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 an d applied to a limited set of U-238-Th-234 disequilibria data in order to examine the relative significance of horizontal versus vertical re moval of chemicals in coastal waters. During an intense scavenging epi sode in September 1993 (> 95% U-238-Th-234 disequilibrium), vertical s cavenging was found to be more important than horizontal transport in both Inner and Outer Casco Bay, Gulf of Maine. However, in May 1994 th e two-dimensional model suggested that onshore horizontal dispersion o f Th-234 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 U-238-Th -234 disequilibrium. The radionuclide (Pb-210(xs), Th-234(xs), Be-7) r ecord of the underlying sediments provided supporting evidence for ons hore horizontal transport of chemicals. The highest sedimentary invent ories for all three radionuclides were found at the stations nearest t o the coast. As anticipated from their relative particle-affinities, t he ''regional boundary-scavenging'' indicator Be-7/Th-234(xs) was high est at the coastal boundary. The application of the two-dimensional Th -234-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, t he model results suggest that about half of the pyrene and benzo[a]pyr ene introduced to Portland Harbor, ME may be settling locally and that the remainder is exported to offshore locations. The approach introdu ced here, coupling information on particle-mediated vertical scavengin g, chemical phase distribution, and tide-induced horizontal dispersion , should provide a useful mechanistic framework for elucidating quanti tatively the dispersal of a wide range of geochemically and environmen tally important chemicals in the coastal ocean. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.