Ra and Th adsorption coefficients in lakes—Lake Kinneret (Sea of Galilee) “natural experiment”

Abstract

The adsorption rate constants of Ra and Th were estimated from empirical data from a freshwater lake and its feeding saline springs. We utilized the unique setting of Lake Kinneret (Sea of Galilee, northern Israel) in which most of the Ra and Th nuclides are introduced into the lake by saline springs with high 226Ra activities and a high 224Ra/ 228Ra ratio of 1.5. The mixing of the Ra enriched saline waters and freshwater in the lake causes the 224Ra/ 228Ra ratio to drop down to 0.1 in the Kinneret due to preferential adsorption of 228Th. These conditions constitute a “natural experiment” for estimating adsorption rates. We developed a simple mass-balance model for the radionuclides in Lake Kinneret that accurately predicted the Ra isotope ratios and the 226Ra activity in the lake. The model is comprised of simultaneous equations; one for each radionuclide. The equations have one input term: supply of radionuclides from the saline springs; and three output terms: adsorption on particles in the lake, radioactive decay and outflow from the lake. The redundancy in the analytical solutions to the mass balance equations for the relevant nuclides constrained the values of Ra and Th adsorption rate constants to a very narrow range. Our results indicate that the adsorption rate constant for Ra is between 0.005 d −1 and 0.02 d −1. The rate constant for Th is between 0.5 d −1 and 1 d −1, about fifty to a hundred times higher. The estimated desorption rate coefficient for Ra is about 50–100 times larger than its adsorption rate constant. The mass-balance equations show that the residence times of all Ra isotopes ( 226Ra, 228Ra, 223Ra, 224Ra) and of 228Th in the lake are about 95, 92, 14, 6 and 1 d, respectively. These residence times are much shorter than the residence time of water in the lake (about 5.5 y). The steady state activity ratios in Lake Kinneret depend mainly on the adsorption rate constants, decay constants, the outflow rate from the lake and the activity ratios in the saline springs. The activity ratios are independent of the saline springs flow rate.