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During the past three decades, many attempts were made to keep sediments and nutrients out of the lake. The techniques used to intercept nutrients and sediments include infiltration trenches, retention basins, wetlands, vegetated slopes, and drop inlets. These all go by the term, Best Management Practices or BMPs.
And they all do some good. But, the question that bugs many people is: How good are they? Well, there isn't a whole lot of information available to answer that question. Still, to implement the TMDL program, researchers and resource managers need to know.
Eric Strecker, with the Portland Office of GeoSyntec Consultants, is one of the principal investigators developing an International BMP database for the EPA. For a BMP to be included in the database, information about it must be collected according to strict protocols. For example, performance of the BMP can't just be judged by a single "grab" sample (e.g., one scoop of water).
Information should include very careful sampling and analysis, watershed conditions, precipitation, and design details. A guidance document on BMP monitoring protocols, about a hundred pages long, has been prepared. With those strict guidelines, only 199 BMPs nationwide have been included in the database to date. Some 30 more BMPs are currently being evaluated for addition to the list.
Interested in more information? Check the BMP database Web site Š www.bmpdatabase.org.
For budgetary reasons, BMP designers often try to use one kind of BMP to treat water for all the pollutants of interest. Yet, Strecker finds that a single BMP might not do the whole job. Some BMPs are better for removing suspended sediments. Others might be better for heavy metals or for nutrients, and so on. In a recent document, Strecker urges designers to use treatment trains (a series of different BMPs) when possible.
For the last several years, Strecker has been studying Tahoe Basin BMPs. He found that the Tahoe City Watershed Treatment System (the wetland next to the Albertson's supermarket) removed sediment and phosphorus much better than the average of some 20 other Tahoe Basin BMPs. It was also better than the average of the best performing BMPs in the national database. The other 20 Tahoe BMPs compared favorably with those in the national database.
Strecker is also modeling the use of BMPs for the control of pollutant-laden flows from the basin's intervening zones. These are zones between streams that drain runoff directly to the lake. Urban runoff from some of these intervening zones is quite troublesome and not easy to treat. A final report on those results is due in a few months.
An important aspect of preventing lake pollution, according to Strecker, is reducing runoff volumes. He points out that by reducing the flow by half, for example, you also reduce the pollutant load to the lake by half, even if the concentrations remain the same. You also reduce the volumes of water draining into streams. And that reduces stream bank erosion, which in turn, reduces sediment flows to the lake.
Increased flows are the result of urban development, with its increase of impermeable coverage. So, Strecker tries to understand how much water would have run off an area before it was urbanized. Decreasing runoff to that amount would be the ideal goal.
Strecker looks at how much precipitation results in runoff, infiltration, and evapotranspiration (evaporation plus plant transpiration) now and before urbanization. He finds that the real change from natural conditions is how much of the current runoff water had actually been evapotranspired before urbanization. Strecker believes it was a significant fraction, and that transpiration should be enhanced in modern BMPs.
Would a lot of vegetation increase evapotranspiration from retention basins? "I think plants can help," Strecker said. "But I think what helps most are plants with woody stems. They help keep the soils open. I think the biggest key to increasing evapotranspiration is having some good organic soils that would soak up water and dry out afterwards. And the vegetation can help by keeping the air moving through the soil a bit."
A common type of BMP is to infiltrate runoff to reduce flows to the lake. But, Strecker points out, that causes much more infiltration of water than occurs in natural landscapes. And infiltrating all of that water may not be a good thing. For example, nitrate-laden water, infiltrated deeply into the ground, will probably seep into the lake eventually.
There's much more on this topic. Tune in next week for Part Two.
Comments or questions? Send them to basinwatch@sbcglobal.net.
Leo Poppoff is a retired atmospheric physicist with NASA and has been a member of the Tahoe Regional Planning Agency's advisory planning commission since 1983. He is also a former member of the Lahontan Water Quality Control Board.
And they all do some good. But, the question that bugs many people is: How good are they? Well, there isn't a whole lot of information available to answer that question. Still, to implement the TMDL program, researchers and resource managers need to know.
Eric Strecker, with the Portland Office of GeoSyntec Consultants, is one of the principal investigators developing an International BMP database for the EPA. For a BMP to be included in the database, information about it must be collected according to strict protocols. For example, performance of the BMP can't just be judged by a single "grab" sample (e.g., one scoop of water).
Information should include very careful sampling and analysis, watershed conditions, precipitation, and design details. A guidance document on BMP monitoring protocols, about a hundred pages long, has been prepared. With those strict guidelines, only 199 BMPs nationwide have been included in the database to date. Some 30 more BMPs are currently being evaluated for addition to the list.
Interested in more information? Check the BMP database Web site Š www.bmpdatabase.org.
For budgetary reasons, BMP designers often try to use one kind of BMP to treat water for all the pollutants of interest. Yet, Strecker finds that a single BMP might not do the whole job. Some BMPs are better for removing suspended sediments. Others might be better for heavy metals or for nutrients, and so on. In a recent document, Strecker urges designers to use treatment trains (a series of different BMPs) when possible.
For the last several years, Strecker has been studying Tahoe Basin BMPs. He found that the Tahoe City Watershed Treatment System (the wetland next to the Albertson's supermarket) removed sediment and phosphorus much better than the average of some 20 other Tahoe Basin BMPs. It was also better than the average of the best performing BMPs in the national database. The other 20 Tahoe BMPs compared favorably with those in the national database.
Strecker is also modeling the use of BMPs for the control of pollutant-laden flows from the basin's intervening zones. These are zones between streams that drain runoff directly to the lake. Urban runoff from some of these intervening zones is quite troublesome and not easy to treat. A final report on those results is due in a few months.
An important aspect of preventing lake pollution, according to Strecker, is reducing runoff volumes. He points out that by reducing the flow by half, for example, you also reduce the pollutant load to the lake by half, even if the concentrations remain the same. You also reduce the volumes of water draining into streams. And that reduces stream bank erosion, which in turn, reduces sediment flows to the lake.
Increased flows are the result of urban development, with its increase of impermeable coverage. So, Strecker tries to understand how much water would have run off an area before it was urbanized. Decreasing runoff to that amount would be the ideal goal.
Strecker looks at how much precipitation results in runoff, infiltration, and evapotranspiration (evaporation plus plant transpiration) now and before urbanization. He finds that the real change from natural conditions is how much of the current runoff water had actually been evapotranspired before urbanization. Strecker believes it was a significant fraction, and that transpiration should be enhanced in modern BMPs.
Would a lot of vegetation increase evapotranspiration from retention basins? "I think plants can help," Strecker said. "But I think what helps most are plants with woody stems. They help keep the soils open. I think the biggest key to increasing evapotranspiration is having some good organic soils that would soak up water and dry out afterwards. And the vegetation can help by keeping the air moving through the soil a bit."
A common type of BMP is to infiltrate runoff to reduce flows to the lake. But, Strecker points out, that causes much more infiltration of water than occurs in natural landscapes. And infiltrating all of that water may not be a good thing. For example, nitrate-laden water, infiltrated deeply into the ground, will probably seep into the lake eventually.
There's much more on this topic. Tune in next week for Part Two.
Comments or questions? Send them to basinwatch@sbcglobal.net.
Leo Poppoff is a retired atmospheric physicist with NASA and has been a member of the Tahoe Regional Planning Agency's advisory planning commission since 1983. He is also a former member of the Lahontan Water Quality Control Board.
