Lake Tahoe Water Quality - A Clear Solution to Pollution
Posted September 5, 2012
As someone who considers themselves about 50% scientist (science literate and science curious, but not quite fluent), I am always very appreciative of efforts to communicate science in understandable language. Every so often, a publication comes along that perfectly addresses both my desire for science and my craving for jargon-free prose and descriptive visuals. Four weeks ago, the UC Davis Tahoe Environmental Research Center released its annual State of the Lake report – a report that does just that.
One of the few science-based publications written for an audience of non-scientists, the annual report is relevant to our work here at NRDC for a variety of reasons, ranging from climate change to green infrastructure and urban stormwater management. And although the term “secchi depth” may never make typical dinner-table conversation (beyond a small subset of scientists and those who love them), the State of the Lake report is full of wonderful, easy to understand charts and graphics displaying how key ecosystem health indices have changed over time. In particular, this report directly speaks to my recent experience encountering a green roof along the shores of Lake Tahoe.
Among many findings, the State of the Lake report presents data indicating that the long-term trend in declining Lake Tahoe winter water clarity is showing marked improvement. Water clarity is measured by secchi depth – the depth at which a white dinner-plate-sized disk is no longer visible from the surface of the water. In 2011, winter secchi depth increased to 84.9 feet, a 12-foot improvement over 2010. This change is graphically – and conceptually – displayed in the image below (yes, that is a “UC Davis” research vessel floating on the top of the lake above the annual secchi disks.)
One hypothesis for the cause of this improvement is a reduction in the load of fine particles from urban stormwater entering the lake. Although the report is quick to include the caveat that a comprehensive, regional urban stormwater monitoring plan is needed to determine if recent investments in stormwater projects have indeed been the cause of reduced particle loads, it is consistent with research and real-world experience that proper stormwater management can provide significant and quantifiable visible improvements to our environment.
While the majority of Americans don’t live along the shores of an iconic alpine lake, findings from the State of the Lake report have implications for everyone living in urban and suburban areas. For example, while only 10% of Lake Tahoe’s watershed is urban, 16% of the total nitrogen, 39% of the total phosphorus, and a whopping 72% of the fine sediment particles found in Lake Tahoe originate in the urban watershed, making their way into the lake via stormwater.
The numbers regarding the source of pollution are even more striking in the nation’s more urbanized areas. In New York City, for example, excess stormwater is one of the biggest sources of water pollution in the city. The problem is not that it rains on our cities. The problem is the miles and miles of pavement and impervious surfaces that cause water to run off our streets and into our sewers and waterways. See, for example, the fact that the non-urban watershed and stream channel erosion contribute only 13% of Lake Tahoe’s fine sediment particles, despite making up 90% of the total watershed. In contrast, stormwater that runs off roofs and roads takes with it pollutants, trash, and chemicals. In some cases, this urban stormwater overwhelms sewer systems, triggering the discharge of untreated sewage and polluted runoff into the closest waterbody.
It doesn’t have to be that way.
Green infrastructure – urban landscapes that absorb water where it falls (like nature does) rather than treating water like a waste product – can help dramatically decrease the amount of stormwater runoff and the threat it presents to our rivers, streams, lakes and bays. Solutions such as bioswales, rain gardens and wetlands can reduce runoff volume by 73 to 99 percent. Another green solution that helps reduce runoff is green roofs, not unlike the Vikingsholm green roof located along Emerald Bay, Lake Tahoe. A green roof with a three to four inch soil layer can generally absorb between one half to one inch of rainfall from a given storm event. These nature-inspired solutions to urban runoff are cheap, effective, and technologically viable today. And from the perspective of a 50% scientist, 100% urban-resident-that-doesn’t-want-my-water-tainted-with-sewage-or-trash, they make intuitive sense.
A View of Lake Tahoe’s Famed Clear Blue Water (Photo by Kelly Coplin)