Aller (1977, 1978, 1980) and Aller and Yingst (1978) have demonstrated that the presence of well-irrigated worm-tubes and other animal burrows in sediments can significantly alter the rate of exchange of solutes between porewater and the overlying waters. Specifically, the burrows modify the geometry of the porewater system such that solutes can diffuse toward or away from either the sediment-water interface or the burrow. The burrow constitutes an additional boundary source or sink...