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Water Resources Investigations

Contamination of a
Surficial Sand
Aquifer-A Case Study
By Bernard J. Franks

stored in potentially leaky surface impoundments include organic chemicals from wood-preservative processes; most of these wastes contain creosote, a complex distillate of coal tar, and pentachlorophenol (PCP).

Owing to the large volume of creosote potentially contaminating local groundwater systems, a wood-preserving plant at Pensacola, Florida, was chosen by the USGS for detailed investigations (fig. 1). The plant was chosen because of its long uninterrupted history (from 1902 to 1981) of discharging wastewaters into unlined surface impoundments, the availability of preliminary data collected by the USGS in cooperation with the Florida Department of Environmental Regulation, and the high probability of transferring the findings to other sites where similar problems exist.

The plant site is located near downtown Pensacola in Escambia County,

Since 1982, teams of U.S. Geological Survey researchers have been studying selected ground-water contamination sites to advance the understanding of hydrologic processes affecting the movement of contaminants in ground water. Over 400 commercial wood-treatment plants are active in the United States; many of them discharge their wastes into onsite impoundments that, in turn, leak into underlying surficial aquifers. Contaminants commonly

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


Florida, and consists of about 18 acres about 1,500 feet north of Pensacola Bay near the entrance to Bayou Chico (fig. 2). It is underlain by a surficial sand and gravel aquifer that is the sole source of water supply for the city of Pensacola. The site, which lies within a recharge area, has all of the characteristics that contribute to a high vulnerability for aquifer contamination: a thin unsaturated zone overlying an unconfined, highly permeable, surficial sand aquifer in a humid climatic zone. One factor that limits the extent of contamination at the Pensacola site is the presence of a shallow confining layer along the coast, which inhibits contaminant movement downgradient from the impoundments.

Downward movement of water in the aquifer is limited both by the physical presence of the confining layer, which acts as a barrier to vertical movement, and by higher water pressures below the confining layer than above the layer.

During the years of plant operation, the wood-treatment wastes were discharged into unlined ponds hydraulically connected to the surficial aquifer. Over the years, large but unknown quantities of the waste have infiltrated through the soil down to the water table. The wastes have separated into two distinct phases: a denser-than-water hydrocarbon phase that moves vertically downward until intercepted by a confining layer, and an

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Extent and Character of Contamination

aqueous phase that includes the watersoluble components of creosote.

Movement of the water-soluble contaminants in the subsurface is controlled by ground-water flow that is generally directed southward toward Pensacola Bay. Flow velocities range from 0.1 to 3.0 feet per day. Flow paths and velocities are locally influenced by the distribution of the confining silts and clays. A small drainage ditch south of the two unlined impoundments intercepts shallow ground-water flow and has a marked effect on the configuration of the water-table surface and also on contaminant transport.

The U.S. Environmental Protection Agency included the Pensacola site on the Federal “Superfund” list of hazardous waste sites in 1982 owing to phenolic compound concentrations in excess of 10 milligrams per liter (mg/L). In 1983, an EPA Emergency Response Team drained the ponds, treated the liquid, and placed a clay cap over the former impoundments (figs. 3, 4). This was done to alleviate a potential public health hazard caused by abnormally high water levels that caused contaminated water to spill over the embankments and flow southward along natural drainage paths, directly into Pensacola Bay. The primary source of subsurface contamination, the dense hydrocarbon phase that had already entered the aquifer, was not affected by this cleanup operation.

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, methane, ammonia, 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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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

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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