The nitrogen and oxygen isotopic compositions of nitrate in pore water extracts from unsaturated zone (UZ) core samples and groundwater samples indicate at least four potential sources of nitrate plumes in groundwaters at the USDOE Hanford Site in southcentral Washington. These nitrate sources are microbially produced nitrate from the soil column (delta15N of 4 to 8 o/oo, delta18O of -9 to 2 o/oo), 2) nitrate in buried caliche layers (delta15N of 0 to 8 o/oo, delta18O of -6 to 42 o/oo), 3) nitric acid in low-level disposal waters (delta15N approx. equal to 0 o/oo, delta18O approx. equal to 23 o/oo ), and 4) co-contaminant nitrate in high-level radioactive waste from plutonium processing (delta15N of 8 to 33 o/oo, delta18O of -9 to 7 o/oo). The isotopic compositions of nitrate have been analyzed from 91 groundwater wells with concentrations up to 763 mg/L NO3-. Estimates based on isotopic mixing indicate that even the wells with the highest nitrate concentrations contain at least 30% of nitrate from natural sources in the UZ. These data indicate that major nitrate plumes resulted from flushing nitrate out of the UZ during disposal of low-level wastewater and nitric acid, rather than migration of the nitrate associated with high-level radioactive UZ contamination.