INVESTIGATING MECHANISMS FOR COAL ASH TRANSPORT INTO MOUNTAIN ISLAND LAKE, NORTH CAROLINA

The Riverbend Steam Station was located along the banks of Mountain Island Lake, a cooling water reservoir and drinking water supply for the city of Charlotte, North Carolina. The Riverbend Station operated as a coal powered steam plant for over 80 years until it was decommissioned in 2013. In 2021, gravity cores and grab samples were collected from Mountain Island Lake to assess the presence of coal ash deposited in the lake. To determine the mechanism of transport of coal ash into Mountain Island Lake, various hydrologic data were compiled from the United States Geological Survey and National Oceanic and Atmospheric Administration including historical flood, drought, and hurricane data for the region, and stream gage, discharge, and precipitation data for the surrounding area and tributaries. To assess the ash content within the lake sediment, point counting was conducted on smear slides from a gravity core in a shallow cove near the ash pond overflow. A Leica DLMP polarizing microscope was used to point count samples at every centimeter. The percentages of six different coal ash morphologies were analyzed separately to determine the total coal ash percentage in a sediment sample. A sedimentation rate for the core based on Cesium-137 and Lead-210 allowed for estimation of the age of the coal ash peaks.

Streamflow and precipitation data were compared to peaks in coal ash to determine if there was a relation between hydrologic events and coal ash sedimentation. Coal ash increased above background in the years 1975, 1985, 1992-1993, and 2003, which coincide with high precipitation events that fell within wet years as classified by the NOAA Standardized Precipitation Index. Matching peaks in high precipitation events and coal ash percentage would suggest that during rain events, coal ash was released from the wet storage ponds by the plant as they were inundated by surface runoff and overtopped. Based on topographic and Federal Emergency Management Agency flood maps, it is unlikely that lake levels historically rose high enough to affect the wet storage ponds. Less defined peaks in coal ash are likely due to resuspension or bioturbation of bottom sediment. The amount of coal ash observed in the lake sediment decreased after the ash in the storage ponds was removed to a lined landfill between 2017 and 2019.