Use of geochemical tracers to determine the sources for Riverine sediment with different sizes in a forest-agricultural watershed, northern Iran

Abstract

Because sediment sources, topography, and runoff vary, sediments generated in different parts of a watershed differ in size and properties, which affects the transport of various elements and contaminants. However, the variability of sediment particle size originating from various sources has not yet been considered. Therefore, the current research aimed to identify and separate sediment sources with different sizes of 0–2000 μm in the Kasilian representative watershed in Northern Iran. In this research, 40 soil samples were taken from different sediment sources of natural forest (10), hand planted forest (4), rangeland (14), and agricultural lands (8). In addition, 4 samples of suspended sediments were taken during two rainfall events in two monitoring stations of MS1 and MS2 in the middle and outlet of the studied watershed, respectively. In addition, four samples of fine-grained bed sediment were taken upstream of the river to determine the river bed's contribution as a sediment source. Then, the sediments were separated into four size classes of <20, 20–40, 40–63, and >63 μm. To select the optimal tracers for sediment fingerprinting, statistical analyses, including range tests, Kruskal-Wallis, and discriminant function analysis, were used, and finally, sediment sourcing was performed for each sediment size class using the FingerPro package in R software. The results showed that in both monitoring stations and two rainfall events, with the increase in sediment particle size from <20 to >63 μm, the mean contribution of river bed in sediment yield increases from ⁓4 to ⁓70 % and decreases from ⁓24 to ⁓4 %, respectively. Although the contribution of agricultural lands in <20-μm sediments was 45 % and 49 % in MS1 and MS2, respectively, its contribution in >20-μm sediments was insignificant. In MS1, the contribution of rangelands in sediment yield decreases with the increase in the size of the sediment particles. However, due to the location of a large part of the rangelands in the vicinity of the outlet of the watershed, its contribution to MS2 does not show a clear trend with the increase in the size of the sediment particles. This research provides a clear picture of the sediment transport processes of different sizes in the watershed to be used in soil conservation plans.