remediation of groundwater contamination through conjunctive use of landfill gas management and soil...

Remediation of Groundwater Contamination through Conjunctive Use of Landfa Gas Management and Soil Vapor Extraction

William Clistev Shevvie Hibbs

Sherrie Hibbs, P.E., provides oversight and direction for the City of Statesboro’s solid waste management programs and general environmental engineering activities. Her responsibilities include monitoring solid waste transfer station construction and operation, oversight of the Lakeview Road landfill site closure and associated corrective actions, related contractor activities (specification, bidding, and award>, and coordination of these programs with regulatory agencies‘ requirements and recommendations. William Clister, a Professional Geologist with Golder Associates (with over 26 years of landfillgas, soil vapor extraction, and site remediation experience) provides technical support for various landfill site related assessments and corrective actions through- out the United States, Canada, the Middle East, and China. His work experience includes assessment of the groundwater impact potential of landfill gas and volatile organics migration at municipal solid waste landfills and development of numerous gas management and utilization programs. He is currently deneloping a life-cycle bioreactor method of leachate and landfillgas management directed towards improving landfill operations by accelerating airspace recovery and reducing postclosure care costs.

72e recently completedpe formance testing oj-a conjunctive-use operation has demonstrated the upplicability of combined landfillgas and Volatile Oqanic Compound (VOC) removal with overall contaminant migration management. ?he extraction of EandfiLl gas (with subsequent control of off-site gas migration) and the removal of VOCs from the landJlilgas/soilairenvironment will minimize VOC solubilization in the underlying groundwater. This, in turn, willpernit cost efficient management of contaminant migration at the site. 0 1998 John Wiley G Sons, Inc.

As with many municipal landfills permitted prior to the promulga- tion of Subtitle D, the City of Statesboro’s Lakeview Road landfill is unlined and has affected the local water table aquifer immediate to the landfill footprint. Moreover, the landfill is under the watchful eye of the Georgia Environmental Protection Division, as locally-derived solvent and pesticide wastes have been disposed here. The impact of the site on the local groundwater quality (through both leachate and landfill gas migration) and the nature of this impact have resulted in the inclusion of the site on the Georgia Hazardous Site Inventory (HSI) list.

This HSI listing has proven to be both a hardship and a help. The governments of the City of Statesboro and Bulloch County (in which the landfill is located) must now meet unusually stringent post-closure monitoring and corrective action standards. However, in association with this listing, the city and county are eligible for the reimbursement of certain remediation costs.

In addition ro the regulatory involvement engendered at this site, the interest of the Coastal Savannah River Authority (CSRA) has developed due to the location of the site and its potential as a regional transfer facility. Even though CSRA interest would remove the responsibility of site mitigation from the city and county, it would also effectively remove them from any decision-making role regarding site afteruse and development. Maintain- ing strict control over site conditions and mitigation activities was determined to be necessary in order to protect their interests and responsibilities.

CCC 1051 -5658/98/0901007-I 6 0 1998 John Wiley ti Sons, Inc.

7

WILLIAM CLISTER SHERRIE HIBBS

Thus, a conjunctive remedy such as landfill gas (LFG) management coupled with enhanced volatile organic compounds (VOCs) removal provided an acceptably responsive measure for protection of public health and the environment. This also enabled selection of the most cost-effective mitigative action, preserving city and county authority over closure operations and postclosure care.

The landfill site (located near the City of Statesboro, Georgia, as shown in Exhibit 1) received solvent and pesticide-contaminated wastes during the 1970s and 1980s. These wastes were received prior to state or federal regulations prohibiting the placement of such materials in an unlined municipal solid waste (MSW) landfill. The site received MSW and construction and demolition (C&D) waste until its closure in May 1997.

The landfill serves a growing city and county populace and a major state university. Industry in the landfill service area is varied and plentiful, including such operations as metal milling and fabrication, agricultural services (herbicides and cotton/peanut plant pesticides), and wool treatment (pesticide application). Due to the nature of these wastes and the age of the landfill, these industries appear to have contributed significantly to the source of the groundwater contamination at the landfill.

Although originally located in an agricultural area of the county, the landfill is now bordered on two sides by subdivisions. A creek separates one subdivision from the site, while a wetlands and second creek lie

Exhibit 1. Location of Lakeview Road Landfill

8 REMEDIATION/WINTER 1998

REMEDIATION OF GROUNDWATER CONTAMINATION THROUGH CONJUNCTIVE USE OF LEG MANAGEMENT AND SVE

Of particular note is the effort expended in communicating these alternatives and costs to the city and county so that informed decisions could be made.

downgradient from the site and in the path of contaminant migration. Due to the proximity of the residential developments, the potential for public use of the wetlands area, and the contamination of the underlying water table aquifer by certain solvents and pesticides, the contaminants migrating from the site must be contained and minimized.

