alternative method of groundwater sparging for petroleum hydrocarbon remediation

Alternative Method of Groundwater Sparging for Petroleum Hydrocarbon Remediation

Todd W Schrauf Patrick]. Sheehan Leslie H. Pennington

Todd X ScbrauJ C G. IK P., i s a principal bydrogeohgist, and PatrickJ. Sbeeban is an environmental scientist, witb Wasatcb Environmental, Inc., in Salt Lake City. Leslie H. Pennington, P.E., i s tbe president and founder of Wasatcb Environmentd

An alternative method of in-situ groundwater sparging, termed density-driven convection (patentpending), ispresented. i%is method has been successfully used to remediate eight underground storage tank releases involving a wide distillation range ofpetroleum hydrocarbons (gasoline to waste oil;) and in a variety of site soils (clay to sandy gravel).

Application of the density-driven convection method is detailed in a case study. 'Ihe system, installed to remediate a gasoline and diesel release from an underground storage tank, was opated and monitored for a period of one year. Monitoring data indicate reductions in totalpetroleum hydrocarbon concentrations in groundwater and in soil. Concentrations of aromatic hydrocarbons (benzene, toluene, ethylbenzene, xylenes, and naphthalene;) also decreased in both media. Stimulation of natural biodegradation, the prima y mechanism of removal, occurred rapidly. Natural biological activity gradually declined over the subsequent 150 days. After one year of operation, the sparging system has achieved or is rapidly approaching the regulato y cleanup goals for both soil and groundwater, including reduction of dissolved concentrations below maximum contaminant levels established under the Safe Drinking Water Act.

Groundwater sparging involves the injection of air into the saturated subsurface to (1) increase oxygen supply and promote natural aerobic biodegradation and (2) effect removal of volatile compounds by in-situ air stripping. Groundwater sparging is a relatively new and increasingly popular method for in-situ remediation of petroleum hydrocarbons, although, to date, there has been limited documentation of the method's implementation or success.

In this article, an alternative method of implementing groundwater sparging is presented, and the effectiveness of the method is demonstrated using a case study. Although we have successfully applied the sparging method described to a total of eight commercial sites to date, our discussion is limited to a single site. This site was selected for discussion based on the relative magnitude of the release and amount of monitoring data available.

REMEDIATION~~INTER 1993/94 93

TODD W. SCHRAUF PATRICK J. SHEEHAN LESLIE H. PENNINGTON

Figure 1. Schematic of Pressured Injection System Combined with Vapor Extraction.

COMPRESSOR m

REMEDIATION~~INTER 1993/94

ALTERNATIVE METHOD OF GROUNDWATER SPARGING FOR PETROLZUM HYDROCARBON REMEDIATION ~~

The method presented has proven to be an economical and effective method of removing petroleum hydrocarbons, including gasoline, diesel, and waste oil from a variety of soils, ranging from clay to sandy gravel.

DISCUSSION OF GROUNDWATER SPARGING METHODS

Pressurixed iqjectwn has been described as “a crude air stripper in the subsurface,

The most commonly reported method of groundwater sparging (Angell, 1992; Brown and Jasiulewicz, 1992; Loden and Fan, 1992; Looney et al., 1992; Marley, 1992; Martin et al., 1992) introduces air into the saturated subsurface by pressurization of vertical or horizontal wells completed below the water table surface (Figure 1). Published diagrams of this system typically show the air introduced as producing an inverted cone of bubbling air passing upward through the porous media surrounding the injection well. Pressurized injection has been described as “a crude air stripper in the subsurface, with the saturated soil column acting as the packing” (Angell, 1992).

Pressurized injection can prove problematic in fine- to medium- grained soils because of the existence of capillary forces in the surrounding porous media. Middleton and Hiller (1990) have stated that airflow through saturated porous media follows irregular pathways and does not form bubbles. Middleton and Hiller also indicate that pressurized injection is suitable only for materials with hydraulic conductivities greater than .001 c d s e c (3 ft/day). Wehrle (19%) found air would rise from hydraulic uplift alone only in coarser-grained materials (d, > .8 mm). Jasiulewicz and Hildebrandt (1992) indicate that low-permeability barriers or formations may result in lateral spread of the contamination.

Our studies indicate that in porous media a pressurized air bubble forms around the well injection point. As injection pressure increases one of the following will occur:

with the saturated soil column acting as the packing.”

The surrounding porous media liquify in an inverted cone-shaped region around the well, allowing bubbling to occur (in fine-grained cohesionless soils) The air flow is channeled within the porous media surrounding the well (in coarse-grained soils) The air bubble expands as an induced hydraulic fracture, which parallels the ground surface at shallow depths (in most fine-grained soils)

In summary, pressurized injection is suitable only for coarser-grained materials and results in the channeling of airflow without bubble formation.

