Saline Ponds to Mitigate Contaminants

Introduction

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The Salton Sea (Sea) is the largest lake in California. The Sea is a critical stopping point for birds along the Pacific Flyway – a 5,000-mile route from the Arctic to South America. More than 400 species of birds make use of the Sea, including the federally endangered Yuma Ridgway’s rail, Western snowy plover, black-necked stilt, Western sandpiper, and American avocet. Water levels at the Sea are maintained by irrigation runoff in the Imperial and Coachella valleys and local rivers, however, levels are declining due to natural evaporation and reduced runoff. As a result, decreased habitat and increasing salinity levels in the Sea water and high selenium levels in agricultural drainage water threaten the many species at the Sea.

Constructing wetlands is a restoration alternative that could offset the loss of habitat. However, constructing wetlands at the Sea is complex because healthy wetlands require comparatively clean water - with less salt and less selenium than exists in the Sea and in agricultural drainwater. High salinity can affect many species by slowing growth rates, reducing survival rates and lowering reproductive capacity. High selenium can become toxic to fish and wildlife if it is bioaccumulated by aquatic organisms and can result in deformed embryos and reproductive failure. 

In 2006, the Bureau of Reclamation and the U.S. Geological Survey explored the feasibility of creating shallow saline habitat ponds (SHP) using blended water to mitigate salinity and selenium risks to wildlife. A 50-hectare experimental complex of four interconnected SHPs was constructed at the southeastern shoreline of the Sea and was flooded with a mixture of highly saline Sea water and agricultural runoff water from the Alamo River. Site preparation methods, costs, and durability of maintaining SHPs were evaluated along with effects to bird and aquatic invertebrate species, and interactions among water and sediment chemistry. Results were compared to four reference sites that included ponds sustained by water from the Alamo River, ponds sustained by variable water intakes from the Sea and drains, marsh sustained by agriculture drainwater, and ponds sustained by Colorado River water.

Key Issues Addressed

Over the past several decades, changes in agricultural water use practices by farmers have gradually diminished inflow into the Sea. The Sea is a terminal lake with no outlet to the ocean, so water that enters it can only depart through evaporation, leaving salts behind. High salinity and selenium in the agricultural runoff water can be toxic and harm wildlife populations. As Sea levels lower, it is also anticipated that dust from the drying lakebed will contribute to poor air quality, human health impacts, and loss of economic development opportunities, which will result in the loss of habitat for birds, fish, and other wildlife that depend on the Sea. A new landscape, that includes managed wetlands, must be engineered that provides dust mitigation and habitat for migratory birds.

Project Goals

• Assess chemical, nutrient, and contaminant characteristics in water and sediment for created shallow saline wetlands and explore management options
• Determine community composition and contaminants in avian food sources
• Determine contaminant concentration, especially for selenium, in black-necked stilt eggs and post-hatch habitat use and survival
• Describe adverse effects of contaminants on avian populations

Project Highlights

Future Planning: Increased human-health and ecological hazards are predicted as inflow reduces over time. Current restoration planning alternatives are based on mitigating these future hazards. 

• Quality Bird Habitat:Surveys were conducted at the SHP and reference sites at the Salton Sea and in San Diego Bay every two weeks to document bird numbers, diversity and patterns of use. Bird use of the SHP site was not significantly different than reference sites across all guilds of birds. Foraging accounted for the most common (62%) activity documented, demonstrating that the SHP provides food resources for migratory birds.
• Selenium in Stilt Eggs: Selenium concentrations in fresh black-necked stilt eggs varied over time among groups. Concentrations of selenium in stilt eggs collected from the SHP were significantly higher than reference site eggs in 2006 and 2008, but did not differ between sites in 2007. A large percentage of fresh stilt eggs collected from the SHP (47 percent) and reference sites Freshwater Marsh and Morton Bay (39%) exceeded the predicted 6.0 microgram/gram egg selenium toxicity threshold during the study. In contrast, no eggs collected from reference sites exceeded the toxicity threshold. Selenium apparently did not affect embryo malpositioning that in turn affects hatchability.
• Black-Necked Stilt Chick Survival:Survival of 132 newly hatched stilt chicks was monitored using telemetry radio tags. Mortality was approximately 2.5 times higher for chicks hatched at the SHP compared to reference sites. Selenium concentrations determined from sibling eggs probably had no effect on survival. However, the probability of death increased nine times from the failure to migrate from the SHP. The most frequent cause of mortality was predation followed by unknown deaths, canals, and crack entrapment. Chicks that could not escape steep sided canals bordering the SHP were likely depredated by aquatic predators (largemouth bass, catfish, and bullfrogs). Death from avian predators coincided with the establishment of a nesting gull-billed tern and California gull colonies. Death due to entrapment occurred when chicks migrating from the Ponds 1 and 2 fell into deep (about 0.5m) cracks in the dry lake beds of Ponds 3 and 4.
• Chick Use of Habitat:Post-hatch stilt chicks left the SHP within 3 days for surrounding freshwater habitats. The search for lower salinity and/or increased vegetation (for cover from predators) were important factors influencing chick-movement during the pre-fledgling life stage.
• Pupfish Habitat: The federally endangered desert pupfish colonized the SHP site from an unknown source and made use of the lower salinity ponds (<70 ppt). It is believed that the shallow nature of the pond allowed predatory birds to locate fish. The pupfish, because of cryptic coloration and tendency to stay low in the water column, were less detectable than other species. Populations of competing species, such as sailfin molly and gambusia, were reduced allowing desert pupfish to thrive. At the time of project decommission the suitable ponds held an estimated one million pupfish.
• Invertebrate Community Structure: Invertebrate community structure structure
  Something temporarily or permanently constructed, built, or placed; and constructed of natural or manufactured parts including, but not limited to, a building, shed, cabin, porch, bridge, walkway, stair steps, sign, landing, platform, dock, rack, fence, telecommunication device, antennae, fish cleaning table, satellite dish/mount, or well head.
  
