BORON AND STRONTIUM ISOTOPIC CHARACTERIZATION OF COAL COMBUSTION RESIDUALS: VALIDATION OF NOVEL ENVIRONMENTAL TRACERS

In the US, approximately six hundred power plants generate 136 million tons of Coal Combustion Residuals (CCRs) annually. Boron and strontium have been identified as good indicators of CCRs through experimental leaching tests, but only a few studies have utilized their isotopic compositions in CCRs. This study investigates the geochemical and isotopic signatures of CCRs from a variety of coal sources, and evaluates their environmental applicability as tracers for water contamination. Eight coal-fired plants were sampled for CCRs that were investigated through controlled leaching experiments under varying conditions (acidity, solid/water ratio). The data show that CCR leachates have distinctive strontium isotopic (⁸⁷Sr/⁸⁶Sr) ratios reflecting the original coal sources: Appalachian (0.7098-0.7111), Illinois (0.7114-0.7125), and Powder River (0.7122) basins (with slight overlap of the Powder River and Illinois Basins). In contrast, the boron isotope composition (d¹¹B values) was nearly all depleted (-18 ‰ to -4 ‰) (with one exception of CCRs from the Illinois basin ranging -7 ‰ to +8 ‰) relative to the NBS-SRM boron standard. Post-combustion treatment through flue gas desulfurization (FGD) appeared to change the original Sr isotopic composition of CCRs with significantly lower ⁸⁷Sr/⁸⁶Sr ratios (0.708 relative to the ash ⁸⁷Sr/⁸⁶Sr 0.7108). Leaching experiments with variable pH values resulted in varying ⁸⁷Sr/⁸⁶Sr ratios, but only a slight variation in d¹¹B values. The leaching experiments were validated in environmental sampling from two case studies (1) surface water and pore water from a river system affected by the 2008 TVA coal ash spill in Kingston, TN; and (2) systematic monitoring of ten CCR effluent discharges in NC and TN, including effluents, surface waters, and pore water. The environmental sampling revealed elevated concentrations of Sr and B from CCR effluents and contaminated waters, as well as distinctive B and Sr isotopic signatures that mimic the CCR leachate composition, while different from background waters and other potential sources of contamination. The combination of the geochemical indicators from the laboratory experiments and field sampling (surface and pore water) provides a unique and practical identification method for tracing the impact of CCRs on the environment.