UVB System A second reported approach is the UVB (Unterdruck-Verdampfer-

Brunnen) system (Herrling et al., 1991) developed in Germany, which uses a subsurface pump and dual screen assembly to create a convection cell around a groundwater well (Figure 2). With the UVB system, oxygenation and air stripping of the water are performed inside a portion of the wellbore

REMEDIATION/WINTER 1993/94 95


Figure 2. Schematic of UVB System Combined with Vapor Collection.

96 REMEDIATION~~INTER 1993/94

ALTFXNATWE METHOD OF GROUNDWATER SPABGING FOR PETROLBUM HYDROCARBON REMEDIATION

In general, gmundwofer sparging waa the principal method of groundwater remediation employed.

assembly. Hence, both contaminant removal and oxygen distribution are affected by solute transport of the circulating groundwater. The UVB system provides improved control over groundwater circulation and can be used to distribute both oxygen and inorganic nutrients. The chief disadvantages of the UVB system are the relative complexity and large diameter of the wellbore assembly, both of which contribute to installation and maintenance costs.

Density-Dtiven Convection A third approach is the density-driven convection system (patent

pending) presented in Figure 3 and developed by Wasatch Environmen- tal, Inc. This method does not attempt to inject air into the soil pore space under pressure, thereby avoiding the disadvantages of pressurized injection. Instead, water inside the wellbore is aerated directly by injecting air at the base of the wellbore. The injected air bubbles rise upward in the wellbore, creating a turbulent frothing action. The rising air bubbles air- strip contaminants from the water and increase dissolved oxygen content of the water (to about 10 mg/l). The aeration process also acts as a groundwater pump, pushing aerated water upward through the wellbore and out the upper well screen and drawing resident groundwater from the surrounding aquifer into the base of the well screen. The density-driven convection system retains the advantages of groundwater circulation and transport created by the UVB system but replaces the more complex mechanical pumping system with density-driven flow. The result is a simple small-diameter installation that is virtually maintenance free.

FIELD APPLICATION OF GROUNDWATER SPARGING The density-driven convection method of groundwater sparging

described above was successfully applied to a total of eight commercial sites. Characteristics of these sites, including the highest pre- and post- remediation petroleum hydrocarbon concentrations, are summarized in Table 1. Inorganic nutrients (such as nitrogen and phosphorus) were not supplemented at any of the eight sites. In general, groundwater sparging was the principal method of groundwater remediation employed. Vapor extraction (soil venting) was used in combination with groundwater sparging at two of the eight sites. Groundwater circulation (groundwater extraction with upgradient reinjection and no surface water treatment) was used at two sites. Detailed discussion of the largest of the eight sites is presented below.

Site Description and Background The site is located in Ogden, Utah, which is situated on alluvial basin

fill near the eastern edge of the Basin and Range province. The encoun- tered soils consist of interbedded silty sands (SM) and poorly graded fine gravel (GP) underlain by a silty clay (CL) aquitard at a depth of about 18 feet.

The water table surface ranges from about 5 to 11 feet below ground surface due to seasonal fluctuations. The shallow groundwater is


TODD W. SCHRAUF PATRICK J. SHXEHAN LESLIE H. PENNINGTON

Figure 3. Schematic of Density-Driven Convection System with Vapor Extraction.

PRESSURIZED BLOWER MOTOR OR

COMPRESSOR VACUUM

VACUUM PUMP

I I I I

98 REMEDIATION/WINTER 1993/94

ALTERNATTve METHOD OF GROUNDWATZR SPARGING FOR PETROLEUM HYDROCARBON REMEDIATION

charged by direct precipitation and subsurface flow from the Wasatch Mountains to the east. Average hydraulic conductivity of the shallow unconfined aquifer is estimated at 190 Wday, The shallow groundwater flows to the north-northwest under a horizontal hydraulic gradient of .004 ft/ft. The estimated groundwater pore velocity is 2 ft/day.

Discovery of a release of petroleum hydrocarbons at the site occurred during the removal of three underground storage tanks containing gasoline and diesel fuels in December 1990. Subsequent site investigations were conducted between December 1990 and August 1991 to delineate the extent of petroleum hydrocarbon migration. The hydrocarbon plume measured about 100 feet in width by 300 feet in length as shown in Figure 4. The estimated volume of affected soil was 6,700 cubic yards.

