Biogeochemistry of anaerobic lacustrine and paleosol sediments within an aerobic unconfined aquifer

Abstract

The geochemistry and the distribution and abundance of anaerobic bacteria were determined for sediments sampled in a deep borehole in south‐central Washington. The sampled sediments consisted of a 12‐m‐thick lacustrine sequence underlain by 8 m of paleosol grading into 5 m of silty sands, within an aerobic unconfined aquifer otherwise composed of transmissive sands and gravels. Concentrations of porewater sul‐fate varied systematically with depth, reaching a minimum of 3.9 mg L ‐1 in the central portion of the lacustrine sequence. Lacustrine sediments contained up to 1 wt% total organic carbon, whereas other sediments contained less than 0.2 wt% organic carbon. Fermentative bacteria were present throughout the sampled sequence, and were assumed to be responsible for primary degradation of organic carbon. Dissimilatory iron‐reducing bacteria (DIRB) were at maximum abundance where bioavailable Fe(III) and organic carbon were present at favorable combined concentrations. Sul‐fate‐reducing bacteria were culturable in two zones with few DIRB, where sulfate and organic carbon were sufficiently available. Evidence of iron oxidizers or methanogens was lacking in all samples. The fine‐grained nature of the lacustrine sediments, their low hydraulic conductivities, the persistence of organic carbon within them for 5–8 million years, and the concentration gradients of electron acceptors in lacustrine porewaters indicated that bacterial oxidation of sediment organic matter was ongoing within these sediments, albeit at very low rates. Low hydraulic conductivity may have constrained the mobility of bacteria and electron donors and acceptors. Microenvi‐ronments within the sediments, within which bacterial activity was limited by the flux of fermentation by‐products and the availability of electron acceptors, could thus explain the continued presence of organic carbon and bioavailable Fe(III).