The effects of seasonal groundwater level fluctuations on the contamination characteristics of total petroleum hydrocarbons (TPH) in soils, groundwater, and the microbial community were investigated at a typical petrochemical site in northern China. The measurements of groundwater and soil at different depths showed that significant TPH residue was present in the soil in this study area, especially in the vicinity of the pollution source, where TPH concentrations were up to 2600 mg kg(-1). The TPH concentration in the groundwater fluctuated seasonally, and the maximum variation was 0.8 mg L(-1). The highest TPH concentrations were detected in the silty clay layer and lied in the groundwater level fluctuation zones. The groundwater could reach previously contaminated areas in the soil, leading to higher groundwater TPH concentrations as TPH leaches into the groundwater. The coincident variation of the electron acceptors and TPH concentration with groundwater-table fluctuations affected the microbial communities in groundwater. The microbial community structure was significantly different between the wet and dry seasons. The canonical correspondence analysis (CCA) results showed that in the wet season, TPH, NO3(-), Fe(2+), TMn, S(2-), and HCO3(-) were the major factors correlating the microbial community. A significant increase in abundance of operational taxonomic unit J1 (97% similar to Dechloromonas aromatica sp.) was also observed in wet season conditions, indicating an intense denitrifying activity in the wet season environment. In the dry season, due to weak groundwater level fluctuations and low temperature of groundwater, the microbial activity was weak. But iron and sulfate-reducing were also detected in dry season at this site. As a whole, groundwater-table fluctuations would affect the distribution, transport, and biodegradation of the contaminants. These results may be valuable for the control and remediation of soil and groundwater pollution at this site and in other petrochemical-contaminated areas. Furthermore, they are probably helpful for reducing health risks to the general public from contaminated groundwater.
Slow-releasing oxygen materials were prepared to overcome some limitations regarding the low dissolved oxygen (DO) concentration and the low efficiency of in-situ purification in groundwater. Tests on slow-releasing oxygen materials that could be used to change the reductive environment in groundwater by slowly releasing oxygen were carried out. Oxygen-releasing laboratory experiments were conducted to monitor changes in DO concentration, pH, and total dissolved solids (TDS) in aqueous solutions. The adsorption of the materials on total Fe and Mn were also analyzed. The experimental results showed that the oxygen-releasing status of materials was stable at 15 mg/L after 20 d for fixed-shape materials. Paraffin wax shells and a KH2PO4 pH regulator facilitated the lowering of pH. The oxygen-releasing process followed the quasi-second-order kinetic model, and the oxygen-releasing rate constant K was 1.28, 1.51, and 1.97 (mg/(L·h)) for silt, silty sand, and fine sand medium, respectively. The larger the seepage medium particles were, the faster the pH and TDS dropped. Adsorption experiment results showed that adsorption on total Fe and Mn were well simulated by the Langmuir nonlinear isothermal adsorption equation. The maximum adsorption capacities of the materials on total Fe and Mn were found to be 0.708 mg/g and 0.438 mg/g, respectively.
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