Impact of heavy oil-polluted soils on reed wetlands

The impacts of heavy oil-contaminated soils on a reed wetland were studied during a 3-year field experiment in China's Liaohe Oilfield. Contaminated soils with a 30% heavy oil concentration were spread in the reed wetland in April of the first 2 years with 0.2, 2, 6, 18, and 0 kg of oil-polluted soil m⁻² for 4 reed beds and a control. In the third year no polluted soil was spread in the wetland. Results indicated that removal efficiencies in 0–80 cm soil layers were between 88 and 92% in the first 2 years, and up to 96% in the third year. The soil profile analysis pointed out that in the third harvest season, there was little residual heavy oil in soil layers 60–80 cm deep, with most of heavy oils removed in the 0–20 cm surface layer, thus avoiding additional pollution of the deep soil layer. Furthermore, contaminated soils had beneficial impacts on soil physiochemical indices of chloride (Cl⁻), pH, and organic matter in the 0–20 cm surface layer, as well as allowing total nitrogen (TN) and total phosphorus (TP) in the 0–20 cm surface layer to recover within the last 2 years of operation. At the end of this experiment, all these indices in the soil profile (0–80 cm) followed the same trend as those in normal soil. During the first harvest season, reed biomass in the wetland increased with increasing heavy oil pollution loading. In the last two harvest seasons, reed biomass followed the same trend, i.e., at the highest and lowest contaminated soil levels (18 and 0.2 kg oil-polluted soil m⁻² soil, respectively), reed biomass in reed beds increased with time, and resulted in levels higher than in the control. In contrast, at middle contaminated soil levels (2 and 6 kg oil-polluted soil m⁻² soil) reed biomass followed an inverse trend similar to that experienced by the control. Reed health results suggested that contaminated soils had no obvious adverse effects on reed height and number of leaves, and no significant effect on the eco-physiological indices of reeds, including cellulose, pentose, lignose, length and width ratio of cellulose, and width of cellulose. There was also no effect on germination percentages from below-ground rhizomes, but some inhibition on the germination process. In order to analyze heavy oil uptake and distribution within reeds, a ¹⁴C-hexadecane tracer experiment was conducted in 2003. Results indicated that after a growing season, heavy oil concentrated mainly in the below-ground root of reeds.