"REDOX PUMPING” IN THE NEAR SURFACE MISSOULA AQUIFER ON THE FLOOD PLAIN OF THE CLARK FORK RIVER: ARSENIC RELATED CHEMISTRY

Arsenic transport in groundwater is evaluated at Eko Compost, a site with measurable levels of arsenic, organic carbon and nutrients in the ground water. The compost facility is located adjacent to the Missoula wastewater treatment plant on a contaminated floodplain of the Clark Fork River in the Missoula Valley, Montana, USA. This site was evaluated over two years for hydrological and chemical characteristics. A series of potentiometric surface maps was created over time and hydraulic conductivity and ground water movement were characterized. Water samples were also collected monthly and results from chemical analyses of the waters were contoured over the site map to evaluate chemical and hydrologic transport.

Arsenic, organic carbon, iron and sulfate increased across the site in summer after water table elevations rose in spring. ‘Redox pump’ mechanisms were characterized in two locations, where reducing conditions with high levels of iron and organic carbon liberated anomalously high concentrations (60 to150 ug/L) of dissolved arsenic in spring. The source of arsenic appeared to be buried contaminated flood sediments at the site. A conceptual model is presented where the chemical character of the water was influenced vertically by the layer of the sediment that contained the top of the water table, and laterally by the chemical character induced by the path of the groundwater.

Hydraulic conductivities (K) of around 1100 ft/day were estimated for sections of the uppermost layer of the aquifer, K values were higher near the river, and lower in wells finished in the organically enriched zones. During spring and early summer a local flow pattern was described that is seasonally different from the established regional pattern. Two distinct hydrogeologic occurrences were observed during runoff season while the aquifer was recharging 1) A direct connection developed between the aquifer and the river at the Eko Compost backwater, and 2) potentiometric maps showed flow direction in the aquifer through backfilled channels of organically enriched areas that had been used as sludge ponds for the nearby water treatment plant.

Our findings indicate that both hydrogeology and chemical transport at this site were heavily influenced by the human altered landscape.