Numerical reproduction of the seasonal variation in dissolved uranium in Lake Biwa

Abstract

The seasonal variation in dissolved uranium (DU) concentrations in the epilimnion of Lake Biwa, the largest lake in Japan, was successfully reproduced by geochemical calculations. The DU in Lake Biwa's epilimnion was calculated using an ion-exchange-equilibrium model. The model used the water-DU simple mixing based on monthly observations of water sources (river water, rainwater, evaporation, and groundwater) and thermocline depths from previous studies (Mochizuki et al. (2016)), the adsorbed uranium (AU) on the soil surface based on work by Saito et al. (2021), and the chemical equilibrium of major chemical species such as H, C, and Ca. The total uranium (TU = AU + DU), which determines the peak DU level in summer, and cation-exchange capacity of the soil surface (CEC_Zp), which determines the DU decrease in winter, were optimized by reproducing the DU measurements. The DU value calculated by PHREEQC with the ion-exchange-equilibrium model can be expressed as the sum of three dissolved chemical species (DU_eq), which constitute on average 97% of the DU calculated by PHREEQC. DU_eq values were decomposed and the contribution of each factor was ranked by comparing their seasonal variabilities. The complexation equilibrium had the greatest variability and was proportional to the reciprocal of the cube of hydrogen ions. This indicates that the seasonal variation in DU is caused mainly by variation in pH. The peak in the DU observations lagged behind the peak in the pH observations by approximately 1 month. However, DU_eq did not reproduce this peak delay. Therefore, we assumed that when DU fails to reach the ion-exchange equilibrium concentration and the peak delay occurs, it follows a first-order reaction, and the rate coefficient is determined by the reproducibility of the DU measurements. The DU_lim, as the adsorption/desorption rate-limited DU, improved the reproducibility of the DU and the delay after the pH peak. This implies that the delay of the DU peak after the pH peak represents the lag from equilibrium to a first-order rate reaction caused by the soil uranium adsorption/desorption rate.