© 2005 by Leo Poppoff

November 17, 2005

Basin Watch

by

Leo Poppoff

GROUNDWATER AND NEARSHORE TURBIDITY

Desert Research Institute scientist Kendrick Taylor cruised around the
lake during the summers of 2000 and 2001, and the springs of 2001 and
2002 in a small boat equipped with special measuring instrumentation and
positioning equipment. Taylor was measuring and mapping turbidity in the
waters just offshore. This is the part of the lake that we see while
driving around the basin or relaxing on the beach. It's also the region
where pollution that's carried by streams and groundwater, or that runs
directly off the land, enters the lake.

Taylor's surveys revealed a number of high turbidity spots around the
lake. Most of these "hot spots" were offshore of developed areas. The
springtime measurements were made during snowmelt runoff periods. So,
hot spots offshore of developed areas would be expected. But, what about
hot spots found during late summer, when there was no runoff?

Taylor suggested that nutrient-rich groundwater flow might be the cause
of persistent hot spots. Groundwater nutrient sources could be residuals
of past sewage disposal practices, leaching of abandoned septic systems,
sewer leaks, fertilizer use, or ground disturbance. Of course, this was
just a hypothesis. But, it was a good, rational hypothesis and deserved
to be checked out.

So, the USGS (U. S. Geological Survey) and the Tahoe Regional Planning
Agency decided to investigate groundwater near one of these hot spots to
see if they could confirm Taylor's idea. That study was just reported by
USGS hydrologist Kip Allander. He investigated the groundwater
characteristics of a small, highly developed south shore watershed
drained by Bijou Creek. It's a diverse watershed, containing urban
development, a golf course, and drainage from a ski resort. One of the
persistent hot spots observed by Taylor was offshore of this watershed.

According to Allander "the hypothesis tested in this reconnaissance
study is that nutrient concentrations in ground water in the Bijou Creek
watershed are greater than in the Lake Tahoe basin in general, and that
ground water in the study area flows toward and discharges into Lake
Tahoe. Discharge of nutrient-rich water to Lake Tahoe might explain why
there is elevated turbidity in the nearshore zone of the Bijou Creek
watershed area."

Four springs and eighteen wells were sampled during the late summer of
2003. The water samples were analyzed for a number of nutrients --
ammonia, nitrates and nitrites, organic nitrogen, phosphorus,
orthophosphate and biologically-available iron. Water temperature, water
levels, pH (acidity) and specific conductance (a measure of dissolved
solids) were also recorded. Nutrient concentrations were compared with
groundwater elsewhere around the lake, including areas that had little
or no development or offshore turbidity hot spots.

Was the hypothesis confirmed? Well, partly, but there was also a big
surprise. The water level data showed that groundwater was indeed
flowing toward the lake, as was hypothesized. Nutrient analysis showed
that nitrogen species and biologically-available iron were much higher
than the average groundwater values around the lake. That was also
expected.

But, concentrations of phosphorus species were much lower than
groundwater averages around the lake. That poses a problem in trying to
explain the summer hot spot off of the Bijou watershed. And this is why.

Nutrients stimulate the growth of algae. Still, algae need the right mix
of nutrients to prosper. In Lake Tahoe, algae get more than enough
nitrogen needed to match the amount of phosphorus available to them. If
they could get more phosphorus, however, they'd be able to use more of
the nitrogen -- and the algal colony would increase.

So, in Lake Tahoe, phosphorus triggers algal growth. In technical
jargon, the lake is said to be phosphorus limited. For nutrients to
stimulate algal growth and produce the turbidity hot spot in front of
the Bijou watershed, much more phosphorus would be needed than was
found. So, where does that leave us?

The idea of scientific research is to ask questions of nature and look
for the answers, objectively. Research doesn't always come up with the
answers we expect because nature doesn't give up answers easily. So,
we're left with some intriguing, unanswered, questions.

Why is the phosphorus level in the Bijou watershed so much lower than
ground water around the lake? Why is nitrogen so high -- is it the
leaching of abandoned septic systems, leakage from sewer lines, past
sewage disposal practices, and/or fertilizer use? Is the water offshore
of Bijou different from water in the middle of the lake? Is it actually
nitrogen limited, instead of phosphorus limited?

Since the funding for Allander's study was limited, it wasn't possible
to pursue the questions spawned by the research. But, it seems to me
that these are important questions. Perhaps someone will come up with
the money needed to finish the job.

Questions or comments? Send them to basinwatch@sbcglobal.net