During the latter stages of the site investigation, selected soil and groundwater samples were analyzed to determine bioremediation feasibil- ity. Measurements included bacterial plate counts, inorganic nutrient concentrations, biochemical oxygen demand (BOD), and chemical oxygen demand (COD). Background microbial populations were relatively low for both total heterotrophic and hydrocarbon-degrading bacteria. As both oxygen and nitrate were severely depleted (less than .5 mg/l and .01 rng/l, respectively), and BODKOD ratios were relatively high (.4 to .8), the low level of microbial activity was attributed to a lack of electron acceptors.

System Installation The system was designed and a corrective action plan (CAP) was

prepared in September and October 1991. The CAP was approved by the Utah Division of Environmental Response and Remediation (DERR) in November 1991. The system was installed in January and February 1992, and operation was begun in March 1992.

Cleanup objectives for groundwater are based on maximum contaminant levels established under the federal Safe Drinking Water Act. The key design criteria for the system included

Ckanup oiliectives for groundwater are b e d on mwimum contaminant levels established under the federal Safe Drinking Water Act. I--

* The presence of site structures, including a neighboring office building covering the tip of the plume The concern of potential health effects within the neighboring office building from migrating product vapors The sensitivity of the adjoining landowner to potential property damage and/or loss of his office tenant The mitigation of further petroleum hydrocarbon migration The minimization of remedial costs and operation

The remediation system consisted of three principal components: a groundwater circulation (pumping) system, a groundwater sparging system, and a vapor extraction system. A plan view of the remedial system as installed is shown in Figure 4.

Groundwater pumping was selected to provide hydraulic containment of the contaminant plume. The pumping system also provided improved circulation of oxygen supplied by the sparging system by increasing the


TODD W. SCHRAUF 0 PATRICK J. SHEEHAN 0 LESLIE H. PENNINGTON

Table 1. Summary of Field Applications of Groundwater Sparging.

Ogden, Utah Near Closure Diesel and 30,000 Sand and Gravel 12 Months GW' Gasoline Soil

Ogden, Utah Remediation Complete Diesel Fuel 9,000 Sand and Gravel 20 Months GW Soil

Salt Lake City

Salt Lake City

Salt Lake City

Salt Lake City

Salt Lake City

Salt Lake City

Remediation Complete

Remediation Ongoing


Pilot Study

Remediation Ongoing


'GW - Groundwater zN/A - not applicable

Gasoline 500 Sand

Sand

6 Months

Diesel and 2,000 9 Months Gasoline

Gasoline 2,400 Sand and Silt 6 Months

Silt and Clay 8 Months Gasoline 8,000

Diesel 1 ,Ooo Clay 22 Months

Waste Oil 10,Ooo Clay 15 Months

GW Soil

GW Soil

GW Soil

GW Soil

GW Soil

GW Soil

groundwater flux through the shallow aquifer. Groundwater was extracted from three downgradient wells (EWlB, EW2B, and EW3B) and reinjected without surface treatment into an upgradient injection gallery. The injection gallery was installed in the pea gravel backfill of the original tank excavation. The combined pumping rate from all three extraction wells was 10 gpm. Pneumatic submersible pumps were used for groundwater extraction. Air feed lines and water discharge lines were placed in conduit


ALTERNATIVE METHOD OF GROUNDWATER SPARGING FOR PETROL~UM HYDROCARBON REMEDIATION

190 1 , < 5

30 11,200

15 67

2.1 N/A

0.56 N/A

118 N/A

110 51,000

N/A N/A

4.7 7.8

0.049 N/A

0.78 0.1

0.19 N/A

0.34 N/A

12.9 N/A

0.055 <o. 1

N/A N/A

N/Az N/A

N/A N/A

N/A N/A

N/A N/A

N/A N/A

N/A N/A

N/A N/A

190 N/A

19 6.3

15 13

<0.02 <2.0

0.16 N/A

<0.02 N/A

22 N/A

<0.02 1,100

N/A N/A

<0.005 0.1

0.008 <o. 1

< o m 2 <0.1

0.076 N/A

<0.002 N/A

1.29 N/A

<0.002 <o. 1

N/A N/A

N/A N/A

N/A N/A

N/A N/A

N/A N/A

N/A N/A

N/A N/A

N/A N/A

3.7 N/A

Soil concentrations below required cleanup levels. Groundwater concentrations below MCLs except naphthalene which is approaching MCL. Average TPH concentration reduced by 86% and 99.7% in soil and groundwater respectively.

Soid concentrations below required cleanup levels. Upgradient source of contaminants including chlorinated solvents migrating onto site and affecting groundwater concentrations. TPH and benzene in upgradient groundwater reduced by 70% downgradient of sparging area.

Groundwater and soil concentrations of benzene and TPH reduced to below detection limits after 6 months of operation.

