Injection ConfiningZone Lithology and Chemistry

The injection zone consisted of multiple Upper Cretaceous strata of sand, silty sand, clay, and some thin beds of limestone (see Figure 20.14). The clay confining layer was about 30 m (100 ft) thick.

Sand & gravel i-^-"i Clay

Limestone f//i Crystalline rock

FIGURE 20.14 Diagram showing construction features and lithologic log of North Observation Well, Wilmington, NC. (From U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste: A Reference Guide, EPA/625/6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.)

The TDS concentration in the injection-zone formation water was 20,800 mg/L, with sodium chloride the most abundant constituent.

20.7.4.3 Chemical Processes Observed A number of chemical processes were observed at the site178,179:

1. The waste organic acids dissolved carbonate minerals, alumino-silicate minerals, and iron/manganese-oxide coatings on the primary minerals in the injection zone.

2. The waste organic acids dissolved and formed complexes with iron and manganese oxides. These dissolved complexes reprecipitated when the pH increased to 5.5 or 6.0 because of neutralization of the waste by the aquifer carbonates and oxides.

3. The aquifer mineral constituents adsorbed most waste organic compounds, with the exception of formaldehyde. Adsorption of all organic acids except phthalic acid increased with a decrease in waste pH.

4. Phthalic acid was complexed with dissolved iron. The concentration of this complex decreased as the pH increased because the complex coprecipitated with the iron oxide.

5. Biochemical waste transformation occurred at low waste concentrations, resulting in the production of methane. Additional microbial degradation of the waste resulted in the reduction of sulfates to sulfides and ferric ions to ferrous ions.

When the dilute waste front reached the North Observation Well in June 1972 microbial populations rapidly increased in this well, with methanogenesis being the major degradative process.180 Elkan and Horvath170 found greater numbers and species diversity of microorganisms in the observation well, which contained dilute wastes, than in the observation well, which was uncon-taminated. In laboratory experiments, however, DiTommaso and Elkan180 found that bacterial growth was inhibited as the concentration of waste increased and could not decompose the waste at the rate it was being injected.

This case study illustrates the importance of dissolution/precipitation reactions in determining waste- reservoir compatibility. Adsorption was observed to immobilize most of the organic constituents in the waste except for formaldehyde. As with the Monsanto case study, biodegradation was an important process when wastes were diluted by formation waters, but the process became inhibited when undiluted waste reached a given location in the injection zone.

20.7.5 Case Study No. 5: Illinois Hydrochloric Acid-Injection Well 20.7.5.1 Injection-Facility Overview

This case study is an example of a well blowout resulting from the neutralization of acid by carbonate rock. Kamath and Salazar181 and Panagiotopoulos and Reid182 both discuss the same incident. Although they do not specify the location, Brower and colleagues183 identify the site as the Cabot Corporation injection well, near Tuscola, Illinois.

The waste hydrochloric acid (HCl) injected at the site was a byproduct of a combustion process at 1633°C (2972°F). When not recovered, the acidic stream was dumped into holding ponds where it was cooled to about 24°C (75°F) before injection. The concentration of injected acid typically varied from 0.5 to 5% HCl, but ranged as high as about 30%. (The pH of injected acid that back-flowed during one blowout incident ranged from 0.5 to 1.3.)

The injection well was cased to a depth of about 1495 m (4900 ft) and extended into dolomite to a total depth of 1617 m (5300 ft). Injection began in the early 1960s and averaged around 340 L/min (90 gal/min). The natural fluid level was 60 m (200 ft) below the wellhead, and wastes were injected using gravity flow; that is, the pressure head of the well when filled to the surface with fluid was sufficient to inject fluids without pumping under pressure.181

Between 1973 and 1975, several blowouts caused surface water pollution and fish kills. The most serious occurred in 1975 after unusually high concentrations of HCl (ca. 30%) were injected intermittently for several weeks. The well refused to accept additional acid under gravity flow. At first the operators thought the well bore had become plugged, and they pumped a concentrated calcium-chloride solution down the hole to dissolve precipitates that might have formed. Shortly thereafter the well tubing broke, pressure suddenly rose to 37 kg/cm2 (450 psi), and a section of the upper tubing was ejected through the wellhead along with acid and annulus fluids. Backflow was stopped for a while by draining cold water from a fire hydrant into the well at 190 L/min

(50 gal/min). The well erupted again the next day, however, with a 3-m (10-ft) gusher discharging at 946 L/min (250 gal/min). The blowout was brought under control 2 d later when a blowout preventer was installed.

20.7.5.2 Injection/Confining-Zone Lithology and Chemistry

The injection zone was a cavernous dolomite, and the native groundwater was very saline, with TDS levels ranging from 21,000 to 26,000 mg/L. No information was provided on the confining layer, but it is discussed in the work by Brower and colleagues183 in detail.

20.7.5.3 Chemical Processes Observed

The HCl dissolved the dolomite, forming carbon dioxide (CO2) gas. Under normal circumstances this gas remains in solution, but if the temperature of the acid or the acid concentration exceed certain limits, CO2 evolves as a gas and accumulates in the upper portion of the cavity. The escape of even small amounts of CO2 into the injection pipe can serve as a driving force to reverse the flow of the injected liquids, because as the CO2 rises, pressure decreases and the gas expands.

