REDOX PUMPING IN THE NEAR SURFACE MISSOULA AQUIFER ON THE FLOOD PLAIN OF THE CLARK FORK RIVER: SURFACE WATER AND GROUNDWATER INTERACTIONS AND ARSENIC RELATED CHEMISTRY AT A COMPOST FACILITY NEAR A WASTEWATER TREATMENT PLANT.
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,
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
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
Our findings indicate that both hydrogeology and chemical transport at this site
were heavily influenced by the human altered landscape.
Advisor:Dr. Johnnie N. Moore
School:The University of Montana
School Location:USA - Montana
Source Type:Master's Thesis
Date of Publication:08/07/2008