Geological study could shed light on Tahoe's future

Researchers are planning one of the most exhaustive geologic studies of Lake Tahoe to date - one that could impact scientists' understanding of lake clarity and climate.

Although the project is in the design stage, scientists hope their results will help public agencies keep the lake blue, as well as assess the threat of landslides, earthquakes and tsunamis at Lake Tahoe. Hoping to begin in 2007 or earlier, researchers will collect information by taking cores of sediment from the shores and the bottom of the lake, and by studying how soundwaves are transmitted through the ground and water.

Through their research, scientist hope to reconstruct a two million year record of the tectonic and climatic forces that shaped the region.

"Lake Tahoe is a North American record," said Kenneth Verosub, professor of geology at the University of California, Davis. He said that because Lake Tahoe is so deep and has a history that extends beyond two million years, it may set the standard for historical climate study in North America.

Tahoe is one of the few sites around the world deep enough to yield a significant historical sample of sediment layers. Those layers can tell scientists how old the region is, as well as when periods of drought occurred and how long they lasted, Verosub said.

By taking coring samples from the lake, scientists can establish a timeline by dating layers of ash from known volcanic events.

"If we can find (through coring samples) a record of continuous accumulation, it could conceivably be a very long record," said Rich Schweikert, professor of geology at the University of Nevada, Reno, and one of the scientists working on the study.

Previous core samples analyzed by a UNR team revealed thin, sandy layers of sediment during dry years and thicker layers during the wetter years, he said.

"We have found evidence for a 70-year drought," Verosub said.

Locally, the research could influence where future structures are built as well as the kind of environmental measures scientists recommend, Verosub said.

Earthquake activity is another topic the scientists hope to make headway on via the study. Schweikert cites debris near a collapsed scar at McKinney Bay and the West Shore landslide.

Core samples could help determine when and how frequently the earthquakes occurred - if in fact earthquakes caused these events.

Multiple samples at a variety of locations will be important to establish patterns, and the patterns could represent an average number of years between earthquakes of a magnitude 7 or higher, which have been found in some of Tahoe's fault lines.

Scientists also hope to answer questions about lake clarity.

"By looking at sedimentation in great detail, I believe we can answer the question, 'What is the most normal state for the lake?'" Schweikert said.

Verosub said the research could help scientists define a standard to which Lake Tahoe should be restored.

"To what do we restore the lake? To a hundred years ago, when it was covered by a foot of sawdust from logging?" he said.

Schweikert said taking core samples from South Lake Tahoe will present an opportunity for scientists to resolve a controversy about contaminants such as MTBE, or Methyl Tertiary Butyl Ether, a chemical compound used as an additive to replace leaded gasoline. Although MTBE was cleaned from Lake Tahoe gas station leakage, there is concern that leakage within deeper layers could occur and contaminate ground water.

"Some say the contaminated layers have impermeable layers of sediment between them, and therefore, there will be no further leakage," Schweikert said. "Others say there is evidence for at least two permeable layers below South Lake Tahoe."

Scientists have been looking at including other water bodies in the study, like Flathead Lake in Montana, the African rift lake in Tanzania and Kenya and Lake Baikal in Russia.

Although the project has received $28,000 in funding from Drilling, Observation and Sampling of the Earth's Continental Crust, a consortium of 52 universities that supports scientific coring studies, the entire project could cost $2 million to $5 million.

A workshop planned for September at Lake Tahoe will include about 60 scientists, who will work out the details and timeline of the project. There will be opportunities for public input and comment as the project progresses.