A MASS-BALANCE APPROACH TO ASSESSING ARSENIC TRANSPORT THROUGH THE HYPORHEIC ZONE OF A MINE-INFLUENCED MOUNTAIN STREAM

In a montane headwater stream in Southwest Virginia, we are investigating the biogeochemical controls on the transport of arsenic through the hyporheic zone (i.e., zone of interaction between surface and groundwater). At the site, arsenopyrite was mined from 1903 to 1919 and waste piles of roasted ore remain adjacent to the stream. Arsenic concentrations in the stream are heavily influenced by the location of the waste piles and increase exponentially with distance downstream from them. Total dissolved arsenic in the stream varies from 0.002 to over 5 mg/L with the vast majority (>90%) occurring as arsenate (i.e., As(V)).

By monitoring arsenic concentrations in groundwater, hyporheic water, and stream water, we are characterizing spatial and temporal variation in arsenic fluxes by employing a mass-balance approach. Dilution gauging by solute injection paired with continuous discharge measurements collected from flumes stationed both upstream and downstream of the mine are being used to quantify groundwater inputs to the stream. Vertical hydraulic gradients along the stream bed indicate that that the stream is gaining throughout the study reach. Groundwater arsenic concentrations paired with groundwater discharge measurements will be used to determine the arsenic load (concentration*discharge) entering the hyporheic zone. The load leaving the study reach will be measured to determine if the hyporheic zone is retaining or releasing arsenic to the stream. Employing this mass-balance approach will allow us to determine the importance of this groundwater-surface water interface in the transport of trace elements.