California Department of Transportation

Division of Environmental Analysis (DEA) LINKS

square icon

Adobe Acrobat Reader is required to view and print PDF files. Adobe Acrobat is required to edit online forms and save them to your computer.

Get Acrobat Reader Image


Last Updated: Thursday, January 5, 2012 1:08 PM

Stormwater Treatment Pilot Studies Lake Tahoe

The quality and clarity of Lake Tahoe's naturally pristine water have been threatened in recent years. Clarity is decreasing at the rate of about one foot per year.

The problem is intensified when nutrients such as nitrogen and phosphorus are carried into the lake by soil erosion and air pollution. The soil particles decrease the clarity of the water, and the nutrients speed up the growth of algae, which also clouds the water. Eroding soils, stormwater runoff, and increased development around the lake all contribute to the problem.

Stringent standards for water quality at Lake Tahoe become effective in 2008. Caltrans is testing alternative methods of treating stormwater runoff to meet these new standards. These standards are part of the National Pollutant Discharge Elimination System (NPDES) permit requirements for stormwater discharge from Caltrans roads and facilities.

This study involves small-scale testing of a variety of treatment processes designed to collect pollutants before they can reach lake waters. In addition, full-scale pilot tests will be performed based on information from the small-scale tests.

This photo shows how stormwater (above) is clarified by coagulation, flocculation and settling (below).

Monitoring stations provide data on stormwater pollutants such as suspended particles (turbidity), various forms of nitrogen and phosphorus, iron and other metals, and oil and grease.Some pollutants are in particle form and can be removed by settling and filtration. The efficiency of these processes is improved by chemical coagulation and flocculation, which cause small particles to clump together. The best chemical coagulant and dosage is determined through laboratory jar testing.After jar testing, selected coagulants are used in small-scale treatment units that include settling tanks and filters containing various kinds of granular media (see photo right). Some pollutants are dissolved and must be removed either by precipitation (conversion to a solid form) or by ion exchange. Most of the small-scale treatment units are gravity-flow systems.

Show diagrams...

Caltrans has deployed small settling tanks (sand traps) to capture sand used on roadways during snowy winter months. Caltrans is monitoring the effluent quality of sand traps as one of many efforts in the basin to improve water quality. In addition, Caltrans constructed two full-scale activated alumina filters during the 2003 summer construction season. Influent and effluent water quality will be monitored during 2003/2004.


The pilot study facility for the small-scale studies, located at the Caltrans South Lake Tahoe Maintenance Station at Meyers, has earned Caltrans a Certificate of Appreciation for the"2002 Best of the Basin Awards Program" by the Tahoe Regional Planning Agency (TRPA).

To view reports on this subject CLICK HERE


Caltrans is partnering with California State University Sacramento and University of California Davis on these pilot studies. The testing facilities are also available for use by other pollution control researchers.


Division of Environmental Analysis, Stormwater Unit
California Department of Transportation
P.O. Box 942874, MS-27
Sacramento, CA 94274-0001
Email: Division of Environmental Analysis, Stormwater Unit