| The purpose of this research was to investigate the mechanisms of arsenic release and geochemistry within the Warm Springs Ponds Operable Unit (WSPOU), near Anaconda, Montana. Dissolved arsenic concentrations at the outlet of the WSPOU rise during the Summer and Fall months. The purpose of this study is to examine the mechanisms of arsenic release.;The following mass balance equation summarizes our current understanding of how arsenic moves through the WSPOU system: Jsw-in + Jgw-in + Jdiffusion = Jsw-out + Jgw-out. In this equation, J is the flux of arsenic (e.g., mole/day) into or out of the ponds, sw stands for surface water advection, gw stands for groundwater advection, and Jdiffusion is the flux of arsenic diffusing upwards from sediment pore water into the overlying surface water.;A series of field experiments conducted in Pond 2 and the West Wet Closure (WWC) were used to estimate the advective and diffusive flux of arsenic into the overlying water column. Iron-stained sediment along the south shore of both ponds indicates probable areas of upwelling groundwater. A combination of piezometers and seepage meters fashioned from sawn-off 55-gallon plastic drums were used to estimate the advective flux in these areas. The diffusive flux was determined indirectly, by use of sediment pore water samplers (peepers). Peepers give a vertical array of closely-spaced, small-volume samples of pore water in the top 25 cm of sediment. The vertical gradient in solute concentration extracted from the peeper cells is then combined with Fick's First Law to estimate the diffusive flux.;Seepage meters provided a specific flux of around 0.5 L/m2/day, which was used to estimate the total groundwater seepage into Pond 2. The total groundwater seepage multiplied by the geometric mean concentration of arsenic found in groundwater (piezometer) samples yielded an advective flux of 1.4E+4 mg As/day. Using Fick's First Law and the arsenic gradients obtained from the peepers, and making the assumption that the rate of diffusion of As from the sediment into the overlying water is constant over the entire pond complex, the total diffusive flux of arsenic is estimated to be 4.5E+5 mg/day. Thus, the diffusive flux appears to be more important than the groundwater advective flux.;Results obtained from this project suggest that both the diffusive and advective flux of arsenic are significant factors in the overall mass balance equation for arsenic, especially at low flows. For example, if the flow at the pond system outlet (SS-05) drops to 10 cfs, which is often the case in late summer, then the predicted increase in As concentration as water passes through the pond complex is around 20 microg/L. |
