Phosphorus (P) inputs to human-impacted watersheds have more than doubled over the last century in response to the use of fertilizers, detergents, and P additives in a range of products, from animal feed to motor oil. Although mass balance studies consistently show P inputs exceeding outputs in human-impacted areas, the forms and relative magnitudes of legacy P accumulation are still not well understood. In the present work, we have developed a parsimonious, process-based model, ELEMeNT-P, that pairs soil P dynamics with both erosion processes for simulation of surface P transport and a travel time-based approach for simulation of transport and retention along subsurface pathways. To drive the model we have developed a more than 100-year trajectory of watershed P inputs to the Grand River Watershed, Canada’s largest watershed draining directly to Lake Erie. Using this long-term input trajectory, we attempt to answer the following questions: (1) What is the magnitude of legacy P accumulation across the watershed? (2) What are the most important landscape reservoirs for legacy P accumulation, e.g. upland soils, reservoirs, groundwater? (3) To what extent is legacy P impacting current water quality? (4) How long will it take to achieve desired reductions in P loading?
Results/Conclusions
Our results show that since 1900, the Grand River Watershed has served as a net sink of P, with an estimated accumulation of more than 480 ktons of P across all landscape compartments. This storage total for legacy P is more than 16 times greater than the approximately 30 ktons P that have left the watershed via riverine output since 1900. While the majority of post-1900 legacy P (~80%) has accumulated in soil, significant quantities (~12%) have also accumulated in reservoirs and riparian zones, where it may be more available for remobilization under changing climate regimes. Our results also suggest that while legacy P sources will continue to persist as important sources of P to surface waters, better management of livestock manure and even greater efficiencies in wastewater P removal provide a means of achieving meaningful reductions in P loading. Future simulations suggest that while 40% reductions in P loading in Lake Erie watersheds may be possible under aggressive management scenarios, it may take on the order of 50 years for policy goals for ecological recovery of Lake Erie and other affected water bodies to be achieved.