With ongoing site closure, the city must comply with state regulations (and Subtitle D) requiring corrective action to contain and mitigate groundwater contamination and offsite LFG migration. Upon closure, the site would require LFG management; i.e., LFG control would be required in addition to a groundwater corrective action. Moreover, the regulatory agency considered a geomembrane cap plus groundwater gradient control. This in addition to LFG management was anticipated to cost as much as six million dollars.

The city (representing both itself and the county) recognized these potential exposure possibilities and undertook to mitigate the associated adverse impacts. To aid in this undertaking, the city and county were eligible to receive reimbursement for corrective actions (up to a maximum of two million dollars) through the HSI fund. To obtain this aid, the regulatory agency had to approve the corrective action, with such approval linked to the expediency and effectiveness of the corrective action, without undue complication and aftercare requirements. The most cost-effective and practical approach to this mitigation proved to be the conjunctive-use corrective action described in this paper.

The city, taking the lead in the site remedy and having limited funds for overall site activities, was hesitant to allocate additional funds for site closure. A typical remedy and containment option might include a low-permeability plastic membrane cap on the surface of the landfill, a groundwater extraction and treatment operation and/or a barrier wall and groundwater interceptor trench at the landfill perimeter. This would total more than six-million dollars; thus, the city could not fund both closure and corrective action. However, the use of a LFG/soil vapor extraction (SVE) conjunctive-use alternative provides an opportunity for contaminant source reduction without the added expense of a geomembrane cap-and without the expense and uncertainty of hydraulic control of the groundwater flow system. The costs of the LFG/SVE alternative were less than one-third of the cost of the other options and are eligible to be reimbursed by HSI.

Of particular note is the effort expended in communicating these alternatives and costs to the city and county so that informed decisions could be made. Coordination and communication with the elected bodies of the city council and county commission were crucial to the advancement of the project and the long-term cost savings that will be realized by its implementation.

TEST OF CORRECTIVE ACTION PROCEDURES AND VOC REDUCTION

In order to demonstrate to the regulatory agency and to the city council and county commissioners the effectiveness of the proposed conjunctive LFG/SVE approach, an LFG extraction and air-induction field test was

REMEDIATION/WINTER 1998 9


conducted at the Lakeview Road landfill on February 4,1797. The test was conducted to assess the application of SVE and induced, limited air intrusion coupled with typical LFG extraction as a corrective action for minimization of volatile contaminant sources. In addition, the enhanced degradation of less volatile contaminants (e.g., pesticides) within the landfill waste was also assessed. This test developed from the need to obtain basic design criteria for the SVE/LFG system and to obtain approval of the state regulatory agency of this method as an overall corrective action for the site. Upon obtaining these data, which proved the viability of the LFG/SVE approach, the efficiency and time requirements of this corrective action were examined. With this information, an economical complemen- tary corrective action (LFG management plus WE) could be presented to the city council and county commissioners in lieu of costly geomembrane capping and groundwater pump-and-treat methods.

Contaminants of Concern The potential lateral and vertically-downward movement of solubi-

lized VOCs through the groundwater flow system and the potential exposure of the public to these contaminants are the principal concerns of both the regulating agency and the city. The SVE wells and the LFG management system are to be used conjunctively to mitigate this impact by encouraging VOC volatilization within the landfill-intercepting these volatiles before they can migrate offsite and by disposing of the extracted VOCs by incineration.

This method will also enhance in situ biodegradation by encouraging the bacteria within the waste to respond to a “feast-or-famine” condition. The bacteria accelerate their activities to meet favorable conditions and thereby produce greater volumes of LFG and improved mass flux and volatilization. When allowed to feed anaerobically-without the cycled

This method will also enhance in situ biodegradation by encouraging the bacteria within the waste to respond to a ‘feast-or- famine” condition. impact and hindrance of intrusive air-facultative and methanogenic

bacteria exhibit an overwhelming recovery response. Each cycle of extended or intermittent LFG interception and SVE followed by a short- lived air intrusion may successively strengthen the bacterial population and enhance biodegradation of the waste and the contaminants they contain. Production of specific enzymes by these bacteria may also help degrade the chlorinated hydrocarbons (and possibly certain pesticides) present in the waste.

