A Method for Source-load Allocation of Nutrients in Agricultural Watersheds

Identification of pollutant sources is critical to solving water resource contamination problems. Non-point sources of agricultural pollution provide substantial challenges to quantifying and allocating the sources of contaminants to streams. A method is presented for identifying the spatial variability of nitrogen and phosphorus sources and allocating proportional responsibility for source-reduction. The method is applied to data at scales ranging from hydrologic regions (2-digit hydrologic accounting units) of the Mississippi drainage basin to the public land survey grid in two small (14-digit) watersheds. A mass balance of nutrient sources and losses is estimated using georeferenced data derived from national to local digital data bases. Nitrogen excess is estimated by balancing sources associated with inorganic fertilizer, manure, crop fixation, mineralization of organic matter, and atmospheric redeposition of ammonia with losses from crop harvest, plant senescence, denitrification, and volatilization of manure and inorganic fertilizer. Phosphorus sources from inorganic fertilizer and manure are balanced with losses due to crop harvest. Allocation in regional units allows targeting of major pollutant source areas while smaller aggregation areas define greater ranges of source-loads useful for specific allocation. Manure sources control the distribution of excess nutrients at many scales, particularly in watersheds with uniform cropping systems. Absolute values of excess N sources provide substantially different allocation patterns than proportional values of total source-loads. Selection of aggregation scale is critical to source-load allocation needed to define TMDLs, monitor loads, and establish water-quality remediation strategies. >http://www.nstl.gov/pubs/burkart/trends/index.html</a>