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Competency Area 6: Watershed hydrology AEM

PO 59. Explain the pollutant delivery process, and describe the relationship of nutrient budgets and total maximum daily loads (TMDL) to Non Point Source pollutant loading.

Pollutant delivery requires a pollutant to be 'available' when water is moving over or through the soil and landscape. Soil erosion is an example of soil being available (by dislodgement) when water is flowing. Generally, pollutants that are readily soluble in water and that have a low soil-water adsorption partition coefficient (do not readily attach to soil) can be transported in either surface runoff or in water percolating through the soil. Nitrate-nitrogen is an example of this and can thus easily be leached to groundwater. Pollutants that have high soil-water adsorption partition coefficients (easily attach to soil) are more readily transported in surface runoff, but are typically removed as water infiltrates and percolates through the soil. Phosphorus would be an example of a pollutant with a high adsorption partition coefficient. However, soils with macropores may have a limited ability to absorb pollutants with high adsorption partition coefficients because of the limited interaction of the percolating water with the soil particles. Thus, the importance of nutrient budgets is to minimize nutrient pollutant availability when water is flowing over or through the landscape. It is typically easier to manage nutrients, than to manage excess flowing water. When nutrients are applied in excess of crop nutrient requirements, the opportunity increases for pollutants to be available and transported off-site to other receiving water bodies. The off-site transport and diffuse loss of pollutants from storm water runoff (from agriculture and other sources) is termed nonpoint source pollution.

The total maximum daily load (TMDL) is the maximum amount of a pollutant that a water body can receive and still meet water quality standards. Federal and state regulatory agencies establish the maximum allowable point and nonpoint source loading for designated stream reaches, based on characteristics of stream flow, ability of the stream to process pollutants, and designated uses. Nutrient budgets or developing and implementing comprehensive nutrient management plans (CNMP's) is a methodology for agricultural producers to minimize the off farm loss of excess available nutrients to reduce nutrient pollutant loads to designated receiving streams and waterbodies. With implementation of CNMP's, nutrient losses and nonpoint source pollutant loads can be reduced, and TMDL targets can be achieved.