Composition of estuarine colloidal material: major and trace elements

Colloidal-sized material from the Patuxent River estuary, Maryland, was analyzed for more than 30 elements by instrumental neutron activation analysis. Sample data from stations ranging in salinity from 0.04 g/kg to 12 g/kg indicate that the winter colloidal material is dominated upstream by poorly crystallized clay minerals and iron oxide, but above a salinity of 10 g/kg, it consists primarily of carbonaceous material. Winter colloidal material decreases non-conservatively seaward from 29 to 0.6 mg/l. The summer colloidal material, however, is carbon-rich throughout the estuary and the amount of colloidal material in the upper water column does not change appreciably with increasing salinity. Many trace elements (Cs, Hf, Rb, Se, Th, V and the REE) covary with Al and probably are associated with the clay component. Other elements (As, Ba, Sb, Se, U and Zn) are considerably enriched relative to Al and may covary with the organic component. The results suggest that the freshwater winter colloidal system is a product of continental weathering processes, whereas the summer colloidal material is derived primarily from estuarine biological processes.