Creating a Gem

A pond originally intended as a storage facility for incoming wastewater becomes a magnet for waterfowl and a favorite spot for wildlife watchers
Creating a Gem

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An unintended consequence of a decision made in the 1980s by the Sewerage Agency of Southern Marin, Calif., turned out to be beneficial to the agency’s urban wastewater treatment plant and the surrounding community of Mill Valley.

Faced with too much inflow to the agency’s 3.6 mgd biotower/trickling filter wastewater treatment plant during the winter rainy season, officials created two dry ponds for temporary storage of wastewater. The ponds were built between the plant and the entrance to a nearby multi-use community park. Although plant general manager Steve Danehy wasn’t on the staff at the time, he knows of the ensuing controversies and the results.

“No one could have predicted this kind of outcome,” says Danehy. One pond serves the dual purpose of providing temporary storage of wastewater, while normally holding clean effluent to sustain plants and wildlife habitat.


Built for storage

Effluent leaves the plant through a 36-inch pipe near the ponds, starting a 6-mile trip to final discharge in San Francisco Bay. However, during a dry season just after a plant upgrade, a break developed in the effluent pipeline. While the leak was being repaired, final effluent was diverted and temporarily stored in the dry south pond.

“It took a while to sort out the cause of the leak, which was attributed to an outside contractor,” Danehy says. “During the time it took to negotiate action on the repair, waterfowl and shorebirds discovered the pond, and it turned into a nesting habitat.” Cattails and other native plants took hold, and suddenly the pond became a popular destination for birdwatchers and nature lovers, who unofficially called it Gem Pond.


Usage conflicts

Controversy erupted in the early 1990s when a group asked the plant to create a skateboard park by cementing over the dry north pond area. But those who enjoyed the south pond objected to the potential for noise and disturbance to wildlife. The dispute was settled when the sewerage agency found it could not get insurance for a city-sponsored skateboard park.

Over time, vegetation began to overrun the south pond, and the agency’s board reconsidered whether the 2-acre pond was suited for dual use. A proposal to drain it and return it to its original purpose drew another public outcry. This time the agency created a management plan for maintenance of the pond that includes mowing and periodic clearing of cattails.

In 2000, the board was confronted with the need to increase the pond’s wastewater storage capacity. “The ponds are earthen, and over the years the sides settled,” says Danehy. With little objection from activists, they decided to raise the berm around the pond instead of permanently lowering the normal level of treated water and threatening the wildlife habitat.

Since then, a sign has been installed in a kiosk that lists more than 120 species of native and migratory birds pond visitors have seen there. And a change in insurance regulations has allowed a skateboard park to be built near the Mill Valley Middle School, away from the pond.


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