LESSONS FROM GEOGENIC MANGANESE CONCENTRATIONS IN DRINKING WATER SOURCED FROM KARST AQUIFERS APPLIED TO ANTHROPOGENIC MANGANESE CONTAMINATION PASSIVE REMEDIATION

A substantial fraction of the global population sources drinking water from karst aquifers, which are susceptible to contamination due to the high connectivity between ground and surface waters. We investigated the risk of manganese (Mn) exposure via drinking water within the Shenandoah Valley, containing karst and non-karst aquifers, to determine if aquifer rock type and overlying soils impact the relative risk of Mn exposure. We collected water and soil samples, supplementing data from the US Geological Survey and Virginia Household Water Quality Program, totaling 1,934 wells and springs. We find aqueous Mn concentrations from karst are lower in Mn than non-karst, with <4.9% of wells and springs in limestone and dolostone aquifers exceeding 100 ppb Mn, while >20% of wells in sandstone and shale aquifers exceed 100 ppb Mn. These results, along with soil Mn oxidation states obtained via Mn K-edge X-ray absorption near edge structure spectroscopy (XANES) indicate that the observed decrease in aqueous Mn in karst may be due to the more oxic conditions stabilizing Mn in the solid phase. Similarly, coal mine drainage (CMD) passive remediation utilizes flow through ponds lined with carbonate rocks to facilitate the microbially-mediated oxidative precipitation of aqueous Mn(II) to solid Mn(III/IV) oxide minerals. With CMD a pervasive issue in Appalachia (adjacent to the Shenandoah Valley), understanding the role these liner rocks play in controlling water geochemistry and subsequent Mn remediation is critical. Thus, we investigated relevant liner rocks (limestone, dolostone, forsterite, zeolite, and quartz sand) with and without the Mn-oxidizing fungus, Stagonospora sp. SRC1lsM3a, to assess their impact on Mn remediation. While zeolite rapidly removes Mn (>95% remediation within 24 hours), the solid-associated Mn is adsorbed and readily desorbs with changing water chemistry. Other liner rocks exhibit a reliance on fungi to achieve effective Mn remediation. XANES spectra reveal that the solid-associated Mn in the myco-dolostone system is a mixture of Mn(II) and Mn(IV), while substantially more Mn(IV) is present in the more effective myco-limestone system. Thus, risk patterns of Mn exposure in karst and non-karst terrain mimic those for liner rocks in CMD passive bioremediation, with Mn remaining more stable in the solid phase when associated with carbonates.