WHY CONSTRUCTED WETLANDS?
The Alameda County Flood Control and Water Conservation District and the Alameda County Clean Water Program have been working to develop different natural and constructed wetlands for the purpose of removing pollutants from storm water. This effort resulted from mandates issued under the 1987 Federal Water Quality Act that modified the 1972 Clean Water Act to address problems associated with storm water pollution. Storm water is any surface water runoff from either rain or artificial water flow (i.e. watering the lawn). This law is commonly known as the National Pollutant Discharge Elimination System (NPDES). Previous law addressed only industrial and sewerage pollution problems. Many times there are pollutants from the surrounding watershed that may include heavy metals, pesticides, oil and grease.
Alameda County constructed a prototype marsh known as the Demonstration Urban Storm Water Treatment (DUST) Marsh in the early 1980’s near Coyote Hills Regional Park. The purpose was to test the effectiveness of artificial wetlands on urban water treatment. Water from Crandall Creek was diverted into a small debris basin and then into two ponds. Water from the two ponds combine in a third pond before it heads off to the North Marsh of Coyote Hills Regional Park.
After seven years of conducting rather complex scientific investigations at the DUST Marsh scientists from the county determined that wetland systems were effective at removing pollutants, and they produced a safe environment for wildlife.
THE TULE PONDS PROJECT
The Alameda County Flood Control and Water Conservation District acquired the site in the 1950’s and has used it since as a flood detention basin. In 1998, they undertook construction of the Tule Ponds Project to develop a wetlands-based storm water treatment system at the 14-acre site in Fremont. The project involved excavation of three ponds, construction of gravel pathways and three pedestrian bridges all within the ruderal area. These new ponds were connected by a channel and designed so that runoff from the urban watershed flows into the first of the three ponds via an underground storm drain system and then consecutively through the next two and then the existing sag pond. The incoming water is treated for pollutant removal by natural processes that typically occur in wetlands.
Throughout the Tule Ponds there are different strategies to help clean the water. Log booms are used to skim the water to trap floating debris and prevent it from continuing on to the bay and obstructing other waterways. The boom also acts as baffle to restrain the movement of surface water and promotes the mixing of resident water with incoming runoff, a process that has been documented as significantly reducing the effects of toxic pollutants. Areas have been created within the Tule Ponds to help filter water. Gravel and pebble-sized rocks are used as a natural filtering system.
As a result of the Tule Ponds Project, the Tyson Lagoon site is unique in now having natural and constructed wetlands combined into a single ecosystem. One function of the water treatment ponds is the slowing of the water to allow heavy metals to bond with clay particles which then drop out of the water column and accumulate in bottom sediments at a depth where anoxic conditions limit the number of animals which might otherwise be exposed to the metals. The result is cleaner, healthier water for downstream receiving waters such as San Francisco Bay and, potentially, the Pacific Ocean.
The suspended particles range from heavy metals; such as copper (Cu), Lead (Pb) and Zinc (Zn), to simple things like sediment. The heavy metals can be toxic to a variety of organisms. In addition, the flora in the system serves as a nutrient uptake mechanism reducing the occurrence of algae blooms that can reduce the oxygen levels creating potentially lethal conditions for downstream aquatic organisms. This condition of eutrophication causes aquatic systems to deteriorate in terms of biodiversity resulting in reduced habitat value, including making the water unusable for human recreation. The suspended sediment can prevent the penetration of sunlight to the underwater organisms. This reduced sunlight condition can prevent photosynthesis resulting in the reduction of plants available for nutrient uptake and essential habitat value for other organisms. On the other hand, nutrients, including fertilizer and pet wastes, can cause extended growths of water plants in areas where open water space is necessary for fish and other water organisms to survive and reproduce. Oil products, usually from vehicles, often reach levels that are toxic when ingested by animals such as fish and birds as well as damaging gills, feathers, and other bodily organs potentially causing death directly or indirectly by making them ill and susceptible to predation. Also, it may cause reduced reproductive capabilities. Lead comes from emissions from diesel and gasoline, paints, and wood staining products. Zinc is from automobile tires, paints, and wood staining. Copper is from plumbing, electroplating processing wasters, brake pads in automobiles, and algaecides. These materials are hazardous to the animals both in and out of the water as well as the plants in the surrounding waterway.
Prior to construction the pond was tested for heavy metals including copper, cadmium, silver, chromium, lead, mercury, selenium, and zinc. It was also tested for chlorinated pesticides, PCB’s, diazinon, chlorophyrifos, carbamate pesticides, and other organophosphates. The sediment up to 110 cm was tested for these components. Cadmium, chromium, copper, lead, and zinc were higher than the Ontario Sediment Guidelines, which is a baseline concentration which represents 95% of the organisms can tolerated.
Data is still being collected to determine the success of this project. Time will only tell.