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Physical, chemical, and biological aspects of subsurface organic waste injection near Wilmington, North Carolina

Professional Paper 987
By J.A. Leenheer, R.L. Malcolm, and W.R. White

Abstract

From May 1968 to December 1972, an industrial organic waste was injected at rates of 100 to 200 gallons per minute (6.3 to 12.6 litres per second) into a sand, gravel, and limestone aquifer of Late Cretaceous age by Hercules Inc. located near Wilmington, North Carolina. This report presents both field and laboratory data pertaining to the physical, chemical, and biological effects of waste injection into the subsurface at this particular site, a case history of the operation, predictions of the reactions between certain organic wastes and the aquifer components, and descriptions of the effects of these reactions on the subsurface movement of the wastes.

The case history documents a situation in which subsurface waste injection could not be considered a successful means of waste disposal. The first injection well was used only for 1 year due to excessive wellhead pressure build-up above the specified pressure limit of 150 pounds per square inch (10.3 bars). A second injection well drilled as a replacement operated for only 5 months before it too began to have problems with plugging. Upward leakage of waste into shallower aquifers was also detected at several wells in the injection-observation well system. The multiple problems of plugging, high pressures, and waste leakage suggested that the reactive nature of the waste with the aquifer into which it was injected was the primary reason for the difficulties experienced with waste injection.

A site study was initiated in June 1971 to investigate waste-aquifer interactions. The first stage of the study determined the hydrogeologic conditions at the site, and characterized the industrial waste and the native ground water found in the injection zone and other aquifers. The injection zone consisted of multiple permeable zones ranging in depth from about 850 to 1,000 feet (259 to 305 metres) below land surface. In addition to the injection zone, aquifers were found near depths of 60, 300, 500, and 700 feet (18, 91, 152, and 213 metres) below land surface. The aquifers from 300 feet (91 metres) down to the injection zone were flowing artesian with the natural pressure of the injection zone being 65 feet (20 metres) above land surface at the site.

The dissolved solids concentration in the native ground water increased with depth to an average value of 20,800 mg/l (milligram per litre) (two-thirds that of seawater) in the water from the injection zone. Sodium chloride was the major dissolved solid, and all of the ground water below 300-feet (91-metres) depth was slightly alkaline.

Dissolved organic carbon of the industrial waste averaged 7,100 mg/l and 95 percent of the organic carbon was identified and quantified. The major organic waste constituents in order of decreasing abundance were acetic acid, formic acid, p-toluic acid, formaldehyde, methanol, terephthalic acid, phthalic acid, and benzoic acid. Prior to injection, the waste was neutralized with lime to pH 4 so that the major inorganic waste constituent was calcium at a concentration of 1,300 mg/l.

The second stage of the site study involved the observation of waste-aquifer interactions at various wells as the waste arrived and passed by the wells. Water samples obtained from three observation wells located 1,500 to 2,000 feet (457 to 607 metres) from the original injection well gave evidence for biochemical waste transformations at low waste concentrations. Gas that effervesced from these water samples contained up to 54 percent methane by volume. Ferrous iron concentrations as high as 35 mg/l, hydrogen sulfide gas, and sulfide precipitates were additional indicators of biochemical reductive processes in the subsurface environment. Approximately 3,000 organisms per millilitre were found in uncontaminated ground water from the injection zone whereas in waste-contaminated wells, the number increased to levels as high as 1,000,000 organisms per millilitre. High concentrations of waste were found to be toxic to microorganisms. Most of the organisms isolated from uncontaminated wells were facultative, aerobic genera whereas the population changed to anaerobic strains in the contaminated wells. Methanogenic bacteria of the genus Methanobacterium and genus Methanococcus were isolated in pure culture from ground-water samples in which methane was found.

The relative ratios of formic acid, p-toluic acid, and terephthalic acid to acetic acid were lower in these ground-water samples than in the injected waste indicating degradation or sorption of formic, p-toluic, and terephthalic acids relative to acetic acid during the period of waste travel to these observation wells. The construction of the screened section of the observation wells allowed dilution of the waste and internal circulation of ground water so that it was impossible to determine quantitative waste concentrations in the various waste-receiving zones within the injection zone.

Highly contaminated ground-water samples obtained from five observation wells located near (50 to 150 feet) (15 to 46 metres) the injection wells gave evidence for waste dissolution of aquifer carbonates and iron oxides. These samples contained carbon dioxide gas, calcium concentrations to 3,900 mg/l, and iron concentrations to 310 mg/l. Organic complexation as well as acid dissolution was suspected to be the cause for the high iron concentrations. There was no microbiological activity apparent in these wells and samples.

Concurrent with and after the site study, a laboratory study was conducted in which waste was injected into cores of aquifer material obtained from the injection zone. The laboratory injection pressure was that of the hydrostatic pressure found in the injection zone. When a known volume of waste was injected into a core, the acidic waste initially dissolved the carbonates, and sesquioxide coatings on the primary minerals as evidenced by high concentrations of iron, aluminum, silica, and manganese. Iron concentrations as high as 200 mg/l were obtained, but this dissolved iron was eventually reprecipitated further on in the core when the pH of the waste rose to 5.5 to 6.0 because of neutralization of the waste by aquifer carbonates and oxides. Exhaustive leaching of a core by the acidic waste quantitatively dissolved the aquifer carbonates and removed approximately 12 percent of the extractable iron.

Sorption of the waste organic compounds upon the aquifer mineral constituents was found for all the waste organic acids. Formaldehyde was not sorbed. Sorption increased as the pH of the waste decreased with the exception of phthalic acid. Phthalic acid was complexed with dissolved iron, and its concentration decreased as the pH of the waste increased because it coprecipitated with the iron hydroxide precipitate. The waste solution was supersaturated with respect to terephthalic acid, and this constituent was found to be both highly adsorbed and precipitated in the core.

At the conclusion of this study, a conceptual model was constructed which by combining the results of the field and laboratory studies, detailed the various stages of injected waste reactivity and movement in the subsurface from the injection well to the edge of the waste front. The excessive pressure build-up in the injection wells was thought to be the result of a number of factors: reprecipitation of aquifer constituents initially dissolved by the acidic waste, precipitation of terephthalic acid, formation of carbon dioxide and methane gases, and the relatively low permeability and porosity of the injection zone. The leak problems were thought to arise from the dissolution of the cement grout around the casing by the waste acids of the injection wells and certain observation wells.

Leenheer, J.A., Malcolm, R.L., and White, W.R., 1976, Physical, chemical, and biological aspects of subsurface organic waste injection near Wilmington, North Carolina: U.S. Geological Survey Professional Paper 987, 51 p.