There is some disagreement as to which parameter is most critical to gas blowout. Based on analysis of CO2 phase behavior at different temperatures and pressures, Kamath and Salazar181 concluded that gas blowout becomes hazardous if the temperature of the injected HCl exceeds 88°F. Panagiotopoulos and Reid182 concluded that HCl concentration is the critical factor and that HCl concentrations exceeding 6% will evolve CO2 gas and create a blowout hazard. Both sets of investigators explained the circumstances of this case study in terms of their respective models.

20.7.6 Case Study No. 6: Texas Petrochemical Plant 20.7.6.1 Injection-Facility Overview

This case study involves an unnamed petrochemical plant located about 15 miles inland from the Texas Gulf Coast, described by Donaldson and Johansen.184 It illustrates two approaches to injecting incompatible wastestreams to prevent well plugging by precipitation: surface treatment and multiple injection wells.

The plant began full-scale operation in 1962 and produced acetic, adipic, and propionic acids; acetaldehyde; butanol; hexamethyldiamine; vinyl acetate; nylon; and other chemical products from petroleum-base stocks. The effluent was collected at waste treatment facilities as two separate mixtures. Because mixing two wastestreams produced considerable precipitation, the wastestreams were processed and injected separately into two wells.

Organic constituents in the first wastestream totaled about 14,000 mg/L (acetaldehyde, acetal-dol, acetic acid, butanol-1, butyraldehyde, chloroacetaldehyde, crotonaldehyde, phenol, and propionic acid) and about 5200 mg/L inorganic constituents. The pH ranged from 4 to 6, and TDS ranged from 3000 to 10,000 mg/L.

The second wastestream contained amines and nitrates generated from the manufacture of nylon, hydrocarbon solvents used in processing, and other minor constituents. Organic constituents (amyl alcohol, cyclohexane, dodecane, hexanol, 1-hexylamine, 1,6-hexylamine, methanol, and valeric acid) totaled about 4700 mg/L. Inorganic constituents in the second wastestream totaled about 21,350 mg/L, including 7500 mg/L nitrate and 4600 mg/L nitrite. The second wastestream was basic, with a pH ranging from 8 to 10. The composition of the wastes changed over time when processes changed or a new unit was installed. Several new process wastes (unspecified) that were incompatible with either wastestream were made compatible by adjusting the pH and diluting them.

Injection began in both wells in mid-1963. The injection zone for Well No. 1 was 13.7 m (45 ft) thick beginning at about 1037 m (3400 ft) below the surface. Well No. 2 was located 824 m (2700 ft) north of Well No. 1, and the injection zone was located between 991 and 1083 m (3520 and 3550 ft).

Donaldson and Johansen184 mention no monitoring wells at the site. About 6 years after injection began, pressure interference from the two injection wells was observed. During the same period, the fluid front from Well No. 1 was about 223 m (730 ft) from the well bore.

20.7.6.2 Injection/Confining-Zone Lithology and Chemistry

The injection formation was loosely consolidated, fine-grained Miocene sand. The confining strata between the base of the freshwater aquifer and the injection zone included about 366 m (1200 ft) of relatively impermeable shale and clay beds with individual zone thickness ranging from 3 to 75 m (10 to 245 ft).

20.7.6.3 Chemical Processes Observed

Well head pressures increased when injection was stopped at Well No. 1 for more than 24 h, apparently caused by a combination of precipitation reactions and backflow of sand. Injecting a slug of brine after every period of interrupted flow solved this problem. Movement of the main organic constituents (n-hexylamine, butanal, butanol, and phenol) was assumed to be slowed by adsorption. This conclusion was based on laboratory adsorption experiments by involving a different geologic formation (Cottage Grove sandstone); no direct observations were made of the injected waste. For current hazardous waste injection wells in Texas, the reader can refer to Texas Environmental Profiles web site for on-line resources for the State of Texas.185

NOMENCLATURE

B

Bacterial concentration

C

Concentration of adsorbed substance in solution, ^g/mL

Ct

Dissolved plus adsorbed phase concentration of compound C, mol/L or ^g/L

[H+]

Concentration of hydrogen ion, mol/L

k

Langmuir coefficient related to adsorption bonding energy (mL/g)

ki

Empirical biodegradation rate constant

ka

Acid-catalyzed hydrolysis rate constant, L/mol/s

kb

Base-catalyzed hydrolysis rate constant, L/mol/s

kB

First-order biodegradation coefficient

kB2

Second-order biodegradation coefficient

kH

Specific hydrolysis rate constant, L/s

kn

Natural hydrolysis rate constant for the pH-independent reactions of a chemical

with water, L/s

K

Empirical coefficient

Kd

Distribution coefficient

Koc

Organic-carbon partition coefficient

Kp

Partition coefficient

N

Empirical coefficient

[OH]

Concentration of hydroxide ion, mol/L

R

Rate of hydrolysis, mol/L/s or ^g/L/s

S

Amount adsorbed (^g/g solid)

^max

Maximum adsorption capacity (^g/g soil)

¿1/2

Half-life of a substance

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