Challenges of accounting for non-point sources in a TMDL evaluation

Recent implementation of nutrient total maximum daily loads (TMDLs) for several East Coast water bodies has left many local governments ‘ahead of the curve,’ being forced to develop compliance strategies with limited TMDL knowledge, and with guidance issued from State and Federal environmental agencies still in the early stages. Accounting for nutrient contributions from non-point sources is the greatest area of uncertainty in performing a TMDL evaluation and yet the non-point source contributions for many water bodies greatly outweigh the point source contributions. The goal of this project was to establish a method to allow local governments to accurately account for non-point source nutrient contributions in order to effectively make planning decisions necessary to achieve the water quality goals of the TMDL. Examining modeling techniques used by three different municipalities, it was found that critical to accurate non-point source modeling was existing water quality data, consistent modeling assumptions, and continuing water quality monitoring to improve model assumptions. Optimal methods for conducting non-point source modeling will vary depending on the location, existing water quality data, water quality monitoring resources, and comfort levels of the municipality.

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