Multi‐year tracking of sediment sources in a small agricultural watershed using rare earth elements

Abstract

Rare earth elements (REEs) have been successfully used as a sediment tracer, but the REE technique has never been used for studying sediment sources for a multi-year period. A nearly four-year field experiment was conducted on a small agricultural watershed near Coshocton, OH, USA, to assess the applicability of the REE technique for a multi-year period and to evaluate the relative contributions of sediment sources in the watershed. Tracer depletion and tracer enrichment ratio (ratio of the tracer concentrations in sediment to the concentrations in the soil in the areas of application) were evaluated to examine the applicability and accuracy of the technique. A minimum of 91 per cent of the mass of the applied elements was still available on any individual morphological element at the end of the experimental period. The tracer enrichment ratio varied from 0·4 to 2·3, and it was not significantly related to time. The relative contributions of six morphological elements within the watershed were evaluated as proportions to total sediment yield. The relative contribution of the lower channel was significantly increased as a function of the amount of sediment yield, while that of the lower backslope was significantly decreased. The relative contribution of the lower channel significantly decreased as a function of cumulative sediment yield, while the contributions of the shoulder and the upper backslope significantly increased. Our results showed that the REE technique can be used to track sediment sources for a relatively long period with two limitations or potential sources of error associated with a selective depletion of tracers and a contamination of downslope areas with tagged sediments from upslope areas. Copyright (Polyakov, 2002;. The watershed was subdivided into six different morphological elements (toeslope, upper and lower backslopes, upper and lower channels and shoulder; Figure 1), and each morphological element was tagged with a different element. Sediment translocation was examined by collecting sediment that left the watershed with surface runoff and by spatial sampling of the soil surface. The previous works (Polyakov, 2002; reported that the channel elements had a higher sediment delivery ratio than did the backslope and shoulder elements, due to high sediment transport efficiency in the channel compared to the slopes.Although the previous works (Polyakov, 2002; have revealed the feasibility of using the REEs as a sediment tracer under field conditions, the applicability of REEs for successive storms for a multi-year period has never been tested. The sediment yield from WS 127 was collected for nearly four years after the original application of the tracers, and the REE concentrations in the sediment were measured to evaluate the relative contribution of each morphological element to sediment yield from the watershed. The total mass of depletion of the elements and the tracer enrichment ratio, which was defined as the ratio of the tracer concentrations in sediment to the concentrations ...

Assessing hillslope-channel connectivity in an agricultural catchment using rare-earth oxide tracers and random forests models

“…The REO tracers were applied using a standard lawn spreader that was calibrated to disperse 500g of mixture on a stretch of 10 meters on flat, even terrain (Table 1). In contrast with earlier studies using REOs spread with a lawn spreader (Kimoto et al, 2006;Polyakov et al, 2004), applied tracers were not mixed into the ground by disking or tillage because: (i) we wanted to keep the oxides within the applied areas, (ii) we did not want to make any changes to normal farming practices, and (iii) the main focus of this study was on the assessment of hillslopechannel connectivity, and, therefore not on calculating exact amounts of displacement of sediments. This choice is supported by Deasy and Quinton (2010), who found that the incorporation of the REO tracers through disking caused high sediment yields for the first events after disking.…”

“…However, this hypothesis needs to be further validated using field data. Field data that need to be gathered to test the hypothesis are sediment tracing data either after every event, and/or tracking sediments over multiple seasons and years (Kimoto et al, 2006). In addition, sediment volumes within the channel need to be quantified to obtain a closing sediment budget.…”

“…Rare-Earth oxides (REOs) are types of tracers that are actively applied to the soil. REOs occur naturally in soils in small concentrations, but are applied to the soil at 10-100 times the background concentrations by lawn spreaders or by spraying (Deasy and Quinton, 2010;Kimoto et al, 2006;Polyakov et al, 2004).…”

Effects of soil type on leaching and runoff transport of rare earth elements and phosphorous in laboratory experiments

“…The depletion rate (Kimoto et al 2006) of the elements indicates the loss rate of REEs and P in different types of soils. This is calculated with the following formula (Kimoto et al 2006):…”

Rare earth elements and relation between their potential bioavailability and soil properties, Nidda catchment (Central Germany)

“…The concentrations of REEs in soils and the degree of REE fractionation is part of many geochemical investigations (e.g. Jifeng 1998;Aide and SmithAide 2003;Kimoto et al 2006;Laveuf et al 2008;Mao et al 2009). REE's occurrence patterns are commonly used as an indicator for weathering rates in soil profiles (Henderson 1984).…”