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

Abstract

Processes controlling the distribution and natural attenuation (NA) of phenol, cresols and xylenols released from a former coal-tar distillation plant in a deep Triassic sandstone aquifer are evaluated from vertical profiles along the plume centerline at 130 and 350 m 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 changing source history with releases of contaminants from different locations. Organic contaminant distribution in the aquifer is determined more by site source history than degradation. Contaminant degradation at total organic carbon (TOC) concentrations up to 6500 mg l −1 (7500 mg l −1 total phenolics) is occurring by aerobic respiration NO 3-reduction, Mn(IV)-/Fe(III)-reduction, SO 4-reduction, methanogenesis and fermentation, with the accumulation of inorganic carbon, organic metabolites (4-hydroxybenzaldehyde, 4-hydroxybenzoic acid), acetate, Mn(II), Fe(II), S(−II), CH 4 and H 2 in the plume. Aerobic and NO 3-reducing processes are restricted to a 2-m-thick plume fringe but Mn(IV)-/Fe(III)-reduction, SO 4-reduction, methanogenesis and fermentation occur concomitantly in the plume. Dissolved H 2 concentrations in the plume vary from 0.7 to 110 nM and acetate concentrations reach 200 mg l −1. The occurrence of a mixed redox system and concomitant terminal electron accepting processes (TEAPs) could be explained with a partial equilibrium model based on the potential in situ free energy (Δ G r) yield for oxidation of H 2 by specific TEAPs. Respiratory processes rather than fermentation are rate limiting in determining the distribution of H 2 and TEAPs and H 2 dynamics in this system. Most (min. 90%) contaminant degradation has occurred by aerobic and NO 3-reducing processes at the plume fringe. This potential is determined by the supply of aqueous O 2 and NO 3 from uncontaminated groundwater, as controlled by transverse mixing, which is limited in this aquifer by low dispersion. Consumption to date of mineral oxides and SO 4 is, respectively, <0.15% and 0.4% of the available aquifer capacity, and degradation using these oxidants is <10%. Fermentation is a significant process 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. Under present conditions, the potential for degradation in the plume is very low due to inhibitory effects of the contaminant matrix. Degradation products correspond to <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 expand. Degradation is likely to increase only after contaminant concentrations are reduced and aqueous oxidant inputs are increased by dispersion of the plume. The results imply that transport processes may exert a greater control on the natural attenuation of this plume than aquifer oxidant availability.