Impact of Impervious Cover on Base Cations in Green Infrastructure Soil

Abstract:

The transition from gray to hybrid green-gray infrastructure systems introduces ecological complexity and variability into the highly engineered urban environment and there is high potential for unintended consequences. One emerging concern is the potential accumulation of deicer salt in green stormwater infrastructure (GSI). In cold climates, deicer salt that is applied to roadways and sidewalks to protect pedestrians and drivers has the potential to accumulate in GSI and impact its hydrological function. Excess accumulation of salt-derived cations can decrease soil infiltration rates and plant water uptake, both hydrological processes promoted by GSI. Due to the potential risks of deicer salt on GSI performance and the absence of relevant data we collected data on the spatial and temporal variability of cations in urban soils. Soil samples were collected in October and November 2019 across a gradient of impervious area in 17 GSI practices and 1 site each of yard, garden, forest and floodplain land uses. Sites with no expected salt accumulation were also sampled and are referred to as reference sites (e.g., forest and prairies). Samples at 10- and 50-cm depth were analyzed for cations (Ca, Na, Mg), conductivity, and pH. Cation concentrations were higher in urban sites than reference sites. Mg and Ca were 100 times higher than Na. Mg and Ca may be derived from (1) imported soil in GSI and/or (2) eroded material from the urban landscape. Because GSI soils are enriched in Mg and Ca, Na derived from deicer salt may not have the opportunity to bind to cation exchange sites or is leached readily by stormwater. Conductivity was highly variable within and across sites; differences with impervious cover were inconclusive. pH was lower in the reference sites compared to the urban sites. Lower pH in reference soils may be associated with lower cation exchange capacity, which could explain the lower concentrations of Na, Mg, and Ca. These results improve understanding of ecohydrological systems in engineered, urban environments and provide initial data that can be used to optimize GSI design and operations and maintenance in cold winter climates.