Identifi cation of Suspended Sediment Sources Using Soil Characteristics in a Semiarid Watershed

Soil Sci. Soc. Am. J. 72:1102-1112 doi:10.2136/sssaj2007.0066 Received 15 Feb. 2007. *Corresponding author ([email protected]). © Soil Science Society of America 677 S. Segoe Rd. Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. The identifi cation of sediment sources in watersheds is important from a number of agricultural and environmental perspectives. Soil losses in the United States have been estimated at 51.5 Mg ha−1 yr−1 (Pimentel et al., 1995), which results in productivity losses and increased production costs as fertilizer, pesticide, and irrigation application rates must be increased to compensate for erosion losses. The increased use of chemical amendments can further degrade environmental and water quality as offsite accumulation of nutrients and pesticides are enhanced. In 1995, the estimated annual cost for such erosion-induced problems was $37 billion (Pimentel et al., 1995). Given this potential cost, scientifi c approaches are needed to relate sediment properties back to specifi c soil conditions in watersheds to better defi ne sediment routing processes, identify source areas of pollutants, and identify needed changes in management systems to resolve excessive runoff and erosion problems. This is especially important in arid and semiarid regions, where soil erosion can be considered a more serious problem, relative to humid regions, because the soils are generally shallow and less well-developed, with inadequate vegetative cover. Direct monitoring and fi ngerprinting are the two primary approaches used in sediment source identifi cation research. Direct monitoring uses technology such as erosion pins, runoff troughs, automated suspended sediment samplers, and manually collected grab samples (Sutherland and Bryan, 1989) to measure sediment yields from potential source areas for determining their relative contribution to overall sediment loads in a watershed. However, since direct monitoring methods cannot be used at scales larger than small drainage basins, they are not useful for distinguishing between source types either within or between individual storm events (Slattery et al., 1995). The other approach, commonly referred to as fi ngerprinting, is best suited for this purpose. Fingerprinting relies on suspended sediment properties for which equivalent values exist in the watershed soils and streambank materials (Slattery et al., 1995). Soil and sediment properties used in this approach include: clay mineralogy, color, chemical composition, radionuclide concentrations, and a range of magnetic susceptibility parameters. The clay mineralogy of suspended sediment has been used to infer source areas (Neiheisel and Weaver, 1967; Klages and Hsieh, 1975; Wall and Wilding, 1976), but using this approach as a singular method can be problematic because clay minerals are F. E. Rhoton* USDA-ARS National Sedimentation Lab. Oxford, MS 38655