In-lake monitoring conducted over the last 40+ years reveals a steady trend of loss in the Lake's transparency and clarity. Consequently, NDEP included the waterbody on its 303(d) List of Impaired Waterbodies due to exceedances in the clarity standard. To address this problem, NDEP is collaborating with the California Regional Water Quality Control Board, Lahontan Region (Lahontan Water Board) to develop and implement the Lake Tahoe Total Maximum Daily Load (TMDL). Initiated circa 2000 and completed and approved by USEPA in August 2011, the Lake Tahoe TMDL was a science-based effort to: better understand the causes of the loss in lake clarity; determine how much pollution needs to be reduced to reinstate historic clarity; and develop a workable, cost-effective implementation strategy to do so.

The scale of the Lake Tahoe TMDL is a testament to the importance of this national treasure. Roughly $10 million was invested to develop the plan which involved the cooperation and participation of over 200 scientists and professionals from federal and state resource management agencies, local governments, consulting firms, education and research institutions and public stakeholders. Consequently, the Lake Tahoe TMDL is one of the, if not the, most robust TMDL in the nation. Below is a summary of the major steps and findings of the Lake Tahoe TMDL. Alternatively, you may wish to view slide presentation or watch the Charting the Course to Clarity video instead.

In order to better manage the enormous undertaking of the Lake Tahoe TMDL, it was necessary to break the effort into three distinct phases:

Phase 1 – Pollutant, Source and Loading Capacity Analyses

Phase 1 involved the establishment of a comprehensive research program to answer the following questions:

• What pollutants are causing Lake Tahoe's clarity loss?
• What are the sources and quantities of each of these pollutants?
• How much of each pollutant can Lake Tahoe accept and still reach the clarity goal?

The analyses indicate that the primary pollutants controlling clarity are fine sediment particles and the nutrients phosphorous and nitrogen. Fine sediment particles (FSP) cloud the water while nutrients fuel algal growth. Although each affects the distance that light is able to penetrate into the water column, the analysis indicates FSP, particularly those less than 16 micrometers, appear to be a more important control on clarity than nutrients due to their light scattering effect. The vast majority of FSP and phosphorous entering the Lake are derived from the urban area. Roadways in particular are bad actors because they accumulate pollutants , facilitate FSP production through vehicular pulverization and roadway deterioration, and effectively route runoff to local waterways. Modeling results suggest that a 65% reduction in FSP, accompanied by reductions in nitrogen and phosphorous of 10% and 35% respectively, is necessary to meet the TMDL numeric target. Approximately half these load reductions are needed to meet the Clarity Challenge, an interim target of 80 feet annual average Secchi disk depth to be achieved within 20 years. The Clarity Challenge is an important goal because once attained scientists can state with confidence that the trend in clarity loss has been reversed and we are now moving in a clear path of restoring Lake Tahoe to its historic clarity.

Detailed information can be found in the Lake Tahoe Total Maximum Daily Load Technical (Tech) Report [341 Pages - 5.8MB] or, view this slide presentation [29 Pages - 1.4MB] for an overview of the key results of the science, research and monitoring that was conducted for Phase 1.

Phase 2 - Load Reduction and Restoration Planning Analyses

The following questions were answered under this phase of the TMDL:

• What are the options for reducing pollutant inputs to Lake Tahoe?
• What strategy should be implemented to achieve the clarity goal?

The Pollutant Reduction Opportunity (PRO) Report [March 2008: 451 Pages - 11 MB] was a basin-wide analysis which quantified load reductions achievable from implementing various levels of pollutant control efforts. Together the Technical and PRO Reports provide the scientific and technical basis for the development of a recommended implementation strategy capable of achieving the clarity goals.

The recommended TMDL implementation strategy was born from a public/stakeholder input process carried out between the fall of 2007 through the spring of 2008. The process was designed to evaluate the political and social acceptability of the strategy alternatives, with the goal of crafting a preferred strategy that received broad stakeholder support. While the recommended strategy calls for load reductions from the atmosphere, stream channels and the forested areas, it emphasizes FSP reductions through the control, treatment and prevention of urban stormwater. This stems from the conclusions that urban stormwater represents both the greatest source of these pollutants as well as the greatest opportunity to achieve needed load reductions.

The analysis suggests that while broader application of conventional pollutant controls will be beneficial, more innovative and advanced controls are required in order to meet the Clarity Challenge. Examples of such controls include, but are not limited to, alternatives to roadway abrasives applications, advanced roadway sweeping practices using the latest technology and equipment, and conveying stormwater to local or regional facilities featuring enhanced treatment through biological or chemical processes.

More information is available in the Integrated Water Quality Management Strategy (IWQMS) Project Report [395 Pages -9.7MB] or view this slide presentation for an overview of the results and outcomes of the PRO and IWQMS projects [26 slides 1.8MB ].

Phase 3 – Implementation, Progress Assessment and Adaptive Management

Phase 3, the current phase, represents the transition from the science-based policy formation phases to the implementation and performance evaluation phase. Under this phase, the following questions will be addressed:

• How will the strategy be implemented?
• How will progress be assessed?

The recommended strategy will be implemented by local government agencies, as well as state, regional and federal regulatory and land management agencies through their respective programs. NDEP will evaluate the need to establish agreements with implementing agencies in all source categories to implement the TMDL. However, because implementation success is heavily dependent upon the urban stormwater source category, NDEP intends to enter into Memoranda of Agreement (MOA) with the Nevada urban stormwater jurisdictions. These include Douglas and Washoe Counties and the Nevada Department of Transportation. The pollutant load reduction milestone schedules identified in the TMDL will be incorporated into the MOA; however, it should be noted that NDEP is committed to working with our constituencies to ensure that milestones are considered realistic and workable. More information on urban stormwater source category implementation is available on the Lake Clarity Crediting Program webpage.

Through the TMDL Management System, NDEP and the Water Board will collaboratively conduct periodic assessments of progress toward the interim Clarity Challenge and TMDL numeric targets to inform decisions such as whether load allocations, milestones and/or implementation strategies and actions need to be adjusted. These decisions will be accomplished in a collaborative manner between the Lahontan Water Board and NDEP to the extent possible.

In addition, monitoring programs will be a key part of evaluating progress. Condition assessment and effectiveness monitoring at the BMP and catchment scales will serve to verify if actions are having the intended effects. Furthermore, tributary stream and lake clarity status and trend monitoring will continue in order to quantify load reductions at the regional scale and assess the Lake's response to these actions.