Hydrochemistry of pollutant attenuation in groundwater contaminated by coal tar wastes

Abstract

AbstractThis paper describes a preliminary hydrochemical study of a Triassic Sandstone aquifer in the UK East Midlands which is contaminated by coal tar waste products from a former coal-carbonization plant. Processes controlling the attenuation of inorganic species within the identified plume are examined, supported by equilibrium and reactive transport modelling of contaminant fate. The study forms part of a more detailed research programme examining the long term fate of pollutants from this site in the aquifer. Groundwater beneath the site is polluted with NH4 (up to 350 mg l-1), SO4 and NVOC from the plant and nearby coal mining operations. Concentrations of organic chemicals within the plume are not significant. The aquifer is calcareous, (Fe) oxide-rich and aerobic, and geochemical modelling suggests that the native groundwater chemistry is primarily controlled by carbonate equilibria. Anaerobic conditions are sustained within the plume by microbial oxidation of the NH4 and NVOC load which is coupled to the reduction of native Fe oxides. Sulphate is transported conservatively within the plume. The NH4 load is primarily attenuated by cation exchange reactions with the aquifer sediments. This is supplemented by nitrification at the plume margins under conditions of higher oxygen flux which generates locally elevated NO3 concentrations and depressed pH in the contaminated groundwater. Elevated alkalinity within the plume is consistent with dissociation of the ammonia waste stream during groundwater transport and is an important control on the increased porewater Ca and Fe load mobilized by cation-exchange and redox reactions. A conceptual model of the key processes affecting contaminant fate within the plume is presented and the implications for aquifer clean-up are discussed.