Using radiometric fingerprinting and phosphorus to elucidate sediment transport dynamics in an agricultural watershed Jasmeet Lamba,* K. G. Karthikeyan and A. M. Thompson Department of Biological Systems Engineering, University of Wisconsin-Madison, Madison, WI, 53706, USA Abstract: The major goals of this study were to determine stream bed sediment erosion/deposition rates, sediment age, percent ‘new’ sediment, and suspended sediment origin during two storm events of contrasting magnitudes (11.9 mm over 5 h and 58.9 mm over 39 h) using fallout radionuclides (excess lead 210 – 210 Pb xs and beryllium 7 – 7 Be) and link the nature and type of sediment source contributions to potential phosphorus (P) off-site transport. The study was conducted in cropland-dominated and mixed land use subwatersheds in the non-glaciated Pleasant Valley watershed (50 km 2 ) in South Central Wisconsin. Fine sediment deposition and erosion rates on stream beds varied from 0.76 to 119.29 mg cm À2 day À1 (at sites near the watershed outlet) and 1.72 to 7.72 mg cm À2 day À1 (at sites in the headwaters), respectively, during the two storm events. The suspended sediment age ranged from 123 ± 12 to 234 ± 33 days during the smaller storm event; however, older sediment was more prevalent (p = 0.037) in the streams during the larger event with suspended sediment age ranging from 226 ± 9 to 322 ± 114 days. During the small and large storm event, percent new sediment in suspended sediment ranged from 5.3 ± 2.1 to 21.0 ± 2.9% and 5.3 ± 2.7 to 6.7 ± 5.7%, respectively. In the cropland-dominated subwatershed, upland soils were the major source of suspended sediment, whereas in the mixed land use subwatershed, both uplands and stream banks had relatively similar contributions to suspended sediment. In- stream (suspended and bed) sediment P levels ranged from 703 ± 193 to 963 ± 84 mg kg À1 during the two storm events. The P concentrations in suspended and bed sediment were reflective of the dominant sediment source (upland or stream bank or mixed). Overall, sediment transport dynamics showed significant variability between subwatersheds of different land use characteristics during two contrasting storm events. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS sediment sources; suspended sediment; sediment fingerprinting; land use; phosphorus; sediment resuspension Received 1 February 2014; Accepted 5 November 2014 INTRODUCTION Fine sediment eroded from agricultural landscapes plays an important role in the quality and health of surface waters (Walling and Fang, 2003; Devereux et al., 2010) and is considered a major non-point source pollutant. Reducing non-point source pollution through the imple- mentation of best management practices (BMPs) has received considerable attention since the promulgation of the Clean Water Act in 1972. However, in the past four decades, several non-point source watershed projects have reported little or no improvement in water quality following BMP implementation (Meals et al., 2010; Jarvie et al., 2013; Sharpley et al., 2013). For example, no significant decrease in phosphorus (P) levels was observed after BMP implementation in the La Platte River and St. Albans Bay agricultural watersheds located in northwestern Vermont, USA (Meals, 1996). In both La Platte River and St. Albans Bay watersheds, water quality monitoring was continued for about 5 years after the implementation of BMPs. The impact of upland management practices and land use changes on downstream water quality is quite complex. Sediment eroded from uplands can take several days to years to reach the watershed outlet. The sediment eroded may deposit permanently or temporarily within the watershed where hillslope gradient declines (e.g. at the base of a slope, on flood plains, or within channels and depressions) (Walling, 1983). Fine sediment depos- ited on stream beds can create a ‘legacy’ effect (i.e. effect of historical/past sediment deposited on the stream bed on the current water quality status), resulting in a ‘lag time’ between implementation of BMPs and achievement of desired water quality goals (Osmond et al., 2012). For example, resuspension of stream bed sediment (deposited during the pre-BMP implementation period) could continue to contribute to suspended sediment load during the post-BMP implementation period, thereby creating a legacy effect. A better understanding of sediment transport dynamics in stream channels at various spatial *Correspondence to: Jasmeet Lamba, USDA-ARS, Pasture Systems & Watershed Management Research Unit, Building 3702 Curtin Road, University Park, PA 16802, USA. E-mail: jul59@psu.edu HYDROLOGICAL PROCESSES Hydrol. Process. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.10396 Copyright © 2014 John Wiley & Sons, Ltd.