Oxygen and hydrogen isotope data acquired from 114 water samples collected at monitoring wells in the Baldwin Park Operable Unit, an Environmental Protection Agency Superfund Site in the San Gabriel Valley Groundwater Basin, reveal evidence for mixing of variable amounts of artificial recharge in the basin. Two stages of sample collection were carried out, (1) just before the Southern California rainy season and (2) at its end, to determine the influence of winter precipitation in the recharge process. No differences were observed in the isotopic compositions of waters collected during these stages of sample collection, suggesting insufficient recharge rates to effect change in the isotopic composition of the aquifer waters. Five possible sources of recharge are evaluated: (1) the Colorado River aqueduct (d¹⁸O = -7.0, dD = -70), (2) the California State Water project (d¹⁸O = -8.3, dD = -60), (3) the San Gabriel River (d¹⁸O = -6.3, dD = -45), (4) the Santa Fe Reservoir evaporation basin (d¹⁸O = -1.3, dD = -20), and (5) the product of tertiary treatment (d¹⁸O = -4.2, dD = -25). Most groundwater d¹⁸O values are between -8 and -6 with accompanying hydrogen isotope values (dD values between -53 to -40) that result in a data array that plots along the meteoric water line. These data indicate: (1) minimal evaporation for the recharge waters and (2) the source for most groundwaters in the aquifer is variably mixed local San Gabriel River water and State Water Project water. Aquifer waters from shallow depth collected from wells near the Santa Fe Lake evaporation basin have higher isotopic compositions (d¹⁸O = -5.5 to -4.5, dD = -40 to -35) that plot along a mixing line between waters from San Gabriel River and Santa Fe Lake. A zone of low-¹⁸O water (d¹⁸O ~ -8) is centered at a depth of approximately 200 meters, suggesting these are unmixed State Water Project waters. In general, waters from above 125 feet below the ground surface have higher dD and d¹⁸O values than those below that depth. There was no isotopic evidence for the recharge of Colorado River aqueduct water. Tertiary treatment water may have played a minor role in the recharge of the San Gabriel Valley aquifer.