The contaminants and their concentrations in the MSW are reflected in leachate samples that include 1,100 parts per billion (ppb) of methyl ethyl ketone (MEK) as well as vinyl chloride concentrations ranging from 15 ppb to 17 ppb. Other VOCs detected in groundwater and leachate at the site include acetone, benzene, chlorobenzene, ethylbenzene, tetrachloro ethylene (PCE), trichloroethylene (TCE), 1 ,l-dichloroethane (l,l-DCA), 1,l-dichloroethylene (l,l-DCE), dichloro- methane (methyl- ene chloride) and cis-l,2-dichloro-ethylene (cis-l,2-DCE). These compounds can be volatilized and extracted from the landfilled waste and the contaminated groundwater by enhanced LFG interception, by W E coupled with LFG recirculation, and by controlled air-sparging.


REMEDIATION OF GROUNDWATER CONTAMINATION THROUGH CONJUNCTIVE USE OF LFG MANAGEMENT AND SvE

Implementation of Conjunctive-Use Approach The factors which affect the desorption of a VOC from contaminated

soil or waste and the subsequent volatilization of that VOC into the soil- air/LFG within the negative-pressurefield of a SVE well are:

the soil/waste adsorptivity; 0

The speed with which this desorption and subsequent extraction occurs is a function of the porosity, moisture content, and bulk density of the soil and waste fill-as well as the operating characteristics of the SVE wells and the conjunctive operation of the LFG management system.

If air intrusion is enhanced, the anaerobic activity of methanogenic bacteria within the landfill will be restricted by the intrusion of atmospheric air into the SVE operations area. By limiting the widespread introduction of atmospheric air into the waste fill, aerobic conditions should not predominate. However, where air intrusion is induced, the facultative bacteria in these areas and (to a lesser extent) the methanogenic bacteria are driven by their intent to survive to become more aggressive biodegraders and gas-producers each time the debilitating air intrusion is removed. This approach can be used to enhance the production of the enzyme methane

the soil-water partition coefficient; and the solubility and vapor pressure of the VOC.

monooxygenase by methanotrophic or possibly facultative bacteria, thereby accelerating the degradation of specific chlorinated hydrocarbons at the contaminant source. Care must be taken to avoid over-aeration of the waste and subsequent development of unacceptable temperatures and conditions within the landfill such that the methanogens and facultative bacteria will not recover and thrive.

Test Well Construction and Design Criteria Four SVE wells were installed, three of which were employed as

monitoring wells during the performance test. The SVE pump well “B” and the three monitoring SVE wells (A, C, and D) were constructed using a mobile hydraulic drilling rig equipped with 8-inch outside diameter, hollow-stem augers. The monitoring wells A, C, and D and standpipes Pe (east) and Ps (south), located as shown onExhibit 2, were used to monitor the development of negative pressures and induced recharge effects within the radius of influence (ROI) of the pumped well.

By maintaining a controlled gas flow across the contaminated waste and subsequently to the operating SVE wells, the performance test was conducted with minimal adverse impact to the methanogenic bacteria within and outside of the test area. Optimum aeration (by both LFG and atmospheric “make-up” air) of the VOC-contaminated waste was achieved.

Care must be taken to avoid over-aeration of the waste and subsequent development of unacceptable temperatures and conditions within the landfill such that the methanogens and facultative bacteria will not recover and thrive.

Corrective Action Plan and Application The use of SVE methods in conjunction with typical LFG management

is a modification of the original corrective action plan (CAP) as proposed for the site. That is, SVE will be applied in conjunction with clay capping

REMEDIATION~~INTER 1998 11


By manipulating both the SVE and LFG systems a t the site, appropriate negative pressure radii of influence (ROIs) were developed within the landfill.

Exhibit 2. Location of LFG/SVE Wells and Monitors

..

Passivc Vent Trench

Lakeview Road Well Ps Landfi 11

[-------I - 150 feet

of the landfill and construction of a LFG management system, both considerations of the original CAP, but without additional groundwater gradient control or manipulation of the hydrogeologic system. Require- ments specific to the CAP were met while still addressing the alternatives necessary to produce a cost-effective and measurable remedy.

By implementing SVE and LFG controls conjunctively with application of the clay cap, not only can both offsite gas migration and onsite LFG emissions be reduced to acceptable concentrations, but a progressive reduction in the concentrations of the source contaminants within the site can also be effected. This will occur through the reduction of surface water infiltration by the clay cap, resulting in minimized leachate production, accompanied by reductions in the source concentrations of the principal groundwater contaminants (i.e., minimization of the VOC sources by enhancing removal of those sources with the LFG).

