GEOGENIC ARSENIC DISTRIBUTION IN METAMORPHIC BEDROCK AND IMPACTS FROM HISTORIC GOLD MINING OPERATIONS NEAR JULIAN, CA

In northeastern North America, there is growing knowledge of As enrichment in metamorphic rocks leading to elevated concentrations in groundwater. As such, concentrations of As up to 243 mg kg^-1 in the Julian Schist have prompted the investigation of As in the environment surrounding the historic township of Julian, CA. Many Julian residents obtain their water from domestic wells installed in bedrock. Historic yet limited groundwater sampling has detected As > 10 ug L^-1 within the aquifer. This study seeks to characterize the As-host minerals in a metamorphosed bedrock aquifer where S is rarely detected (max. 0.01% S) and pyrite is not visible in hand samples indicating that silicate minerals such as biotite, garnet, and cordierite should be considered as probable sources of As. The Julian Schist therefore provides a unique opportunity to increase our knowledge of As-silicate mineral hosts and their controls on As mobility in groundwater.

Additionally, hydrothermal quartz veins hosting historically mined gold locally intrude the Julian Schist and are suspected to further enrich rocks with As. Many abandoned tailings piles are scattered throughout hiking trails where concentrations up to 2,822 mg kg^-1 As (median 543 mg kg^-1 As) have been measured. When mixed with locally collected rainwater and left to equilibrate for 24 h up to 710 ug L^-1 As leached into the previously As-free rain water. These tailings pose a significant risk to local surface waters during rain and snow events and As may be transported downstream of tailings piles. Springs discharging down gradient of abandoned mines are stained bright red and indicate conditions may be suitable for As mobility under changing redox conditions when groundwater discharges at the surface.

There are multiple controls on As mobility in this hydrologic system, and it is expected that As host mineralogy will differ between the bedrock aquifer of the Julian Schist, the As minerals co-occurring with gold, and the weathered and exposed tailings. This study seeks to characterize the nature of these As mineral sources so as to inform local residents and park officials of the possible risks of As exposure through drinking and recreational water use in the Greater Julian region of southern California.