Spatiotemporal Dynamics of Road Salt in a Highway Bioswale: A Comparison of Point and Continuous Monitoring Methods

Abstract

Stormwater entering stormwater control measures (SCMs) from roadways can carry high loads of deicing salt, which can impede vegetation growth. We investigated spatial and temporal patterns of soil conductivity to compare point and continuous monitoring methods, and directly measured plant responses in an SCM that receives runoff from Interstate 95 in Philadelphia, USA. We buried time lapse electrical resistivity tomography (ERT) cables and soil conductivity sensors adjacent to plants, which were monitored for size, leaf chlorophyll, and tissue sodium. Soil sensors showed that porewater conductivity reached high levels (10—20 mS/cm) immediately after February snow events, and that conductivity was consistently higher in topographically low portions of the SCM. These patterns were also seen in the ERT data; however, ERT also identified high spatial heterogeneity in conductivity, despite the presumed uniformity of the soil. Both methods showed that conductivity levels remained elevated through leaf out (~ April), likely explaining why both of the plant taxa we monitored (daylily and feather reed grass) had impaired growth in the topographically low zone, as well as elevated leaf and root tissue sodium. Further, ERT surveys captured changes in conductivities at depth suggesting that plant rooting depth may play a role in winter survival. Although the soil sensors provided extensive temporal data, the ERT data provided a spatial component unachievable by sensors. Given that ERT showed deicing salts remained in the soil long enough to affect plant health, placement of salt tolerant taxa should account for flooding regions and plant root depth.