CONJUNCTIVE OPERATION OF SVE AND LFG MANAGEMENT SYSTEMS

By manipulating both the SVE and LFG systems at the site, appropriate negative pressure radii of influence (ROIs) were developed within the landfill. This permitted atmospheric air recharge through one area of the clay cover, while simultaneously inducing soil-air recharge through the sidewalls at the perimeter of the landfill and enhancing movement of LFG from adjacent landfill cells. Thus, the measured induction of oxygen through the surface of the landfill, coupled with the slow development of a soil-air excursion front into the ROI of the SVE wells. enhances the

12 REMEDIATION~~INTER 1998

REMEDIATION OF GROUNDWATER CONTAMINATION THROUGH CONJUNCTIVE USE OF LFG MANAGEMENT AND SVE

volatilization of VOCs and the biodegradation of other contaminants. The controlled intrusion of atmospheric oxygen through the surface

of the waste fill also enhances the development of methanotrophic bacteria (as opposed to methanogenic microorganisms) in the soil cover. As these bacteria consume methane while using available oxygen as fuel, they make a significant contribution to the control of oxygen entering the landfill. They also permit an increase in the gas flux through the fill.

Deeper within the MSW, “feast-or-famine” cycles may develop through starving, then feeding, the facultative bacterial colonies and methanogens. The production of methane monooxygenase or equivalent enzymes may accompany the air intrusion periods of anaerobic “famine” and the corresponding decrease in LFG production.

Landfill Gas Factors Affecting SVE System Operation Oxygen intrusion and soil-air recharge may be developed by extracting

LFG at an increased rate from an SVE well. To this effect, the pumping rate of an SVE well is designed to be slightly in excess of the LFG production rate as calculated for the area of the landfill where the well is constructed. That is, the SVE well is designed to overpump the landfill LFG “aquifer” (the unsaturated portion of the waste) and create an LFG extraction-vs- production deficit with subsequent controlled oxygen intrusion into the uppermost portion of the landfill.

To better define the rate of SVE appropriate for inducing limited atmospheric air recharge to the SVE zone, an LFG budget was developed using the most recent site capacity (airspace) calculations. The total existing airspace capacity of the site was calculated to be 2.3 million cubic yards (1.7 million m?. Using this value and an estimated in-place wet weight density of 58 pounds per cubic foot, an estimate of the total weight of landfilled waste received at the site was developed. This estimate was subsequently corrected for the percentage of cover soil, as well as for waste decomposition prior to closure.

The current and anticipated LFG production potential of the site was developed by estimating the waste composition (percentage of paper, wood, etc.) based on visual examination of waste samples collected from test pits and boreholes completed across the landfill. The biodegradable portion of the waste (that percent of the waste which may generate LFG) was determined from typical municipal waste analyses presented in the literature and by laboratory analyses of similar wastes collected at other sites assessed by Golder Associates.

Exhibit 3 shows that this municipal solid waste and the construction and demolition materials include about 86 percent total volatile solids, with about 6 percent of this waste comprised of nonbiodegradable volatiles (plastics, rubber, etc.). Of the remaining 14 percent, glass and metal appear to contribute about 6 percent to the total volume of the waste, with other non-degradables (inert waste) contributing the remaining 8 percent. Exhibit 3 presents the estimated composition by weight of a typical tonne of MSW as received at the Lakeview Road landfill over its operating life.

Deeper within the MSW, ‘yeast-or- famine” cycles may develop through starving, then feeding, the facultative bacterial colonies and methanogens.



Other Degradable TOTAL Glass Metal

TOTAL Other Non-degradable

Exhibit 3. Waste Composition and Gas Production

0.9% 85.9% 3.0% 3.0% 8.1%

100.0%

-

Estimated Waste Composition Food Wastes Paper Cardboard Plastics Textiles Rubber Leather Yard Wastes Wood

Estimated Percent Weight

1 .O% 10.0% 12.0% 5.0%

15.0% 1 .O% 1 .O%

10.0% 30.0%

Wet Weight lounds

22 221 265 110 33 1

22 22

221 662

kilograms 10

100 120 50

150 10 10

100 300

30

Jounds kilograms ”,””” 208 252 109 298

22.05 18 77

5 29

94 114 50

135 10 8

35 240

179 81 1846 837

The optimum LFG gas generation rate (GGR) for the site was estimated to be about 8 cubic feet of LFG per pound of in-place MSW (about 500 m3 per tonne) over the life of the landfill. Exhibit 4 presents a typical LFG- production spreadsheet model based on first-order kinetics of organic decay and gas generation. Note that a recoverable LFG production potential of about 680 cfm as opposed to an estimated total theoretical LFG production of about 1,050 cfm is indicated. This suggests a more practical

Exhibit 4. LFG Production Potential and Recoverability

Current Year: Production Years: Waste Receipts up to: No. Years after “Closure”: Annual Production of LFG at Time “t”: Annual Recoverable LFG at Time “t”: Mean Remaining Recoverable LFG: Mean Remaining NMOC Production: Mean Remaining Recoverable Btus: Mean Remaining Recoverable Fuel Oil Equivalent: Mean Remaining Recoverable Kwh Equivalents: Mean Remaining Kwh Value @ US $0.04/Kwh:

1997 60

1996 1

1,042 684 159 107

7.6E+07 544

22,153 $886

cfm@t=yr cfm@ t=yr

cfm kg/day Btu/day gal/day

Kwh/day US dollardday


REMEDIATION OF GROUNDWATER CONTAMINATION THKOUGH CONJUNCTIVE USE OF LFG MANAGEMENT AND SVE

GGR of about 5.2 cubic feet of LFG per pound of in-place MSW (about 325 m3 per tonne) over the life of the landfill.

