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HEALTH CONSULTATION

WESTON WOODS DEVELOPMENT SITE
WHITE BEAR TOWNSHIP, RAMSEY COUNTY, MINNESOTA


I. SUMMARY OF BACKGROUND AND HISTORY

The Weston Woods Development Site (the site) is located in White Bear Township, RamseyCounty, Minnesota. The site is located north of Minnesota Highway 96, and just west ofAllandale Drive, about seven miles north of the City of St. Paul. The site property occupiesapproximately 70 acres and is characterized by rolling hills, ponds and wetlands, and is currentlybeing developed for residential use. The site is located in a growing residential area, with thecommunity of North Oaks located just to the west and newer homes to the east. A map showing the location of the site is presented in Figure 1.

The site is also the location of the Highway 96 Dump, a state Superfund site. The Highway 96Dump operated from the 1920s until 1973, and accepted solid and industrial wastes, includingsome hazardous wastes such as paints, varnishes, oils, and solvents. The Highway 96 Dump wascomprised of two separate disposal areas, known as the North and South Disposal Areas. Otherareas of the property may have been used for the disposal of small quantities of waste as well. Amap showing the historical features of the Highway 96 Dump is presented in Figure 2. TheHighway 96 Dump has been the subject of two Minnesota Department of Health reports, aPublic Health Assessment (MDH 1991) and a Health Consultation (MDH 1993). Groundwatercontamination (primarily volatile organic compounds, or VOCs) from the waste disposed of atthe dump impacted nearby residential wells in nearby North Oaks. In 1989, a groundwaterextraction system was installed by the responsible parties for the Highway 96 Dump to controlthe spread of the groundwater contamination. The system has been effective at controlling thecontamination plume and removing contaminants, and the contaminated groundwater isdischarged to the sanitary sewer for treatment. In 1994, 60 homes with private wells in NorthOaks were connected to a public water supply, eliminating any potential future exposure to thecontaminated groundwater (CRA 2001). Monitoring of other private wells in North Oaks isongoing.

Beginning in 1994, buried drums, contaminated soil, and other hazardous wastes were excavated and removed from the Highway 96 Dump site. Some 400 drums were removed from the North Disposal Area, South Disposal Area, and a pond at the site. Over 300 drums were removed from the South Disposal Area alone (CRA 2001). Some of the drums were empty, some contained solid waste material, and about 10% contained liquid wastes. All remaining waste materials from the South Disposal Area were excavated and transferred to the North Disposal Area, which was then compacted and capped with a layer of clean soil. Levels of VOCs were low to non-detectable in groundwater beneath the former South Disposal Area, which was backfilled with soil from the site (CRA 2001). A sump was installed beneath the North Disposal Area to collect landfill leachate and prevent it from reaching the groundwater. Leachate is produced when rain and snow melt percolate through waste, carrying contaminants with them. Three soil gas vents were also installed in the North Disposal Area (now known as the Consolidated Waste Area) to allow landfill gases (such as methane and VOCs) to escape. Once these activities were completed, the site entered a long-term monitoring and maintenance phase. The MPCA restricted the use of the Consolidated Waste Area itself by filing an environmental restrictive covenant with the property deed that prohibits future development in that area.

In 2001, Mark of Excellence Homes (MEH) proposed to develop the site for residential use. Due to the presence of the Highway 96 Dump on the site, it was entered into the MinnesotaPollution Control Agency's (MPCA) Voluntary Investigation and Cleanup (VIC) Program forproject oversight and liability assurances (MPCA Project Number VP 14310). In November of2001, the MPCA staff requested that MDH review site documents prepared to date along withthe results of soil and air quality monitoring in order to develop conclusions andrecommendations regarding the site and potential public health impacts of the development.

Geology/Hydrogeology

The surface soil at the site consists of a complex mix of unconsolidated glacial deposits rangingfrom clay to gravel, varying in thickness from 50 to 150 feet (CRA 2001). Underlying theglacial deposits is the St. Peter Sandstone, with occasional remnants of the Platteville Limestoneand Glenwood Shale. The St. Peter Sandstone is underlain by the Prairie du Chien formation, animportant regional aquifer.

There are three hydrogeologic units of concern at the site: perched groundwater, anunconsolidated glacial aquifer (known as the Lower Sand aquifer), and the St. Peter aquifer(CRA 2001). Perched groundwater is typically associated with surface basins that collect runoff.It is usually localized and may be influenced by wetlands and other surface features. A perchedgroundwater unit exists beneath both the North and South Disposal Areas, but the two featuresare not connected. Flow in the perched groundwater units is typically downward toward theLower Sand aquifer.

