Experimental kinetic rates of food-chain and waterborne radionuclide transfer to freshwater fish: A basis for the construction of fish contamination charts

A standardized procedure is proposed to obtain from laboratory experiments the kinetic accumulation and release rates necessary to calibrate dynamic m odels to quantify radionuclide direct and trophic transfer in fish. The mod el takes into account the food-chain effect, the feeding rate, and the grow th of organisms. It takes as examples Mn-54, Co-60, and Cs-137 transfer dyn amics through a simple pelagic food-chain (phytoplankton, zooplankton, prey fish, and predator fish). The estimated kinetic rates used in quantifying all the transfers of the three radioactive pollutants through the pelagic f ood chain are compared from the radioecological point of view. For fish, co mparison was based on the calculation of concentration factors referring to direct transfer from water and trophic transfer factors. For the prey fish and the predator fish, direct transfer gave the following order for accumu lation Co-60 < Cs-137 < Mn-54. Values reached at equilibrium in L/kg WW wer e respectively for the prey fish and the predator fish: 8.7 < 27.4 < 107 an d 4.14 < 6.59 < 13.4. For the trophic route, Cs-137 is the most accumulated (TTFeq = 0.485 in 291 days for the prey fish and TTFeq = 1.45 in 17 years for the predator fish). A sensitivity analysis adapted to the case of a chr onic contamination scenario of a watercourse was run. It showed that the ph ytoplankton biomass, the contact time of these drifting particles from a re lease point to the station where they are ingested and the feeding rates of the fish are the most influential parameter with regard to the concentrati on in fish, whatever the trophic level. Contamination charts are constructe d for the predator fish to illustrate the relationship between the most inf luential ecological parameters and the radionuclide concentration in fish f or simple contamination scenarios. They are shown to be effective tools for helping in the choice of the most relevant value of aggregated concentrati on factors (ACFs: radionuclide concentration ratio between the organism and the water, referred to steady-state and to all possible transfer pathways) for a given key ecological situation in a given ecosystem. An example is g iven of a simple chronic release scenario of 1 Bq/L and a phytoplanktonic b loom period. For Cs-137, th, ACF increases with increasing contact time and increasing feeding rate, to nearly 550 L/kg WW at equilibrium. For Mn-54, ACF reaches 65 L/kg WW. For Co-60, the general pattern of the relationship is due to the rapid kinetic rates governing the distribution of the radionu clide between dissolved and solid (phytoplankton) phases with a maximum val ue for ACF of 7.2 L/kg WW for the case study. Analysis of these charts prov ides a basis for overall guidelines for chronic releases in a given waterco urse.