FLOW IN THE UNSATURATED ZONE

In 1981, ground water from a substantial number of municipal and private wells near the Tucson International Airport was found to be contaminated by the organic solvent trichloroethylene (TCE), other organic compounds, and chromium. At this location, a plume of con-taminated ground water, in which TCE concentrations exceed drinking-water standards, is more than 4 mi long and more than 0.50 mi wide. At another location near the Phoenix-Goodyear Airport (formerly Phoenix-Lichfield Airport), concentrations in ground water of TCE as large as 86,000 micrograms per liter (/^g/L), 1,1-dichloroethene (DCE) as large as 241 /^g/L, and chromium as large as 1,300 /jg/L have been reported. In both locations, contamination occurred despite a depth to ground water of about 100 ft. The presence of a thick unsaturated zone does not prevent the eventual migration of a contaminant to the regional ground-water system. Examples such as these have increased public awareness of the need to protect ground-water quality