Catch pond falls short of expectations
The U.S. Geological Survey completed a five-year study in February indicating widespread erosion control techniques in the Lake Tahoe Basin might not be as effective as originally thought at keeping nutrients out of the lake.
The study found that a man-made basin near Cattlemans Trail and Cold Creek changed groundwater flow below it. The large amount of water entering the basin caused the groundwater to flow faster, and carry slightly more nutrients into the creek.
On the plus side, the basin did capture all the sediment from the water flowing into it and the ground did absorb some nutrients. The basin did not affect groundwater health, said the study’s author David Prudic, a hydrogeologist.
Stormwater catch basins look like small ponds and are scattered along Tahoe’s roads and neighborhoods. They capture rain and snowmelt and allow water to seep slowly into the ground.
Hundreds of basins have been built to the tune of almost $600 million in an effort to keep nutrients and sediment from entering Lake Tahoe.
These pollutants are the primary culprits in the lake’s declining clarity.
The Tahoe Regional Planning Agency requires all property owners, public and private, to install erosion control techniques to hold rain and snowmelt on property. The basins are a major part of public agencies strategies to control erosion.
Prudic said previous studies of only surface water found the basins prevented 100 percent of nutrients and sediments from entering streams, creating hope they could solve the problem of Lake Tahoe’s declining clarity.
Lahontan Water Board said the study is more evidence that other options should be explored, such as filtration systems or relying more on plants to suck up nutrients.
“What we have is better than nothing, but it may not be sufficient to save the lake,” said Lahontan’s division manager Lauri Kemper.
TRPA’s executive director John Singlaub was on the same page, but said the basins’ primary advantage is capturing sediment.
“We need to keep looking at what all the options are,” he said. “The natural system itself was able to handle it, and we’ve got to find something that works.”
The basins serve as substitutes to meadows, which were once Tahoe’s natural filtration systems. More than 70 percent of Tahoe’s meadows have been damaged because of development.
The overall effect of the Cattlemans basin was surprisingly positive for the study’s author Dave Prudic, a hydrogeologist. Since groundwater contains nutrients, he expected to find much more entering the creek than he did.
Since the ground below the basin did absorb some nutrients, Prudic said there was no overall increase downstream in concentrations of nitrogen, phosphorous and organic carbon.
“Some scientists were hoping that all the nutrients would disappear (by absorption into the ground), and I don’t think that will happen,” Prudic said. “That was a naive perception, because there are nutrients in the groundwater naturally.”
Prudic estimated the total cost of the study was around $600,000, with funding assistance from El Dorado County.
Several other organizations in Tahoe have followed suit to study groundwater.
The Tahoe Daily Tribune reported last fall that scientists at agencies throughout the lake were concerned the basins might not be the ultimate solution to preventing pollution from entering Lake Tahoe.
South Tahoe Public Utility District and the city of South Lake Tahoe had begun two studies at a cost of $750,000 to examine whether more serious pollutants, such as heavy metals and gasoline compounds, could be entering the ground and affecting groundwater health.
Last fall, scientists at UC Davis, Lahontan Water Board, the U.S. Forest Service, the city and the utility district all voiced concern that not enough was known about the basin’s impacts to groundwater health. The utility district relies solely on deep groundwater wells to provide water to South Shore’s residents.
“We are all looking forward to seeing the results from these studies,” Kemper said. “That’s what everyone wants to know: if we do this, what kind of benefit can we count on?”
Prudic did not find negative impacts to groundwater health.
“The ground is as good a filtration system as you are going to find, so the detention basins are probably not bad,” Prudic said.
To view the abstract for the USGS study, go to http://pubs.water.usgs.gov/sir2005-5260/.
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