Estimating point and non-point source nutrient loads in the Saginaw Bay watersheds

• Multiple databases were used to estimate N and P loading potential for 1987–2002.
• Nonpoint sources contributed about 75 percent of the total P load to Saginaw Bay.
• Total amount of N and P loading declined between 1987 and 2002.
• Nonfarm applications of N and P fertilizers increased for the same period.
• The zip code level data identified more critical P loadings than the county data.

Databases of point sources including combined sewer overflows (CSOs) were acquired from the governmental agencies to map the occurrences and magnitude of the CSOs. Multiple databases of land use, topography, hydrography, soils, and agricultural statistics were used to estimate nonpoint source loading potential in the Saginaw Bay Basin, Michigan. Animal manure production was computed from tabulations of animals by 5-digit zip code area for the census years of 1987, 1992, 1997, and 2002. Fertilizer applications for both urban and agricultural land uses were calculated from county fertilizer estimates for the same periods. Results indicate that point sources from municipalities, industrial sectors and business entities contribute approximately 25% of the total phosphorus load to Saginaw Bay, with the remainder being accounted for by nonpoint source contributions. While the total amount of nutrients (N and P) from animal manure and fertilizer applications and atmospheric deposition declined in the Saginaw Bay Basin, fertilizer applications in non-farmland increased significantly. Estimation of nutrient loading potential at 5-digit zip code level reveals more detailed spatial variation and critical areas of nutrient loading than county level data for implementation of targeted water quality programs.