BIOGEOCHEMISTRY OF ANAEROBIC LACUSTRINE AND PALEOSOL SEDIMENTS WITHINAN AEROBIC UNCONFINED AQUIFER

The geochemistry and the distribution and abundance of anaerobic bacte ria were determined for sediments sampled in a deep borehole in south- central Washington. The sampled sediments consisted of a 12-m-thick la custrine sequence underlain by 8 m of paleosol grading into 5 m of sil ty sands, within an aerobic unconfined aquifer otherwise composed of t ransmissive sands and gravels. Concentrations of porewater sulfate var ied systematically with depth, reaching a minimum of 3.9 mg L-L in the central portion of the lacustrine sequence. Lacustrine sediments cont ained up to 1 wt% fetal organic carbon, whereas other sediments contai ned less than 0.2 wt% organic carbon. Fermentative bacteria were prese nt throughout the sampled sequence, and were assumed to be responsible far primary degradation of organic carbon. Dissimilatory iron-reducin g bacteria (DIRB) were at maximum abundance where bioavailable Fe(III) and organic carbon were present at favorable combined concentrations. Sulfate-reducing bacteria were culturable in two zones with few DIRE, where sulfate and organic carbon were sufficiently available. Evidenc e of iron oxidizers or methanogens was lacking in all samples. The fin e-grained nature of the lacustrine sediments, their low hydraulic cond uctivities the persistence of organic carbon within them for 5-8 milli on years, and the concentration gradients of electron acceptors in lac ustrine porewaters indicated that bacterial oxidation of sediment orga nic matter was ongoing within these sediments, albeit at very low rate s. Low hydraulic conductivity may have constrained the mobility of bac teria and electron donors and accepters. Microenvironments within the sediments, within which bacterial activity was limited by the flux of fermentation by-products and the availability of electron accepters, c ould thus explain the continued presence of organic carbon and bioavai lable Fe(III).