Ground-water flow velocities and dispersivities determined using Chlorine-36 data from 29 years of archived water samples, Snake River plain aquifer, Idaho

The Idaho National Engineering Laboratory (INEL) overlies part of the Snake River Plain aquifer in southeastern Idaho. Since 1953, wastewater containing low-level radiochemical constituents including chlorine-36 ({sup 36}Cl) has been discharged to the environment at the INEL through disposal wells and ponds. The U.S. Geological Survey (USGS), in cooperation with the U.S. Department of Energy, maintains a monitoring-well network to determine hydrologic trends and to describe the fate of contaminants. Since 1966, the USGS annually has archived at least one suite of water samples from these wells. Concentrations of {sup 36}Cl in selected samples from the archive library were determined by accelerator mass spectrometry. First-arrival times of {sup 36}Cl at wells downgradient from selected facilities were determined and preliminary flow velocities of 1.8 to 3 meters per day were calculated. Longitudinal and transverse dispersivities of 100 and 90 meters, respectively, were determined from reconstruction of the {sup 36}Cl plume through time.