The Effect of Common Road Salts and Organic Additives on Microbial Respiration in Urban Streams

Due to significant population growth and urbanization, there has been an increase in impervious surfaces such as roads and sidewalks. During winter, impervious surfaces are often treated with salt to make road conditions safer. However, this has significant impacts on water quality when salt enters urban streams during runoff events. The addition of road salts to urban streams has been shown to have detrimental effects to various organisms across trophic levels and affects nutrient cycling and greenhouse gas release. These findings have made the development of alternative treatments necessary, such as using organic additives, like beet juice, which can decrease the amount of salt applied. Yet, there are few studies exploring the tradeoffs of using new “environmentally friendly” alternatives. With the addition of beet juice comes the addition of carbon and nutrients, like phosphorus. Increasing nutrient and carbon availability can increase microbial respiration rates, in turn lowering the dissolved oxygen concentrations in the water column, which is largely important for water quality. In this study, we compared the response of microbial respiration rates within and between two urban streams of differing base conductivity levels (~319 and ~463 μS/cm, proxy for salinization) with the addition of traditional road salts (NaCl) and organic additive deicers during two different seasons. We collected sediments from two urban streams in Fayetteville, Arkansas and conducted laboratory assays with spiked stream water using low and high NaCl (0.21 and 2.1 g/L Cl-), low and high beet brine (0.25 and 2.5 g/L Cl-), and control. Results from this study indicate that high concentrations of beet brine significantly increased respiration rates for the stream with a greater base conductivity that may be adapted to salt additions (p<0.05). The outcome of this study indicates that while beet brine can decrease the amount of salt applied to the roads, there are significant determinants to this treatment, such as stimulating respiration, that bring into question its “environmentally friendly” label.