Phytoremediation of deicing salt from roadside soils

Abstract

Rock salt, consisting of primarily sodium chloride, is a low-cost and effectivedeicing agent for winter weather events in northern climates. However, most of the salt dissolves in melting snow and ice and eventually finds its way into soil, streams, lakes, wetlands and groundwater, causing negative impacts to aquatic life and plants. Like deicing salt, improper irrigation can also lead to salt contamination resulting in substantial global agricultural and economic losses. While many current forms of soil remediation are costly and disruptive, phytoremediation, on the other hand, is an inexpensive and environmentally friendly method. Phytoremediation uses salt tolerant plants to uptake sodium chloride into their aboveground biomass, which is then harvested and utilized for purposes such as animal feed or bioenergy. This research explores the viability of using salt tolerant plants to remove sodium chloride from Minnesota (MN) roadside soils, thus reducing the overall quantity of road salt in the environment. First, potential plant species were inventoried and screened for their potential for roadside phytoremediation using a decision matrix. Then, some of the highest rated species from the matrix were evaluated for their capability to uptake sodium and chloride from soil in a greenhouse screening experiment. The plants with the highest total salt uptake were the annual species common sunflower and pitseed goosefoot, followed closely by the perennial tall fescue turfgrass with showy goldenrod and weeping alkaligrass trailing slightly behind tall fescue. Perennial species are more desirable for roadside planting and management and therefore may be more likely candidates for phytoremediation of road salt from roadside soils. Another greenhouse study was performed on some commonly grown MN plants. Of the species tested, sugar beet showed the highest concentration of salt in itaboveground biomass with around 20% salt by dry mass when watered with a salt solution of 100 mM NaCl. The promising results of sugar beet led to the investigation of the phytoremediation ability of other beet crops and the potential salt uptake of sugar beet in an agricultural setting. All the beet crops tested showed similar salt uptake to sugar beet in a greenhouse experiment and could therefore also be potential strong candidates for phytoremediation. Testing of beet samples from agricultural soils found that the harvest of the aboveground biomass can extract around 50 g salt/m2 from slightly moderate to moderately saline soils. Also, the high salt uptake by common sunflowers led to a greenhouse experiment comparing common sunflower to the perennial Maximilian, tall, stiff, western, and sawtooth sunflowers. While the perennials had significantly lower salt uptake than common sunflower, they had higher salt extraction than many of the perennials tested in the previous experiments. Using the greenhouse study results, common sunflower was estimated to remove roughly 30 to 50 g salt/m2 in an agricultural setting with moderately saline soil. Two field studies were conducted using species identified as having high potential in the decision matrix. The low salt content of roadside soils at our testing sites led to additional salting of the roadside plots to determine potential salt uptake in a heavily salted roadside field condition. In the first roadside study at a road research facility, the site was not readily accessible throughout the experiment and therefore was not able to be easily maintained. Regardless, common sunflower showed high levels of survival and growth in the two types of plots tested; seeded plots and plots with transplants that were germinated in a greenhouse. In both the seeded and transplant plots, common sunflower showed the highest salt uptake of plants tested with an extraction potential of around 10 g salt/m2. The second roadside study at a different site the following year was more easily maintained through weed control and watering. Common sunflower showed the highest salt extraction potential in the seeded plots with again around 10 g salt/m2. In the transplant salted plots, pitseed goosefoot had much higher salt extraction potential with an average of nearly 50 g salt/m2. Overall, this research has found multiple annual and perennial species that are promising candidates for phytoremediation that should be further investigated for their ability to grow together in a species mixture in various field conditions. Also, it was determined that sugar beet and common sunflower have significant potential for salt remediation of agricultural soils. Therefore, this research has the potential to reduce the salinity of salt contaminated roadside and agricultural soils over time through harvest of aboveground biomass of the highest potential species tested.