Groundwater concentrations of benzene and TPH have been reduced by 78% and 99% respectively; however, benzene concentrations are still above cleanup levels.

Contaminated soil was removed by excavation. Groundwater benzene and TPH concentrations reduced by 99?h and 96% to below detection limits.

Average groundwater benzene concentrations reduced by 78%. Full-scale sparging system to be installed.

Groundwater benzene and TPH Concentrations have been reduced by 96% and 99% to below detection limits. Soil TPH concentrations reduced by 98%; however, soil TPH still above required cleanup levels.

Groundwater contaminants are below required cleanup levels. Soil sampling data unavailable.

pipe located in shallow below-ground trenches. Groundwater sparging was chosen as an effective but relatively

inexpensive method for remediation of the site soils and groundwater. The system would provide in-situ air-stripping treatment of the reinjected groundwater in addition to providing dissolved oxygen for promotion of biodegradation. A total of twelve sparging wells were installed both in the injection gallery and along a line parallel to the direction of groundwater


TODD W. SCHRAUF PATRICK J. SHEEEIAN LESLIE H. PENNINGTON

Figure 4. Plan View of Example Site.

0

v 0

A

e

0 MWll

0 MWB TRENCH LINES

OFFICE BUILDING

APPROXIMATE EXTENT OF CONTAMINATION

LEGEND

Groundwater monitoring well

SHOP BUILDING

EMISSION PIPINO

EMISSION STACK

0 MWll

0 MWB TRENCH LINES

OFFICE BUILDING

APPROXIMATE EXTENT OF CONTAMINATION

LEGEND

Groundwater monitoring well

SHOP BUILDING

EMISSION PIPINO

EMISSION STACK

Vapor extraction well

Groundwater monltorlng/vapor extraction well

Groundwater extraction well

Sparglng well

Post-remedlation sol1 sampling location

APPROXIMATE SCALE

0 90 60 120

0 MW1

I

N


ALTERNATIVE METHOD OF GROUNDWATER SPARGING FOR PETROLEUM HYDROCARBON REMEDIATION

Shutdowns of the groundwater pumping system were required on two occa8ions.. .

flow about 50 to 80 feet downgradient of the injection gallery. This placement of sparging wells permitted aeration of injected groundwater, as well as providing additional oxygen supply downgradient of the source area. Sparging well construction was identical to that shown previously in Figure 3. Individual air feed lines were run to each sparging well in shallow, below-ground trenches.

Compressed air was provided from a trailer containing a 5 hp, 36 cfm air compressor. The trailer also contained pressure controls for the pneumatic pumps and flow rate indicators and control valves for each sparging line. Airflow rates were maintained at about 1 cfm into each sparging well.

A soil venting system was installed to prevent product vapors from entering the neighboring office building and to collect vapors generated by the sparging wells. Soil venting also provided air circulation through the vadose zone, thus removing carbon dioxide and providing additional oxygen to promote natural biodegradation. Vapor extraction points included wells EWlA, EW2A, EW3A, MW4, and MW7. Vapor emissions were vented directly to the atmosphere via a 30-foot-high stack near the adjacent shop building.

Based on a total volume of 6,700 cubic yards of soil contaminated by petroleum hydrocarbons, the cost to design and install the system was less than $30 per yard of affected soil. These unit costs have decreased with our increasing experience in sparging well applications. Additional cost information is provided at the end of this article.

System Operation and Monitoring Required maintenance of the system was minimal and performed

during weekly visits to the site. Typical weekly maintenance included adjustment of sparging well airflow rates and removal of biomass accumulations from the base and walls of the injection gallery. Shutdowns of the groundwater pumping system were required on two occasions, about three and twelve months after system startup, due to overflow at the injection gallery. System shutdowns lasted only a few days on both occasions.

The overflows resulted from biomass buildup in the pea gravel backfill surrounding the injection gallery. On both occasions the injection gallery was moved several feet to cleaner gravels. The bacterial clogging appeared to be related to a type of hydrocarbon-degrading iron bacteria that we have visually observed at several sites in northern Utah and southern Idaho. At a similar site in southern Idaho, the iron bacteria was identified as being of the genera Gullionellu and teptothrix, which contain up to 90 percent iron hydroxide by dry weight.

System monitoring consisted of the following:

Collection of air samples from the venting system emissions stack Collection and analysis of soil gas samples from the vadose zone Measurement of field parameters for each monitoring well Collection and analysis of groundwater samples from selected wells

REMEDIATION/~INTER 1993/94 103

TODD W. SCHRAUF PATRICK J. SHEEHAN L E S ~ H. PENNINGTON

The samples were collected and analyzed in the field for oqygen and carbon monoxide content, using a portable gua detection instrument.

