Sediment Source Assessment along an Urban to Rural Transition using Plutonium Isotopes and Sediment Fingerprint Modeling

Abstract:

Excessive watershed erosion affects water quality, aquatic ecosystem health, and the economic profitability of agricultural landscapes. Recent research regarding urbanization’s effects on sediment transport have investigated the impact of land use change on the source and quantity of sediment delivered to streams, however, there is a lack of study across multiple systems undergoing varying degrees of urbanization. We study five watersheds with vastly different percent urban land use (from least to most urban): Kill Creek (16.7%), Blue River (20.8%), Cedar Creek (24.4%), Mill Creek (67.4%), and Indian Creek (98.3%). We collected weekly time-integrated stream sediment samples over the course of a year and collected samples from six possible sources, including streambank, grassland, woodland, cropland, construction, and residential sediment. All samples were analyzed for plutonium activity using an ICP-MS (Pu-239 and Pu-240). A sediment fingerprinting model was developed to apportion transported sediment to different sources. Results from plutonium tracing indicated different Pu-activities in surface (0.11 ± 0.009 Bq/kg) and sub-surface (0.04 ± 0.042 Bq/kg) sources. Pu-activity was generally higher in agricultural watersheds (0.06 ± 0.007 Bq/kg) when compared to urban watersheds (0.04 ± 0.005 Bq/kg). Our unmixing model showed dominant streambank contributions in all watersheds: Kill Creek (66 ± 23%), Blue River (59 ± 25%), Cedar Creek (70 ± 24%), Mill Creek (72 ± 24%), and Indian Creek (72 ± 23%). Using plutonium isotopes as a tracer has proven successful in this study by distinguishing surface sources from subsurface sources. We find that sediment sourcing in agricultural watersheds is discharge-dependent, but in urban watersheds it is discharge-invariant. During small storms, the dominant source of fluvial sediment in agricultural watersheds is upland surface sediment. As discharge increases, source dominance shifts from upland to in-stream bank sediment. However, in urban watersheds, streambank sediment is always the dominant source, potentially due to impervious surfaces increasing fluvial shear. These results show that the different land uses and hydrologic transportation methods in urban and agricultural watersheds affect the sources of fluvial sediment.