This recoverability (reflecting the combined inefficiencies of the mechanical gas interception system and the inherent inefficiencies of LFG production given the site heterogeneity) is derived from analyses and testing of similar LFG management systems. The recoverable LFG production defines the expected volume of LFG which may be extracted from the site on a reasonably continuous basis (at least for the next 15 to 20 years, as shown by the graphs included in Exhibit 5); i.e., based upon a continuous consumption of degradable organics and the biodegradation rates applicable to the site.

Using the theoretical LFG production capacity ( 1,050 cfm), the gas production per acre per foot of waste depth was calculated. Given about 50 acres of landfill and an average depth-to-base of fill of 28 feet with 12 percent soil cover, this LFG production was determined to be no more than 0.74 cfm per foot of waste (corrected vertical depth) for each acre.

In the test area, the depth of waste was measured at 38 feet. Given this depth, the maximum LFG production rate is about 28 cfm for each acre. Based on the measured ROI for an LFG production well, the well was expected to produce about 28 cfm for each acre multiplied by a negative- pressure interference area of about a 200-foot diameter zone of influence.

Q = (28 ft3/min/acre)(100 ft)’ (nX1 acre/43,560 ft*)= 31.5 ft3/min (1)

However, at a recoverable LFG production potential of 680 cfm, or about 65 percent of the theoretical production, this value drops to about 21 cfm. An average expected short-term LFG production rate (and, therefore, an acceptable LFG performance test rate) may be estimated at about 26 cfm, the average of the maximum production potential and the conservatively estimated recoverable LFG.

Long-term pumping of a specific LFG/SVE well will most likely produce a flow rate comparable to the measured and predicted gas extraction potentials. However, a more liberal method of estimating LFG production potential for each specific area of the site requires use of the

Exhibit 5. Graph of LFG Production

1,200 --

I I I I t i N a a v , P a - r t ; d q ? ; ; ; d 2 %

Elapsed Time



The test results demonstrated the areal and vertical extent of forced-draft induced negative pressures within the landfilled waste and also evidenced a demonstrable increase and relationship between the VOC volatilization rate and the LFG extraction rate.

site’s calculated LFG generation rate or GGR (about 2.31 cubic feet of LFG per tonne per day).

Using a typical ROI of 100 feet for the SVE test area, an in-place waste density of 45 lb/ft3, and a measured depth of waste in that area of 38 feet, the total LFG production (maximum) may be calculated as

(2.31 ft3/tonne/day)(l day/l,440min) (2)

(1 tonne/2,205 lb)(45 lb/ft3)(100 ft)2(.n)(38 ft) = LFG, (3)

for an estimated maximum LFG production of 39 cfm. This is a higher value than previously calculated and suggests that the ROI of the SVE well may be less than 100 feet or that the GGR is slightly higher than anticipated. Given the 65.6 percent adjustment for system and landfill inefficiencies as described earlier, the expected LFG (or SVE extraction well) test rate was expected to be about 26.5 cfm.

Exceeding this flow rate of 21 to 26.5 cfm was expected to induce atmospheric recharge through the existing fill cover. This would be caused by the aggressive negative-pressure interference area, resulting in enhanced VOC volatilization and subsequent reduction in the methane content of the extracted LFG (due to the additional air inflow through the landfill cover into the SVE zone). Restricting the induced air recharge to acceptable levels prevents development of uncontrolled aerobic conditions within the unsaturated portions of the landfilled waste and maintains sufficient levels of methane to assure combustibility of the extracted LFG.

The data presented in Exhibit 6 develop this application further using a typical waste hydraulic conductivity of about 1 x lo3 cm a second (comparable to an intrinsic permeability of about 1 x cm?. The waste characteristics and gas generation rate are those presented on the previously described exhibits. The well construction details are derived from the well logs obtained during the February 1997 SVE well construction program. These data show that the expected well-head pressures and negative-pressure generation (in the SVE/LFG extraction zones) would be about 15 inches water gauge (wg) or about 0.54 psi.

