SUSPENDED PARTICULATE MATTER TRANSPORT IN THE LOWER CAPE FEAR RIVER SYSTEM

Suspended particulate matter (SPM) is an important component of water quality. The presence of SPM diminishes water quality by reducing the light available to aquatic vegetation, and by providing a substrate for many pollutants. SPM levels have been monitored at seven stations in the Cape Fear, Black and Northeast Cape Fear rivers since 1998. These systems regularly experience periods of high sediment loading during which turbidity levels exceed the North Carolina state standards. The long-term objectives of this study are to determine SPM concentration and composition in the river and to relate SPM levels to tidal and meteorological phenomenon.

Two 1-liter water samples were collected monthly on an ebbing tide at each of the seven sampling stations. Samples were collected manually from the upper 50 cm of the water column, stored on ice, and returned to the laboratory where they were filtered through pre-weighed,1 µm glass fiber filters, dried overnight at 60°C, reweighed, and concentrations determined in mg l^-1. Filtered samples were then combusted at 450°C for 4 hours to determine organic content.

SPM data collected in the lower Cape Fear River indicate that precipitation and season interact to control the composition of material being exported to the coastal ocean. Positive correlations between rainfall in the upper watershed and the various SPM components in the Cape Fear mainstem suggest that runoff from the Piedmont contributes to sediment loading in the estuary. Low inorganic SPM concentrations and high organic percentages observed at the Northeast Cape Fear and Black River stations indicate that little particulate matter enters the estuary from these blackwater systems. Tidal processes and seasonal variations in primary productivity, both beyond the scope of this study, mediate SPM concentration and composition in the estuary. Efforts to control particulate loading in the lower Cape Fear River should begin in the upper watershed and should focus on the retention of inorganic components.