Salt Dynamics in prairie wetlands under changing climate

Uri Nachshon, Andrew Ireson, Howard Wheater, Garth van der Kamp, Samuel Ryan Davies

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Impacts of Climate Change

ABSTRACT: ‚The Prairies™ - The semiarid glaciated plains of the North American continent are characterized by undulating terrain, rich in sulfate salts, with ephemeral streams and a large number of wetlands marked by seasonal or semi-permanent ponds. The prairie region supports a diverse community of wildlife species, as well as a major agricultural industry, which are both highly sensitive to the hydrological conditions of the wetlands. Meteorological conditions have large effects on wetland pond volumes, and spatial distributions. In dry seasons the number of ponds is reduced, while heavy rainfall events or prolonged wet periods can result in increased wetland water storage and inter-connectivity, extreme streamflow events and the flooding of many depressions and areas that are usually dry. Over the recent period of settlement, around 100-200 years in Canada and the United States, respectively, land use changes associated with agricultural activities have had major impacts on the hydrological processes. Particularly significant changes are the conversion of native grasslands to cropped land and drainage of wetlands to expand the cropped area. These changes to the hydrology can affect the wildlife that depends on the wetlands, as well as agricultural activity and water resources. Some 40-50% of the area of the prairies does not normally connect to the major fluvial systems in the region, but connectivity can change significantly under extreme rain events. High salt concentrations in subsurface amplify the potential hazard associated with changes in the hydrology, as these changes may result in salinization of the soils, surface and subsurface water. It is well known from other places in the world that changes in the hydrological regime can result in severe environmental and economic problems associated with salt dissolution, migration and crystallization. Therefore, understanding the coupled hydrological, hydrogeological and geochemical system of the prairies under various climatic conditions and land use scenarios and the consequences for salt migration and accumulation is an important aspect of water and land management. Here we explore salt dynamics under normal and wet climatic conditions at a field site near St. Denis, Saskatchewan, using field data from the last 30 years. Measurements include meteorological data, soil moisture, soil salinity, groundwater levels and pond volumes, salinity, and chemical composition. The summers of 2010 and 2012 were exceptionally rainy, resulting in unusually high pond water levels. This large influx of water into the ponds was accompanied by a marked increase in the pond water salinity, contrary to the dilution that might have been expected intuitively. Here we present field measurements which quantify and improve understanding of the subsurface salt dynamic processes and the potential risk of salinization for the prairie environment. It is evident that hydrological processes within the shallow subsurface, between the wetlands and adjacent uplands result in quick and significant salt transport under extreme wet conditions. Changes in salt concentrations of various ponds and shallow ground water were measured under the extreme wet conditions and a conceptual model was proposed to explain the salt dynamics under typical dry and the extreme wet conditions. A numerical model was used to simulate hydrological processes between two adjacent ponds in a typical prairie landscape and results were compared to the field measurements.

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