  Learn more about structure and diversity was most strongly coupled with salinity. Invertebrate composition in lower-salinity ponds (20-60 parts per thousand) was characterized by variable abundance of Capitellidae, Amphipoda, and Corixidae. Conditions in higher-salinity ponds (>60 ppt) were most conducive to Corixidae or Ephydridae. Most Salton Sea reference sites favored Chironomidae, Clitellata, Capitellidae, Amphipoda, Gastropoda, and Ephemeroptera.

Lessons Learned

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This study demonstrated that the design and logistics of the shallow water saline habitat concept was an ecologically viable solution to offset the anticipated problems of a receding Salton Sea. The SHP site was able to quickly establish invertebrates and fish which served as prey for a wide variety of birds. Using blended agricultural tailwater (Alamo River) and Sea water reduced selenium concentrations below levels found in Alamo River water; however, concentrations in all samples are potentially causes for concern. These concentrations were higher than those found at reference sites maintained with Colorado River water, but were similar to concentrations at reference sites on the Salton Sea maintained with Sea water and/or drainwater. For mitigation purposes, habitats using the blended water approach are expected to perform similarly to Salton Sea habitats being lost as the lake recedes. In theory, the unblended water should create conditions much higher in selenium. During this study there was little difference between the blended water and unblended water sites while both were different from the “clean” water sites. Use of unblended water should be explored further as an option at the Salton Sea.

Although the study did not evaluate logistical or mechanical requirements necessary to sustain the ponds, water delivery challenges and flow into and between ponds contributed to seasonal variation in biological and physical parameters measured at the ponds. Many of the problems that arose in this project came from difficulty pumping adequate water to the site. Aside from being expensive and consumptive, pumping requires constant maintenance as pumps and pipelines break. The availability of water may change as sediment intake areas become sedimented. The quality of water may also change as will happen as the Salton Sea becomes saltier. All of this creates issues that must be dealt with if the habitat is to function as intended.

It will be beneficial to diversify the habitat with varying water depths, structural features such as islands and rocky reefs, and different substrate types for invertebrates. This diversity will also encourage aquatic vegetation to create micro-habitats within the water column. Diversity creates resilience.

Avian communities that inhabit built wetlands should be monitored for contaminants. It may be necessary to determine the sources of contaminants, which may include local or distant foraging as well as site use. For mitigation purposes, habitats built with blended water would require additional selenium monitoring to assess risks to wildlife. Managers have the ability to address possible issues by adapting operations to change how the water is blended.

Next Steps

Use lessons learned for habitat creation around the Salton Sea

Funding Partners

• Proposition 50 Agricultural Water Quality Grant Program, California State Water Resources Control Board
• Bureau of Reclamation

Resources

• Salton Sea Ecosystem Monitoring Project
• USGS Salton Sea Website
• U.S. Fish and Wildlife Service, Sonny Bono Salton Sea National Wildlife Refuge

Contact

Tom Anderson, Assistant Refuge Supervisor, USFWS Pacific Southwest Region, tom_w_anderson@fws.gov

Case Study Lead Author

Deanna Morrell, Program Analyst, Bureau of Reclamation, dmorrell@usbr.gov

Suggested Citation

Morrell, D.P. (2020). “Constructing Shallow Saline Habitat Ponds at the Salton Sea to Mitigate Contaminant Risks.” CART. Retrieved from https://www.fws.gov/project/saline-ponds-mitigate-contaminants.