In the 300 Area of a U(VI)-contaminated aquifer at Hanford, Washington, USA,
inorganic carbon and major cations, which have large impacts on U(VI) transport, change on
an hourly and seasonal basis near the Columbia River. Batch and column experiments were
conducted to investigate the factors controlling U(VI) adsorption/desorption by changing
chemical conditions over time. Low alkalinity and low Ca concentrations (Columbia River
water) enhanced adsorption and reduced aqueous concentrations. Conversely, high alkalinity
and high Ca concentrations (Hanford groundwater) reduced adsorption and increased
aqueous concentrations of U(VI). An equilibrium surface complexation model calibrated
using laboratory batch experiments accounted for the decrease in U(VI) adsorption observed
with increasing (bi)carbonate concentrations and other aqueous chemical conditions. In the
column experiment, alternating pulses of river and groundwater caused swings in aqueous
U(VI) concentration. A multispecies multirate surface complexation reactive transport model
simulated most of the major U(VI) changes in two column experiments. The modeling
results also indicated that U(VI) transport in the studied sediment could be simulated by
using a single kinetic rate without loss of accuracy in the simulations. Moreover, the
capability of the model to predict U(VI) transport in Hanford groundwater under transient
chemical conditions depends significantly on the knowledge of real-time change of local
groundwater chemistry
