An assessment of the relative importance of horizontal and vertical transport of particle-reactive chemicals in the coastal ocean

A two-dimensional transport and scavenging model has been developed and applied to a limited set of 238U–234Th disequilibria data in order to examine the relative significance of horizontal versus vertical removal of chemicals in coastal waters. During an intense scavenging episode in September 1993 (>95% 238U–234Th disequilibrium), vertical scavenging was found to be more important than horizontal transport in both Inner and Outer Casco Bay, Gulf of Maine. However, in May 1994 the two-dimensional model suggested that onshore horizontal dispersion of 234Th was substantial. Recognition of this horizontal flux required us to increase the net vertical scavenging flux in Inner Casco Bay by a factor of three over that obtained based only on the local 238U–234Th disequilibrium. The radionuclide (210Pbxs, 234Thxs, 7Be) record of the underlying sediments provided supporting evidence for onshore horizontal transport of chemicals. The highest sedimentary inventories for all three radionuclides were found at the stations nearest to the coast. As anticipated from their relative particle-affinities, the “regional boundary-scavenging” indicator 7Be/234Thxs was highest at the coastal boundary. The application of the two-dimensional 234Th-based transport model to assess the distributional fate of other chemicals was demonstrated for Casco Bay using simultaneously measured polycyclic aromatic hydrocarbons (PAHs). Based on limited PAH data, the model results suggest that about half of the pyrene and benzo[a]pyrene introduced to Portland Harbor, ME may be settling locally and that the remainder is exported to offshore locations. The approach introduced here, coupling information on particle-mediated vertical scavenging, chemical phase distribution, and tide-induced horizontal dispersion, should provide a useful mechanistic framework for elucidating quantitatively the dispersal of a wide range of geochemically and environmentally important chemicals in the coastal ocean.