The Lower Sand aquifer is the first consistent aquifer across the site, and can be found at depthsof 30 to 70 feet below ground. Groundwater flow within this unit is towards the west, in thedirection of North Oaks. The St. Peter aquifer is not separated from the Lower Sand aquifer, andis found approximately five feet below it. The groundwater flow direction in the St. Peter is alsotoward the west. Flow directions in both the Lower Sand and St. Peter aquifers are influencedby the groundwater pumpout system installed at the site.

Site Investigation Activities

MEH began constructing townhomes on the site in 2001. The townhomes consist of tworesidential units side-by-side under one roof. A map showing the locations of the townhomes inrelation to the former Highway 96 Dump site features (namely the Consolidated Waste Area andthe former South Disposal Area) is presented in Figure 3. Because of the potential forundiscovered wastes at the site, prior to beginning construction, MEH was issued a liabilityassurance letter by the MPCA that required MEH to develop and implement a constructioncontingency plan and a methane monitoring plan (MPCA 2001). The Site ConstructionContingency Plan (Bay West 2001a) outlined the procedures to be followed during constructionactivities at the site in the event that waste materials or hazardous substances were unexpectedlyencountered. These procedures included notifying Bay West, MEH's environmental consultant,when wastes were discovered, evaluating the waste materials, and excavation and temporarystorage of the waste materials until a determination could be made on how to dispose of them. In response to MPCA concerns over the whether the contingency plan was being implemented asdesigned (Bay West 2001b), modifications to the contingency plan were proposed (Bay West2001c). The modifications called for meetings with site workers to better inform them of therequirements of the contingency plan, and a requirement for Bay West staff to visit the site everyother day to monitor construction activities firsthand. Some waste materials (concrete, bricks,tree roots, trash, and rusted metal) were encountered during the course of construction at the site,excavated, and disposed of at a local landfill (Bay West 2001b). The exact locations where wastematerials were encountered and excavated were not well described.

Because of the potential for waste materials or contaminated soils to remain on the site, fieldscreening was conducted during foundation and footing excavation for townhomes to be built inthe area of the former South Disposal Area using a hand-held flame ionization detector (FID). AFID is an instrument that measures the concentration of organic vapors in air (Bay West 2002a). When concentrations of organic vapor exceeded 10 parts per million (ppm), the screening actionlevel for the site, a soil sample was to be collected from the excavated soil for analysis forVOCs. Organic vapor concentrations in excess of 10 ppm were found in soils screened fromseveral of the lots (Bay West 2002b). Soil samples showed low levels of one VOC, toluene, inthe soil below the now existing townhome units 13/14 (Block 2), which are located on theformer South Disposal Area. Samples from other lots showed VOCs were not present above thelaboratory detection limit.

Soil borings were drilled at some of the proposed townhome locations in the former South Disposal Area prior to construction so that soil or soil gas samples could be collected (Bay West 2002a). The soil borings were drilled along the centerline of the proposed townhome building, between the general locations of the sump pits for each townhome unit. It was believed that the sumps would be the most likely point of vapor entry; therefore, any VOCs detected in the soil in this area would represent what was available for potential entry into the future structure. The sumps are sealed from the indoor space, and are designed as such to minimize or prevent the infiltration of radon gas into the structures. Soil samples were collected at regular intervals and screened using a FID unit. A soil sample was then collected from the interval with the highest FID reading for laboratory analysis for VOCs. The MPCA subsequently requested that soil gas samples also be collected at the interval with the highest FID reading. Separate probes were drilled to collect the soil gas samples. The soil gas samples were collected over a 24-hour period using a Summa canister, an inert stainless steel canister that captures a known volume of air or gas for laboratory analysis. The samples were analyzed for VOCs using EPA method TO-14. The soil gas samples showed a low level of one VOC, dichlordifluoromethane, beneath the now existing location of townhome units 13/14 (Block 2) at a depth of six to eight feet, and low levels of toluene and methylene chloride beneath the future location of townhome units 15/16 (Block 2; not yet built) at a depth of eight to ten feet.

