Assessing the Geological Sources of Manganese in the Roanoke River Watershed, Virginia

The release of geogenic elements to water supplies is an issue of worldwide concern. Because these elements occur naturally and are often present in a variety of geologic materials, delineating sources and fates of these elements can be challenging. In this study, we examine connections between manganese (Mn) in modern groundwater, bedrock geology, and ores in the Roanoke River watershed of the Piedmont Province, Virginia. In the watershed, Mn concentrations in groundwater are often elevated above secondary drinking water standards. Evaluation of chemical characteristics of groundwater and geologic materials within the region suggests that carbonate-bearing lithologies are likely sources of Mn to groundwater. The inverse correlation of Mn with dissolved oxygen concentrations in groundwater suggests that once released from chemical weathering, Mn persists in groundwater under reducing conditions that develop along flowpaths. Analysis of Mn ores of the James River-Roanoke River Manganese District provides support that the ore deposits are supergene in origin, consistent with previous models. However, in contrast to previous models suggesting ore formation from downward flow of groundwater, our analysis of ore formation in the context of groundwater geochemistry supports an upwelling model of ore formation in which Mn oxides were precipitated near discharge zones. Overall, our results suggest that Mn cycling in the region has been active over geologic time, as Mn-rich groundwater discharges to riverine systems, both past and present. Thus, the processes that formed the Mn ores in the past are still occurring in the modern day.