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.