Global deposits of relatively high 137Cs activity also correspond to the nuclear accidents in Chernobyl, Ukraine in 1986 and Fukushima, Japan in 2011. As its half-life of 30.2 years is similar to 210Pb, 137Cs is often used in parallel with excess 210Pb to identify the sources of sediment. Sediment derived from shallow, surficial erosion, such as through overland flow, would typically have higher amounts of excess 210Pb than sediment from deeper sources that have been isolated from the atmosphere for a longer time. Samples with higher activity readings of excess 210Pb indicate sources from upland/surface RO4929097 price erosion, while samples with lower readings suggest sources from depths that have not recently

been exposed to the atmosphere (Feng et al., 2012). Surficial sources eroded in the uplands and/or floodplains contribute to higher activity levels. Deeper sources, with lower or nonexistent Baf-A1 ic50 excess 210Pb levels, might come from sources that expose and transport sediment, such as hillslope failure or river bank erosion.

Many previous studies have used radionuclides to determine sediment sources (e.g., reviewed in Brown et al., 2009, D’Haen et al., 2012 and Mukundan et al., 2012) for more than 20 years (e.g., Joshi et al., 1991). These studies have used tracers in mountain streams to determine particle transit times (Bonniwell et al., 1999), watershed sediment budgets (Walling et al., 2006), sources of suspended sediments (Collins et al., 1998 and Mukundan et al., 2010), floodplain deposition and erosion (Humphries et al., 2010), and land use changes (Foster et al., 2007). Information for sediment sources derived from 210Pb and 137Cs has also been combined with numerical models to produce sediment budgets for watersheds. Generally,

these studies have used radionuclides and/or other sediment tracers with some combination of transport, mixing, storage, and depositional models with a randomization component (e.g., Monte Carlo simulation) to determine potential contributing sources to the sampled sediment. This approach identifies the often diffuse nature of sediment sources from the sediment sample. For example, numerical modeling elucidated the percent contributions of sediment (and associated Exoribonuclease possible statistical deviations) from various catchment land uses (Collins et al., 2012b and Collins et al., 2012c). However, model limitations include the amount and timing of storage in system (Parsons, 2012), assumptions about unmeasured terms (Parsons, 2012), and the need for validated input data (Collins and Walling, 2004). Like any scientific model, the limitations and assumptions should be recognized to prevent over-reaching. In a previous study, the authors validated the regional correlation between excess 210Pb with urban watersheds and little to none excess 210Pb with channel/bank areas. Feng et al.