National Water-Quality Assessment (NAWQA) Project
VOCs were detected in 14 percent of samples from about 2,400 domestic wells and about 25 percent of samples from about 1,100 public wells. Concentrations were greater than the USEPA maximum contaminant levels in less than 2 percent of wells.
Full study can be found at the Volatile Organic Compounds (VOC) Project Website by accessing the USGS Circular 1292.