IT'S a marsh like any other marsh. Elegantly feathered night herons perch in the drooping trees. Northern shovelers with their rich chestnut flanks dabble among the cattails and bulrushes. Coots skitter on the water before taking flight. But the water drifting slowly through the shallow, man-made pools has been flushed down the toilets and washed down the drains of the neighboring houses.
``It's amazing to me,'' said civil engineer Ed Nute, ``that the birds find [this marsh] here among the oil refinery tanks and right next to the freeway.''
This is the Mountain View Sanitary District's 83-acre managed wetlands, one of two marshes in the San Francisco Bay Area constructed as components of sewage-treatment plans.
Both projects were conceived as alternatives to a requirement that sewage-treatment plants pipe their effluent to ``deepwater outfalls,'' not just discharge it at the shore or into creeks or sloughs.
The marshes create an environmental benefit from waste water that would otherwise degrade the bay. They are an innovation that has had rave reviews from bird watchers and mixed reviews from environmentalists and regulators.
The marsh in Martinez, about 40 miles northeast of San Francisco, serves as a ``polishing pond'' for 1.6 million gallons of effluent produced each day by the district's 15,000 residents. It is also home to thousands of birds that flock to an increasingly rare but critical habitat for fish and birds.
``By building marshes, you're doing two things. You're creating habitat and you can improve water quality,'' said Francesca Demgen, an aquatic biologist who specializes in waste-water wetlands.
The concept is simple. ``In the water [coming out of the sewage-treatment plant], we have a fertilizer: nitrogen and phosphates,'' said Mr. Nute. ``In a lake, it would be detrimental, but in an estuarine system it's beneficial.''
Nute and his father, J. Warren Nute, designed and built the Martinez marsh in 1974. Ed Nute's firm, based in San Rafael, Calif., also built a marsh for the Las Gallinas Sanitary District in San Rafael, in Marin County (see story below). He also worked on a similar marsh in Arcata, 250 miles north of San Francisco.
In Martinez, the effluent flows slowly through a series of carefully monitored man-made freshwater ponds, and then through a less-managed, naturally brackish marsh. ``There's an uptake of nutrients in spring and summer by plants,'' Nute said.
The cattails and bulrushes act as a horizontal trickling filter, said Robert Ludwig, a civil engineer who used to research waste-water wetlands and now works for the California Department of Health Services.
``And the cattail stems in the water column act as a surface for bacteria to attach to,'' he said. ``It is the bacteria that provide the majority of the treatment.''
Those bacteria are eaten by larger cells, which in turn are eaten by zooplankton. Several complex food chains develop that support fish, shore birds, and birds of prey.
The Martinez plant is two miles from the bay. A ``deepwater outfall'' pipe that long, costing millions of dollars, would have been needed to comply with federal and state water-quality requirements.
``The board says that if you can create a net environmental benefit, then you don't need to build a deepwater outfall,'' said Larry Kolb, assistant executive officer of the state Regional Water Quality Control Board. ``Mountain View Sanitary District has done just that.''
San Francisco Bay has lost more than 75 percent of its historical wetlands. Starting in the 1850s, the marshes were diked and drained: some for development, some for salt evaporation ponds, some for pasture.
Wetlands are now recognized as anything but useless. The microscopic creatures teeming in the wetlands are the base of the bay's food chains. ``There are any number of organisms that go in and out of wetlands that need wetlands for part of their life cycle or for part of their daily routine,'' said Ms. Demgen, who has advised districts in the Bay Area, southern California, and Nevada. ``Some fish, like salmon, need both the ocean and the fresh water. For other species, their fry grow in wetlands areas.''
Wetlands are also invaluable as habitat for shore birds and as resting and feeding grounds for the millions of birds migrating on the Pacific Flyway.
The constructed marshes create a ``net environmental benefit'' as a tiny replacement for the tens of thousands of acres of wetlands destroyed in the last century.
The sewage in the marshes is more properly called effluent, according to Richard Bogaert, the full-time biologist/analyst who monitors the marshes in Martinez. It has had what the professionals call ``secondary treatment,'' meaning that 85 to 90 percent of the suspended solids and organic matter has been removed.
Until its discharge into the marshes, the effluent has been through what Nute calls a ``pretty standard process for publicly owned treatment plants.''
Mr. Kolb said, ``This way of treating effluent is land-intensive. The regional board has tended to require sizable wetlands, because we feel that sizable mitigation is needed in lieu of a deepwater outfall.''
The board doesn't allow effluent discharge into existing marshes, Kolb said, because that wouldn't create a ``net environmental benefit.'' It has required districts to construct marshes or restore historical wetlands.
Barry Nelson, program director of the Save San Francisco Bay Association, a 25-year-old Berkeley-based environmental group, is ``cautiously supportive of the exploration of this idea.''
One concern, he said, is the possibility of toxins from the effluent ending up in the bay's ecosystem. ``What's implicit in this is that you're removing contaminants from the effluent. We're concerned about the possible ultimate fate of those contaminants.''
Emy Chan Meiorin, an environmental engineer with the Association of Bay Area Governments, has studied plant and animal life in a constructed marsh in Fremont, on the southeastern end of the bay. In a December 1986 report on an experimental marsh that filters storm-water runoff, she looked for toxins such as heavy metals, oil and grease, and PCBs.
``We did find some elevated levels of some of these metals [chromium, copper, lead, zinc, nickel] in some of the fish,'' Ms. Meiorin said. ``But it was inconclusive whether it came from the runoff source or from being in the water before then. Some of the fish we studied were older than the marsh.''
Meiorin also found that the heavy metal content of the runoff was significantly reduced by the marsh. ``The metals accumulate in the marsh and form fairly insoluble compounds in the sediment layers at the bottom of the marsh,'' she said.
But Meiorin doubts this system can be widely used. The marshes require a lot of land and need to be carefully monitored. In addition, they are appropriate only in largely residential districts without industries that produce toxic wastes. ``It can only be used under very specific conditions,'' she said. ``However, where it's appropriate, it certainly is a workable and less costly alternative.''
Demgen is more optimistic. She estimates that 100 sanitary districts nationwide are already using what she called ``constructed, on-purpose'' wetlands to treat waste water. ``I think this is going to take off. It's in the logarithmic early stages now. It's gaining acceptance and recognition as a process that works.''