USGS Publications Warehouse

Abstract

Crude oil floating at the surface of a shallow aquifer of glacial outwash, near Bemidji, Minnesota, is altered by geochemical processes. Hydrocarbons from the oil are attenuated by several reactions that include aerobic and anaerobic microbial degradation. These degradation reactions result in the development of geochemical facies in the shallow groundwater system. Groundwater most affected by the presence of organic compounds is anoxic, and concentrations of methane, dissolved organic carbon, and total inorganic are high--0.76 millimole/L, 2.9 millimole/L, and 12.3 millimole/L, respectively. The concentrations of chemical species and delta-(13)C isotope values indicate that the plume near the oil lens has become progressively more reducing. Over a 4-year period (1984 through 1987), the concentrations of methane and iron have increased by a factor of > 25. The data suggest that sequential degradation occurs, as predicted by thermo-dynamics: manganese is reduced before iron is reduced, which occurs before methanogenesis. These data provide field evidence that reduction of iron and manganese is an important mechanism of decomposition of organic matter in aquifers. The delta-(13)C values of inorganic carbon of the native groundwater range from -12 ppt to -15 ppt as a result of mixing of soil CO2 with CO2 from the dissolution of carbonates. Non methanogenic biodegradation of oil constituents adds isotopically light CO2 to the groundwater because the oil has a delta-(13)C value of 28 ppt. The delta-(13)C value of inorganic carbon in the reducing zone have become progressively heavier from 1985 through 1987. The maximum change occurs 15 m downgradient from the oil lens, where the delta-(13)C values increased from -21.6 ppt to -5.35 ppt. This change indicates that the plume has become more reducing and methanogenic over time. (See also W90-05059) (Author 's abstract)