The townhomes on the east side of Weston Woods Way (Block 1) were already well underconstruction before the MPCA requested in early May, 2002 that soil borings be conducted andsoil gas samples be collected prior to construction. Presumably, soil screening was conductedduring excavation of the footings and foundation, but no data are available. Only two units(9/10) are located on the former South Disposal Area, however. Indoor air sampling wascompleted in these units as described below. The results of the screening and analysis of soiland soil gas in the Block 2 townhome unit sites located above the former South Disposal Area are summarized in the table below:

Soil Screening and Soil Boring Results, Block 2 Units
Unit #
(Block 2)
Soil AnalysisResults (ppm)Soil Vapor Analysis Results
FID (max ppm, depth)Summa Canister
13/140.11 ppmToluene60 ppm @ 6'-8' 10 ppb Dichlorodifluoromethane
15/16*ND84 ppm @ 8'-10' 20 ppb Methylene chloride,
30 ppb Toluene
17/18*ND42 ppm @ 3'-5'Methane 4% by vol.
19/20*NDNAMethane 4.2% by vol.
21/22ND180 ppmNA

ND = below laboratory detection limits
NA = no sample collected
*Not yet constructed

Due to the depth to groundwater, the fact that a groundwater pumpout system is in operation atthe site, and that the development is served by a public water supply no groundwaterinvestigation activities were proposed or conducted during the development of the site.

Methane Gas Monitoring

Because of the presence of methane gas within the Consolidated Waste Area, the potential existsfor subsurface methane gas migration towards the proposed townhome sites. To address thisconcern, the Site Methane Monitoring Plan (Bay West 2001d) was developed and included the following activities:

  • Monitoring of methane in the Consolidated Waste Area, a buffer zone around theConsolidated Waste Area, and in ambient air during construction activities; and
  • Long-term monitoring of the buffer zone, ambient air, and points of potential methanegas entry into finished townhomes.

Methane monitoring in the Consolidated Waste Area consisted of collecting combustible gassamples from the six permanent monitoring points already in place (GP-1 to GP-6). To monitormethane gas concentrations in the buffer zone, ten additional methane monitoring points (MMP-1 through MMP-10) were installed at 100-foot intervals in August and September 2001 (Foth & Van Dyke 2002). Three additional methane monitoring points (MMP11, MMP-12, and MMP-13) were installed east of the Consolidated Waste Area and wetland in October 2002 (Foth &Van Dyke 2003). The locations of the methane sampling points are shown in Figure 4. Ambientair monitoring was conducted primarily in low areas across the site. Methane gas readings werecollected using a combustible gas indicator (CGI) and a portable meter equipped with a flameionization detector (FID).

To determine if the methane gas detected was from natural sources (such as the wetland) or fromthe degradation of waste in the Consolidated Waste Area, VOC samples were collected using aSumma canister from MMP-6 in September 2001, and from MMP-6 and MMP-8 as well as fromone of the landfill passive gas vents (PV-4) in the Consolidated Waste Area in December 2001. Additional VOC samples were collected from MMP-6, MMP-10, and MMP-12 in November of2002. The results of the VOC analyses were as follows:

Methane Monitoring Probe Summa Canister Results
Sample DateSample PointVOC Analysis Results, ug/m3
9/12/2001MMP-6Toluene, 368 ug/m3
12/10/2001MMP-6Toluene, 498 ug/m3
 MMP-8Toluene, 337 ug/m3
 PV-4Vinyl chloride, 229 ug/m3; Ethyl chloride 938 ug/m3
11/8/2002MMP-6Toluene, 111 ug/m3
 MMP-10ND
 MMP-12ND

The results of the VOC samples from the methane monitoring points show toluene is present inthe soil gas. Other common organic landfill gases were also tentatively identified in some of thesamples, but could not be accurately quantified.

A three-tiered methane monitoring and response system was developed for the site (Bay West2001d). Tier I represented an initial phase of monitoring of the newly installed methanemonitoring points during initial site construction activities to determine if methane gas wasmigrating outside of the buffer zone around the Consolidated Waste Area. Monitoring wouldproceed to Tier II if levels exceeded 0.1% of the lower explosive limit (LEL) of methane gas inair in the monitoring points. Methane gas in air has the potential to explode when itsconcentration falls within the range of 5.5% to 15% by volume. These limits are defined as thelower and upper explosive limits (UEL), respectively. Below the LEL, there is not enough gaspresent to be combustible. Above the UEL, there is not enough oxygen present for combustionto occur.

