All groundwater is vulnerable to contamination from surface sources of pollution; however, some areas are more vulnerable than others. The Oklahoma Water Resources Board (OWRB) developed a map showing the relative vulnerability of groundwater in 30 hydrogeologic basins in Oklahoma that are exposed at land surface, and are thus more likely to be susceptible to pollution. Vulnerability was computed with the DRASTIC index method, developed by the U.S. Environmental Protection Agency (EPA). Based on the DRASTIC indices, the hydrogeologic basins were classified in five groups of relative vulnerability: very low, low, moderate, high, and very high. The vulnerability map shows that the alluvium and terrace deposits are most susceptible to pollution of groundwater; the igneous and low yielding bedrock basins are the least susceptible.