METHODOLOGY AND RESULTS OF THE FIELD PERFORMANCE TEST

The most appropriate configuration of the LFG/SVE test well and monitoring well locations was derived from the results of previous test pit investigations and subsequent standpipe construction in which leachate levels and waste conditions were identified. As discussed previously, the test well locations were selected where the greatest available unsaturated thickness of waste fill and proximity to potential VOC contaminant source areas were available.

The test results demonstrated the areal and vertical extent of forced- draft induced negative pressures within the landfilled waste and also evidenced a demonstrable increase and relationship between the VOC volatilization rate and the LFG extraction rate. The volatilization trends



35.0

24.8

Exhibit 6. LFG/SVE Well Radii and Pumping Rates

24.8 20.2 17.5 15.7 14.3 13.2 12.4 11.7 11.1 34

17.5 14.3 12.4 11.1 10.1 9.4 8.8 8.3 7.8 17

Depth (meters)

5 10 15 20 25 30 35 40 45 50

150

140

130

120

110

100

70

60

50

40

30

20

10

Depth (feet)

Note: At Q of 40 cfm, ROI is about 32 meters, or 104 feet.

associated with PCE, 1,1, 1-TCE, chlorobenzene, benzene, and trans/cis dichloroethylene (trans-DCE and cis-DCE) were established. The impact of oxygen intrusion, vacuum pressure development, and resulting air-flow enhancement within the ROI and across the vapor extraction zone were clearly demonstrated during the test period. Through controlled aeration and volatilization in the contaminant area, increased VOC yields were produced.

Methodology The pumped SVE well was monitored for static and total pressures,

temperature, flow rate, and major LFG components-plus specific VOC concentrations. Carbon monoxide, ammonia, hydrogen sulfide, and hydrogen cyanide were also monitored. The vacuum and static/positive pressures at the well heads were measured using a Dwyer digital manometer. Concentrations of the LFG components (methane, carbon dioxide, and oxygen) were measured using a Landtec GA-90 Infrared Gas Analyzer, while balance gas (nitrogen) was defined as the percent remaining major component. Carbon monoxide, hydrogen sulfide, hydrogen cyanide, and ammonia were measured using appropriate Draeger tubes.

A time-series of VOC analyses were performed in the field using a Foxboro organic vapor analyzer (OVA)/128GC gas chromatograph (GC)



Soil air pressures, temperatures, gas flows, and gas qualities were measured at the pumping well and monitoring wells at regular intervals depending on the degree of change measured at each well.

to identify specific VOCs which were extracted during the 24-hour performance test. Chlorobenzene, vinyl chloride, trichloroethane (TCA), TCE, dichloroethane, and dichloroethene were of most interest-as these compounds have been linked to the heavy-metal wastes and to the pesticides encountered at the site. Well-head flow was monitored using a standard pitot tube to measure velocity pressure, which was subsequently converted to velocity (feet per minute) using average LFG humidity and density.

The SVE pump produced an average pumping rate of 40 cubic feet a minute (cfrn) from Well B. Given an average well-head pressure of 11 inches and a flow rate of 40 cfm (+/- 5 cfrn), the landfill waste appeared to have a moderate permeability with an apparent hydraulic conductivity of about 1 x lo-* to 5 x lo-’ cm/sec. This corroborated the predicted average intrinsic permeability of the wastefill and cell construction of about 5 x 10” cm2 (5.4 x 10.’’ ft’)

Soil air pressures, temperatures, gas flows, and gas qualities were measured at the pumping well and monitoring wells at regular intervals depending on the degree of change measured at each well. Measurements were collected (from monitoring wells Well A [30-foot radius from Well Bl, Well C [90-foot radius from Well BI and Well D 179-foot radius from Well B]) during pumping and until recovery and well-head pressure stabiliza- tion were restored following pump shutdown. The gas probes and groundwater monitoring wells located more than 90 feet from the gas extraction well (monitoring points P (east) and P (south) at distances of 235 feet and 170 feet respectively) were monitored as dictated by the expansion of the negative-pressure ROI.

LFG Pressures and ROI The February performance test demonstrated that controlled

overpumping of the SVE well at 40 cfm (about 10 cfm in excess of a typical LFG extraction rate of 30 cfm for the depth of well and type of waste encountered) would induce natural recharge of atmospheric air through the landfill cover. This recharge effected a movement of air into the landfill within the ROI of the pump well, resulting in the dilution of the typical LFG component concentration (from about 60 percent methane volume,’ volume [v/vl to about 47 percent v/v). A schematic of the pressure drawdown and well ROI 24 hours after test startup is shown by the development of negative (vacuum) LFG pressures depicted in Exhibit 7 .

