Geohydrologic test drilling was conducted throughout Dade County to describe the chemical characteristics of water from geohydrologic units in the surficial aquifer system. Water quality analysis of samples collected from the test wells completed in central Dade County indicates that the Biscayne aquifer (unit A), the upper clastic unit of the Tamiami Formation (unit B), and parts of a limestone, sandstone, and sand unit of the Tamiami Formation (unit C) have been effectively flushed of residual seawater, and contain primarily calcium bicarbonate groundwater. The lower parts of unit C and the lower clastic unit of the Tamiami Formation (unit D) primarily contain calcium-sodium bicarbonate or sodium bicarbonate type water. An analysis of variance test indicates that water in units A and B is not significantly (0.05 probability level) different in composition. Mean concentrations of dissolved solids, sodium, and chloride are significantly different between units A and C, whereas mean concentrations of dissolved solids, calcium, sodium, and chloride are significantly different between units A and D. Groundwater in the surficial aquifer system in northwestern Dade County is more mineralized than groundwater that occurs elsewhere in the county (except in coastal areas affected by saltwater). An analysis of variance test indicates that mean concentrations of dissolved solids, sodium, and chloride at sites in northwestern Dade County, developed in units A and C, are significantly different from the mean concentrations of these constituents at sites throughout the rest of the county. Water in this part of the surficial aquifer system is similar to highly mineralized water found in western Broward County. However, in northwestern Dade County, the groundwater has been diluted to a greater extent by less mineralized recharge water. Overall, groundwater in the four major geohydrologic units of the surficial aquifer system of Dade County is suitable for most uses. Maximum concentrations of sodium, chloride, color, fluoride, iron, and dissolved solids in some areas of Dade County, however, exceed maximum contaminant levels established by the Florida Primary and Secondary Drinking Water Regulations standards. (Author 's abstract)