Groundwater was contaminated with petroleum hydrocarbons (toluene and xylenes) and chlorinated solvents (PCE, TCE) at a Korean national groundwater monitoring station (NGMS). The shallow groundwater was also very alkaline (pH 11–12) with a very high electrical conductivity (EC) of over 3000 μS cm−1. Extensive diagnostic investigation of potential contamination sources and groundwater and stream water quality for the whole surrounding area revealed that the groundwater contamination was spatially restricted to a small area of the monitoring station. Detailed investigation of the small area uncovered the existence of many waste asphalt emulsion containers and various road construction waste materials within a 15 m distance from the monitoring station. All relevant diagnostic and detailed investigation results, including contamination history (coincidence of road construction period with contamination period), vertical EC distribution and surrounding groundwater and stream water quality, indicated that the shallow groundwater contamination of the NGMS was caused by illegal dumping of the road construction waste on the surface at the site. A pulse input of liquid waste containing solvents and petroleum hydrocarbons is suspected as the most plausible cause for the contamination. Sustained high pH of groundwater indicates the possible existence of raw cement product in the subsurface, which requires further investigation.