Commentary : Tijuana Estuary Can’t Be Put on Hold : Environment: Stopgap measures are needed to protect this wildlife refuge until the Mexico-U.S. wastewater treatment plant is built.

The good news is that the United States and Mexico have finally agreed on a wastewater treatment plant for the Tijuana River Valley. The bad news is that it won’t be operating until 1993 or later.

During this lengthy wait, raw sewage--sometimes as much as 20 million gallons a day--will continue to flow into the Tijuana Estuary, an endangered species refuge and one of 17 national estuarine research reserves in the country.

To protect this national refuge and the animals and plants that inhabit it, an interim method of containing the wastewater is desperately needed to prevent irreversible damage.

The Tijuana Estuary is a small and little-known coastal water body. Unlike some of its more prominent cousins, such as Chesapeake Bay, the Tijuana Estuary is not critical to supporting a large commercial fishing industry. Rather, its value is in maintaining a piece of our natural heritage.

More than 75% of Southern California’s coastal wetland habitat has been lost to development. If these last pieces of a unique coastal ecosystem are not managed to support our native species, then we all lose the potential to enjoy the aesthetic, educational and recreational values of the natural world. When too much habitat has been lost, animals and plants face extinction, which is the case for several birds and one plant that depend on the wetlands.

When our wetlands were more plentiful and less disturbed, the Tijuana Estuary provided food and profit, as well as enjoyment. As recently as the 1970s, it was a popular clamming spot, where weekend visitors harvested buckets of food. Bait fishermen were licensed to harvest ghost shrimp. Young halibut used to come into the estuary to feed and grow and once supported sport fishing.

These values have been lost at the Tijuana Estuary--and the primary culprit appears to be the excessive flow of water through the river valley. First, there were the floods of 1978 and 1980, which reduced the populations of clams, ghost shrimp and other invertebrates, and probably also the halibut. Now, the sewage flows from Mexico prevent them from recovering.

Ironically, the wastes and toxins in the sewage, although detrimental, are not the biggest hazard for river valley wildlife. Rather it is the water that carries the sewage. Although it is black and stagnant, this water is considered “fresh,” while the species native to the estuary are marine organisms--they thrive in sea water, which is 3.4% salt. Wastewater often dilutes the estuary’s salt content to near zero, and the sensitive animals either leave or die.

Without sewage inflows, Tijuana Estuary would be as salty as the ocean for most of the year, because sea water moves in and out of the ocean inlet with the daily tides. Only during the winter rainy season would there be measurable inflows of fresh water. During such brief inflows, clams can close their shells and survive; fish can move into refuges and return when the flood is over.

Although rainfall is rare and usually brief, sewage flows year-round.

In addition, the volume of water flowing through the river valley is greater because Tijuana imports water from the Colorado River. Thus, the estuary’s water supply is altered in two ways--there’s a larger volume of fresh water and it flows into the estuary for a much longer part of the year.

In the four to 10 years before sewage treatment becomes a reality for the Tijuana River, sewage flows will continue to dilute the estuary’s salinity. Fish and invertebrate populations will not recover. Like weeds, mussels and clams native to Asia may move into the channels and displace native invertebrates. Birds that nest in the native vegetation and feed on the invertebrates will lose their habitat and food sources.

The effects are not limited to the animals. The Pacific Estuarine Research Laboratory (PERL) at San Diego State University predicts that many areas of native salt marsh vegetation will be displaced. Several patches of weedy plants have already moved into the salt marsh at the expense of the native vegetation. Habitat for the endangered Belding’s savannah sparrow has declined as a result. The future of the entire estuarine ecosystem is jeopardized by waste-water inflows.

But there is a better way to manage wastewater until the new treatment plant is built. The concept is simple: Impound the wastewater upstream of the estuary and discharge it in pulses that are timed to coincide with the outgoing tides. If discharged in pulses, waste water would flow quickly through the estuary, reducing the time and extent of its impact on estuarine organisms. Freshwater marsh plants, such as cattails and bulrushes, would grow well in the impounded wastewater wetland; they would also absorb many of the nutrients and toxic substances in the wastewater. Thus, the water discharged to the estuary would be cleaner than raw sewage.

Although both pulsed discharges and wastewater wetlands have been used elsewhere, their combination, as proposed here, is innovative. On Humboldt Bay, the city of Eureka discharges treated effluent in pulses, to minimize the impact on bay organisms, and the city of Arcata uses a series of waste-water wetlands to cleanse treated sewage further. However, PERL knows of no situation where the two concepts have been combined to accomplish both objectives.

We are convinced that pulsed-discharge, wastewater wetlands would reduce the harm caused by raw sewage flowing into the Tijuana Estuary. PERL has received funds from the California Sea Grant College Program to conduct more research on the concept. What’s needed next is a pilot project. But this solution is not a panacea. Neither is the proposed sewage treatment plant, which will not solve the problem until excess flows are prevented from entering the Tijuana Estuary. The plan to build an ocean outfall would accomplish that purpose, but it will take years to construct.

The ultimate solution is total recycling of wastewater, which would prevent imported fresh water from entering our coastal estuaries. Total reuse would not only eliminate the disposal problem; it would also help solve the county’s critical water-supply problem.