This guest post was written by Rob Rossi, a graduate student in the department of Geology and Environmental Science at the University of Pittsburgh. He was a graduate summer intern of NMRWA in 2015.
Road salt is a common part of winter for many Pittsburgh residents. In Pennsylvania, more than 840,000 tons of road salt (sodium chloride, or table salt) were applied to roadways between 2009 and 2014. Although it helps keep our roads and sidewalks ice and snow free, road salt has unintended consequences. Many people are familiar with the ever annoying winter problems of salt stained clothing or shoes/boots, but the environmental effects of road salt are less obvious. Road salt can have numerous negative effects on the environment, such as increased fresh water and soil salinity, and less obvious effects, such as increased time necessary for rain to soak into the soil. Additionally, when road salt dissolves in highway runoff, these waters have high total dissolved solids (TDS), which can flush roadside soil metals from clay particles (see animated Figure 1). Metals flushed by these reactions can include plant nutrients (e.g., potassium, calcium, magnesium) or toxic trace metals (e.g., arsenic, lead, cadmium).
Figure 1. Animation of a cation exchange reaction. Potassium (K), calcium (Ca), and magnesium (Mg) ions bound to soil clay particles are exchanged by sodium (Na) ions in solution. Mobilized metals are then released into the soil water, and ultimately the ground or surface water.
Road Salt Study in Nine Mile Run
Figure 2. Lysimeters are plastic tubes with a ceramic cup. To collect a soil water sample, a scientist applies a vacuum (arrow) and the lysimeter sucks up soil water (dashed lines) like a straw.
Rob Rossi, a graduate student in the Department of Geology and Environmental Science at the University of Pittsburgh, has been researching the effects of road salt on roadside soils in Nine Mile Run. Specifically, Rob has been analyzing soil and soil water chemistry in samples collected from three roadside soil water sampler “nests”. Each nest is a group of four lysimeters which behave much like giant straws, sucking up soil water samples when a vacuum is applied to the end of the soil water sampler (see Figure 2). The lysimeters collect soil water at roughly 6, 12, 24, and 36 inch depths along a hill slope perpendicular to I-376.
In the soil samples, soil sodium concentrations are highest in soils collected from near the road. Soil sodium concentrations decrease with distance from the roadway, approaching values observed in the local bedrock (see Figure 3). One theory is that high sodium concentrations can be attributed to the minerals breaking down in the bedrock but because sodium concentrations in roadside soils are much higher than sodium concentrations found in the bedrock, minerals in the bedrock breaking down is likely not what inputs sodium to these soils. Instead, the application of road salt to I-376 is likely causing high sodium concentrations in roadside soils.
Figure 3. Sodium concentrations in the sampled top (black), mid (red), and bottom (grey) hillslope soils. The vertical dashed line indicates the average sodium concentration in local bedrock. Parts per million (ppm) is a measurement scientists use to describe the concentration of an element. In other words, if a bucket holding a total of 1 million marbles contained 100 ppm of blue marbles, 100 of those 1 million marbles would be blue marbles.
Sodium concentrations in sampled soil waters peak at different times throughout the year relative to the location along the hillslope (see Figure 4). In particular, the earliest peaks in soil water sodium concentrations occur in the top hillslope soil waters in late February/early March in the intermediate depth (39 and 61 cm depth) soil waters. Additionally, soil water samples from the deepest top hillslope nest have, in general, the highest sodium concentration. While sodium concentrations spike in soil waters collected from all depths of the top hillslope nest station, soil water sodium concentrations peak only in deeper soil waters of the mid hillslope nest. Moreover, the peak in soil water sodium concentrations at the mid hillslope nest do not peak at the same time as when soil water sodium concentrations peak at the top hillslope nest.
Figure 4. Sodium concentrations in top (a), mid (b), and bottom (c) hillslope soil waters collected between October 2013 and November 2014. The light blue box indicates the time of the year when road salt is not applied to roadways.
These patterns in soil water sodium concentrations suggest that the way soil water flows in roadside soils influences the movement of sodium through these soils. Specifically, because the deeper top hillslope lysimeters (i.e., 12, 24, and 36 inch) peak before the shallowest (i.e., 6 inch) lysimeter, high TDS waters likely interact with deeper soils first. High TDS runoff from the highway is often observed to enter the soil column via infiltration (i.e., water percolating downwards through the soil), which produces a peak in sodium concentrations in the shallowest soil waters first. However, because this pattern in soil water sodium concentrations is not observed in samples collected from the Nine Mile Run transect, sodium is potentially transported to deeper soils via lateral flow originating from leaking highway drains and water flow between bedrock layers.
Previous scientific studies have observed that sodium loadings to soils persist beyond the period when road salt is applied to roadways, and this relationship is also apparent at this study site. Specifically, sodium persists as slow moving wave, where peaks in top hillslope soil water sodium concentrations occur within a month of when road salting ends, and peaks in soil water sodium concentrations at the mid and bottom hillslope stations occur later in the year. Thus, the distance from the roadside affects when soil water sodium concentrations will peak, suggesting that sodium is relatively slowly released from roadside soils throughout the spring and summer.
How does road salt affect the water quality of Nine Mile Run?
The results of this study suggest that sodium and metals are continually flushed to stream waters throughout the year. When sodium levels are high, the ecosystem cannot physiologically maintain a salt balance, which affects aquatic organisms living in the stream – particularly plants and animals that are not adapted to high concentrations of ions, and therefore cannot regulate the water and salt content within their cells. This stress can change the diversity of species within the ecosystem. The increased metal loading could impair the stream ecosystem, negatively impacting aquatic life such as fish. Some metals may be either beneficial or toxic, depending on their concentration. The primary mechanism for toxicity to organisms that live in streams is by absorption or uptake across the gills. The metals that are most toxic to aquatic organisms are Copper, Iron, Cadmium, Zinc, Mercury, and Lead.
Figure 5. A conceptual model of how sodium travels through the hill slope soils next to I-376. The color of the arrows indicates the relative timing of when sodium is transported via this flowpath. Blue occurs in mid to late February, dark grey in early March, orange in early May, and red in early August.
Thus, it is likely that road salt application impacts soils down the hillside of I-376, and that the negative impacts of road salt application are not limited to the winter and early spring.