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NPS Assessment

The purpose of this study is to identify and evaluate existing scientific data to quantify nonpoint source (NPS) phosphorus loads in the Spokane River/Lake Spokane watershed. Where data are not available, the study will determine possible data gaps and suggest studies for quantifying them.

This study a three phase effort to identify and quantify NPSs into the Spokane River and Lake Spokane, to identify best management practices (BMPs) to address the NPSs, to evaluate the cost-effectiveness and longevity of the identified BMPs, and to prepare an implementation plan for reduction of NPS based on the selected BMPs.

Nonpoint Source Advisory Committee Participants

  • United States Environmental Protection Agency
  • Washington Department of Ecology
  • Spokane County
  • Spokane County Conservation District
  • City of Spokane
  • City of Spokane Valley
  • Liberty Lake Water and Sewer District
  • Spokane Tribe
  • Idaho Department of Environmental Quality
  • Kootenai County
  • Panhandle Health District
  • City of Coeur d'Alene
  • Coeur d'Alene Tribe
  • Spokane River DO TMDL Oversight Committee
  • Sierra Club - Center for Justice
  • Lands Council
NPS Study Basin Compressed
The Spokane River and Lake Spokane are listed on the 303 (d) list for impairment of dissolved oxygen (DO). Modeling indicates that the main cause of the DO depletion is excess algae in the water bodies. The algae growth is primarily caused by excessive nutrients, with phosphorus being the growth limiting nutrient.

Spokane River DO Total Maximum Daily Load

The Spokane River DO Total Maximum Daily Load (TMDL) process has established a phosphorus discharge target of 10 mg/L for municipal wastewater treatment plants. The low phosphorus concentration is expected to help reduce excessive algal productivity that causes DO concentrations to fall below the water quality standard. The Foundational Concepts for the Spokane River DO TMDL will begin guiding TMDL implementation, and meeting the phosphorus target will require a combination of improved point source wastewater treatment technology and reduced non-point source pollutant loads.

The study area of the NPS study includes the watersheds associated with Coeur d’Alene Lake, the Spokane River and its tributaries Hangman Creek and Little Spokane River, and Lake Spokane.
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