SEASONAL CHANGES IN REDOX CONDITIONS DUE TO FLUCTUATING GROUNDWATER DEPTH LEADING TO CHANGES IN SUBSURFACE MICROBIAL ACTIVITY AT THE OLD RIFLE SITE IN WESTERN COLORADO

The Old Rifle site is located on a floodplain adjacent to the Colorado River in Rifle, Colorado. The site is the location of an intensive research program to track the flow of carbon in the subsurface and its impact on microbial activity in the subsurface. The depth to groundwater at the site is generally 3 m, but during peak runoff and high water levels in the Colorado River in early- to mid-June, the groundwater level can increase by more than 1 m relative to basal flow. The geology consists of around 6 m of Quaternary alluvium composed of quartz and feldspar sands with silts, clays, pebbles, and cobbles, with iron oxide coatings overlying the less permeable Tertiary Wasatch Formation. Within the alluvium are deposits of natural organic matter that create natural bioreduced zones (NBRZ) that have a significant impact on movement of redox sensitive chemical species in the sediments. To monitor biological and chemical processes in the unsaturated zone at the site, 3 wells were installed with soil gas samples and lysimeters at 50 cm depth intervals from the surface to the top of the water table. Sampling from these wells is done on a monthly basis in order to track changes in activity related to seasonal variations in activity due to changing infiltration and variations in groundwater depth.

Preliminary results found that rising groundwater during spring run-off of this year resulted in sharp increases in CO₂ and N₂O concentrations in soil gas at all depths in all three vadose zone wells providing direct evidence of increased subsurface microbial activity. The nitrogen isotope ratios of N₂O in these wells were low and decreasing with time (as low as -28‰), suggesting that nitrification is an important microbial process in these soils. In one well, the groundwater rose above the level of the two deepest sampling points and brought oxygenated water into contact with a NBRZ in the vicinity of the well. Sulfate concentrations in pore water samples increased and the sulfur isotope ratios of the sulfate dropped below -30‰ indicating rapid oxidation of low-³⁴S pyrites in the NBRZ. This sulfate was also accompanied by high uranium concentrations (1.3 µM) possibly also derived from oxidation of reduced uranium in the NRBZ. A more complete data set will be available for discussion at the meeting.