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3-D program puts local water in perspective

South Shore’s primary water supplier has a new tool for its fight against MTBE.

The South Tahoe Public Utility District now has a better understanding of how the lake, South Shore rivers, and creeks as well as its own wells impact the groundwater supply beneath South Lake Tahoe.

A company in Sparks has completed a three-year, $290,000 hydrogeological study of part of South Shore’s aquifer system, the underground porous layer of rock which contains water. The group, AGRA Earth & Environmental, will present the results to the South Tahoe Public Utility District Board of Directors at a meeting today.



“We produced a three-dimensional (computer) model of the northern half of the South Lake Tahoe groundwater aquifer,” said Chris Wessel, senior hydrogeologist for AGRA Earth & Environmental. “It is complete to a point. Modeling is an ongoing process. It will have to be updated.”

The model is supposed to be a predictive tool.




The program will allow STPUD to simulate drilling new wells or increasing the pumping of current wells. The simulation will give district officials an idea of the impact to the aquifer system.

Although the district’s problems with the controversial fuel additive MTBE did not materialize until after the study was under way, STPUD officials believe the program will help them better protect the system of wells.

“One of the huge questions that comes up every time we find a new MTBE release is how is this going to affect wells 200 feet away or 500 feet away? If we close down the Arrowhead well, will it affect the Bakersfield well? What’s the effect of closing this well or continuing to use it?” said Dawn Forsythe, STPUD information officer. “In nature, every action has a reaction. You can’t see underground. You need all the tools you can to make all the best judgments you can, based on what is happening.”

STPUD’s water system supplies 30,000 people through 12,500 connections, and at times, the total population served is as high as 60,000. Because of MTBE, more than one-third of the district’s wells have been closed in the last year and a half. The district’s customers faced mandatory water-usage restrictions for much of last summer and may have to do the same this summer.

One of the reasons the fuel additive MTBE – methyl tertiary butyl ether – has created more problems than many other gasoline contaminants is that it behaves differently in groundwater. While other gasoline compounds adhere to the soil, traveling slowly, MTBE permeates the ground at the same rate as water. In a place like South Shore, where most of the drinking water wells access shallow aquifers, MTBE is even more of a threat.

The study also verified that the level of Lake Tahoe has a significant impact on the amount of groundwater available on South Shore.

“This is very important,” Wessel said. “Lake level has a 10-foot range that fluctuates based on what’s released to the Truckee River. (We’ve always wondered) If the lake level falls, does the groundwater fall, too? We found the lake level has significant impact on groundwater levels.”

The model also revealed that the water capacity of South Shore’s aquifers is much higher than what STPUD uses. The company simulated taking 150 percent of the water STPUD actually does and found the aquifers were not significantly affected.

However, that doesn’t mean STPUD won’t be instituting mandatory water-usage restrictions this coming summer. While the aquifer may be able to provide more water, STPUD’s well system – because so much of it is out of operation – cannot accommodate the increased demand South Shore sees in the summer months.

South Tahoe Public Utility District Board of Directors meets today at 2 p.m. in City Council Chambers, 1900 Lake Tahoe Blvd.


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