Lawrence Berkeley National Laboratory

This study of the hydrogeology of Chicken Creek Canyon was conducted by the Environmental Restoration Program (ERP) at Lawrence Berkeley National Laboratory (LBNL). This canyon extends downhill from Building 31 at LBNL to Centennial Road below. The leading edge of a groundwater tritium plume at LBNL is located at the top of the canyon. Tritium activities measured in this portion of the plume during this study were approximately 3,000 picocuries/liter (pCi/L), which is significantly less than the maximum contaminant level (MCL) for drinking water of 20,000 pCi/L established by the Environmental Protection Agency.There are three main pathways for tritium migration beyond the Laboratory s boundary: air, surface water and groundwater flow. The purpose of this report is to evaluate the groundwater pathway. Hydrogeologic investigation commenced with review of historical geotechnical reports including 35 bore logs and 27 test pit/trench logs as well as existing ERP information from 9 bore logs. This was followed by field mapping of bedrock outcrops along Chicken Creek as well as bedrock exposures in road cuts on the north and east walls of the canyon. Water levels and tritium activities from 6 wells were also considered. Electrical-resistivity profiles and cone penetration test (CPT) data were collected to investigate the extent of an interpreted alluvial sand encountered in one of the wells drilled in this area. Subsequent logging of 7 additional borings indicated that this sand was actually an unusually well-sorted and typically deeply weathered sandstone of the Orinda Formation. Wells were installed in 6 of the new borings to allow water level measurement and analysis of groundwater tritium activity. A slug test and pumping tests were also performed in the well field.