Nutrient and particle load estimates to Lake Tahoe (CA-NV, USA) for Total Maximum Daily Load establishment.

The Lake Tahoe Total Maximum Daily Load (TMDL) requires detailed methodologies to identify sources of flows and pollutants (particles and nutrients) for estimating time-variant loads as input data for the Lake Tahoe clarity model. Based on field data and a modeling study, the major sources of pollutant loads include streams (three subdivisions of this category are urban, nonurban, and stream channel erosion), intervening zones (IZs) (two subdivisions of this category are urban and nonurban), atmosphere (wet and dry), groundwater and shoreline erosion. As Lake Tahoe remains well oxygenated year-round, the contribution of internal loading from the bottom sediments was considered minor. A comprehensive quantitative estimate for fine particle number (< 16 µm diameter) and nutrient (nitrogen and phosphorus) loading is presented. Uncertainties in the estimation of fine particle numbers and nutrients for different sources are discussed. Biologically available phosphorus and nitrogen were also evaluated. Urban runoff accounted for 67% of the total fine particle load for all sources making it the most significant contributor although total urban runoff was only 6%. Non-urban flows accounted for 94% of total upland runoff, but the nitrogen, phosphorus and fine sediment loadings were 18%, 47% and 12%, respectively of the total loadings. Atmospheric nitrogen, phosphorus, and fine particle loadings were approximately 57%, 20%, and 16%, respectively of the total loading. Among streams and IZs, IZ 8000, Upper Truckee River, Trout Creek, Blackwood Creek, and Ward Creek are the top fine particle, nitrogen and phosphorus contributors. The relative percentage contribution of inorganic fine particles from all sources based on annual average for the period 1994-2008 on size classes 0.5-1, 1-2, 2-4, 4-8, and 8-16 µm are 73%, 19%, 5%, 2%, and 1%, respectively. These results suggest clear priorities for resource managers to establish TMDL on sources and incoming pollutants and preserving lake clarity.

Potential toxicity concerns from chemical coagulation treatment of stormwater in the Tahoe basin, California, USA.

Coagulant dosing of stormwater runoff with polyaluminum chlorides (PACs) is used in numerous waterbodies to improve water clarity, but the potential risks of PACs to aquatic organisms in Lake Tahoe, California are not fully understood. To assess these risks, the USEPA 3-species toxicity test and a non-standard fish test using Japanese medaka (Oryzias latipes) were used to determine the toxicity of PAC-treated and non-treated stormwater samples to aquatic species. Stormwater samples were collected from three sites representing runoff from different urbanized areas in May 2004; samples received coagulant dosing using three different coagulants (JC1720, PAX-XL9, Sumalchlor50) at levels optimized with jar testing. Raw stormwaters were toxic to algae and fathead minnows (mortality). Treatment with coagulants increased toxicity to zooplankton (reproduction) and had no consistent effects on the other toxicity metrics.

Stream phosphorus transport in the Lake Tahoe basin, 1989-1996.

Lake Tahoe is undergoing the initial stages of cultural eutrophication due to human alteration of the airshed and watershed. The lake's switch from nitrogen (N) to phosphorus (P) limitation has been attributed primarily to atmospheric N loading. This places an increased importance on controlling watershed movement of P to the lake. A stream water quality monitoring data set consisting of nine streams in the Lake Tahoe basin has been analyzed to characterize the spatiotemporal variation of P delivery to the lake. This data is from the Lake Tahoe Interagency Monitoring Program (LTIMP), which provides scientific data for planning and regulatory agencies to address environmental problems in the Lake Tahoe basin. Results indicate that P delivery (concentrations, loads) varies greatly at interannual, seasonal, and spatial scales. Annual and seasonal total P (TP) concentrations can vary up to three orders of magnitude in a given stream and are strongly associated with suspended sediment. Particulate P is the major form of P transported by Tahoe streams and was strongly correlated with percent surficial geologic deposits, which are primarily located near streams. Tahoe streams with the highest annual P concentrations often had the lowest annual P loads, and visa versa. P loading is greatest during the spring snowmelt (75% of annual average). Potential watershed parameters influencing P delivery to Lake Tahoe have been identified as precipitation, basin area, basin steepness, and road and human development coverage. Results also suggest that human development impacts on stream P loads are most prevalent during high precipitation years. Identification and quantification of stream sediment and P sources such as streambanks and impervious surface is necessary to aid in watershed restoration efforts.