PRELIMINARY CLAY MINERALOGY INVESTIGATIONS OF SEDIMENT FROM A SUBURBAN CATCHMENT BASIN IN THE GEORGE MASON UNIVERSITY ECOLOGICAL OBSERVATORY

Sediment pollution is a major problem in Chesapeake Bay region, especially with respect to phosphate. Detailed mineral-based studies are lacking. The nature of sediment in flood control or catchment basins in the region is particularly poorly understood. Mineralogy of clay rich sediment from the upper 20 cm of a catchment basin within George Mason University's ecological observatory and associated areas was investigated using transmission electron microscopy techniques to assess if any pollution-mineral interactions could be observed. Oxide minerals were found to be significant controls on phosphate. Fe-rich oxide particles are poorly crystalline and have complex and variable chemical compositions. Major components with respective averages are Fe2O3 ( 71.92 wt% ), Al2O3 (11.03 wt % ), SiO2 (13.14 wt % ), CaO (1.44 wt % ) and P2O5 (2.44 wt%). TEM imaging indicates Fe-rich oxide particles are acicular to tabular in habit and commonly are less than 1 micron in length. Crystals are composed of several domains 5 to 10 nm in diameter. Elemental ratios of Fe and Si suggest a single source of the Fe-rich oxides. Although less abundant, poorly crystalline Mn-Fe rich oxides were also observed commonly with irregular textures. These particles contained 1.21 to 2.70 wt% P2O5. Several weight percent SO3 was also observed and some particles had approximately1-2 % ZnO. Oxides appear to play a major role in the sequestration of phosphate in the system and may play a role in the distribution of Zn and perhaps other minerals. Phosphate is believed to be derived from excess lawn fertilizer from housing developments immediately up stream. The origin of Zn and S is unclear but the affinity for Mn-rich oxides suggests a complex redox interaction may be taking place. The relationships observed here may be representative of other local catchment basins and provide additional insight into the nature of pollution in the Chesapeake Bay region.