Eastern Bernalillo County is a historically rural, mountainous area east of Albuquerque, New Mexico. Historically, the primary economic activity consisted of subsistence farming and ranching and support of these activities from small communities. During the last 40 to 50 years, however, the area increasingly has become the site of residential developments. Homes in these developments typically are on 1- to 2-acre lots and are serviced by individual wells and septic systems. Between 1970 and 1990, the population of the area increased from about 4,000 to more than 12,000, and housing units increased from about 1,500 to more than 5,000. Results of analysis of water samples collected from 121 wells throughout eastern Bernalillo County in 1990 indicated that (1) total-nitrate concentrations in 10 samples exceeded the U.S. Environmental Protection Agency national primary drinking-water regulation maximum contaminant level of 10 milligrams per liter as nitrogen; (2) total-nitrate concentrations may be related to the length of time an area has been undergoing development; and (3) large dissolved-chloride concentrations may result from geologic origins, such as interbedded salt deposits or upward movement of saline ground water along faults and fractures, as well as from domestic wastewater disposal. Ground water throughout eastern Bernalillo County was assessed to be highly susceptible to contamination by overlying domestic wastewater disposal because (1) soils in more than 95 percent of eastern Bernalillo County were determined by the U.S. Department of Agriculture Natural Resources Conservation Service to have severe limitations for use as septic-system absorption fields and (2) a fractured carbonate geologic terrane, which typically has large secondary permeability and limited sorption capacity, is at the surface or underlying unconsolidated material in 73 percent of the area. Ground-water-level rises following an episodal precipitation event during July 22-27, 1991, were large and rapid, particularly in the fractured carbonate terrane. This response indicates a significant hydraulic connection between the land surface and the water table.