Final Report: Evaluating Source Grouting and ORC for Remediating MTBE Sites

EPA Grant Number: R828598C809
Subproject: this is subproject number 809 , established and managed by the Center Director under grant R828598
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Gulf Coast HSRC (Lamar)
Center Director: Ho, Tho C.
Title: Evaluating Source Grouting and ORC for Remediating MTBE Sites
Investigators: Rifai, Hanadi , Rixey, William G.
Institution: University of Houston - University Park
EPA Project Officer: Lasat, Mitch
Project Period: September 1, 2000 through August 31, 2004
RFA: Gulf Coast Hazardous Substance Research Center (Lamar University) (1996) RFA Text |  Recipients Lists
Research Category: Hazardous Waste/Remediation , Targeted Research

Objective:

Methyl tertiary-butyl ether (MTBE), a common oxygenate, is used in 30 percent of the nation’s gasoline and usually constitutes 11 to 15 percent of the gasoline product.  Although MTBE has beneficial properties for improving air quality, it also has several characteristics that make it a highly mobile groundwater contaminant.  Recent studies in the literature have documented the presence of MTBE in many of the nation’s groundwater aquifers.  Current research provided evidence of the biodegradability of MTBE, particularly under aerobic conditions.  This has sparked interest in aerobic bioremediation technologies as a possible remedy for MTBE plumes.  One of the emerging oxygen sources for groundwater is a class of compounds known as oxygen releasing compounds (ORC).  These compounds are in powder form and generally present an interesting design dilemma in relation to the amount needed and the delivery method.  ORC have been studied at the laboratory scale and to a lesser extent at the field scale.  Reported results in the general literature have been promising; however, not enough work has been done to answer all the questions related to oxygen delivery and other design variables.

The goal of this research project was to study the effectiveness of ORCs as a source of oxygen when injected in slurry form directly into the soil matrix.  The specific objectives were to determine:  (1) the amount of ORC needed per injection location; (2) the levels of oxygen that can be achieved as a function of distance away from the injection point; and (3) the longevity of the ORC as a source of oxygen.  To address these objectives, an in situ pilot-scale test was undertaken.  The pilot scale consisted of a single ORC injection at a field site in Texas.  An extensive monitoring network was used to monitor the levels of oxygen in the aquifer that resulted from the injection.

In addition to the field injection project, laboratory oxygen release experiments were completed to determine the levels of oxygen that can be released from ORC, as well as the longevity of ORC as an oxygen source.  The laboratory experiments served as a tool to understand the relationship between oxygen release levels and flow rate and to evaluate the potential for ORC to reduce the hydraulic conductivity in the matrix.  The laboratory columns were completed with glass beads and with native soils from the site where the pilot scale oxygen release test was conducted.  An analytical model was used to simulate oxygen release from ORC at the field scale.  The laboratory experiments were used to determine the model variables.

Summary/Accomplishments (Outputs/Outcomes):

In the field experiment, a multilevel monitoring network was installed around a single ORC injection point as mentioned previously.  Approximately 50 lbs of ORC were injected into a 10 ft zone below the groundwater table at a site in Texas.  The experiment was monitored for approximately 8 months.  Results from the experiment confirmed that oxygen is released from ORC, although at lower levels than those observed at other similar field experiments.  Oxygen levels observed in the groundwater were up to 5 mg/L, in contrast with previous efforts that measured levels of up to 20 mg/L.  Although the specific reason for this phenomenon has not been isolated, it is theorized that the aquifer matrix is exerting an inorganic oxygen demand.  The maximum oxygen levels in the experiment were observed almost 2 months after ORC injection.  Additionally, the zone of oxygenation extended to 5 ft away from the ORC injection point.  Oxygen fluxes through the experimental plot were calculated and are shown in Table 1.  The peak flux was calculated 66 days after injection and totaled 80 g of oxygen.  This represents less than 5 percent of the total mass of oxygen present within the ORC.