Monitoring was performed on approximately a weekly basis for the first two months of system operation and monthly thereafter.

The venting system emissions were sampled by collecting air samples from the exhaust stack using carbon tube fiiters and a portable air sampling pump. Field measurements of organic vapor emissions were also made using a portable organic vapor monitor equipped with a photoionization detector.

Soil gas samples were collected from two permanent soil gas vapor monitoring points. The north sampling point (near well MW9) was used as a background or control measurement point. The south sampling point (near the midpoint of the plume) was used to monitor carbon dioxide production and oxygen utilization in the vadose zone. The samples were collected and analyzed in the field for oxygen and carbon monoxide content, using a portable gas detection instrument. The measurement range of this instrument is 0 to 5 percent carbon dioxide and 0 to 25 percent oxygen.

Measurements of water elevation, temperature, dissolved oxygen, and organic vapors (in the wellbore) were made using portable field instru- ments. Organic vapors were measured with a portable organic vapor monitor (OVM) equipped with a photoionization detector, which is insensitive to the presence of methane. The measurement range of the OVM is 0 to 2,000 ppm. Dissolved oxygen content of the shallow groundwater was measured in-situ, using a portable meter that corrects for temperature and salinity. The dissolved oxygen meter has a recording range of .1 to 10 mg/l.

Collected groundwater samples were analyzed by a Utah-certified analytical laboratory for total petroleum hydrocarbons (TPH), using EPA method 801 5 modified, and aromatic hydrocarbons (benzene, toluene, ethylbenzene, total xyIenes, and naphthalene), using EPA method 8020.

Selected groundwater samples were also analyzed for total heterotrophic and petroleum hydrocarbon-degrading (HCD) bacteria. Ground- water samples were diluted in .l percent sodium pyrophosphate dilution blanks. Soil samples were first blended in .1 percent sodium pyrophosphate for sixty seconds to homogenize the sample, then further diluted in the dilution blanks. Enumeration was accomplished by spreading .1 ml of an appropriate dilution onto tryptic soy agar (total heterotrophs) plates and onto mineral salts agar plates (HCD). Gasoline vapors were supplied to the mineral salts agar plates by gasoline-saturated cotton swabs placed in the lids of the petri dishes. These vapors served as the sole carbon source.

Agar plates were incubated at room temperature 20°C and colonies were counted at three days for total heterotrophs and ten days for hydrocarbon degraders. Results are expressed as colony forming units per milliliter (CFU/ml) of water or gram dry weight (CFU/gdw) of soil.

On February 15,1993, after eleven months of system operation, seven soil samples were collected to evaluate changes in petroleum hydrocarbon and inorganic nutrient concentrations and microbial activity. The sample locations are shown in Figure 4 . The objective of this sampling was to determine if declining microbial activity was the result of inorganic nutrient

-


~ T E R N A T T V E METHOD OF GROUNDWATER SPARGING FOR PETROLBUM HYDROCARBON REMEDIATION

Background concentrations of oxygen were below atmoepheric concentrations at syetem etartup, indicating oxygen depletion in the vadoee zone outeide of the immediate plume area.

limitations (no inorganic nutrients were added during system operation) or exhaustion of the carbon food supply. Soil samples were collected from seven locations, including four locations that were sampled prior to system startup. A comparison of measured petroleum hydrocarbon concentrations in soil and groundwater, both prior to and following remediation, are presented in Table 2. Total petroleum hydrocarbon concentrations decreased 99.7 percent and aromatic hydrocarbon concentrations decreased between 92.9 and 99.1 percent in the site soils. Significant reductions in measured groundwater concentrations, 85.9 percent for total petroleum hydrocarbons and between 78.0 and 99.8 percent for aromatic hydrocarbons, were also observed.

Based on an estimated volume of 6,700 cubic yards of petroleum hydrocarbon-contaminated soil, system operation, maintenance, and monitoring costs were less than $10 per cubic yard over the entire life of the system.

DISCUSSION OF RESULTS Operation of the groundwater sparging system has resulted in signifi-

cant decreases in petroleum hydrocarbon concentrations in both soil and groundwater and the site is rapidly approaching or has achieved cleanup guidelines established in the corrective action plan (Table 2). Interpreta- tion of the data collected is presented in the following sections.

Soil Gas and Vapor Emissions Monitoring Results Measurement of carbon dioxide and oxygen content of the vadose

zone soil gas are presented in Figure 5. Carbon dioxide percentages within the vadose zone remained relatively constant through the first 100 to 150 days of operation but declined to background levels about 250 days after startup. Correspondingly, oxygen percentages increased from system startup through 250 days, when they approached background levels. Background concentrations of oxygen were below atmospheric concentrations at system startup, indicating oxygen depletion in the vadose zone outside of the immediate plume area. This may be attributable to lateral migration of petroleum hydrocarbon vapors or natural background microbial activity. Atmospheric concentrations of oxygen and carbon dioxide are 20.95 percent and .033 percent for clean dry air at sea level (Glover, 1992). Based on these observations, microbial activity has decreased substantially after 250 days of operation.

