In August 1997, petroleum products came out of a spring near the wetlands, called Spring Lake, due east of the refinery. FHR constructed a trench uphill of the spring to intercept the oil and contaminated groundwater to prevent oil from reaching the wetlands or river. (The contaminated groundwater is treated at FHR's wastewater plant before being discharged back to the river.) FHR has since extended the trench nearly a mile, so that the river and wetlands are protected across all of FHR's property that adjoins the river.

Geology
Why did the petroleum wind up in a different place and much sooner than experts thought? Because the geology underneath the refinery is very complex, as shown in Figure 1. The first layer - Layer A - under the refinery is a 100 to 150 foot thick layer of soil and gravel. Most of the petroleum is in this layer - it hasn't had enough time to move downward to the other layers. The second layer - Layer B - is a thin layer of groundwater only 20 to 30 feet thick. Petroleum floats on the top of this layer, and is dissolved in it. The top of the second layer (Layer B) fluctuates as much as 10 feet in a year, depending upon how much rain falls and how hard neighboring wells are pumping. As the top of the second layer moves up, it comes in contact with more petroleum in the first layer (Layer A).

Figure 1

Layer B is not used for drinking water, because it is so shallow and yields little water. The water in this layer moves slowly towards the northeast (a few feet a year), and eventually discharges to the river. FHR, the MPCA and experts thought that was where the petroleum products would go. We were mistaken.

Underlying the second layer is a layer of limestone bedrock that also contains water, called the Prairie du Chien aquifer - Layer C. Like the water in Layer B, the Prairie du Chien aquifer also discharges to the Mississippi River. FHR originally thought that the Prairie du Chien sloped uniformly toward the river, and that Layers B and C did not interact. It turns out there is a buried valley under the refinery which extends eastward to the river. In this valley, the Prairie du Chien can actually flow up into Layer B before both layers B and C discharge to the river. (Please note that this valley is not shown in Figure 1; it runs at right angles to the cross section shown.)

In the late 1980's and early 1990's, Layer B dropped low enough, because of a region-wide drought, to allow some of the petroleum contamination to enter the upper levels of the Prairie du Chien (Layer C). Water moves much more quickly in the Prairie du Chien (100's to 1,000's of feet a year), because it contains large cracks, so when the petroleum got into this valley it headed directly east, where it came out in the spring.

Groundwater Remediation
What can be done to clean up the petroleum contamination in the groundwater? Unfortunately, not very much. The soil at the bottom of Layer A contains petroleum that slowly leaches into the groundwater. Once in the groundwater, it is difficult to remove. FHR has removed floating pockets of oil where they have appeared, and has tried pumping and treating the groundwater from areas with high concentrations. Unfortunately, little water can be removed from Layer B, and once pumping stops, the petroleum in the soil continues leaching into the groundwater.

FHR monitors a network of wells and will continue to remove floating pockets of oil if they re-occur and will continue to operate the trench near the river. FHR is removing as much of the oil from Layer A (see Soil Remediation below) as possible. Naturally occurring bacteria living in the soil and groundwater will continue to break down the petroleum. FHR is also experimenting with pumping air into the groundwater, to supply additional oxygen for the naturally occurring bacteria.

Figure 2

Unfortunately, complete remediation will take decades. Figure 2 shows the concentration of petroleum in the groundwater in 1999. The map of contamination will likely continue to look the same for the next 30 or 40 years. Eventually the area of contamination will decrease, as soil bacteria break down the oil.

Soil Remediation
Fortunately, most of the petroleum was in Layer A, and much of this has been removed. In 1998, FHR installed a soil vapor extraction system (SVE). The SVE pulls a slight vacuum on the soil, using a large vacuum pump connected to a series of wells beneath the refinery. This vacuum pulls the lighter, easily evaporated portions of the petroleum out of the soil. The air containing the petroleum is combusted in two large burners to prevent transferring pollution from the soil into the air.

At the same time that petroleum vapors are being removed, air is pulled down into the soil, through the soil surface and through other wells. Bacteria in the soil use the oxygen in the air to help break down the remaining petroleum product.

Figures 3 and 4 show the effect the SVE system has had. Figure 3 shows the concentration of petroleum in the soil before the SVE system was installed in 1997. In some places the soil contained as much as 200,000 parts per million (ppm) petroleum. Figure 4 shows concentrations in 1999 - a year or so after the SVE system was operating. Concentrations have been reduced by about 95%. Even in the worst areas, petroleum concentrations are down to 3,000 to 15,000 ppm.

FHR is managing its production wells in the Prairie du Chien (Layer C) to lower the water table as much as possible in the areas of highest concentration. This will expose as much of Layer A as possible to the SVE system, and remove as much petroleum as possible near the groundwater. At the same time, FHR must be careful not to lower the Layer B too much, so that petroleum doesn't get back into the bedrock valley.

At some point, the SVE system will have removed all the petroleum that can be evaporated from the soil and can be shut down. FHR may continue to operate the SVE system intermittently for a short time after that, to be sure that sufficient oxygen remains for soil bacteria to break down the remaining petroleum. Soil bacteria will eventually remove most of the remaining petroleum.

Prevention
To prevent future leaks to soil and groundwater, FHR has implemented a number of pollution prevention projects under an agreement with the MPCA. Additional pollution prevention measures are included in FHR's Aboveground Storage Tank (AST) permit issued by the MPCA. FHR has added double bottoms to all gasoline, crude oil and other light oil product tanks to prevent oil leaks. FHR has added a number of other pollution prevention steps - to see more, go to the Pollution Prevention page.