AN EVALUATION OF THE GEOLOGIC SOURCES OF MANGANESE IN THE ROANOKE RIVER WATERSHED

Elevated concentrations of manganese (Mn) have been measured in surface and ground water within the Roanoke River watershed of Virginia. As Mn is regulated by the EPA as a secondary drinking water contaminant at 50 ppb, identifying the sources of Mn to the watershed is critical for protecting water supplies. This study investigates the geologic formations of the Roanoke River watershed to evaluate which formations may release Mn to surface and ground waters.

Statistical and spatial analyses were performed on 456 groundwater samples in the Roanoke River watershed from the National Uranium Resource Evaluation (NURE) database. Using information in the NURE database, individual rock formations in the watershed were categorized into lithologic groups, based on criteria such as rock type, petrogenesis, and lithogeochemistry. Although the median Mn concentration in groundwater within the Roanoke River watershed is 22 ppb, groundwater in five geologic regions and seven lithologic groups within the basin have median values above 22 ppb.

Spatially, Mn concentrations in groundwater vary greatly, even within the same geologic formation. However, there are regions with consistently elevated Mn. For example, in Campbell County, 97 percent of the groundwater samples contain Mn that exceeds 50 ppb. These elevated Mn concentrations may be linked to the presence of historic Mn ore deposits within the James River-Roanoke River Manganese District. The Mn ore deposits are believed to have been formed from circulating groundwater due to their predominant localization within saprolite layers. The presence of nearby plutonic rocks and the occurrence of a brecciated quartzite cemented entirely by a Mn-oxide matrix suggests that there may be a secondary source of Mn associated with hydrothermal fluids. Further work will include examining the origin, chemistry, and mineralogy of the Mn ore deposits to illuminate potential links between ore characteristics and water quality.