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Lake clarity study released to Pathway forum

Andrew Pridgen

A quick glance at the vanity plates of cars parked in the North Tahoe Conference Center main lot in Kings Beach Thursday did portend the goings-on inside.

“TAHOBLU” and “CLARITY” were parked just steps away from what some Pathway 2007 participants felt is to date the most significant release of information in the process – The Lake Tahoe Sediments and Nutrients (TMDL) Clarity Model Analysis report.

Participants got a glimpse not only of 30 years of data about the lake’s clarity – compiled by Dr.’s Geoff Schladow and John Reuter of UC Davis and presented by Lahontan RWQCB representative Dave Roberts – but a clear look at what management practices can be implemented to ensure the lake’s health and even resurgence over the next 20 years, the scientists explained.



“We were as skeptical as we (could be),” Reuter said. “I get papers all the time, people use these ‘black box’ methods here they look at three or so years … Policy is made and they turn out not so happy.

“We have 35 years of data: Wet years, not-so wet years; heavy urban (development) years, not-so heavy (development years) – we have a pretty good feel.”



The study does show some consistencies, the scientists explained. The lake’s clarity has diminished steadily since 1965 but has improved in recent years, one slide of researcher’s Power Point presentation showed.

In 1968, the lake’s clarity was approximately 31 meters, the study showed. The clarity dropped to 20 meters in 1996. In 2005, the clarity was back up to around 23 meters.

Clarity, and whether the lake can be defined as “impaired”, is based on measuring the lake’s Total Daily Maximum Load (TMDL) or the calculation of the maximum amount of a pollutant that a body of water can receive and still meet water quality standards.

The EPA uses this measurement to determine what bodies of waters are “impaired”. Lake Tahoe is currently on that list.

The UC Davis scientists explained to the 50-plus in attendance at the Pathway Forum that TMDL measurements suggested “nutrient loading” (runoff from streams, forest fires, urban runoff and auto emissions/road dust) was the problem. But further studies showed that particle loading (like it sounds – the same culprits as nutrient loading but sediments on a more microscopic scale) were also the cause for the problem.

“We found what’s important for clarity is not just the overall number but the size of pollutants,” Dr. Schladow said. “The distribution of the different pollution sizes also affect the lake’s clarity.”

The scientists also determined that TMDL reductions on the order of 30-40 percent “appear sufficient” to restore clarity. The results, the study said, could vary by the nature of what “loads” are reduced (particles or nutrients) as well as which contributing factors to the lake’s clarity are addressed (atmosphere vs. stream etc.)

Perhaps the fact that “wowed” the crowd the most was the notion from scientists that the lake could “very plausibly” be restored over the next two decades.

“It’s not going to take 200 years to restore the lake,” Reuter said. “We’re not going to have to throw everyone out of the basin to do it.

“We’re talking about restoration over a reasonable time scale.”

The scientists were less sure of the money it may take than the timeline, and when asked directly how much (in dollars) it would take to restore the lake – or how they could predict (or guarantee) when the lake could be restored, the scientists could not answer.

“That’s the question, that’s the quest,” Schladow said.

The overall tone of the meeting was positive and upbeat, one TRPA official even commented that this was some of the first “tangible progress” in the Pathway process.

Pathway 2007 is a multi-agency collaborative planning process charged with the mission to chart a vision for the Tahoe Basin for the next 20 years.

Placer County principal management analyst Jennifer Merchant said the TMDL and clarity release will “help how (all communities) in the basin can implement a lot of ideas.”

“We’re focusing in (other parts of) the Pathway process around town centers,” Merchant said. “Here’s an opportunity for (communities) to set standards.”


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