Table 1.  Oxygen Mass and Flux Analysis for Pilot-Scale Field Experiment


Day

Mass (grams)

Flux (g/d) at ML-3 *

Background

44.3

0.42

Day 18

48.75

0.43

Day 36

41.25

0.33

Day 66

80.35

0.66

Day 96

21.53

0.20

Day 126

45.65

0.40

Day 160

44.47

0.43

Day 190

55.73

0.42

*  ML-3 is 1.5 m downgradient of ORC injection point.

Six laboratory experiments (Table 2) were conducted using a glass column that was 18.7 cm in length and had a 4 cm internal diameter.  The column was packed dry with glass beads and ORC or with soil and ORC.  The column experiments were conducted in an upward flow mode.  The effluent from the column was collected in a 20 ml vial.  There were three openings on the vial cap:  one as an inlet, one as an outlet, and the third opening housed a one-quarter inch diameter Foxy T-1000 oxygen probe.  The effluent from the vial was collected for flow and pH measurement.


Table 2.  Experimental Variables for Packed Column Experiments


Experiment

Mass of ORC
(g)

Ratio of Mass of ORC /Mass of glass beads (105-150 mm) or soil

Flow rate
(ml/min)

Duration
min(days)

Size of glass beads above and below the ORC-glass beads mixture

1

95

0.57

0.66

5718 (4)

30-50 mm

2

5

0.20

0.70

6928 (4.8)

30-50 mm

3

15

0.10

0.68

8318 (5.8)

30-50 mm

4

15

0.10

0.18 -1.53

35776 (24.8)

105-150 mm

5
(with soil)

15

0.10

0.76

8654  (6)

Soil Matrix

6
(with soil)

15

0.10

0.77

 

Soil Matrix

 

Results from column experiments 1-3 confirmed that ORC slowly releases oxygen.  In experiments 1-3 (as well as experiments 4, 5, and 6), it took an average of 3 to 4 days for dissolved oxygen (DO) concentrations to achieve a steady level.  Effluent results indicated that column 1 exhibited the highest DO level (12-16 mg/L), and column 2 released the lowest concentration (1-3 mg/L) (Table 3).  This is not surprising as column 1 had the highest amount of ORC, whereas column 2 had the lowest.  In Experiment 3, increases in DO level by 3 to 6 mg/L were observed.  Also, the pH initially rose above 11.0 but later stabilized to around 9.60 in the three columns.  All three columns experienced plugging within 4 to 6 days of experiment initiation and released between 8 to 62 mg of oxygen prior to plugging.  The alkalinity of the feed solution in the third column experiment was found to be less than 5 mg/L; an indication that there was not much carbonate in the solution, and hence there would not be any significant formation of calcium carbonate or magnesium carbonate precipitates in the high pH environment to cause the plugging.  It was concluded that the reduction in flow might be a result of plugging of the pore spaces by magnesium hydroxide precipitates.  Another possible explanation may be the presence of 30-50 mm glass beads above and below the ORC-glass bead (105-150 mm) mixture.  Based on the observed DO levels, columns 1, 2, and 3 are expected to generate oxygen for 666, 248, and 340 days, respectively, in the absence of plugging.  The rates of release of oxygen from ORC ranged between 1.50 x 10-4 to 3.22 x 10-4 g-O2/g-ORC per day.


Table 3.  Results from Column Experiments


Experiment

Flow Rate (ml/min)

Mass of ORC in Column (g)

Change in DO Concentration (DC, mg/L)

Time to plugging
(days)

Release Rate (g-O2/g-ORC per day)

Mass of oxygen Released (mg)

Predicted ORC Lifetime (days)

1

0.66

95.0

12-16

4.0

1.50 x 10-4

43.8

666

2

0.70

5.0

1-3

4.8

3.22 x 10-4

8.0

248

3

0.68

15.0

3-6

5.8

2.93 x 10-4

61.5

340

4

0.75

15.0

8.1

24.8

5.83 x 10-4

130.59

 

4

1.53

5.6

8.96 x 10-4

 

4

0.37

9.6

3.41 x 10-4

 

4

0.18

13.6

2.35 x 10-4

 

