HYPORHEIC TRANSPORT OF ARSENIC IN A MINE-INFLUENCED HEADWATER STREAM: A MASS-BALANCE APPROACH

In a montane headwater stream in Southwest Virginia, we are investigating the role of the hyporheic zone (i.e., zone of interaction between surface and groundwater) in the transport of arsenic. At the site, arsenopyrite was mined from 1903 to 1919 and waste piles of roasted ore remain adjacent to the stream. Groundwater arsenic concentrations near the mine range from 0.05 to over 6 mg/L. Arsenic concentrations in the stream are heavily influenced by the location of the mine tailing piles and increase exponentially with distance downstream from them. Resistivity profiles show areas of high hydraulic conductivity associated with areas of high groundwater arsenic concentrations.

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 a flume 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. The depth of the hyporheic zone (<0.35 m) was delineated by installing multiple shallow subsurface wells (0.2 to 0.7 m below stream bed) and determining percent surface water during injections of Rhodamine WT. Groundwater arsenic concentrations paired with groundwater discharge measurements were used to determine the arsenic load entering the hyporheic zone. Loads necessary to close the mass balance were calculated by determining the stream load leaving the study reach. The difference between the input and output loads was used to determine if the hyporheic zone was retaining or releasing arsenic. Employing this mass-balance approach will allow us to determine the importance of this groundwater-surface water interface in the transport of trace elements.