Chemical and Physical Controls on Microbial Populations in the Bemidji Toxics Site Crude-Oil Plume

By Barbara A. Bekins, Isabelle M. Cozzarelli, E. Michael Godsy, Ean Warren, Mary Ellen Tuccillo, Hedeff I. Essaid, and Victor V. Paganelli

ABSTRACT

Processes controlling the establishment of aquifer microbial populations that degrade organic ground-water contaminants are poorly understood. We provide insight into this problem with a combined data set that includes microbial populations, grain size, pore-water chemistry, and sediment iron content. Data from three vertical profiles through the anaerobic portion of the Bemidji crude-oil plume show similar patterns in the microbial populations. Within each profile, numbers of iron-reducers vary from lows of 10²-10⁴/g sediment to highs of 10⁵-10⁶/g. Areas that are evolving from iron-reducing conditions to methanogenic conditions are indicated by lower numbers of iron-reducers and the presence of culturable methanogens (10¹-10²/g). These conditions are found in areas of high contaminant flux either in the vicinity of the non-aqueous oil or where higher concentrations in the contaminant plume are associated with local increases in aquifer permeability. In all locations where methanogens are found, lower numbers of culturable iron reducers are also present. Moreover, in these areas, significant extractable Fe(III) (>10 µmol/g) is still present, suggesting that the remaining iron on the sediments may be less available for microbial reduction. The methanogenic zones are vertically narrow, ranging from 0.25-1 m thick, but they are laterally continuous extending from the source area to at least 60 m downgradient.