Water quality in the main drinking-water source aquifers of Oahu was assessed by a one-time sampling of untreated ground water from 30 public-supply wells and 15 monitoring wells. The 384 square-mile study area, which includes urban Honolulu and large tracts of forested, agricultural, and suburban residential lands in central Oahu, accounts for 93 percent of the island's ground-water withdrawals. Organic compounds were detected in 73 percent of public-supply wells, but mostly at low concentrations below minimum reporting levels. Concentrations exceeded drinking-water standards in just a few cases: the solvent trichloroethene and the radionuclide radon-222 exceeded Federal standards in one public-supply well each, and the fumigants 1,2-dibromo-3-chloropropane (DBCP) and 1,2,3-trichloropropane (TCP) exceeded State standards in three public-supply wells each. Solvents, fumigants, trihalomethanes, and herbicides were prevalent (detected in more than 30 percent of samples) but gasoline components and insecticides were detected in few wells. Most water samples contained complex mixtures of organic compounds: multiple solvents, fumigants, or herbicides, and in some cases compounds from two or all three of these classes. Characteristic suites of chemicals were associated with particular land uses and geographic locales. Solvents were associated with central Oahu urban-military lands whereas fumigants, herbicides, and fertilizer nutrients were associated with central Oahu agricultural lands. Somewhat unexpectedly, little contamination was detected in Honolulu where urban density is highest, most likely as a consequence of sound land-use planning, favorable aquifer structure, and less intensive application of chemicals (or of less mobile chemicals) over recharge zones in comparison to agricultural areas. For the most part, organic and nutrient contamination appear to reflect decades-old releases and former land use. Most ground-water ages were decades old, with recharge dates ranging from pre-1940 to the present, and with most dates falling within the 1950s to 1980s time span. Several widely detected compounds were discontinued as long ago as the 1970s but have yet to be flushed from the ground-water system. Although large tracts of land in central Oahu have been converted from agriculture to residential urban use since the 1950s, water quality in the converted areas still more closely reflects the former agricultural land. It appears to be too early to detect a distinct water-quality signature characteristic of the newer urban use, although several urban turfgrass herbicides in use for just 10 years or so were detected in monitoring wells and may represent early arrivals of urban contaminants at the water table.