GEOCHEMICAL ASSESSMENT OF METALS IN SEDIMENT FROM THE SAN CARLOS RESERVOIR AND THE GILA, SAN CARLOS, AND SAN FRANCISCO RIVERS, ARIZONA

Analyses of the spatial and chronological variation of potentially toxic metals, Cu, Pb, Zn, and Cd, in sediment in cores collected from the San Carlos Reservoir, Ariz. indicated two significant events when release of metals occurred from the Morenci mine. Radioisotope chronometry (⁷Be, ¹³⁷Cs, and ²¹⁰Pb) of sediment from cores provided a good chronological record back to 1959. Prior to 1959, the reservoir was generally dry and the sediment accumulation rate irregular. The chronological record prior to 1959 was based on the ¹³⁷Cs anomaly associated with atmospheric test at the Nevada Test Site and the historical records showing water levels in the reservoir. Data from the reservoir cores recorded two discrete contamination events: one between 1978 and 1983 and a second one about 1957 when the Cu concentration in reservoir sediment exceeded recommended sediment quality guidelines. At these concentrations, Cu in reservoir sediment should have had an adverse effect on sensitive aquatic and benthic organisms. Concentrations of Zn in sediment deposited during the 1957 event also exceeded recommended sediment quality guidelines.

Lead isotope data combined with geochemical data from a 2M HCl-1 percent H₂O₂ leach of selected sediment samples show two discrete populations. One suite of samples represents the dominant sediment load derived from agriculture in the Safford Valley, and the second reflects sediment load derived from the San Francisco River where the Morenci mine is located. Samples containing elevated Cu concentrations have geochemical and Pb isotope signatures that indicate that the Morenci deposit is the likely source of these metals. Comparison of chemical and Pb isotope data from terrace sediment indicate that agricultural practices in the Safford Valley had resulted in an increased sediment load to the Gila River prior to the completion of the San Carlos Reservoir in 1929.