Long-term monitoring of unsaturated-zone properties to estimate recharge at the Bemidji crude-oil spill site

Ground-water recharge is an important factor affecting the Bemidji, Minnesota crude-oil spill site. About 400,000 liters of crude oil remained in the ground after remediation was completed following the 1979 pipeline break. An automated data logging system was used to measure unsaturated zone properties relevant to estimating recharge and to evaluate their effects on dissolution of the oil. Laboratory and field testing of several soil-moisture probes indicated that the CS615 probe was better suited to estimating recharge in the glacial outwash at the Bemidji crude-oil spill site than the CS605 probes. Both probes are manufactured by Campbell Scientific Inc. The CS615 probe provided dependable and accurate data over long time periods, using a limited power supply, under the extreme weather conditions typical of northern Minnesota. Based on results of the testing, arrays of the CS615 probes, zero-maintenance tensiometers, and thermocouples were installed in the unsaturated zone at the north oil pool in the fall of 1998. Computer simulations indicated that the rate of dissolution from the oil body is linearly related to the recharge rate. Additional multiphase flow model analyses are being conducted to quantify this increased dissolution. Additional model analyses are also being conducted to evaluate how dissolution is affected by recharge that varies in relation to the presence of crude oil in the unsaturated and saturated zones, discontinuous lenses of lacustrine silt and clay, and topography. The VS2DT code is also being used to estimate recharge rates and to evaluate the movement of water through the oil.
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Minnesota Water Research Digital Library
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