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City of Lino Lakes SWMP | February 8, 2013 Draft | 54 Nonpoint Sources Nonpoint source pollution is caused by rainfall or snowmelt moving over and through the ground. R unoff picks up and carries away natural and human -made pollutants, finally depositing them into lakes, rivers, wetlands, coastal waters and gr ound waters. Because of the nutrient (phosphorus) impairments of the Chain of Lakes, phosphorus is the most important nonpoint source pollutant in Lino Lakes. Figure 23 shows the phosphorus loading by subwatershed for existing conditions (2005) and a F ull B uild -out S cenario (FBO) in Lino Lakes as modeled for the Lino Lakes Resource Management Plan (RMP ) usin g PLOAD (pollutant loading model). PLOAD uses a combination of precipitation and anticipated runoff from the land cover by subwatershed to approximate the volume of water delivered to the waterbody in question. A gridded surface was developed in a Geogr aphic Information System (GIS) based on the MN Hydrology Guide (SCS, 1992) to determine the annual precipitation, evaporation, and runoff by watershed. Each land use was assigned an event mean concentration (EMC), which serves to estimate the loading rate of phosphorus from runoff. Using this data, the total phosphorus loading by subwatershed can be estimated. Additionally, t he Rice Creek Watershed District (RCWD ) has created a P8 (Program for Predicting Polluting Particle Passage through Pits, Puddles and Ponds) model for the entire City . This model was created during the development of the Lino Lakes Resource Management Plan (RMP ) and contains a great deal more detail than the PLOAD model. This model was used to assess nutrient loading to wetlands (Figure 13 ) and for the Lino Lakes Chain of La kes Nutrient TMDL (Figure 24 ). This model could also be used to evaluate future water quality projects and development. P8 model inputs are defined by a series of w atersheds and devices that can be used to describe ponds, infiltration basins and other BMPs. Watershed areas, imperviousness and curve number s in the P8 model were defined based on the XP -SWMM H ydrologic and H ydraulic model created in 2007 for the RMP (see also Floodplain portion of Section 3.8 ). Additionally, device inputs such as storage and outlet configurations were created from data in the XP -SWMM model. Fu ture water quality modeling efforts should utilize the P8 model to the greatest extent possible. The current total phosphorus budgets for the C hain of L akes are identified in the Lino Lakes Chain of Lakes Nutrient TMDL (Draft, in process). The draft report includes phosphorus budgets that are based on an average of the data for growing seasons for 2002 -2004.