Tahoe Environmental Improvement
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.
Bin Walls: A bin wall consists of a series of small bin or box like structures constructed of concrete or corrugated steel, that when placed side by side create a wall. The bins are filled with sand or a similar material, which ultimately forms a gravity retaining structure. Some of the walls are faced with a rock-like finish. Upon completing the wall, roads or other structures can be constructed on top of or adjacent to the structure. Galvanized corrugated steel is typically used for wall construction however due to its reflective nature, it is not allowed under most circumstances in the Tahoe Basin. Instead, "weathering steel", which lacks reflective properties is used but deteriorates more rapidly as it is not protected from environmental factors.