HR: 1330h
AN: H32A-0540    [PDF]
TI: Unsaturated Zone Tracer Test at the Bemidji, Minnesota Crude Oil Spill Site
AU: * Herkelrath, W N
EM: wnherkel@usgs.gov
AF: USGS, 345 Middlefield Road, MS 496, Menlo Park, CA 94025 United States
AU: Delin, G N
EM: delin@usgs.gov
AF: USGS, 2280 Woodale Drive, Mounds View, MN 55112 United States
AB: As a part of a study of the subsurface transport and natural attenuation of petroleum hydrocarbon contaminants at the Bemidji, Minnesota crude-oil spill research site, we used aqueous tracers to investigate solute transport from the soil surface through the crude oil-contaminated unsaturated zone to the water table. We applied tracer solution to the soil surface within a 5 by 12 meter tracer test plot that ran from a heavily oil-contaminated area to an oil-free zone. The depth to the water table was about 6 meters. The tracer test plot was instrumented with soil moisture probes, tensiometers, suction lysimeters, and drive-point sampling wells. Sixty liters of solution containing about 6.0E03 mg/l rhodamine WT and 1.0E04 mg/l bromide was uniformly sprayed on the soil surface in October 2001. We monitored subsequent tracer movement in response to precipitation by obtaining water samples weekly using the suction lysimeters in the unsaturated zone and the drive point wells in the saturated zone. Rhodamine concentrations were measured in the field using a fluorometer, and bromide concentrations were measured in the lab using ion chromatography. The time required for rhodamine tracer to reach the water table was 340 +/- 26 days. Travel times for bromide were about the same as for rhodamine, but the bromide data were less useful because the maximum bromide concentrations observed in the wells were close to background values. Rhodamine travel times through the oily unsaturated zone were not significantly different from the travel times through the oil-free unsaturated zone. However, the peak rhodamine concentrations found in ground-water samples obtained below the oil zone were an average of 3 times larger than the peak rhodamine values beneath the oil-free zone. We hypothesize that the rhodamine was adsorbed less in the oil-contaminated zone than in the oil-free zone because iron-containing minerals that absorb rhodamine have been largely removed from the oily sediments as a consequence of reducing conditions caused by the natural microbial degradation of the oil. It is also possible that the tracer solution flowed through the hydrophobic soil in the oil-contaminated area primarily in preferential pathways, bypassing much of the rhodamine-adsorbing sediment.
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2003 Fall Meeting