ESTIMATION OF SEDIMENT SOURCES IN AGRICULTURAL AND URBAN ENVIRONMENTS USING CHEMICAL TRACERS

Nationally, sediment is a concern as related to several important issues including water quality, aquatic habitat, and reservoir sedimentation. This abstract describes results of sediment-source investigations for a large agricultural watershed and a small urbanizing watershed in northeastern Kansas.

To estimate suspended-sediment sources within the agricultural 2,900-km² Perry Lake watershed, samples of channel-bank sources, surface-soil sources (cropland and grassland), and reservoir bottom sediments were collected, analyzed, and compared. A nested approach was used in which five representative subwatersheds were selected for sampling that ranged in size from 20 to 50 km². Samples were analyzed for nutrients, carbon, 25 trace elements, and the radionuclide cesium-137.

Total nitrogen, total phosphorus, total organic carbon, and cesium-137 provided the best ability to discriminate between channel-bank and surface-soil sources of sediment in the Perry Lake watershed. Using these constituents, it was determined that sediment contributions from channel-bank and surface-soil sources varied among the subwatersheds. However, for Perry Lake, channel-bank sources were predominant indicating that these sources increased in importance with increasing distance downstream in the larger watershed.

To investigate suspended-sediment sources in the urbanizing, 160-km² Mill Creek watershed, surface soils, channel banks, and suspended sediment were sampled and analyzed for nutrients, carbon, 25 trace elements, and four radionuclides. Although concentrations or activities of cobalt, nitrogen, selenium, total organic carbon, cesium-137, and excess lead-210 were significantly different among surface soils and channel banks, variability in source estimates among constituents and sites precluded estimation of suspended-sediment sources. Redistribution of soil horizons by urban construction, enrichment of constituent concentrations during sediment transport, and the inability to accurately represent rill and gully erosion were possible factors affecting source estimates.

Chemical tracers are useful for estimating sediment sources in some environments. However, to date, a universally applicable tracer or suite of tracers has not been discovered and may not exist.