The test data show that the general negative-pressure influence zone and the approximate recharge area (atmospheric air induction zone) were well defined; the air-induction radius was about 33 feet while the estimated radius of the gas extraction influence zone was about 150 feet. The effective ROI of the pumped well (where gas interception was assured) measured at least 110 feet. The test data showed that about 8 cfm of atmospheric air (about 20 percent of the total volume extracted from the SVE test well) was induced through the landfill cover. This is the expected recharge of atmospheric air based on a cover permeability of 1 x cm/sec and a waste permeability of 5 x lo-* cm/sec.

REMEDLATION/WINTER 1998


Exhibit 7. Pressure Drawdown and ROI Development

2 , 1

d f

f

0 5

2 0 v1

- 0 5

- Monitoring Wcll A Monitoring Wcll C ... Monitoring Wcll 0

+Monitoring Wcll P-E -Monitoring Well P-S

214197 2/4197 215197 215197 2/5/97 215197 15:OO 21:OO 3:OO 9.00 15:OD 21:OO

Time

The variations in static and absolute pressures versus time as measured at the monitoring wells and temporary probes during the February 1997 SVE performance test show satisfactory correlation. That is, Well A (30-foot radius from Well B), Well C (90-foot radius), Well D (79-foot radius) and monitoring points P (east) and P (south) (located at distances of 235 feet and 170 feet respectively) show generally decreasing absolute pressures) with increasing distance from the pumped well PW B. Wells located about 100 feet from the pumped well developed similar drawdown conditions after only a few hours of pumping. This indicates that the effective ROI (or most efficiently manipulated portion of the negative-pressure zone) is at least 100 feet in this part of the landfill.

LFG Components and VOC Concentrations The methane, oxygen, and cis-DCE concentrations measured at the

pumped well during the performance test are listed on graphs of concentration versus time included in Exhibit 8. These data show that methane gas in the LFG extracted from Well B decreased from 59.8 percent v/v to 46.8 percent v/v over the period of the test. A corresponding increase in oxygen concentrations (0.3 percent v/v oxygen to 3.8 percent v/v over the 25-hour test period) also occurred.

Recorded static pressures reflected barometric pressure variation and were not a reflection of well-flow manipulation, as static pressure at the pumped well was maintained within 11 percent of the maximum pressure recorded after the first hour of operation. Minimal fluctuation in flow volume was observed. Changes in methane and oxygen concentration in the extracted gas and fluctuation in carbon monoxide and hydrogen sulfide concentrations generally reflected air intrusion through the landfill cover or localized capture of a specific gas (e.g., hydrogen sulfide).



Exhibit 8. Variation in Gas Concentrations at Well B

I00 0000 h CI

c L P - I0000 E

8 100000

._ c, 2 0 1000 - L: p 0.0100 0 ; 00010

c1 0 0001

I .. - - . - -

-500 000 5.00 1000 15.00 20.00 2500 3000

Time (Test Start at 0.00) (hrs)

Samples of LFG collected at the pumped well (in one-liter Tedlar bags) at specific times during the performance test were analyzed for VOCs using a portable gas chromatograph (GC). A single confirmation sample was collected from the pumped well and from each monitoring well (Wells A, B, C, and D) and analyzed for Appendix I VOCs.

The laboratory analyses were generally confirmatory of the ranges of major LFG component concentrations and indicated that certain VOCs (toluene and xylene in particular, plus benzene, 1, l-dichloroethane, cis- 1,2-dichloroethene (cis-DCE), ethylbenzene, tetrachloroethylene (PCE), and trichloroethylene (TCE)) were most susceptible to SVE. Based on the field GC analyses, the volatile compound cis-DCE showed the greatest response to the LFG extraction and SVE enhancement (oxygen intrusion, consumption within the landfill cover and gas flow development).

With the exception of benzene, the nonmethane organic compounds (NMOCs) developed significant responsiveness to the LFG extraction and controlled oxygen diffusion into the ROI of Well B. An initially moderate VOC removal commenced upon LFG extraction, but significantly increased as the oxygen concentration in the extraction zone increased. Similarly, as the oxygen content stabilized, the VOC removal rate flattened and subsequently reflected barometric pressure events only.

VOC Removal Rates A typical Mw of 86 (n-hexane) and a presumed maximum VOC mass per

unit volume of 2.35 x 10j mg/m3 were assumed (based on the range of VOC concentrations measured during the performance test and similar VOC extraction tests conducted at other sites). The maximum amount of VOCs in the LFG extracted during the February 1997 test was then approximated at 750 ppm. The EPA-suggested NMOC values for a municipal solid waste landfill site such as the Lakeview Road landfill vary from 595 ppm to 1,170 ppm.



