RADIONUCLIDES FOR TRACING DISTRIBUTION AND HISTORY OF COAL ASH CONTAMINATION IN LAKE SEDIMENTS

Coal ash is known to contain elevated levels of naturally occurring radionuclides compared to common raw coals, soils, and sediments. A recent study shows that coal ash has been constantly released into freshwater reservoirs adjacent to coal-fired power plants across North Carolina, resulting in abnormally high concentrations of toxic elements (As, Se, Cd, Tl, etc.) in the sediments and posing chronic risks to aquatic ecosystems. Here we report the activities of both naturally occurring (²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²²⁸Ra) and man-made (¹³⁷Cs) radionuclides in sediments of Hyco Lake (near Roxboro, NC), where sediment cores were retrieved from 4 stations, including 1 upstream and 3 downstream relative to the location of coal ash ponds. We aim (1) to determine the sediment chronology, and (2) to track the spatial and temporal variations of radionuclides and their relationships with coal ash contamination in lake sediments. Lake history since the onset of its impoundment (~ 1964) was reconstructed within all 3 downstream cores by the continuous downcore decay of unsupported ²¹⁰Pb and maximum fallout of ¹³⁷Cs, further validated by a lithology change marking the transition from pre-reservoir to post-reservoir sediments. Consistent with their much greater coal ash impact, lake sediments in the downstream cores were characterized by slightly lower ²²⁸Ra/²²⁶Ra activity ratios (mean = 0.72 ± 0.25) and higher total Ra activities (²²⁸Ra + ²²⁶Ra, mean = 73.3 ± 13.6 Bq/kg), compared to the upstream sediments (0.86 ± 0.18, 70.2 ± 6.3 Bq/kg). Additionally, the downcore distribution of ²³⁸U/²²⁶Ra activity ratios in the downstream sediments was correlated with that of coal ash-derived trace elements (r > 0.65, p < 0.005). Unlike the secular equilibrium between ²³⁸U and ²²⁶Ra in fresh coal ash, there was a notable disequilibrium (i.e., ²³⁸U/²²⁶Ra > 1.5) in the coal-ash impacted downstream sediments deposited between the mid-1970s and early 1990s, corresponding to the highest enrichment of toxic elements and lowest Mn contents during this period. This is likely due to the selective leaching of ²²⁶Ra and/or retention of ²³⁸U under anoxic conditions prevailing in the lake sediments. Overall, our study demonstrates the applicability of radionuclides for tracing both the spatial and temporal distribution of coal ash contamination in lake sediments.