Could Repaving Roads Be a Water Quality BMP?
In partnership with El Dorado County and Texas Southern University, the Tahoe Resource Conservation District (Tahoe RCD) is monitoring stormwater runoff from Elk’s Club Drive to quantify the amount of fine sediment that comes from the road itself. Samples are taken of the runoff water and sent to the laboratory for source apportionment analysis. This lab test uses microscopic markers to determine what portion of the fine sediment in the runoff comes from native soil (road shoulder erosion), traction abrasives (road sand), and asphalt (the road itself).
Currently, Elk’s Club Drive is in poor condition. It is covered in cracks and potholes. Not only is a degraded road bad for public safety, it is difficult to recover road sand and other material through road sweeping. Road sediment is a large portion of the fine sediment that ends up in Lake Tahoe, reducing its famed clarity. Early results show that up to 30% of this fine sediment includes asphalt (aggregate and binder). El Dorado County will be repaving Elk’s Club Drive this summer. After it is repaved, the Tahoe RCD will continue to monitor the runoff from the road for an additional year. If data shows that repaving roads contributes to improved water quality (less sediment), then improved pavement condition could be recognized as a water quality Best Management Practice (BMP). This would not only be identified for garnering credits for the Lake Tahoe TMDL Clarity Crediting Program (https://clarity.laketahoeinfo.org/), but also potentially opening up water quality improvement funds for road maintenance. New roads would be beneficial for public safety, vehicle maintenance costs, aesthetic appeal, driving pleasure, road maintenance and sweeping operations, long term durability, snow removal operations, stormwater quality, and lake clarity.
A washoff simulation was conducted in April to analyze runoff under controlled conditions. Over 5,000 gallons of water were released on Elk’s Club Drive and sampled at the terminal location. In a rainstorm, water runs off the road shoulder contributing native soil in runoff. In a snowstorm, there is likely to be high concentrations of road sand on the road and in runoff. The controlled simulation excluded native soil and road sand from the procedure, and focused runoff on the road surface only. Early results show peak turbidities (cloudiness) occurring with peak flows, indicating that sediment is being continually supplied by the road and suggesting that the road itself is eroding. The sediment in the samples will be subjected to the source apportionment analysis to confirm if this hypothesis is correct. Though the data is not yet in, the Tahoe RCD and El Dorado County are hopeful that, in the future, investments in new roads will be viewed as a win-win for the community and the lake.
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