The Bisbee-Naco Area is in southeastern Arizona and northeastern Sonora, Mexico. Annual rainfall averages about 17 inches. The basin fill is the principal aquifer and supplies about 95% of all water for domestic purposes. The total groundwater pumped in 1985 was about 6,500 acre/ft. Of that amount, 2,200 acre-ft was used for domestic use and the rest was used for irrigation and livestock. Domestic use is expected to increase 20% by the year 2000. Water levels at the Bisbee well field have remained virtually unchanged since 1953, but water levels have declined about 25% in areas east and southeast of the well field. The native groundwater in the area is a calcium bicarbonate type and generally is suitable for domestic use. Groundwater down-gradient from a mine tailings pond contains 650 to 850 mg/l of sulfate. Recharge occurs naturally through direct infiltration or rainfall along the mountain fronts and through subsurface inflow from adjacent areas. Some water is recharged through direct infiltration from a mine tailings pond, sewage ponds, septic systems, and urban runoff. The potential for contamination exists from mine tailings, sewage, and urban runoff. No other sources of drinking water are readily available in the immediate area. Potential alternate sources of drinking water are the aquifers underlying Sulphur Spring Valley and San Pedro River valley. (Author 's abstract)