Ineffective Natural Attenuation of Degradable Organic Compounds in a Phenol-Contaminated Aquifer

Two detailed vertical profiles through a complex plume of phenolic contaminants in a Triassic sandstone aquifer show that natural attenuation by biodegradation and dispersion is active but very slow. The plume has a microbially active aerobic and NO3 reducing fringe that is less than 2 m thick at both 150 and 350 m downstream of the source. The anaerobic core has evidence of active bacterial populations and degradation at total organic carbon (TOC) concentrations up to at least 1400 mg/L (1800 mg/L total phenolics), although gross half-lives are more than 50 years. There is evidence from the same locations of Mn, Fe, and SO4 reduction, with the latter inhibited by the pollutant matrix and not significant at concentrations more than 1000 mg/L TOC. Degradation of these contaminants in this aquifer is influenced by a range of environmental factors, including the chemical toxicity and pH of the contaminant matrix, and inputs of electron acceptors into the plume by dispersion. The results show that the plume is likely to grow under the present conditions, despite the biodegradable nature of the organic pollutants and availability of suitable electron acceptors. Vertical profiles have proved a cost-effective method of understanding the evolution of the plume.