When most people think of Lake Tahoe, a picture of an alpine lake with exceptional water quality and clarity comes to mind. As a matter of fact, Lake Tahoe is known as one of the three clearest alpine lakes in the world. The NDOT faces the challenge of assisting in the protection of that clarity; however, while trying to maintain it, we must also maintain our roads. Cars emit pollutants and during the process of maintaining our roads in the winter, NDOT uses sand and salt to melt the snow in an effort to keep the roads safe for the traveling public. These sediments and other pollutants are then washed off the road, down the adjacent embankments by rain or snowmelt and ultimately into the lake. Our goal is to minimize the amount of all sediments and pollutants that actually reach the lake. It is a constant challenge that we must face year round.
Each year, the University of California, Davis checks the clarity of Lake Tahoe. They check the clarity
by using what is called a Secchi disk; it's white and about the size of a dinner plate. The disk is
attached to the end of a waterproof measuring tape and then lowered down into the lake to measure the
clarity. When the disk cannot be seen anymore that is called the "Secchi depth." As you may or may
not know, the clarity of Lake Tahoe has been declining at a steady rate of about 1 foot per year. In
2002, you could only see down below the water surface about 78 feet.
One of the ways that NDOT is trying to meet the goal of minimizing the amount of pollutants that reach the lake is employing water treatment facilities on all its roads. Water treatment facilities vary in type and size and have similar functions. A few of these facilities are:
Infiltration Basin:
An infiltration basin is basically a large man-made depression in the ground that helps trap
sediment. Unlike the two water treatment facilities shown below, an infiltration basin "fills up" with
a sediment-water mix and then over time, the water portion of the mixture seeps into the ground from the
basin, leaving behind the sediment and other trapped material. However, like other water treatment
facilities, maintenance crews clean out these basins on an as needed basis.
Sediment Traps:
A sediment trap allows sediment to fall out of the incoming water and be stored at the bottom of the
trap, until NDOT maintenance crews can clean it out. Crews clean out the traps once or twice a year. The
traps are basically a series of below ground storage structures with a drop inlet or grate on the surface.
Treatment Vaults:
Once the storm water flows into a treatment vault, the structure allows both fine and coarse sediments
to settle to the bottom of the vault, while floatable and petroleum products rise and become trapped
underneath a barrier until cleaned out by NDOT maintenance crews.
The NDOT will continue to focus on improving the roadway water quality that reaches the lake.
