The University of Sheffield
Catchment Science Centre

Processes controlling the distribution and natural attenuation of phenolic compounds in a deep sandstone aquifer

Steven F. Thornton, Sean Quigley, Michael J. Spence, Steven A. Banwart, Simon Bottrell, and David N. Lerner

ABSRACT

Processes controlling the distribution and natural attenuation of a complex mixture of phenolic compounds (phenol, cresols, xylenols) released from a coal-tar processing plant in a deep Triassic sandstone aquifer are evaluated from vertical profiles along the plume centerline at 130m and 350m from the site. Up to four groups of contaminants (phenols, mineral acids, NaOH, NaCl) form discrete and overlapping plumes in the aquifer. Their distribution reflects a complex transient source history with episodic releases of inputs. The organic contaminants form two plumes, which result from releases of compositionally similar fractions at different concentrations. Contaminant distributions in the aquifer are determined more by site source history than degradation. Contaminant degradation at TOC concentrations up to 6500 mg l-1 (7500 mg l-1 total phenolics), is occurring by aerobic, NO3-reducing, Mn/Fe oxide reducing, SO4-reducing, methanogenic and fermentative processes, with the accumulation of inorganic carbon, organic metabolites (4-hydroxybenzoic acid, 4-hydroxybenzaldehyde), acetate, CH4 and H2 in the plume. Aerobic and NO3-reducing processes are restricted to a 2m thick plume fringe but Mn/Fe oxide reducing, SO4-reducing, methanogenic and fermentative processes occur concomitantly in the plume. Dissolved H2 concentrations in the plume vary from 0.7-110nM l-1 and acetate concentrations reach 200mg l-1. The occurrence of a mixed redox system and concomitant TEAPs could be explained with a partial equilibrium model based on the potential in situ free energy (Gr) yield for oxidation of H2 by specific TEAPs. The distribution of H2 and TEAPs in this plume cannot be explained using empirical H2 concentration ranges and competitive exclusion of microbially mediated redox processes. Respiratory processes are rate limiting in determining the distribution of H2 and TEAPs. The kinetics of respiratory processes apparently determines the dynamics of H2 oxidation in this system more than the rate of fermentation. Most (min 90%) contaminant degradation occurs by aerobic and NO3-reducing processes at the plume fringe. This potential is determined by the supply of aqueous O2 and NO3 from uncontaminated groundwater, as controlled by transverse mixing, which is limited in this aquifer by low dispersion. Consumption of mineral oxides and SO4 is respectively <0.15% and 0.4% of the available capacity in the aquifer, and degradation using these oxidants is <10%. Fermentation processes are significant in contaminant turnover, accounting for 21% of degradation products present in the plume, and indicating that microbial respiration rates are slow in comparison with fermentation. The potential for degradation in the plume is very low due to inhibitory effects of the contaminant matrix. Degradation products correspond to no more than 22% mass loss over the life of the plume, providing a first order plume scale half-life >140 years. The phenolic compounds are biodegradable under the range of redox conditions in the aquifer and the aquifer is not oxidant limited, but the plume is likely to be long-lived and to grow. Degradation is likely to increase only after increased dilution, suggesting that transport processes exert a greater control on the natural attenuation of this plume than oxidant availability

Schematic plan and section of the site, showing location in UK, general observation wells, multilevel samplers and location of the plume.