Melody Caron et al. Sediment fingerprinting in the Lower Little Bow River Sediment fingerprinting in the Lower Little Bow River using Cs-137 as a tracer Melody Caron Department of Soil Science, University of Manitoba David A. Lobb Department of Soil Science, Watershed Systems Research Program, University of Manitoba Jim J. Miller Lethbridge Research and Development Centre, Agriculture and Agri-Food Canada Kui Liu Department of Soil Science, University of Manitoba Phillip N. Owens Environmental Science Program, University of Northern British Columbia The Lower Little Bow River watershed is a source of irrigation water for southeast Alberta. However, sediment in the river can be detrimental to both irrigation pump equipment and the health of the river. In order to understand and manage how sedi- ment moves through a watershed, a study of sediment sources was conducted along a reach of the Lower Little Bow River employing fingerprinting techniques. The radionuclide cesium-137 was used as a tracer to determine whether the sediment in the river is being generated by surface or subsurface erosion processes. The main sources of sediment were found from subsur- face erosion of an irrigation return-flow channel, coulee walls, and the river’s stream banks. This demonstrates that subsurface erosion is the principle generator of sediment in the Lower Little Bow. Through measurement of multiple points along the reach, the study also determined that the composition of the suspended sediment load did not change as it moved downstream in the river. Keywords: soil erosion, sedimentation, sediment fingerprinting, cesium-137, irrigation, Alberta Introduction tion, where an increase in nutrient concentration causes exces- The sources of suspended sediments in watersheds can be identi- sive algal growth on the surface of water (Barthod et al. 2015). fied through the technique of fingerprinting using environmental Moreover, contaminants such as industrial chemicals, metals, tracers. Employing this technique, Davis and Fox (2009) dem- and pathogens can be transported within watersheds by sediment onstrated that nutrient-bound sediments within a watershed can acting as a vector. As a result, the amount and nature of sediment cause significant deleterious effects on the aquatic ecosystem and its transport can have a significant impact on the ecology, as well as water quality. This decline is linked to eutrophica- food web, mortality, reproductive stress, and economic value of This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited. Correspondence to: Melody Caron, Department of Soil Science, University of Manitoba, 13 Freedman Crescent, Winnipeg, MB R3T 2N2 Email: [email protected] Prairie Perspectives: Geographical Essays 2016, 18: 50–56 50 ISSN 1911-5814 Melody Caron et al. Sediment fingerprinting in the Lower Little Bow River a watershed ecosystem (Koiter et al. 2013a). In the Lower Little Bow River (LLBR) considered in this study, the large amount of sedimentation occurring is a concern for these reasons. More sediments will result in turbidity and siltation, creating oppor- tunities for nutrient transfer and damaging equipment such as irrigation pumps. In order to manage and improve the health of sediment-affected watersheds, it is essential to discover the sources of those sediments. Study area The focus area for this study was the LLBR watershed, located within the Oldman River basin of southeast Alberta, Canada (Figure 1). The study was conducted in a 2565-hectare sub- catchment located at 112°37’30”W, 50°00’00”N that was origi- nally part of an Agriculture and Agri-Food Canada Watershed Evaluation of Beneficial Management Practices (WEBs) proj- ect (AAFC 2013). Weather in the LLBR watershed is charac- terized by strong chinook winds and a regional average annual Figure 1 precipitation of approximately 386 mm a year (AAFC 2013). Map showing the Lower Little Bow River with the four sediment Its watershed is a mixed grass sub-region with geology mainly collection sites LBW1, LB4-14, LBW4, and LB4 relative to the Up- dominated by sand and coarse gravel and its surface material is stream and Irrigation Return Channel sediment samples mostly glacial till (Miller et al.2011). The classifications of soils (Map source: © Her Majesty the Queen in right of Canada) found in the study area are Orthic Dark Brown Chernozems, Rego Dark Chernozems, and Orthic Regosols (Rahbeh et al. 2013). Agricultural uses within the watershed include cereals, forages, and intensive livestock and cow-calf operations (AAFC tributing to the suspended sediment load in a watershed, these 2013; Rahbeh et al. 2013). natural tracers are measured both in potential sources and in the The Travers Reservoir, rainfall, and irrigation return flow are sediment (Davis and Fox 2009). major factors affecting the flow rate within the LLBR (Little et Radionuclides are environmental tracers than can label soil al. 2003). Water for irrigation in