The Tucson Basin is a semi-arid alluvial basin in southeastern Arizona in which the Santa Cruz River, an ephemeral stream, flows south to north with its flows resulting directly from rainfall. The City of Tucson discharges treated sewage effluent into the bed of the Santa Cruz and to some irrigated farms. Previous investigations indicate that sewage effluent is recharging the Tucson Basin Aquifer with the water spreading horizontally in the Fort Lowell Formation. The ¹⁸0/¹⁶0 ratios determined in water samples by the author support the findings of these previous investigations. Sewage effluent had an average δc0-18 value of -7.9 per mil and water samples from the north Santa Cruz wells had an average δc0-18 value of -9.3 per mil. Up hydraulic gradient, the ¹⁸0/¹⁶0 ratios are lighter indicating that sewage recharge water has mixed with ground water. In the case of one well in the mixed zone, it is calculated that approximately 70 percent of the water comes from sewage recharge and 30 percent from normal ground water. Recharge water spreads horizontally in the Fort Lowell Formation up to two miles on each side of the river. The δc0-18 values of water samples from the south Santa Cruz wells averaged -8.9 per mil and compared closely to the average δc0-18 values for summer flows in the Santa Cruz River of -8.2 per mil.