5

0.76

15.0

13.6

6.0

9.92 x 10-4

95.27

98

6

0.39

15.0

16.6

ND

6.22x 10-4

67.57 up to t = 10202 min

161


ND - Not determined

In Experiment 4, 15.0 g of ORC increased the DO levels to between 5.6 and 13.6 mg/L, depending on the column flow rate.  The column did not plug up until day 25 of the experiment.  Increasing the flow rate caused a decrease in the released oxygen concentration, whereas decreasing the flow rate caused an increase in dissolved oxygen.  This is to be expected because a lower flow rate allows more contact time between the flowing water and the ORC.  A total of 136 mg of oxygen was released prior to plugging.  Initially, the pH in the effluent rose above 10.8, but later in the experiment, the pH varied between 9.8 and 10.5.

As can be seen from Table 3, when the flow rate was doubled, the oxygen release rate increased by 1.54 (ratio of 8.96 x 10-4 and 5.83 x 10-4) and the effluent concentrations decreased by 2.5 mg/L because of the shorter contact time between ORC and water.  Similarly, reducing the flow rate by one-half caused the oxygen release rate to decrease by 0.59 and the effluent concentrations to increase by 1.5 mg/L relative to the base flow and so on.  The variation in release rates as a function of flow velocity was modeled using a power law.  By extrapolating the model to lower flow rates, it was possible to estimate the ORC release rate for the pilot-scale field experiment.  At a groundwater velocity of 0.04 ft/day at the project site, the ORC release rate was calculated to be around 3.89x10-5 g-O2/g-ORC per day.  The dependence of release rates on the flow rate suggests that there are mass transfer limitations associated with oxygen release from ORC particles.  As it took longer for plugging to occur in Experiment 4, it was concluded that the relatively faster plugging observed in earlier experiments might be a function of the use of different size glass beads above and below the ORC-glass bead mixture.

Column 5 lasted for 6 days before the glass column cracked as a result of build up of excess pressure.  ORC increased the DO levels by 13.6 mg/L in the column effluent at a flow rate of 0.76 ml/minutes.  Based on the observed DO levels, column 5 is expected to generate oxygen for 98 days in the absence of plugging.  The rate of release of oxygen from ORC was calculated to be 9.92 x 10-4 g-O2/g-ORC per day and the mass of oxygen released was 95.2 mg.  Data from Experiment 6 indicated that oxygen levels increased by 16.6 mg/L at a flow rate of 0.39 ml/min.  The oxygen release rate was calculated to be 6.22 x 10-4 g-O2/g-ORC per day and the mass of oxygen released was 65.57 mg up to t equal to 10,202 minutes.  At this observed release rate, ORC would have lasted for 161 days in the absence of plugging.

A two-dimensional continuous line source model with steady state conditions was used to compare the concentrations observed in the field to those predicted using the model.  The model predictions indicated that ORC would increase the DO levels by a 1 to 6 mg/L in the test network.  This compared well with the overall increases of 1 to 4 mg/L that were observed in the field experiment.  The model assumes a continuous source without any oxygen sink, whereas ORC will deplete with time in the field and there might be oxygen sinks in the subsurface.

Journal Articles:

No journal articles submitted with this report: View all 7 publications for this subproject

Supplemental Keywords:

pollution prevention, waste treatment, site remediation, advanced treatment technologies, bioremediation, hazardous waste treatment, HSRC,, RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Waste, Water, Contaminated Sediments, Remediation, Health Risk Assessment, Hazardous Waste, Ecology and Ecosystems, Groundwater remediation, Hazardous, sediment treatment, advanced treatment technologies, in situ remediation, in situ treatment, contaminant dynamics, biodegradation, MTBE, contaminated sediment, source grouting, BTEX, chemical contaminants, emissions, contaminated soil, natural attenuation, in-situ treatment of chlorinated solvents, treatment, environmental engineering, hazadous waste streams, contaminated groundwater, contaminated aquifers, air emissions, groundwater, aquifer remediation

Relevant Websites:

http://dept.lamar.edu/gchsrc/

Progress and Final Reports:

Original Abstract
  • 2001 Progress Report
  • 2002
  • 2003

  • Main Center Abstract and Reports:

    R828598    Gulf Coast HSRC (Lamar)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
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