Tier II monitoring would consist of continued sampling of the methane gas monitoring points. Tier III monitoring would be implemented if methane gas concentration continued to exceed0.1% of the LEL and would consist of the addition of monitoring within completed townhomeunits near the Consolidated Waste Area at likely points of entry (utility pipes and cracks in theconcrete floor or wall in basement areas). If significant methane gas levels were observed in themethane monitoring points (10% LEL or greater), the installation of a soil gas interceptor trenchin the buffer zone would be considered. Passive vents in the interceptor trench would be used toremove the soil gas and prevent its migration towards the townhome units. At all levels ofevaluation, sampling would stop after four consecutive (typically quarterly) rounds of samplingwith results below 0.1% of the LEL.

The long-term methane monitoring plan, to be implemented after all construction activities are completed, is to consist of (Bay West 2001d):

  • Re-sampling of the same sampling points during the first frost event; and
  • Continued monitoring of sampling points and the townhome basements not eliminatedfrom further monitoring due to zero readings during four previous quarterly samplingevents.

Sampling of the methane monitoring points began in September 2001 (Foth & Van Dyke 2002). High concentrations of methane (greater than 10% of the LEL) were initially observed in someof the methane monitoring points in the buffer zone, particularly MMP-5 through MMP-10. Avacuum was applied for up to one week to some of the monitoring points to determine the effecton methane concentrations. Due to the levels of methane observed, a soil gas interceptor trenchwas constructed in October 2001. The trench extended to the depth of the uppermostgroundwater unit. When the soil gas interceptor trench was installed, some of the methanemonitoring points collapsed, and had to be reconstructed. The location of the soil gas interceptortrench can be seen in Figure 4. Because of the continuing detections of methane gas well inexcess of 0.1% of LEL in the methane monitoring points, Tier III monitoring was implemented.

The weekly methane monitoring results for points MMP-5 through MMP-10 for the period of September/October 2001 to April 2003 are presented graphically in Figures 5 and 6. The installation of the initial passive soil gas interceptor trench did not appear to prevent methane gas from reaching the methane monitoring points. Methane gas concentrations in probes MMP-8, MMP-9, and MMP-10 remained relatively constant after installation of the initial soil gas interceptor trench. In March of 2002, the trench was converted to an active gas collection system by the installation of four vacuum fans (Foth & Van Dyke 2002). The conversion of the system to an active gas collection system also appeared to have had little positive effect, perhaps due to the lack of an effective seal on top of the trench or other factors. Methane gas concentrations in some points appeared to be rising after the conversion of the system to active gas collection. Modifications to the system were proposed to try to improve its function (Foth & Van Dyke 2002).

Six additional push probes were also advanced in July 2002 to determine if landfill gas wasmigrating via a utility trench beneath Greenhaven Drive. Methane gas was not detected in theutility trench, or south of Greenhaven Drive. Push probes were also advanced in other areas ofthe development to look for the presence of methane gas in October and November, 2002 (Foth& Van Dyke 2003). Low levels of methane gas were found in some of the push probes.,including one located "35 feet east of MMP-6". It is not clear where this boring was placed. InNovember 2002, the soil gas interceptor trench was reconstructed and deepened to try toimprove its performance (Foth & Van Dyke 2003). The trench was deepened to better interceptlandfill gas at the depth at which it was migrating, and is of passive design.

The reconstructed passive trench appears to have had a positive effect on methane gas concentrations in probes MMP-5 through MMP-10, although concentrations continue to approach the LEL of 5.5% in two probes (MMP-6 and MMP-8). Probes MMP-1 through MMP-4 have traditionally had low to non-detectable levels of methane gas. The reconstruction of the trench has apparently had little effect on the low levels of methane gas found in newly installed probes MMP-11, MMP-12, and MMP-13. These probes, located across the wetland from the Consolidated Waste Area, are likely picking up low levels of naturally generated methane gas from the wetland. MEH's consultant, Foth & Van Dyke, has proposed that the probes continue to be sampled on a bi-weekly basis in 2003 to monitor the effectiveness of the reconstructed methane gas interceptor trench (Foth & Van Dyke 2003).

To determine if methane gas was infiltrating the storm sewers, levels were measured at up to sixlocations on five separate occasions in September and October of 2001, and February of 2002. The locations of the samples are shown in Figure 4. No methane gas was detected in any of thesamples.

