Measuring Uranium Distribution within Samples from a Contaminated Fractured-rock Aquifer in Livermore, California

Abstract

Uranium is a heavy metal and radionuclide reported in groundwater worldwide. Dissolved and colloidal uranium at high concentrations is a threat to human and ecosystem health due to its radioactivity and chemical toxicity. The geochemical distribution of uranium and its co-association with major and trace elements (Al, Si, P, Ca, Fe, Pb) was studied at the Lawrence Livermore National Laboratory (LLNL) munitions test site (Livermore CA, USA). Historically, the Firing Facility at Site 300 within LLNL released a total of 1.6 × 10–6 GBq (4.3 × 10–8 Ci) of 234U, 1.3 × 10–7 GBq (3.6 × 10–9 Ci) of 235U, and 1.3 × 10–5 GBq (3.4 × 10–7 Ci) of 238U in particulate form. Cores, below and hydraulically downgradient of the site were collected from the Neroly sandstone aquifer from below the water table at 15 m to over 150 m. Despite previous high-explosive activity, radiation levels have been measured at 1% of the allowable drinking water level for alpha particles at the site. To access the possible sequestration and sorption capacity of the aquifer, sediments were collected for a six step selective sequential chemical extraction (SSE) to operationally define pools of U sinks. The SSE showed U was mainly bound in silicates (4.50 µg g-1), with approximately 10% associated with organic matter and sulfides (0.472 µg g-1). It was also observed, to a lower extent, in Fe and Mn oxyhydroxides and phosphate, attributed to U originating from the munition activity. Selective sequential extractions showed U was mostly bound in silicates, and generally immobile; and approximately 13% of the total U was retained in pools that could undergo geochemical release to porewaters. Therefore, we hypothesize that the uranium in groundwater is less than the retention capacity of the aquifer, and that the sandstone has excess retention capacity to immobilize uranium. Characterization of the retention capacity is important for quantifying the amount of uranium susceptible to re-mobilization due to variations in water chemistry and by determining sufficient retention capacity, assists in prediction of off-site transport of uranium.