Fertilizer runoff from healthy, well maintained lawns is minimal when the ground is not saturated or frozen.
Runoff from lawns is typically 5% of precipitation if the soil is not saturated of frozen and the lawns are not maintained on severe slopes. Nutrient runoff in urban areas appear to be related to runoff volume which can be reduced by maintaining dense lawn and possibly by creating swales or berms between vegetated areas. Nitrogen and Phosphorus should not be applied to lawns when soils are saturated or frozen or when plants are not actively growing. December-January 2012, page 18, Tennessee Turfgrass.