~

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Using a maximum anticipated mass extraction rate of about 2.35 x lo3 micrograms per liter (about 2.4 x lo3 mg for each m3 of extracted LFG), the weight of VOCs extracted each day for each cfm of LFG from a single SVE well can be calculated as follows:

The coordination of these activities (LFG management and groundwater remediation) provides a significant cost savings for cities and counties engaged in site closure and corrective action. These savings are estimated to be about 2.5 million, a 60 percent savings over the cost of other alternatives. Valuable tax dollars, which otherwise might be expended on

These savings are estimated to be about 2.5 million, a 60 percent sauin@ Over the cost of other

At a rate of 20 cfm per well (the minimum expected LFG extraction rate) and 0.2 pounds of VOCs each day per cfm, the maximum VOC removal rate is estimated to be about 4 pounds per day per well. A well field of 60 wells can be expected to remove about 240 pounds of VOCs for each day of operation given optimum conditions (i.e., optimum temperature, moisture, oxygen intrusion, ROI overlap, and gas flow).

The Lakeview Road landfill SVE/LFG system should develop an average VOC removal rate of at least 25 percent of this value, or 60 pounds per day. This includes mechanical inefficiencies inherent to an active gas management system, to the need for offsite LFG migration control along the landfill perimeter (resulting in locally-degraded LFG quality), and to the heterogeneity of the landfilled waste and variable-cell construction.



cated city and county personnel and experienced supporting professionals is crucial to successful implementation of this method.

As various cities and counties attempt to streamline their construction and regulatory processes, increasing demands are being placed on their engineering staff as well as on their consultants to apply innovative and cost-conscious thinking to municipal projects. Forward-looking approaches, coupled with proactive and creative thinking, are necessary for cost- effective solutions to the environmental problems now facing many cities, as well as the environmental concerns and corrective action requirements now being addressed by many cities and counties.

REFERENCES Brown, K.S. and Clister, W.E. (1993). Design procedures for landfill gas interception, collection, and extraction systems pursuant to the proposed EPA regulations for the control of landfill gas. National Solid Waste Management Association Waste Tech '93, Marina del Ray, California.

Clister, W.E. and Booth, F.R. (1992). In situ treatment of hydrocarbon contaminated soils using soil vapor extraction and enhanced biodegradation, Proceedings A&WMA/EPA Symposium on In Situ Treatment of Contaminated Soil and Water, Cincinnati, Ohio.

Clister, W.E., Brown, K.S., and Khurana, S. (August 1994). Analysis of the impact of a landfill gas management system on exotic gas migration at a hypothetical municipal solid waste landfill site. 26th Mid-Atlantic Industrial and Hazardous Waste Conference, University of Delaware, Newark.

Ehlers, W., et al. (1969). Lindane diffusion in soils. Theoretical considerations and mechanism of movement, Soil Science Society of America Proceedings, 501-504.

Heuckeroth, D.M., Eberle, M.F., and Rykaczewski, A. 1995. In situ vacuum extraction/ bioventing of a hazardous waste landfill. In situ aeration: Air sparging, bioventing, and related remediation processes, (pp. 341-349), Columbus, OH: Battelle Press.

Howard, P.H. (1990). Fate and exposure data for organic chemicals (Volume l) , Chelsea, MI: Lewis Publishers.

Laquidara, M.J., Leuschner, A.P., andwise, D.L. (1986). Procedure for determining potential gas quantities in an existing sanitary landfill. In Wat. Sci. Technology, 18(12), 151-162.

Spencer, W.F., et al. (1982). Review. Behavior of organic chemicals at soil, air, water interfaces as related to predicting the transport and volatilization of organic pollutants. Environmental Toxicology and Chemistry, 1, 17-25.

Tchobanoglous, G., Theisen, H. and Vigil, S. (1993). Integrated solid waste management engineering principles and management issues, New York: McGraw-Hill.

Van Zanten, Bart and Scheepers, Martin J.J. (March 1995). [Modelling] of landfill gas potentials, 18th Annual Landfill Gas Symposium, (pp. 85-111). Silver Springs, MD: Solid Waste Association of America.

Vogel, T.M., Criddle, C.S., and McCarty, P.L. (1987). Transformations of halogenated aliphatic compounds, Environmental Science and Technology, 21(8), 722-736.

Westinghouse Savannah River Company (1996, April). Sanitary landfill in situ bioreniediation optimization test final report (U). Washington, DC: US. Department of Energy, WSRC-TR- 96-0065, Rev. 1, 126 pp.

Whalen, S.C., Reeburgh, W.S., and Sanbeck, K.A. (1990). Rapid methane oxidation in a landfill cover soil. Applied and Environmental Microbiology, 56, 3405-341 1.


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