Rapid industrial development along Thailand’s eastern seaboard has increased pollution and environmental degradation in this densely populated region of the country. A growing problem is illegal disposal of industrial waste in abandoned borrow pits in many locations, especially in the capital Bangkok and its neighboring provinces. Previous work has identified the most likely illegal dumping sites in Chachoengsao Province. The focus of this work was to establish an impact assessment of potential dumping sites using a geographic information system (GIS) in order to create a patrolling and monitoring program for areas at highest risk. The impact assessment is based upon exposure and sensitivity factors of the illegal dumping sites. The exposure factor attempts to quantify the likelihood of illegal dumping; it includes distances from borrow pits to the nearest villages and highways, as well as the areas of the borrow pits. The sensitivity factor assesses the probability of environmental contamination; it includes site topography, ground permeability, aquifer characteristics, and distances to the nearest rivers. GIS was used to compute distances and to overlay other site characteristics. Variables within exposure and sensitivity factors were weighted based upon previous studies and recent expert analysis to compute exposure and sensitivity indices. Maps were then created in GIS for particular exposure and sensitivity factors, and overall impact assessment maps were constructed to indicate zones with low, medium, and high risk. These assessment maps could be adopted by local and regional governmental agencies, as well as local citizens, as part of preventive measures to conserve the local community and environment. Identification of high-risk areas can contribute to the initiation of a monitoring and patrolling program to prevent illegal dumping and preserve environmental and natural resources.