The Tijuana Slough National Wildlife Refuge (NWR) is a coastal plain estuary located in San Diego County, California.  It represents one of the few remaining relatively undisturbed tidally influenced wetlands in southern California.  Several rare and endangered species are located at the estuary including the California least tern, the salt marsh bird's beak, and the light-footed Clapper rail.  The slough has been subject to contamination from a variety of sources in the Tijuana River watershed such as municipal/industrial discharges from Mexico, landfills, agriculture, nonpoint source runoff, and illegal dumping in the United States and Mexico. The Sweetwater Marsh NWR was established as a mitigation bank for several large water resource related projects in San Diego County.  It is located close to the Tijuana Slough and encompasses the largest remaining wetland area in San Diego.  It has also been subject to contamination from a variety of sources including storm water runoff and industrial/municipal sources.  This study took place from 1988 to 1992 and was aimed at monitoring the contaminant levels in various animals, plants, and sediments of the area.  The contaminant concentrations accumulating in food items of many birds was also monitored.  The contaminant concentrations relative to threshold values for specific contaminants were examined and the concentrations between the two sites were compared.

Methods:  Organisms were collected at eight sites on Tijuana Slough:  the mouth of the Tijuana River, the Headwaters, the bridge at Dairy Mart Road, Hollister Bridge, Sunset Avenue, South Beach, Grove Road, and Reem Air Field and at four sites on in Sweetwater Marsh:  Gunpowder Point, the mouth of the Sweetwater River, Paradise Creek, and F&G Marsh. Organisms collected included invertebrates, plants, fish, and birds.  The invertebrates collected were razor clams, yellow shorecrabs, ghost shrimp, mud crab, fiddler crab, African clawed frogs, striped shorecrab, and mussels.  The plants collected included  water hyacinth, brown algae, Sueadea, spikerush, Fundulus, cattail, pickleweed, and bullrush.  The fish collected were mosquito fish, California killifish, longjaw mudsucker, sailfin molly, mullet, goby, Pacific staghorn sculpin, and topsmelt.  The birds collected were coot liver, wigeon liver, black-necked stilt liver, and clapper rail egg. The organisms collected were tested for the following contaminants:  polynuclear aromatic hydrocarbons (PAHs), Polychlorinated biphenyls (PCBs), Dichlorodiphenyltrichloroethan (DDT), Dieldrin and Endrin, Arsenic, Cadmium, Chromium, Copper, Lead, Mercury, Nickel, Selenium, and Zinc.

Results and discussion: Most organic and inorganic compounds tested were found to be below threshold levels in the various plants, fish, sediments, birds, and invertebrates sampled.  However, a few compounds were above threshold levels and should be considered when evaluating the overall contamination of the two sites. The following inorganics were found to be above low effect thresholds (the sediment concentration indicating a level of contamination that has the potential to affect some sensitive benthic organisms) in the sediments at Tijuana Slough NWR; cadmium, chromium, copper, mercury, lead, nickel, and zinc.  Only zinc exceeded the level above which adverse effects are likely, so the impacts of these elements on the biota are not likely to be major.  The organic compound p,p'DDE was found to be above NST ERL levels (the concentration below which adverse effects are seldom expected) and was equal to the ERM (the sediment concentration above which adverse effects are likely) concentration in 1989.  These concentrations could also have moderate levels of effects on biota.  Based on the concentrations found, minor to moderate impacts to benthic invertebrates are possible. The only organic to exceed the low effects threshold for Sweetwater NWR was p,p'DDE in 1992. The organics and inorganics in invertebrates at Tijuana Slough were all within the known acceptable level.  At Sweetwater NWR  total PCB and copper concentrations in invertebrates were above dietary thresholds for birds, so additional monitoring is necessary to determine if the potential for impact exists. The organics and inorganics in plants collected at Tijuana Slough were also all within acceptable bird dietary threshold levels. At Sweetwater NWR copper was below levels toxic to plants in 1989, but above levels in 1990.  Impacts on plants due to copper toxicity include induced iron chlorosis, thick roots, and the inability to put forth new roots.  There were no detectable organics in the plants collected at Sweetwater NWR.  The plant samples were also analyzed in consideration of their being food resources for birds.  Lead was below the acceptable dietary threshold for birds in 1989, but above it in 1990. In the fish at Tijuana Slough, zinc was found to be slightly elevated above the normal level each year.  This can have an impact on growth, reproduction, and survival of the fish.  Fish were also examined as an important dietary resources for birds, and it was found that all organics and inorganics were within acceptable threshold levels.  At Sweetwater Marsh NWR zinc was found to be above normal threshold levels in all four years sampled. Only birds from Tijuana Slough were collected and their liver tissues were sampled for contaminants.  Selenium was found to be above the level identified for reproductive impairment in 1987 and 1988.

Comparisons between the two sites:

Summary: It appears that Sweetwater Marsh NWR had higher levels of environmental contaminants than did Tijuana Slough.  The concentrations of most organic compounds and inorganic elements appeared to be below biologically significant threshold levels.  However, long-term bioaccumulation of contaminants can result in impacts and is a cause of concern.

Recommendation: Additional sampling of trace metals at Sweetwater Marsh NWR is recommended due to the elevated concentrations of these contaminants that were found fairly consistently in the samples. Minimal sampling at Tijuana Slough recommended to confirm the benefits of the International Wastewater Treatment Plant and the South Bay Ocean Outfall which take sewage out of the Tijuana River for treatment and offshore discharge theoretically resulting in decreases in contaminant inputs to the Tijuana Slough NWR. Ongoing service participation in watershed management and pollution prevention efforts should be made to facilitate the reduction of contaminant leads into these two refuges and enhance the benefits of the IWTP and SBOO for Tijuana Slough NWR by reducing nonpoint sources of contaminants. (return to top)

  
Learn more by reading the following full report: King, E., Roberts CA.  Evaluating Environmental Contaminants at Tijuana Slough National Wildlife Refuge and Sweetwater Marsh National Wildlife Refuge.  USFWS March 2000.

Return to the Carlsbad Field Office Reports

Visit the: Pacific Region - Environmental Contaminants-Investigating and Monitoring

Visit the: USFWS- Pacific Region Ecological Services Home Page