely to include studies at the upstream and downstream margins of the lagoon to determine whether the wetland is producing or consuming FIB; measurement of the export rate of FIB from a small constructed wetland in Mission Bay (San Diego); and another breach study at Malibu Lagoon (Los Angeles County).

Contact: Stanley B. Grant,, 949-824-7320,