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In the terrestrial compartment, the availability of metal compounds for uptake by biota can differ from site to site and may change over time due to many processes, including weathering and (de)sorption processes.

It should also be noted that Kd values are accurate only during an equilibrium state, which is difficult to reach for metals in the environment. As a consequence, part of the metal present in the solid phase may be encapsulated in the mineral fraction and is therefore not available.

Hf element is considered for terrestrial compartment. The IAEA Technical Reports Series No. 364 (Handbook of Parameter Values for the Prediction of Radionuclide Transfer in Temperate Environments, 1994) publication reported Kd values for Hf within the range of 1500 to 5400 L/kg, with a Kd of 2500 L/kg for all soils in general.

These values and IAEA assessor were considered reliable enough to be used for HfCl4 in a read across approach, as the common metal form Hf is retrieved in soils.

The typical value was considered: Kd of 2500 L/kg. This coefficient confirmed a high adsorptivity of the substance on particulate matter and low mobility in soils, leading to a poor availability to biota via interstitial water.

Moreover, local exposure for the soil compartment could be due to deposition of particles emitted in the atmospheric compartment, and no volatilization is assumed for metal compounds. There is then no way to expect soil exposure to HfCl4 and all required endpoints for the dossier are waived.