Phosphorus run-off from urban lawn care is a significant water quality concern in many US suburban communities. Many state and municipal governments in the USA adopt urban fertilizer laws or ordinances that prohibit the use of phosphorus-containing lawn fertilizers. This study assesses the long-term water-quality impacts of two phosphorus-reduction scenarios in a suburban watershed in Central New Jersey. While Scenario I reduces the phosphorus fertilizer application rates by 25%, Scenario II completely eliminates phosphorus fertilizer for urban lawns as required by such laws, rules or ordinances. The long-term water-quality impacts were estimated using the Soil and Water Assessment Tool and then compared to the total maximum daily load (TMDL) water-quality standards using the load duration curve approach. Scenario I will decrease total phosphorus (TP) load to streams by 15.34%. Scenario II achieves greater reduction in TP load to streams than Scenario I, but results in excessive nitrogen run-off and violation of the TMDL standard for total nitrogen. While many regulatory measures focus on reducing phosphorus run-off by prohibiting the use of phosphorus in urban lawn fertilizer, they should be carefully implemented to balance the nutrient needs to maintain healthy lawns and avoid such unintended consequences.