The U.S. Geological Survey (USGS) is developing SPARROW models (SPAtially Related Regressions On Watershed Attributes) to assess the transport of contaminants (for example, nutrients) through the Pacific drainages of the United States (the Columbia River basin; the coastal drainages of Washington, Oregon, and California; the Klamath River basin; the Central Valley of California, and the west slopes of the Sierra Nevada Mountains). SPARROW relates instream water quality measurements to spatially referenced characteristics of watersheds, including contaminant sources and the factors influencing terrestrial and aquatic transport. The number of people with on-site wastewater treatment (primarily septic tanks) is a potential factor affecting nutrient delivery to streams. The spatial data set “Population with On-Site Wastewater Treatment within the Pacific Drainages of the United States (2010)" represents the number of people that did not have access to centralized municipal wastewater treatment in 2010. This data set was created by disaggregating census block populations to developed land and retaining those populations that were outside of the service boundaries for municipal wastewater treatment plants.