Field-Scale Phytoremediation of Mixed Contaminated Site in Chicago, USA

Big Marsh is one of the largest expanses of wetland within the Calumet region. The site, which is representative of many other unrestored wetland sites in this region that have been significantly altered by the steel industry and decades of legal and illegal dumping, has been massively altered from original conditions by industrial filling. These fill materials, as well as the soil and surface water have been found to be contaminated with both organic (polycyclic hydrocarbons) and inorganic contaminants (heavy metals). Therefore, the wetlands at Big Marsh are greatly in need of restoration efforts. The large size of the site as well as the shallow contamination make the implementation of phytoremediation technique as the most feasible and sustainable restoration technique to the remediation of the site with mixed contamination. The use of plants to restore areas impacted by industrial activities enhances soil structure and microbial activity, stimulating the biodegradation processes in the soil. The objective of the present work is to study the feasibility of the field – scale implementation of phytoremediation technology in a mixed contaminated site at Big Marsh, and study the final fate of the contaminants (heavy metals and polycyclic aromatic hydrocarbons) in the soil of the three different Areas of Concern found to be representative of the different ecotypes present at Big Marsh: slag disposal area, wet meadow and upland area. In addition, the enhancement of the phytoremediation technique by amending the soil with compost is also evaluated. The study duration extended to three growing seasons. During the first season, replicate test plots are prepared by tilling and homogenizing the soil, sediments and fill material. Soil is also amended with compost only at the slag disposal area. A total of 9 native and restoration plant species specific for each area are planted, and their survival and growth is monitored during two growing seasons. At the end of the second and third growing seasons, sampling of soils and plants is performed, and polycyclic aromatic hydrocarbons and heavy metals are analyzed. Additionally, sequential extraction is performed in all soil samples to determine the fractionation and mobilization of heavy metals in the soil throughout the experiment. The results showed that only 4 out of the 9 species planted at the slag disposal area, and 1 out of the 9 species planted at the wet meadow and upland area survived at the end of the experiment. All of the surviving plants except the one at the wet meadow area were herbaceous species and prairie grasses. A decrease in PAHs concentrations in the soil of the slag disposal area, and non-detectable levels of PAHs in the above and below ground plant samples was found. While no changes were observable in the soil concentration of PAHs at the wet meadow area and upland area, concentration of those contaminants in the roots of their surviving species was detected. However, the concentration of PAHs in shoots and leaves was also undetectable. Overall, no changes in the concentration of heavy metals in the soil of any of the three areas of concern are observed, except for Manganese, which decreases in the soil of the upland area. Concentrations of heavy metals were detected in the roots of all the surviving species analyzed but not in stems and leaves, except for Manganese, that was uptaken by the plant. Results from sequential extraction showed that the exchangeable fraction of the metals in the soil at each experimental area was very small, indicating that these metals have very low mobility. In the case of Mn, results from sequential extraction showed that it was mainly retained in the Fe and Mn oxides – bound fraction, what makes this element more bioavailable. Overall, the native grasses showed the best survival rates, and in combination with compost amendment at the slag disposal area showed the best performance. Additionally, the addition of compost amendment seemed to enhance the process of biodegradation of PAHs in the soil, and buffered the negative impact of high concentration of toxic metals in the soil that could potentially cause phytotoxicity.