Evaluation of ground-water quality in the Cortaro area, Arizona

Chemical analyses of samples from 120 wells in the Cortaro area show that most of the area is underlain by ground water of a quality suitable for most agricultural and domestic purposes. Some wells near the Rodger Road sewage treatment plant and a septic tank area west of the Santa Cruz River, however, yield water with concentrations of nitrate exceeding the U.S. Environmental Protections Agency maximum contaminant level of 45 milligrams per liter for public water supplies. In general, concentrations of nitrate and boron significantly exceed background levels for the two ions in ground water downgradient of the Rodger Road sewage treatment plant. The high nitrate and boron concentrations are indicative of sewage-effluent recharge. The areal distribution of nitrate, boron, and other chemical constituents suggests that Santa Cruz River underflow and the recharged sewage effluent do not follow the Santa Cruz River channel but rather appear to follow high transmissivity zones in the aquifer. Records of historic ground-water quality indicate that the chemical character of ground water in the Cortaro area has changed with time. In general, ground water has degraded in quality since 1944 beneath the Santa Cruz River flood plain; whereas, east of the river the ground-water quality has remained relatively unchanged. The changes in ground-water quality seem largely related to discharge of sewage effluent.hydrology collectio