AN ADVECTION DIFFUSION-MODEL FOR RN-222 TRANSPORT IN NEAR-SHORE SEDIMENTS INHABITED BY SEDENTARY POLYCHAETES

A porewater solute transport model is proposed to explain the dissolved radon distributions in four profiles from a near-shore, marine site. The macro-infauna at the site is dominated by Clymenella torquata, a sedentary deposit feeding polychaete that actively irrigates its burrow. The model includes both an advective component that takes into account the (e.g.) C. torquata mechanism of burrow irrigation as well as enhanced diffusion-like transport brought about by mobile infauna. Because it explains the existence of subsurface radon minima with a constant source function and is based on the known irrigations habits of C. torquata, the model provides good fit to observed radon profiles and may be applicable to other solutes that display depth dependent minima. Sediment advection velocities, individual burrow irrigation rates, and enhanced diffusion coefficients are estimated based on a minimization of chi-squared for model fits to the Rn-222 data of each profile. The net advection velocity ranged from 0.19 to 0.91 cm d-1, and enhanced diffusion ranged from 13 to 30 times effective molecular diffusion. These values are comparable to those determined by previous investigators using other transport models in similar sediments. The range of burrow irrigation rates (0.16-0.78 ml/h/individual) is somewhat lower than the average rate for C. torquata that had been determined in the laboratory.