© 2001 by Leo Poppoff

May 17, 2001

Basin Watch

by

Leo Poppoff

CONSTRUCTED WETLANDS CAN WORK

Natural wetlands are an important resource in the Tahoe Basin. They trap
nutrients and sediments, and thereby cleanse water before it enters the
lake. Of course, wetlands aren't perfect. Occasionally, nutrients are
flushed out of wetlands. Still, many folks believe that wetlands were
responsible for Lake Tahoe's recovery after the clear cutting of its
watershed in the late 1800s.

Unfortunately, many of the basin's wetlands disappeared during urban
development. It's been estimated that something like three-fourths of
Tahoe's wetlands had been destroyed. So now, we're trying to construct
new wetlands, hoping to restore some of the former water cleansing
capabilities of the watershed.

Yet, experience with constructed wetlands hasn't been encouraging. New
wetlands in cold winter areas have been especially troublesome.
Nonetheless, the potential value of wetlands stimulates folks to
continue trying to make them work.

Many years ago, Tahoe City wanted to improve parking and traffic and
upgrade its downtown with sidewalks. Tahoe's regulatory agencies
required Tahoe City to treat its urban runoff as a condition for
approval of the improvement project.

It was decided that runoff would be collected and treated in a wetland.
A catch basin and wetland complex was built on a vacant parcel next to
Albertson's at the Tahoe City "Y." It began to receive drainage from
Tahoe City in 1998. The wetland was a part of a \$6 million Placer
County drainage and water quality improvement project, which also
included drainage lines, grease traps and oil separators. The wetland
portion was funded mainly by the California Tahoe Conservancy.

After the wetlands had a couple of years to mature, Alan Heyvaert and
his associates at the UC-Davis Tahoe Research Group investigated its
nutrient and sediment removal efficiency. The results are very
encouraging.

Drainage from Tahoe City flows first into a sediment catch basin sized
to contain the runoff from a storm that might produce one-inch of
precipitation in one-hour. Then it trickles slowly through a constructed
wetland to provide time for adequate water treatment. The wetland and
catch basin were planted with a mix of native sedges and rushes found in
other Tahoe Basin wetlands. Treated water then flows to the lake.

Not all drainage from Tahoe City flows to the wetland. The project
treats storm runoff from the commercial area above Highway 28, between
Highway 89 and Grove Street.

Water flowing into the catch basin was monitored and analyzed for
nutrient and sediment loading. Similar monitoring was done at the outlet
of the catch basin and the inlet to the wetlands. Effluent was also
analyzed just before it flowed to the lake. In addition to suspended
sediment, the researchers looked for total phosphorus, dissolved
phosphorus, soluble reactive phosphorus, nitrates and ammonia. The
reactive portion of dissolved phosphorus is most readily consumed by
algae.

TRG scientists monitored the wetland during the period of March 2000 to
April 2001.

Compared to runoff from undisturbed forest, the Tahoe City drainage, on
average, contained 29 times as much suspended sediment, 15 times as much
total phosphorus, 8 times as much dissolved phosphorus, 22 times as much
soluble reactive phosphorus, 17 times as much ammonia, and 344 times as
much nitrate.

The inflow to the wetlands isn't constant. As might be expected, flows
peak during the spring snowmelt. Suspended sediment and nutrient levels
increase rapidly and drastically during the same period.

Heyvaert and his colleagues found that treatment through the catch basin
and wetlands reduced pollutant levels significantly. Sediment levels
dropped by a factor of 10, total and dissolved phosphorus by a factor of
5, soluble reactive phosphorus and ammonia by a factor of 6, and
nitrates by a factor of 7. Sediment and nutrient loads were reduced by
80 to 90 percent.

Another look at the figures shows that sediment and nutrient levels
weren't reduced to undisturbed forest values. But the bottom line is
that effluent from the constructed wetlands meets standards set by the
Lahontan Water Quality Control Board. That's a remarkable achievement.
As a bonus, the wetlands provide a habitat for birds.

During the next years and decades, vegetation that sops up nutrients
will mature. And, rich wetland type soils with more microbial activity
will develop. Microbial action is especially useful for treating
nitrogen compounds. We would hope, then, that wetland cleanup efficiency
will increase with time.

Now, the question is: how many wetlands can be constructed in the basin?
Urbanized areas around the lake don't have much vacant, level land
downhill of development. Much of the level land in the basin, including
former wetlands and meadows, has been developed.

On the other hand, urban runoff supplies an estimated one-third of the
total phosphorus and one-fifth of the dissolved phosphorus loading to
the lake, according to Tahoe Research Group scientists. So, it's well
worth finding ways to treat it -- and constructed wetlands will be very
important tools in that attempt.