Restoring lake clarity to 100 feet may be more difficult than previously thought

A 50 percent to 60 percent reduction in pollutants entering Lake Tahoe will be required to return the lake to its famed 100 feet of clarity, researchers announced on Thursday.

That reduction in pollutants is substantially greater than what was thought needed for 100 feet of clarity just 14 months ago.

But at that time, a significant source of fine sediment was not included in the model used by the Lahontan Water Board and Nevada Division of Environmental Protection.

In July 2006, a preliminary model indicated such a clarity increase could be achievable within 20 years if the amount of nitrogen, phosphorus and fine sediments going into the lake were immediately reduced by 35 percent.

Fine sediment contributions to the lake from urban areas, yet to be included in the preliminary model, underlie the call for greater pollutant reductions, according to John Reuter, associate director of the UC Davis Tahoe Environmental Research Center.

Studies over the past year, looking at fine sediment in dust and water runoff from roads, residential communities and commercial development, show these urban areas contribute 72 percent of the fine particles entering the lake.

While nitrogen and phosphorus contribute to algae growth and are still a factor in the clarity decrease, the nutrients "are a really small part of the equation," Geoff Schladow, director of the UC Davis Tahoe Environmental Center, said on Thursday.

The tendency of fine sediment to become suspended in the upper reaches of the lake's water column and cloud visibility have been receiving more attention in recent years.

"If this is the source of the problem, this may be part of the solution," said Jon Reuter, referring to the fine sediment contribution of urban areas.

Researchers presented the most recent model projections during a Pathway 2007 stakeholder session held at Lake Tahoe Community College on Thursday.

Pathway 2007 is a process to update the Lake Tahoe Basin's 20-year regional plan, and stakeholders represent wide range of perspectives, from government agencies to business interests and environmental organizations.

Over the coming months, the stakeholders will be asked to weigh the costs and benefits for different methods of reducing fine sediment, nitrogen and phosphorus from urban runoff, forest runoff, groundwater, stream channel erosion, shoreline erosion and atmospheric deposition.

While the model, a measure of the impact of pollutants known as Total Maximum Daily Load, or TMDL, is the most comprehensive scientific understanding of Lake Tahoe's clarity decline to date, holes remain.

"We don't have all the answers," said Harold Singer, executive director of the Lahontan Water Board near the beginning of the Thursday's session. "We constantly have to be looking at better ways to fix this problem."

How effective flood plains are in removing pollutants from streams and effects of fire in the basin on lake clarity are two of the topics that deserve further study, according to researchers on the project.

Results from air-quality monitoring showing the amount of fine sediment reaching the lake from September's Plumas County fire, as well as the effects of a recent prescribed burn near Incline Village, are expected to be obtained in the next several months, Schladow said.

Water quality monitoring data from Angora Creek, collected since the Angora fire, is also expected to be analyzed this winter, according to Lauri Kemper, a supervising engineer with the Lahontan Water Board.

The next Pathway Forum Stakeholders Session, in which draft strategies to reduce pollutant loads will be discussed, will take place Oct. 25 at the North Tahoe Conference Center in Kings Beach.