To determine if methane gas was infiltrating any of the newly completed townhomes, methane gas readings were collected from the basements and sump baskets of several of the units as they were completed (Foth & Van Dyke 2002). Readings were taken with two separate direct-reading meters to provide the lowest possible detection limits, and the units were sealed with minimal ventilation for between 44 and 62 hours prior to taking the readings to promote the accumulation of soil gas. The maximum detections in the units tested were as follows:

Townhome Unit Methane Gas Readings
BlockUnitDateSealing Period,(hours)Sample Location% of theLEL
19 12/24/0161Sump Basket0.018
11012/24/0161.5Sump Basket0.009
111 2/15/0244Sump Basket0.00
112 2/15/0244.25Sump Basket0.005
113 2/15/0244.5Bathroom0.002
114 2/15/0244.75Utility Room0.004
221 4/8/0247.9 1st Floor Bathroom0.016
222 4/8/02511st Floor0.025

Note that the units are expressed as a percentage of the LEL, which itself is a percentage (5.5%)of methane gas in air. Only very low levels of methane gas were found in this screening. Nodata has have been presented for some of the units near the Consolidated Waste Area

Indoor Air / Soil Gas Evaluation

Air samples were collected within individual townhome units at varying stages of construction totest for the presence of soil gases from the former waste disposal areas at the site (Bay West2002c). The samples were collected under conditions designed to encourage the infiltration ofsoil gas, i.e. no ventilation, sealing the structure as much as possible for a minimum of 48 hours,and collecting the samples during periods of low barometric pressure (28.8 inches of mercury orless). The air samples were collected instantaneously from the sump basket, mechanical room,or other areas of the townhome using stainless steel Summa canisters, and analyzed for VOCsusing EPA method TO-14.

The data from the VOC samples are presented in Table 1. The concentration of some individualVOCs exceeded either the Health Risk Values (HRVs) established by MDH for contaminants inair, or screening criteria known as Interim Screening Concentrations (ISCs). The HRVs arepromulgated standards designed to be protective of human health from exposure to contaminantsin air. There are only a limited number of chronic HRVs; ISCs were developed for contaminantsfor which there is no HRV as a way to determine if contaminant concentrations in air have thepotential to pose an unacceptable human health risk from long-term exposure. They aredesigned for screening purposes only, and are not intended to represent site-specific cleanupcriteria.

Site Visit

MDH staff visited the Weston Woods Development Site on January 8 and April 10, 2003. Thesurrounding land use is primarily residential and commercial. The site is bordered on the northby railroad tracks, and on the south by state Highway 96. The city of North Oaks lies to thewest, and residential and commercial properties and Interstate 35 lie to the east.

The site is currently being developed for residential use. Phases 1 and 2 of the development are nearing completion, Phase 3 is under construction, and work is yet to begin on Phase 4. Due to the environmental issues, the construction on ten of the lots in Phase 2 (located south of Greenhaven Drive) has been postponed and these lots remain undeveloped. The location of these postponed units can be seen in Figure 3. Of the completed townhomes, many appear to be occupied.

The Consolidated Waste Area is dotted with monitoring wells and gas vents, and is not fenced. A paved recreational trail has been constructed that runs partially around its perimeter. It is designated as a "proposed future city park" on some maps of the development. The monitoring wells and associated groundwater pumping equipment are locked and appear to be in good condition. The gas vents are completed 8-10 feet above grade, and would be difficult to reach without a ladder. The adjacent wetland, which had been partially drained in an effort to improve the performance of the gas interceptor trench, is now filled again. The trench itself runs along the south edge of the Consolidated Waste Area, and is adjacent to and below the portion of the paved recreational trail that runs along Greenhaven Drive.


II. DISCUSSION

Several current and potential future residents of the Weston Woods development have expressedconcern about the proximity of the development to the former Highway 96 Dump, the methanegas and other contaminants found in some samples collected at the site, and the potentialimplications on public health and property values. The MPCA (with input from MDH) preparedan information sheet in January of 2002 entitled "Environmental Conditions in 2002 at theHighway 96 Dump Superfund Site" to address some of these concerns (MPCA 2002). Thedeveloper agreed to provide this information sheet to the current homeowners in thedevelopment, and to prospective purchasers of new townhomes. Concerns continued to beexpressed to MDH even after publication of the MPCA information sheet, however. The publichealth implications of potential exposure to the various affected media at the site (soil,groundwater, landfill/soil gas, and indoor air) are discussed in more detail in this section.

