Using Trace Elements as an Indicator of Materials Eroded from Surface Soils

Book excerpt: Abstract: Surface materials eroded during land disturbance are increasingly recognized as an important non-point source contaminant that affects water quality and increases the infilling rates of waterways. This study examines: 1) the use of trace element concentrations as a tracer of materials eroded from soils, and 2) the relation between changes in land use and sediment yield. This study examines adjoining drainages (Eversole Run, ER and the unnamed tributary, UT) that empty into an embayment of O'Shaughnessy Reservoir (built in 1920, northwest of Columbus, Ohio). Agriculture was the dominant land use in each drainage prior to1920. Residential subdivisions dominate present land use in the UT drainage, while the ER drainage remains predominantly agricultural. ER's drainage is three times larger than that of UT. Soil samples were collected from both drainages and sediment cores were collected directly offshore each stream in the embayment. Samples were analyzed for trace and major elements using x-ray fluorescence. Sediments were also analyzed for 137Cs activity. Cu, Ga, Rb, V, Zn and Zr show consistent behaviors between soils and sediment. In non-disturbed soils, concentrations increase from upper to lower soil horizons for five of these six elements; Zr shows the opposite pattern. Elemental concentrations are similar in both the upper and lower soil horizons of disturbed soils (from residential subdivisions), approximating the mean concentration in non-disturbed soils. The sediment thicknesses offshore UT and ER were 1.62 meters and 0.73 meters, respectively, with the 137Cs peak (representing 1963-64) at 0.94 and 0.21 meters downcore, respectively. In both cores, intervals with higher trace element concentrations were deposited during times of increased residential subdivision development. Lower trace element concentrations were deposited during periods when the upper horizon of non-disturbed soils was eroded. Sediment accumulation offshore UT since 1963-64 is four times larger than accumulation off ER, due to increased erosion in the UT drainage during construction of residential subdivisions. Because of the difference in drainage areas, sediment yield (mass eroded/drainage area/time) from the more heavily developed UT drainage since 1963-64 is estimated to have been approximately ten times larger than the yield from the ER drainage.