Potential Impacts of Legacy Lithium Mining on Water Quality in North Carolina

Abstract

As the search for new lithium deposits continues and new mines become permitted in North Carolina, understanding the potential water quality impacts of legacy lithium mining is important to enable sustainable lithium extraction. This project aims to evaluate the possible impact of the legacy of lithium mining on the quality of surface water and groundwater in the vicinity of legacy lithium mining and the Carolina Tin-Spodumene Belt (TSB) region of North Carolina. As part of this project, we conducted extensive sampling and analyses of the major and trace elements (50 elements) as well as strontium isotopes (87Sr/86Sr ratios) of 99 stream water and 93 groundwater samples in 6 sampling campaigns in a legacy lithium-pegmatite mining district in North Carolina. Analysis of the quality of surface water downstream from outfalls of two legacy lithium mines (Hallman Beam and Kings Mountain) and an active lithium processing site (Livent) shows marked increases in the overall salinity (reflected by Electrical Conductivity; EC as well as high calcium and sulfate), lithium (>1000 μg/L), rubidium (>25 μg/L), and cesium (>10 μg/L) immediately downstream from the outfalls in comparison to the background levels in local surface water. Further downstream, these concentrations decrease to the background levels. No other potential contaminants were observed and impacted streams were characterized by neutral pH. Elevated levels of lithium and rubidium in the impacted stream waters were associated with higher 87Sr/86Sr (radiogenic), Rb/Sr, and low K/Rb ratios reflecting the natural composition of the lithium pegmatite rocks and from processing facilities, distinct Ca/SO4 (molar) ratios (~1) reflecting gypsum dissolution, likely from gypsum generated during lithium processing. Overall, the concentrations of lithium in streams associated with legacy lithium mining and processing were higher than background streams within the Carolina Tin-Spodumene Belt (TSB) region, which were higher than lithium levels in common streams in the Piedmont region of North Carolina. We observed a high correlation (R2=0.97, p<0.01) between EC and lithium in the stream waters, suggesting a novel and rapid methodology to monitor lithium occurrence in surface waters near lithium mining and processing sites through using real-time EC measurements as a proxy for lithium occurrence in surface waters. Analysis of groundwater show systematically higher lithium, rubidium, and cesium in groundwater underlying legacy lithium mining (Kings Mountain) relative to the regional groundwater from TSB region, which in turn were higher relative to common groundwater in the Piedmont region of North Carolina. However no other contaminants such as arsenic were detected in the groundwater underlying the legacy lithium mine. In contrast, in the northern section of Gaston County, we have identified a cluster of wells with high levels of contaminants, including arsenic (up to 346 μg/L), lithium (890 μg/L), and cesium (46 μg/L). A previous study in the area has also identified a cluster of high arsenic levels and suggested a naturally occurring phenomena of groundwater contamination from geogenic sources. We observed direct correlations between 87Sr/86Sr and lithium as well as Rb/Sr and low K/Rb ratios in the apparent geogenic contaminated groundwater. These correlations suggest that intensification of water-rock interactions with lithium pegmatite rocks that has caused mobilization of lithium into the groundwater. On the other hand, arsenic contamination in groundwater in North Carolina is typically derived from interaction with other rock types such as metamorphic volcanics and mica schist. We posit the that the mix of aquifer rocks in that area that includes mica schist and lithium pegmatite would generate a unique groundwater chemistry in which As is elevated from interaction with the mica schist rocks, while the lithium, 87Sr/86Sr, Rb/Sr, and K/Rb signatures are affected by reactions with the lithium pegmatite rocks. While the apparent geogenic groundwater contains high arsenic levels, the regional TSB groundwater has typically low arsenic concentrations. A parallel study of the quality of groundwater in the vicinity of legacy lithium mining of Kings Mountain show systematically elevated levels of lithium, rubidium, and cesium, and yet, negligible levels of arsenic or other EPA regulated contaminants. Overall, while lithium in groundwater in the TSB region can be derived from both naturally occurring (geogenic) source and the legacy of lithium mining and processing, arsenic appears to exclusively occur in geogenically-sourced groundwater. Therefore, we posit that the ratio of arsenic to lithium in groundwater can be used to detect the possible origin of groundwater and links to legacy lithium mining and processing. The distinction between naturally occurring contamination and water-quality impact from legacy lithium mining is the key component of this research. Our geospatial analysis reveals that the groundwater quality is not dependent on the proximity to the legacy lithium mining (Hallman Beam and Kings Mountain) and the active lithium processing site (Livent) in the TSB region. Therefore, wells located near these lithium mining and processing sites were not affected and the quality of the TSB groundwater is primarily controlled by naturally occurring processing and reactions with aquifer rocks, some containing lithium pegmatites with distinctive geochemical and isotopic fingerprints.