Spatially Explicit Modeling of Land Use Specific Phosphorus Transport Pathways to Improve TMDL Load Estimates and Implementation Planning

Diffuse pollution from urban stormwater and agricultural runoff are among the leading causes of water pollution in the USA. A process-oriented, stakeholder-driven research approach was implemented in the small heterogeneous watershed of St. Albans Bay, Vermont to model the relative load of phosphorus from all sources, including diffuse transport pathways, and compared to goals and assumptions outlined by a Total Maximum Daily Load (TMDL) developed for phosphorus in Lake Champlain. Mass-balance and dynamic landscape simulation models were used to describe the distribution of the average annual phosphorus load to streams (10.57 t/year) in terms of space, time, and transport process. The majority of the phosphorus load comes from two subwatersheds dominated by clay soils, Stevens and Jewett Brooks. Dissolved phosphorus in surface runoff from the agricultural landscape, driven by high soil phosphorus concentrations, accounts for 41% of the total load to watershed streams. Direct discharge from farmsteads and stormwater loads, primarily from road sand wash-off, are also significant sources. Results reported in this study could help target watershed interventions both in terms of the types and locations of recommended best management practices (BMPs). The study offers an approach to attaining TMDL diffuse pollution targets in a cost-effective and participatory manner and could be replicated for other TMDL processes around the country.