The contribution of atmospheric pollutants to declining water clarity in the Lake Tahoe Basin

Starting in the second half of the 20^th century, decline in Lake Tahoe’s water clarity and degradation in the basin’s air quality have become major concerns due to its unique scenic features.  Gaseous and particulate nitrogen (N) and particulate phosphorus (P) loading via direct atmospheric deposition and sediment transport to the lake have also been implicated as responsible for its eutrophication and decline in water clarity.  Estimates suggest that atmospheric N deposition contributes half of the total N loading to the lake, while atmospheric P deposition contributes one-quarter of the total P loading.  In order to improve both air quality and, as a consequence, water quality, it is necessary to develop an understanding of the origin of the atmospheric pollutants.  Once this is accomplished, it is possible to implement cost-effective strategies to reduce this impact.  This paper summarizes the findings of a series of studies performed by research groups in the basin to determine the levels and sources of ambient air pollutants.  The studies found that the pollutants most closely connected to the decline in water quality come largely from within basin sources, as opposed to those coming from the Central Valley and upwind urban areas of California.  These results indicate regulators need to control N, P, and sediment emissions within the Tahoe basin in order to reduce the impact of atmospheric pollutants on water quality.