Measurements of organic vapor emissions from the soil venting system are presented in Figure 6. These data indicate a rapid decline in petroleum hydrocarbon emissions with little to no indication of measurable vapor levels two months after system startup. These data indicate that physical removal of contamination through volatilization was not a primary mechanism in the remedial system operation.

Groundwater Monitoring Results Groundwater monitoring data were collected from up to twelve

monitoring wells on site. For the purposes of visual comparison, data from


TODD W. SCHRAUF . PATRICK J. SHEEHAN . LESLIE H. PENNINGTON

Figure 5. Oxygen and Carbon Dioxide Soil Gas Concentrations.

0 50 100 150 200 250 3 TIME SINCE SYSTEM STARTUP (days)

A) OXYGEN

10

0 TIME SINCE SYSTEM STARTUP (days)

B) CARBON DIOXIDE

wells MW3, MW4, MW5, and MW7 were averaged and displayed. These wells were chosen because they were located along the axis of the plume (area of highest petroleum hydrocarbon concentrations) and were sampled during all monitoring events.

The groundwater monitoring results include field measurements of


ALTERNATIVE METHOD OF GROUNDWATER SPARGING FOR PETROLXUM HYDROCARBON REMEDIATION

Figure 6. Emissions from Vapor Extraction System.

water elevation, temperature, and dissolved oxygen as summarized in Figures 7 and 8. The groundwater temperature follows the expected seasonal trend with peak temperatures occurring during August and minimum temperatures occurring during March. Higher groundwater temperatures should favor increased microbial activity and indeed microbial activity generally peaked about 50 to 150 days after startup. The decline in microbial activity after 150 days is the result of several factors but may be at least partially related to declining groundwater temperature.

The groundwater elevation also exhibits a seasonal fluctuation, reaching its highest level in July. Peak levels are related primarily to surges of groundwater recharge during the spring thaw in the Wasatch Mountains to the east. The arrival time of this seasonal peak is typically delayed relative to the distance of the site from the recharge area. During the last (March 1993) monitoring round, a sharp increase in water level was observed. This increase is probably associated with unusually high direct local recharge. This local recharge is a result of heavy snowfall for northern Utah last winter (fourth highest on record) combined with rapid melting of the local snow cover during a period of relatively warm weather and moderate rainfall in early March. There appears to be an inverse correlation between groundwater elevation and dissolved petroleum hydrocarbon concentrations for the first 220 days of operation.

Measurements of dissolved oxygen in the shallow groundwater (Figure 8) indicate a net decrease between upgradient (well MW1) and downgradient (well MW6) measurements, which may be attributable to increased biological oxygen demand within the zone of contamination. Dissolved


TODD W. SCHMUP PATRICK J. SHEEHAN LESLIE H. PJNNINGTON

Figure 7.

92 ! 1 I I I I I I 5 0 50 100 150 200 250 300 350 400

TIME SINCE SYSTEM STARTUP (days)

Figure 8. Oxygen Concentration in Groundwater.

0


ALTE~ATXVE METHOD OF GROUNDWATER SPARGING FOR PETROLSUM HYDROCARBON REMEDIATXON

Figure 9. Microbial Activity in Groundwater.

TIME SINCE SYSTEM STARTUP (days)

n o I

oxygen was typically above background concentrations within the inter- mediate plume area due to introduction of oxygen by groundwater sparging. Immediately following system startup (about 25 days), dissolved oxygen concentrations increased briefly and rapidly to about 2.2 ppm. The subsequent rapid decrease in dissolved oxygen may be caused by corresponding rapid increases in microbial populations and associated biological oxygen demand. Dissolved oxygen concentrations fluctuated between .23 and 1.0 ppm through 280 days after startup, then increased dramatically over the last 100 days. The final increase in dissolved oxygen suggests that microbial activity and associated biological oxygen demand have decreased substantially within the saturated subsurface after 280 days.

Microbial activity was also monitored by performing plate counts on collected water samples during the course of operation. Plate counts of total heterotrophic and hydrocarbon-degrading bacteria are presented in Figure 9. Both HCD and total plate counts rose sharply following system startup, peaking at 50 and 120 days after system startup, respectively. Plate counts subsequently declined steadily during system operation, reaching preremediation levels about 280 days after system startup.

