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Groundwater pollution remediation and protection 259

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Monitored natural attenuation of a crude oil spill, Bemidji, Minnesota
7. 4

A demonstration site for monitored natural attenuation (MNA)
within the United States Geological Survey Toxic Substances
Hydrology Program (http://toxics.usgd.gov) is located near
Bemidji, Minnesota, where a buried pipeline located in a glacial
outwash plain ruptured in 1979 spilling crude oil into the subsur-
face. The oil is entrapped as a residual non-aqueous phase in the
vadose zone and also forms two bodies of oil floating on the water
table. The largest oil body was estimated to contain 147,000 L of
oil in 1998. As shown in Fig. 1a, the oil forms a long-term, continu-
ous source of hydrocarbon contaminants that dissolve in and are
transported with the groundwater. Microbial degradation of the
petroleum hydrocarbons in the plume has resulted in the growth of
aquifer microbial populations dominated by aerobes, iron-reducers,
fermentors and methanogens (Fig. 1b). The biodegradation reac-
tions cause a number of geochemical changes near the dissolved
aqueous plume which include decreases in concentrations of
oxygen and hydrocarbons and increases in concentrations of dis-
solved iron, manganese and methane (Fig. 1c).
Modelling of the natural attenuation processes simulates initial
aerobic degradation followed by the development of an anoxic
zone in which manganese and iron reducers and methanogens
begin to grow, consuming solid phase Mn(IV) and Fe(III) and releas-
ing dissolved Mn(II), Fe(II) and methane (Fig. 1c). The modelling
predicts that 40% of the hydrocarbon degradation occurs aerobic-
ally and 60% anaerobically. Combined with field data and the
measurement of microbial populations, the results suggest that the
natural attenuation capacity of the glacial outwash sands is being
slowly consumed by depletion of the intrinsic, electron-accepting
capacity of the aquifer (Bekins et al. 2001b; Cozzarelli et al. 2001).

Fig. 1 Illustration of natural attenuation of crude oil
contamination by aerobic and anaerobic biodegradation in a
glacial outwash aquifer located near Bemidji, Minnesota. (a) The
1995 concentration of BTEX compounds defines the extent of
contamination in the aquifer for a vertical cross-section along
the plume axis. (b) The cross-section shows the distribution of
microbial populations inferred from most probable number data.
(c) Modelled and observed concentrations versus time plots for a
well positioned at the water table, 36 m downgradient from the
contaminant source and illustrating the loss of oxygen and
production of reduced electron acceptors (Fe(II), Mn(II) and
methane) during the temporal evolution of redox conditions in
the aquifer. After Bekins et al. (2001a).

hydrogeologic settings; and second, the superposi- factors considered to control the groundwater pollu-
tion of a relative rating system having the acronym tion potential are: depth to water (D); net recharge
DRASTIC. Inherent in each hydrogeologic setting (R); aquifer media (A); soil media (S); topography
are the physical characteristics that affect groundwater (slope) (T); impact of the vadose zone (I); and hydr-
pollution potential. The most important mappable aulic conductivity of the aquifer (C). The numerical