At sites of groundwater contamination. predictions of contaminant behavior and evaluation of remedial strategies depend on identification and characterization of the geochemical processes affecting contaminant migration. Heavy-metal loadings to waters and sediments by leachate from the Golbasi waste disposal site in Ankara, Turkey, have been evaluated quantitatively using hydro-geochemical modeling. The groundwater of the waste disposal area. characterized by high concentrations of Ca, K, Cl, Cd, Pb, Zn, Cu, B, Ni, SO4, and NO3, contaminates the waters and sediments in the down-gradient area, Eymir Lake and a swamp along the flow path. An advective mass transport duration is similar to15 years for unretarded contaminants to move from the waste disposal well area to the southern shoreline of Eymir Lake. Mixing calculations suggest that the down-gradient groundwater is formed by mixing of 40 to 72% up-gradient groundwater and 28 to 60% waste-disposal-area groundwater, as well as Eymir Lake surface-water ion concentrations formed by mixing of down-gradient groundwater (3%-25%) and swamp-water ion concentrations (75-97%) along the flow path. An evaluation of the changes in concentration of trace ion-related precipitation/dissolution and exchange reactions between water and sediments for the formation of both Eymir Lake surface-water composition and the down-gradient groundwater composition indicate considerable trace-ion content of the clays (exchangers) and related reactions in the system. These results suggest that the amounts of contaminants removed from or added both to the down-gradient groundwater and to surface waters, through mixing, dilution, and evaporation processes are. rather small. The amounts of ions in the waters at the present stage of the contamination process are predominantly governed by exchange and dissolution/precipitation reactions.