Measurements of dissolved petroleum hydrocarbon concentrations in the center of the groundwater plume are presented in Figure 10. Data are presented for both total petroleum hydrocarbons and the five aromatic hydrocarbons monitored. In general, a11 dissolved petroleum hydrocarbon concentrations exhibited the same pattern over time as described below:

109

TODD W. SCHRAUF PATRICK J. SHZEHAN LESLIE H. PENNINGTON

Figure 10. Petroleum Hydrocarbon Concentration in Groundwater.


ALTERNATIVE METHOD OF GROUNDWATER SPARGING POR PETROLEUM HYDROCARBON REMEDIATION

Concentration increased slightly over the first 30 days of operation Concentration decreased dramatically between about 30 and 100 days of operation Concentration increased slightly to strongly between about 120 and 250 days of operation (with all but benzene and TPH reaching preremediation levels) Concentration decreased steadily after 250 days of operation

The increase in dissolved petroleum hydrocarbon concentrations over the first 30 days is most likely attributable to disturbance of the subsurface equilibrium caused by the sparging and pumping operations. Concentra- tions then subsequently decreased as microbial activity and associated biodegradation rates increased. The cause of the increase between 120 and 250 days of operation is less well understood and could be a result of any combination of the following factors:

Increased natural surfactant production with increasing microbial activity, resulting in increased desorption of petroleum hydrocarbons from the aquifer soils Decreased microbial activity due to a seasonal drop in groundwater temperature Increased desorption of petroleum hydrocarbons due to seasonal increase in water table elevation Increased competition from non-hydrocarbon-degrading bacteria (total bacterial counts peaked at 150 days versus 50 days for HCD bacteria), resulting in reduced hydrocarbon degradation rates Reduction in HCD microbial activity due to inorganic nutrient limitation (nitrogen or iron) or toxic by-product formation

A comparison of initial (preremediation) and final dissolved concentrations of petroleum hydrocarbons is summarized in Table 2.

Soil Sampling Results Soil samples collected after nearly one year of operation exhibited

significant reductions in measured petroleum hydrocarbon concentrations (Table 2). All seven soil samples collected in the remediated area contained petroleum hydrocarbon concentrations below cleanup objectives established by the Utah DERR, and five samples did not contain detectable concentrations of petroleum hydrocarbons.

Plate count measurements of microbial activity generally indicated the highest concentration of HCD bacteria in areas of residual hydrocarbon contamination. Conversely, samples in which HCD bacteria were not detected were generally associated with areas of no detectable hydrocarbon contamination.

COST COMPARISON The cost to install, operate, and monitor the density-driven convection

system of groundwater sparging is presented in Figure 11 as a function of

111

TODD W. SCHRAUP PATRICK J. SHEEHAN LESLIE H. PENNINGTON

Table 2. Comparison of Pre- and Post-Remediation Concentrations. (All concentrations in mg/kg or mgfl)

Cleanup Percent Pre- Post- Reduction Pammeter Remediation Remediation Goal

Groundwater Samples (Average Of W d S Mw 3,495, and 7)

Total Petroleum Hydrocarbons

Benzene Toluene Ethylbenzene Total Xylenes

Naphthalene

51 7.2 Not Established 85.9

1.3 0.004 0.005 99.7

2.4 0.005 2 99.8

0.78 0.085 0.7 89.1

2.5 0.55 10 78.0

0.18 0.024 0.020 86.7

Soil samples (Average of four locations)

Total Petroleum Hydrocarbons 555 1.6

Benzene 2.0 <o. 1

Toluene Et hylhenzene

Total Xylenes

1.4 0.1

5.7 0.025

37 0,35

Naphthalene Not Available <o. 1

30 99.7

0.20 95.0

100 92.9

70 99.6

1000 99.1

2.0 Not Established

the areal extent of the plume. These costs are taken from the six largest sites listed in Table 1. Factors influencing the indicated total remediation costs included shallow depth to groundwater (less than fifteen feet at all of the sites) and the integration of other technologies, including soil venting (one site); groundwater circulation (three sites); and excavation/disposal of highly contaminated source area soils (two sites).

Approximate costs for the described groundwater sparging system are $30 per cubic yard for system design and installation. System monitoring and maintenance cost an additional $10 per cubic yard. Reported costs to implement in-situ bioremediation range from $66 to $1 23 per cubic yard of material treated (OTA, 1989). When hydrogen peroxide is used for oxygen supply, the cost is nearly doubled (Geselbracht et. al., 1986). Hence, the described system is cost-effective in comparison to other bioremediation technologies.

