Number range CHAPTER© 2003 by Leo Poppoff

November 13, 2003

Basin Watch

by

Leo Poppoff

A POTENTIAL TOOL FOR TAHOE'S RESOURCE MANAGERS

How will we know if the money and time spent on Tahoe's watershed
restoration efforts are paying off? Well, we can keep track of water
clarity in the middle of Lake Tahoe. That, after all, is what all this
fuss is about. Still, it takes a while for the lake to respond. By the
time we're convinced that clarity has improved, we might've spent all
the restoration funds without knowing if we had used the best
techniques.

Watching mid-lake clarity is very important, but it doesn't give
resource managers a clue about where algae stimulating nutrients and
soil particles are coming from. But there might be a way to learn more
quickly how well restoration efforts are going, and get some clues about
the locations of pollutant sources.

A very large portion of the pollutants in the lake are washed in from
the watershed. The first part of the lake that they affect is the water
just offshore. Eventually, pollutants move out to deeper portions of the
Lake and affect the Lake's clarity. But, the effect on nearshore water
is very rapid. It should be apparent after every storm or snowmelt. So,
if resource managers could keep an eye on this part of the lake, they
could get an idea of which areas they should be

working on, and they could tell if what they're doing uphill is working.

Though this has been suspected for quite a while, the occasional
sampling and measurements done over the years really didn't confirm it.
When the environmental standard for nearshore turbidity was evaluated a
couple of years ago, it was judged to be one of the few environmental
standards that had been achieved at Tahoe.

However, recent research by Professor Kendrick Taylor casts doubt on
previous evaluations. Professor Taylor is with the University of
Nevada's Desert Research Institute (DRI). The research was done for the
Tahoe Regional Planning Agency (TRPA) and funded by California's
Lahontan Water Quality Control Board, Nevada's Department of State
Lands, DRI and TRPA.

Taylor fitted a boat with sampling gear and toured the shoreline. Water
was collected by a probe in the bow of the boat, and pumped through a
device called a nephelometer. A nephelometer measures the amount of
light scattered by particles in the water. The more particles there are
in the water flowing through the nephelometer, the more light is
scattered. As Taylor's boat moved along the shoreline, the
instrumentation provided a continuous record of the cleanliness of the
water while a GPS kept track of the locations of the measurements.

The good news in Taylor's report is that "most of the nearshore water in
Lake Tahoe is very clean." The bad news is that water in front of
developed areas is not so clean. "Hot spots" exceeding the threshold
value for turbidity were recorded in the Tahoe Keys and near the mouth
of the Upper Truckee River. Persistently high turbidity levels were
found offshore of South Lake Tahoe and Tahoe City. High turbidity was
measured occasionally off Kings Beach and Incline Village. Turbidity of
waters off undeveloped areas such as Deadman's Point and Rubicon Point
was consistently low.

Turbidity can be caused by particles of soil or algae -- or both. To see
if some of the nearshore turbidity was caused by algae, Taylor surveyed
the area offshore of Tahoe City and Star Harbor with a fluorometer, an
instrument that detects and measures the chlorophyl content of water. He
found that elevated concentrations of chlorophyl (and, therefore, algae)
coincided with high turbidity.

The preliminary surveys that Taylor reported in 2002 were made in the
summer. He found that high turbidity was associated with developed areas
around the lake. But, Taylor wasn't able to tell what might have caused
these high turbidity areas. There was no rain, so they couldn't have
been caused by urban runoff. Taylor suggests that groundwater seepage
from urbanized areas might explain his observations. Another possibility
is that pollutants settle out in the nearshore area during winter storms
and are then resuspended by boat traffic or wave action.

Currently Taylor is researching turbidity changes during and after
storms, when runoff occurs. The work isn't complete, but it indicates
that runoff from urbanized areas does increase nearshore turbidity.
Correlating turbidity measurements directly with runoff could be tricky,
however, because they're composites of effects by runoff and wave
action.

Nonetheless, Alan Heyvaert, Tahoe Research Group scientist who studies
urban runoff, believes that "over time we should see a decrease in
near-shore turbidity with success of stormwater BMPs, just as we hope to
eventually observe it in the pelagic zone." Larry Benoit, who heads the
water quality program at TRPA, explains that the agency is interested in
ways to monitor turbidity continuously. They're looking into using
instruments suspended from buoys and monitoring water pumped from the
lake by utility districts. Progress is limited by budget constraints.

Much more work is needed, but it seems like the idea of giving resource
managers a tool to evaluate restoration work is well worth pursuing.

Send any comments to basinwatch@sbcglobal.net