the local area is supplied from particles through the process of adsorption to clay minerals and the river itself, as well as by the Lethbridge Northern Irriga- organic matter, both of which have high specific surface areas. tion District (Little et al. 2003; Rahbeh et al. 2011). Irrigation This adsorption between radionuclide fallout particles and clay is a cause of concern in the LLBR due to its potential for sedi- minerals or organic matter can then be measured as a value for ment delivery to the river through irrigation return flow chan- the activity of the radionuclide within soil or sediment samples nels (NRCB/CEAA 1998). While irrigation is valuable to ag- (Koiter et al. 2013a). One readily available radionuclide in the ricultural producers, its contribution to sediment loading of the environment is 137Cs, which has a half-life of 30.2 years. Its pres- river can have detrimental effects as it can impair pumps used ence can be linked back to a peak deposition over the period downstream of irrigation return flow channels, necessitating re- of 1950s–1960s when Cold War thermonuclear weapons test- pairs and leading to decreased crop growth and herding. ing released them into the atmosphere (Krishnappan et al. 2009; Koiter et al. 2013b). As a result, the presence or absence of 137Cs Cesium-137 (137Cs) as a tracer in soil can be an indicator in distinguishing surface from subsur- Sediment fingerprinting has been used in order to determine bet- face materials, determining whether land has been disturbed or ter management practices for controlling non-point source pol- remains untouched, and revealing whether soil has been deposit- lutants within watershed ecosystems. For example, surface run ed or eroded (Koiter et al. 2013a). For instance, Smith and Blake off is considered a non-point source because of its ability to con- (2014) showed that an important step in sediment fingerprinting tribute sediment and or nutrients from a large area rather than a is statistically differentiating the properties that identify target single point source such as a channel. This requires management sediment sources; as 137Cs settled onto the landscape, the radio- techniques that can be applied over a larger area rather than nuclide only adsorbed into 10–15 cm of surface topsoil. This management of a single point. By understanding what sources range provides the statistical capability to distinguish between are contributing to the sediment within a river better manage- surface soils, which will show adsorption of 137Cs, and subsur- ment practices can be applied to control their negative effects. face materials which will not. 137Cs also demonstrates the highly Fortunately, there are many different biophysical and geochemi- conservative properties necessary for use as a tracer, resulting cal properties of sediment and soil that can be used as tracers from its non-exchangeable behaviour in the soil matrix (Li et al. for linking sediments back to their sources (Koiter et al. 2013a; 2010). Thus 137Cs can be uniquely identified to differentiate be- 2013b). In order to determine the proportion of each source con- Prairie Perspectives: Geographical Essays 2016, 18: 50–56 51 ISSN 1911-5814 Melody Caron et al. Sediment fingerprinting in the Lower Little Bow River tween surface and subsurface sources of sediment contributing irrigated fields and the erosion of the channel itself result in a to suspended sediment in an aquatic system. large amount of sediment being delivered to the river. 137Cs was used in this study to pursue two objectives: first, to examine 137Cs activity in the sediment as it moves downstream Gamma ray spectrometer methods in the reach, and second, to determine if surface or subsurface Sediment and soil samples were analyzed for 137Cs activity us- erosion is the main cause of sediment influx into the Lower Lit- ing gamma ray spectrometry (Wallbrink et al. 1999). Analysis tle Bow River. was carried out for up to 24 hours per sample depending upon the amount of soil used and the concentration of 137Cs. Samples were placed in containers and packed to ensure a uniform geom- Methods and materials etry and bulk density. The height of the sample in the container and the gamma detector efficiency for that container geometry Methods of sediment and soil collection were recorded. Depending on the amounts of sample available, Two general sample types were taken in this study: first, source containers with different geometries were used. The amount of samples were used to gain information about soil at potential activity was determined from the gamma spectrum obtained erosion sites contributing to the river, and second, suspended from each sample and in order to account for decay, each mea- sediment samples were taken in the water once the materials surement was corrected back to a single date based on the first had entered. The sites chosen for suspended sediment collection year of sampling: 1 January 2009.