SUMMARY AND CONCLUSION Groundwater sparging is a cost-effective method for remediation of

petroleum hydrocarbons by promotion of natural aerobic biodegradation

112 REMEDIATION~INTER 1993/94

ALTERNATIVE METHOD OF GROUNDWATER SPARGING POR PETROLBUM HYDROCARBON REMEDIATION

250- I I I D

200-

150 m

loo----- I B

---- II_ ~- 50 I

m 0

Figure 11. Air Sparging Cost Versus Plume Size.

AREAL EXTENT OF PLUME (sq.ft.) (Thousands)

and in-situ air stripping. Of the available groundwater sparging methods discussed, density-driven convection provides significant advantages, including the following:

The method is applicable to both fine- and coarse-grained soils. The method does not produce hydraulic fracturing or promote significant contaminant spreading. The method is inexpensive and easy to implement and maintain.

The method has resulted in significant reductions in petroleum hydrocarbon concentrations without inorganic nutrient supplementation in field applications at eight sites. Field applications included gasoline, diesel, and waste oil releases in both fine- and coarse-grained soils.

Application of the density-driven convection system has demonstrated the ability to reduce petroleum and aromatic hydrocarbon concentrations below regulatory cleanup goals, including maximum contaminant levels for drinking water, within approximately one year of operation. The primary mechanism of removal was enhancement of natural biodegradation processes. Despite the presence of volatile constituents, significant hydrocarbon vapor emissions were not created by the sparging system.

REFERENCES

Angell, K.G. 1332. “In Situ Remediation Methods: Air Sparging.” National Environmental Journal, February, pp. 20-23.



Brown, R.A., and F. Jasiulewicz. 1992. “Air Sparging: A New Model for Remediation.” Pollution Engtneerfng 24(13): 52-55.

Geselbracht, L.D., T.A. Donovan, and R.J. Greenwood. 1986. “Realistic Cost Estimates for Alternative Remedial Actions of Contaminated, Unsaturated Soils and Underlying Aquifers.” In Proceedings N W W . . ! I Confetwzce on Petroleum Hydrocarbons and organfc Chemf- cals in Ground Water: Pranentton, Detectton andRestoratton. Worthington, OH: National Water Well Association.

Glover, T.J. 1592. Pocket Ref Morrison, CO: Sequoia Publishing.

Herding, B., J. Stamm, and W. Buermann. 1991. “Hydraulic Circulation System for In SitU Bioreclamation a n d o r In Situ Remediation of Strippable Contamination.” In Hinchee, R.E. and R.F. Olfenbuttel (eds.), In Stfu Btomlamatton. Stoneham, MA: Butterworth-Heineman.

Jasiulewicz, F. and W. Hildebrandt. 1992. “Cleaning Up Military Bases.” 7be Military Engineer, September-October, No. 552, pp. 6-9.

Loden, M.E., and C. Fan. 1992. “Air Sparging Technology Evaluation.” In Nattonal RED Conferenceon the Contml ofHazardousMaterlals(San Francisco, February 4-61, Greenbelt, MD: Hazardous Materials Control Research Inst.

Looney, B.B., D.S. Kaback, and J.C. Corey. 1992. “Field Demonstration of Environmental Restoration Using Horizontal Wells.” Btoventfng and Vapor Extraction Uses and Applfca- tfons in Rernedfatfon Operations. Air and Waste Management Assoc., Satellite Seminar, April 15.

Marley, M.C. 1992. “Removing Gasoline from Soil and Groundwater through Air Sparging.” Remediatton 2(2): 121-31.

Martin, L.M., RJ. Sarnelli, and M.T. Walsh. 1992. “Pilot Scale Evaluation of Groundwater Air Sparging: Site-Specific Advantages and Limitations.” In National R&D Conference on the Control of Hazardous Materfals (San Francisco, February 4-6). Greenbelt, MD: Hazardous Materials Control Research Inst.

Middleton, A.C. and D. Hiller. 1990. “In Situ Aeration of Groundwater-A Technology Overview.” Conference on Preventfon and Treatment of Soil and Groundwater Contamt- natfon f n the Petroleum Refining and Dtstrlbution Industry, Montreal, Quebec, October.

Office of Technology Assessment (OTA). 1989. Comfng Clean: Superfund’s Problems Can Be Solwd, OTA-m-433, U.S. Congress, Washington, DC.

Wehrle, K. 1990. “In Situ Cleaning of CHC Contaminated Sites: Model-Scale Experiments Using the Air Injection (In Situ Stripping) Method in Granular Soils.” In Arenak, F., M. Hinsenveld, and W.J. van den Brink (eds.), Contumfnated Solk ’90. Netherlands: Kluwer Academic Publishers.


alternative method of groundwater sparging for petroleum hydrocarbon remediation

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