Project Specifics

The California Regional Water Quality Control Board (Water Board) has designated Lake Tahoe as an Outstanding National Resource Water under the US Federal Clean Water Act. Lake Tahoe has been losing its famed clarity since the late 1960s, and the Water Board and Nevada Division of Environmental Protection jointly created a phased Lake Tahoe total maximum daily load (TMDL) to address pollutants of concern.

As part of the Phase II TMDL, public and private proponents of redevelopment projects needed to design strategies to control loads of nutrients and fine particulates from urbanized regions of the Tahoe Basin. They also needed to evaluate the performance of these strategies. The tool developed to meet this need is the Pollutant Load Reduction Model (PLRM). The first version was released in 2009, and the second version was released in 2015.

Geosyntec Scope of Services

Geosyntec Consultants and Northwest Hydraulic Consultants jointly developed PLRM, a pollutant load model that can be used to estimate pollutant load reductions associated with water-quality-improvement projects in the Lake Tahoe Basin.

To do this, the team took the following steps:

• Evaluated model options and developed a load-reduction modeling framework that integrates stormwater best management practices (BMPs)
• Developed technical algorithms
• Wrote software code to implement, test, and release the model for public use

The PLRM is a custom version of the USEPA SWMM 5.1 stormwater model that is linked to a Tahoe-specific database containing preprocessed soils, land uses, meteorological data, and default input values. To evaluate project alternatives, the tool uses continuous simulations of pollutant load generation and reduction associated with hydrologic and pollutant source controls and centralized treatment controls. Improvements and enhancements added in version 2 include a built-in, open-source Geographic Information System (GIS) engine that allows users to intersect drainage areas, soils, and land uses within the model. This version allows users to automatically create hydrologic elements without the need for additional GIS software and includes enhanced results reporting and scenario comparisons.

Notable Accomplishments

Version 1 of PLRM has been used extensively by state and local agencies in the Tahoe Basin to support stormwater BMP planning and TMDL compliance assessment. PLRM has become the fundamental tool for estimating load-reduction credits for meeting wasteload allocations for fine sediment particulates. Version 2 includes refinements to the input geodatabase and user interface, updates to the user documentation, upgrades to the underlying SWMM 5.0 engine to version 5.1, and a GIS toolbox that significantly reduces model setup time. The model has been rigorously tested by the project team and is now released as an open-source tool for public use.

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