The relationship between transportation and air quality has been widely studied while little did we know about transportation and water quality in economic papers, despite its importance emphasized by the Clean Water Act. This study estimates the effect of annual average daily traffic (AADT) on lead pollution in surface water across the United States. The results show that one million more vehicles within 2 miles of the water will lead to 1.95 ug/L increase in lead concentration and the effect is mainly localized. The total estimated increase in lead in water due to heavier traffic amounts to 1.33 ug/L from 2011-2017, accounting for about 45% of the sample mean lead concentration. This paper then confirms that the mobilizing effect dominates the diluting effect, meaning the heavy storm events will result in worsening highway runoff quality for lead. This study also indicates that the effect is larger in rural areas than urban areas. Finally, policy implications are discussed to deal with the negative externalities brought by the transportation sector.