QUANTIFYING FINE SEDIMENT SOURCES IN THE NORTHEAST BRANCH OF THE ANACOSTIA RIVER USING TRACE ELEMENTS AND RADIONUCLIDES

Suspended sediment is a water quality problem in the Chesapeake Bay. Fine sediment blocks light necessary for subaquatic vegetation growth, resulting in loss of habitat for fish and other organisms. This project identifies sediment sources in an urban watershed, the Northeast Branch of the Anacostia River (in Washington, D.C. and Maryland; drainage area = 188.5 km²), which delivers sediment directly to the Chesapeake Bay. The target of this research is fine sediment that remains suspended, rather than bulk soil that includes larger particles, which do not travel downstream in the suspended fraction.

Using GIS analysis and a field soil survey, potential sources of sediment were identified and sampled. Source groups included stream bank, floodplain, and upland landscape positions, and street residue. Suspended sediment was collected at the watershed outlet during storm events for comparison with sediment sources. Source sediment and suspended sediment samples were characterized using the following techniques: radionuclide (Cs-137 and K-40) analysis by gamma-ray spectrometry, trace-element analysis by ICP-MS, and particle-size analysis by laser particle-size analyzer. Eighteen sample sites were chosen to represent tributary source areas. Comparisons were made between landscape positions, tributaries to the main stem, physiographic region (Piedmont and Coastal Plain), and the two subwatersheds. The amount of enrichment of various metals in watershed sources and suspended sediment was determined by calculating enrichment ratios, which are ratios of the normalized concentration of elements in the sample relative to their average normalized concentration in Earth's crust.

Elements that separate banks from other sources include Cs-137, K-40, TiO², and Au. Elements that separate the west and east subwatersheds were differentiated from each other by Hf, Nb, Ta, Th, U, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Elements that show enrichment relative to the Earth's crust, and thus are likely of anthropogenic origin, include Hf, Zr, Cd, Ag, and Se. This method provides a concise summary of complex environmental factors.