Strontium isotopes as tracers of coal combustion residue in the environment

Abstract

We evaluated the application of Sr isotopes as tracers of coal combustion residues in the environment. Strontium isotopes, unlike light stable isotopes, do not fractionate and can be used to monitor interactions between coal combustion residue and hydrogeologic systems. The primary objective of this investigation was to determine if Sr isotopes could detect the presence of flyash and other effluents produced by coal combustion in the environs surrounding a coal-fired power plant in the southwestern United States. Analyses of coal, flyash and bottom ash indicated a 87Sr/ 86Sr range of 0.70883 to 0.70972 for these residues. This range in 87Sr/ 86Sr exceeds the error of an individual analysis (±0.00002) and represented our criteria to establish evidence for impact, i.e., any sample whose 87Sr/ 86Sr ratio lies within this range received Sr from fossil fuel (FF) residue. Natural background materials (groundwater, soil, rocks) have 87Sr/ 86Sr ratios which exceed 0.710 and are therefore markedly distinct from the FF residue. Our analytical techniques were developed to selectively leach the flyash-derived Sr from soils. This makes the detection of trace quantities (< 0.1%) of FF residue possible. Soil Sr leachate analyses suggest that FF residues have impacted on-site soils to depths of 51.5′ (15.5 m). The 87Sr/ 86Sr ratios of soil leachates below a FF waste evaporation pond lie within the range of the FF residue. This supports our selection of the Sr isotopic range defined by the FF residue as our criteria for impact. Off-site soil analyses show that surface (0–1 cm) soils collected from as far as 12.5 km in all geographic directions have interacted with flyash. Detailed analyses of deeper soils (1–5 cm; 5–10 cm) indicate that flyash impact varies with depth. Plants, such as the brittlebush, growing on such soils uptake Sr from both the flyash and soil; their 87Sr/ 86Sr ratios represent mixtures of the two components. Our data suggest that groundwater has not been severely impacted by FF residue in most cases. Waste evaporation pond waters are more commonly impaired. Temporal variations in the 87Sr/ 86Sr ratios of groundwater and waste evaporation pond water were observed. The data may be used to assess the quality of these waters over time.