Field-scale assessment of weathered hydrocarbon degradation by mixed and single plant treatments

Abstract

Phytoremediation is gaining recognition as a viable treatment option for hydrocarbon contaminated soil. However, there is a recognized need for field research to corroborate the findings of controlled environment studies, particularly in cold climate regions. In this study, a two-year field trial was conducted at a weathered hydrocarbon flare-pit site in southeastern Saskatchewan, Canada. Three plants commonly used in phytoremediation mixes (tall wheat grass, Altai wild rye (AWR) and alfalfa), a mix of all plants, and non-planted controls were established and sampled regularly over two growing seasons. Total petroleum hydrocarbon (TPH) concentration and microbial parameters, including endophytic and rhizosphere degrader populations, were measured at 6-week intervals during the growing seasons. Significant differences occurred in degradation trends in the first growing season, with AWR promoting greater than 50% TPH degradation while no cumulative degradation occurred in mixed plant or control treatments. Some of the discrepancy in degradation potential was related to microbial population dynamics. Results show that AWR selectively recruits endophytic hexadecane degraders in response to high TPH concentration ( r, 0.795; p < 0.01) and then maintains these communities during times of environmental stress. During local drought, when most plants experienced significant decreases in rhizosphere and endophytic degrader communities, AWR supported up to 100 times more endophytic hexadecane degraders than the other plants. It is probable that the increased degradation seen in AWR treatments is related to its ability to act as a refuge and hence subsequent source for hydrocarbon degrader communities. As both plants and microbial communities mature, these discrepancies in degradation potential are decreased and cumulative TPH degradation increases in all treatments. Although this study shows that the use of mixed plant treatments may initially hinder the achievement of remediation goals, extenuating factors, including the increased desorption of previously unextractable hydrocarbons, may cause an underestimation of the actual amount of degradation occurring in all treatments.