Road salt soil contamination and remediation methods using halophytes
In northern latitudes, road salt (sodium chloride; NaCl) application is leading to soil and freshwater salinization, and having widespread environmental impacts. Canada applies over 5 million tonnes of road salt annually in order to maintain road safety during the winter. As there is currently no cost-effective alternative to sodium chloride, there is a need to develop a method of mitigating its detrimental environmental impacts. Phytoremediation offers a sustainable, in situ management option for road salt contamination whereby plants take up the salts and are then harvested to remove this contaminant from the environment. Three native halophytes (salt-tolerant plants) in the genus Atriplex were studied to determine their suitability for the phytoremediation of road salt. The Atriplex spp. accumulated 18-55 mg Na+/g dry weight (DW) and 41-64 mg Cl-/g DW when grown for a two-month period in soil spiked with NaCl to simulate a very highly salt-contaminated roadside (4620 µg/g Cl-). Based on extraction rates, it would theoretically take 6, 19, and 9 growing seasons for A. patula, A. hortensis, and A. canescens to remove all salt from a less highly contaminated area (1540 μg Cl-/g). Analysis of roadside soil samples from a minor highway in southern Ontario indicate that current chloride levels are actually an order of magnitude lower than this scenario (mean of 71 ± 89 μg Cl-/g in April 2019) when salt is applied as a 30% NaCl, 70% sand mixture. Thus, roadsides with relatively low salt input could be remediated in a shorter timeframe. In order for phytoremediation to be effective, it is important to understand how road salt migrates through soil. A mesocosm study was completed to investigate the movement of Na+ and Cl- in roadside soil following the spring melt. Analysis of the soil profiles revealed that Na+ was largely held in the top layer (0-5 cm), with 87% retention in the soil profile with a high salt application rate (300 kg/km). Chloride mainly followed the movement of porewater, with a much lower retention in soil (11% retention). Thus, for phytoremediation, halophytes should be planted early in the spring in order to access the chloride before it leaches into groundwater. The results from this thesis can be used to develop best practices for the phytoremediation of road salt.
URI for this recordhttp://hdl.handle.net/1974/26671
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