Identifying Sources of Suspended Sediment using Radionuclides in an Agricultural Watershed in South Central Wisconsin

Abstract

Phosphorus (P) is an essential nutrient for plant and livestock growth. However, P loss in agricultural runoff can increase the frequency of toxic algal blooms and fish kills in receiving waters. Agricultural P loss occurs in both dissolved and particulate (sediment bound) forms. Suspended sediments play an important role in the transport of particulate P from fields to surface waters. Implementing appropriate management practices to control soil erosion and subsequent sediment delivery requires quantification of the relative contribution of sediment sources (e.g. stream bed, stream bank and upland areas under various land uses). Sediment fingerprinting using atmospheric fallout radionuclides can be used to apportion sediment sources, and thus provide valuable guidance for management decisions. Due to their long half-lives, the fallout radionuclides 137Cs and unsupported 210Pb are ideally suited for evaluating sediment transport processes that occur over long time scales. This fingerprinting method is independent of soil and rock type and can be used to differentiate between surficial and channel sources of suspended sediments. The objective of this study was to identify sources of in-stream suspended sediment in an agricultural watershed using the atmospheric fallout radionuclides 137Cs and 210Pb. The study was conducted in the non-glaciated region of southwestern Wisconsin in the Sugar Pecatonica River Basin, which is part of the Upper Mississippi River Basin. The watershed is approximately 5000 ha in size and contains primarily agriculture, forest, and grass land cover. The average watershed slope is about 11% with silt loam soils. Fieldwork included collection of both source materials (upland, streambed, and stream bank) and in-stream suspended sediments. In-stream suspended sediment samples were collected monthly for four months using passive time integrated in-stream tube samplers (Phillips et al., 2000). The samplers consist of a 10.2 cm diameter PVC tube with 0.4 cm diameter inlet and outlet, and collect a sample that is statistically representative of the grain size distribution in small streams. All source material samples were collected from the top 2.5 cm. Upland soil samples were collected from fields that represented various combinations of land use, soil type, and slope within the watershed. Upland samples were collected in a 20 m x 20 m grid with 5 m spacing and composited for analysis. Representative samples were also collected from the top 2.5cm of stream beds and eroding stream banks. All samples collected were stored at 40 °C and analyzed for organic matter content (percent volatile solids) and 137Cs and unsupported 210Pb. Radionuclide analysis was done through low background gamma counters. Over a four month period (mid-April through mid-August, 2010), results indicate that approximately two-thirds of in-stream suspended sediment originated from eroding stream banks and the remainder from upland areas. Within the upland categories (cultivated, pasture, woodland, grassland), cultivated lands followed by woodlands were significant contributors to in-stream sediments.