Numerical estimation of the error of the biodiffusion coefficient in coastal sediments

We have developed a time-dependent model to test the accuracy of the classical method used to calculate biodiffusion coefficients from 7Be and 234Th profiles in coastal sediments. The model simulates activity profiles influenced by either seasonal or episodic fluxes of radionuclides to the sedimentewater interface and calculates the apparent biodiffusion coefficient from these disrupted activity profiles. The error induced by a steady-state approximation is then evaluated from the difference between the apparent Db and the actual coefficient initially introduced into the time-dependent model. The relative error reaches significant values with the set of parameters typically encountered in coastal sediments, reaching values as high as 163% with 7Be and 74% with 234Th. The error is inversely proportional to the variations of the flux to the sedimentewater interface and it is more sensitive to the amplitude of the flux than to the actual value of the biodiffusion coefficient. The classical procedure used to determine biological mixing remains valuable for coastal environments and the error is less than a factor 2. Cautions are advised if strong variations of the flux to sedimentewater interface exist.