LACUSTRINE SEDIMENT CORES RECORD AN EVOLVING HISTORY OF COAL ASH CONTAMINATION IN LAKES WITH COAL-FIRED POWER PLANTS IN NORTH CAROLINA

Hyco and Mayo Lakes were constructed in 1964 and 1983, respectively by damming small Piedmont streams for cooling water for the Roxboro and Mayo coal-fired power plants. Hyco Lake, 15.2 km² in area, averaging 11 m deep is surrounded by expensive homes and serves as a recreational lake for swimming, boating and fishing. Mayo Lake, 11.3 km² in area, averaging 9.1 m deep is surrounded by a forested buffer. Construction of the reservoirs and adjacent coal ash disposal ponds are observed on aerial photographs. Gravity cores recovered the lake sediment history upstream and downstream of the plants. Chronologies were based on ²¹⁰Pb, ¹³⁷Cs and a distinctive lithology change between lake and pre-lake sediments. Sedimentation rates varied from 0.71 cm/yr in Hyco Lake to 0.32 cm/y in Mayo Lake, producing high-resolution records. Coal ash within sediments was detected and quantified using optical point counting, combined with magnetic susceptibility and trace element geochemistry. Six coal ash morphologies with particle sizes of 4 to 100 µm were identified, including spherical particles that were clear, black (ferrospheres), and orange, plerospheres, and both clear and opaque amorphous particles.

Coal ash was identified throughout all cores, including up to 10% in pre-lake sediment and upstream of the plant in Mayo Lake. In Hyco Lake, prior to the late 1970’s, core samples contained up to 15% total ash, dominated by magnetically enriched amorphous particles. Prior to enactment of the Clean Air Act, small spherical particles were emitted to the atmosphere and not retained as waste at the plant. The coarse and amorphous ash may have been used in plant construction and/or for agricultural purposes allowing transport into lakes. In Hyco Lake from the 1980’s through 2018, several discrete ash peaks can be correlated between cores collected 5 km apart. They contain spherical ash and elevated trace elements, including As and Se. The timing coincides with peak discharges recorded by USGS gages on the inflow streams, suggesting breaches of ash ponds during flooding. Larger, irregular, and dense particles settled out of suspension during transport resulting in a sorted deposit near the dam. This study illustrates the usefulness of the lacustrine sediment archive to record the history of contaminant releases near power plants.