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Water Resources

Investigations

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Figure 2. Aerial photograph of the Pensacola site, 1982. (Photograph courtesy Florida Department of Transportation.)

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Figure 3. Part of the EPA Emergency Response Team, shown flocculating the pond sediments after draining the impoundments before capping the ponds in 1983. (Photograph by Bernard J. Franks, U.S. Geological Survey.)

Contamination from the woodpreserving plant has resulted in the generation of anaerobic (oxygen-free) leachate that has had a significant effect on the water chemistry of the aquifer. In the contaminated water, the pH is about 5.4, the dissolved-solids concentration is about 350 mg/L, and many of the water-soluble components of creosote are found in concentrations near their limits of water solubility. In addition, hydrogen sulfide, methammonia, iron, nitrogen, and dissolved organic carbon all show marked increases in concentration in the aqueous phase downgradient from the impoundments. In contrast, the uncontaminated ground water has a pH of about 6.0, concentrations of dissolved solids generally less than 150 mg/L, and dissolved oxygen concentrations greater than zero. Uncontaminated water is free of organic contaminants, hydrogen sulfide, methane, and ammonia and contains low concentrations of iron, nitrogen, and dissolved organic carbon.

Contamination exists in two of the three permeable zones present in the upper 300 feet of sediments. A plume in the water-table zone is 15 to 30 feet below land surface and above a shallow clay lens, and a

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Figure 4. Clay cap overlying the dewatered impoundments. Since the photograph was taken, the cap has been sodded and revegetated. (Photograph by Bernard J. Franks, U.S. Geological Survey.)

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Figure 5. Vertical extent of ground-water contamination, as measured by total phenol, in

deeper plume is 50 to 100 feet below land surface and below the clay lens (fig. 5). The contamination extends about 1,200 feet downgradient from the impoundments and includes the water-soluble components of creosote, as well as inorganic compounds and gases resulting from degradation of the organic compounds within the aquifer. The waste plumes are much less extensive, both areally and vertically, than expected from calculations based on measured ground-water gradients. For the plume in the water-table zone in particular, this attenuation is partly a result of ground water discharging into a drainage ditch that in turn discharges directly into Pensacola Bay.

Ongoing research at the site includes evaluation of the hydrogeology and groundwater flow system, definition of the extent of organic compounds found in the aquifer, investigations of microbial and inorganic processes active in the subsurface, and a

FEET

milligrams per liter, at the Pen

sacola site.

study of the effects of contaminants on the ecology of Pensacola Bay. These studies and concurrent Superfund-related activities at the site are being used to document the extent of contamination and to support discussion of feasible restoration activities at the site. Results to date include the following:

The hydrogeology of the underlying aquifer can be represented by three horizontal layers corresponding to permeable zones within the sand and gravel aquifer: a water-table zone, a shallow confined zone, and a deeper confined zone. Contamination is limited to the upper two zones because of a combination of the distribution of the clay layers and the upward hydraulic gradient that prevails near the coast south of the impoundments. The simulated movement of ground water is relatively rapid (as much as 3 feet per day) through the surficial sands.

• The aquifer is highly contaminated by organic compounds, including organic nitrogen compounds such as quinoline (as much as 90 mg/L), double-ring aromatic compounds such as naphthalene (as much as 15 mg/L), and phenolic compounds such as 3,5-dimethylphenol (as much as 13 mg/L). The organic nitrogen compounds undergo extremely rapid attenuation to virtually nondetectable levels (less than 0.01 mg/L) within 400 feet of the source. Most of the other compounds persist to

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