Soil Contamination

During excavation of townhome foundations, soils were visually screened, and potentiallycontaminated soils separated for further screening using an organic vapor detector. Contaminated soils were to be removed from the excavation. Residual soil contamination mayremain at the site at low levels in a few locations, but is unlikely to represent a potential healththreat from direct human contact given its location and likely depth below ground. It may act asa low-level source of VOCs in soil gas, however. The waste materials in the Consolidated WasteArea were covered by two feet of fill, and are also unlikely to represent a direct public healththreat unless disturbed.

Groundwater Contamination

No VOCs are present above the laboratory detection limits in two monitoring wells locateddowngradient of the former South Disposal Area, indicating that if groundwater contamination ispresent, it is very limited in extent. The two wells, MW-14D and MW-9B, are located in thelower sand and St. Peter aquifers, respectively. No monitoring wells are completed in theperched groundwater unit located in the area of the South Disposal Area. The perchedgroundwater in this area may be intermittent and non-contiguous, making installation of apermanent monitoring well difficult. It is unlikely that contaminated groundwater is presentbeneath any proposed townhome locations in the former South Disposal Area.

It also does not appear at this time that the contaminated groundwater plume associated with theConsolidated Waste Area extends beneath the area of the proposed Phase 4 of the WestonWoods Development, which is located just southwest of the Consolidated Waste Area. This islikely due to the effectiveness of the groundwater pumpout system. It is unlikely that thegroundwater plume will serve as a future source of VOC vapors in soil gas in this area, unlessthe groundwater pumpout system is shut down. The MPCA will continue to provide oversight ofthe contaminated groundwater plume and treatment system.

The Weston Woods development is served by the White Bear Township public water supply,and does not rely on private wells for drinking water purposes. There is no public health threatassociated with consumption of the contaminated groundwater at the site, since it is not beingused for drinking water purposes.

While contaminated groundwater may at times discharge to one or more of the many ponds orwetlands present at the site, VOCs present in the groundwater would quickly volatilize to the air.Contact with surface water is unlikely to result in significant exposure to contaminants.

Landfill / Soil Gas

Landfill gas is composed of a mixture of gases, primarily methane (45-60%) and carbon dioxide(40-60%) (ATSDR 2001). It also can contain lesser or trace amounts of a variety of other gases,including nitrogen, oxygen, ammonia, sulfides, hydrogen, carbon monoxide, and various VOCs. It is produced in landfills or dumps by the bacterial decomposition of waste, volatilization ofcontaminants present in the waste, or by chemical reactions between different waste materials.The generation of landfill gas via bacterial decomposition follows predictable phases that areinitially driven by the availability of oxygen. The rate and volume of landfill gas generated isbased on several factors, including the composition of the waste, the age of the waste deposit, thepresence or absence of oxygen, the moisture content of the waste, and the temperature of the waste (ATSDR 2001).

As described previously, methane gas in air is potentially explosive at concentrations betweenapproximately 5.5% and 15% by volume. This is especially of concern in enclosed structures,where due to a lack of ventilation, it can accumulate until it reaches explosive concentrations. Carbon dioxide is heavier than air, and can accumulate in low areas where there is littleventilation. At high enough concentrations, it can cause suffocation as it displaces the air. Someconstituents of landfill gas, such as ammonia and hydrogen sulfide can have unpleasant odors,even at low concentrations. Exposure to specific VOCs in air at high concentrations or for longperiods of time can be related to a variety of adverse health effects, including nervous systemeffects, liver and kidney damage, or cancer.

Landfill gases are capable of migrating through the pore spaces in soil for some distance awayfrom a waste deposit. While there is a natural tendency for gases that are lighter than air (suchas methane and many VOCs) to move upward, this upward movement can be inhibited by densecover materials or frozen soil conditions. Landfill gases may then migrate in a more lateraldirection, following the "path of least resistance." The three main physical factors that influence the migration of landfill gases are (ATSDR 2001):

  • Diffusion - the natural tendency for a gas to reach a uniform concentration in a givenspace, moving from areas of high concentration to areas of low concentration. Becausegas concentrations are usually higher within the landfill, gases tend to move to thesurrounding soils where gas concentrations are lower.
  • Pressure - gases also tend to migrate from areas of high pressure to areas of low pressure,a process known as convection. As gases are generated in the landfill through theprocesses described above, the pressure increases relative to the surrounding soils.
  • Permeability - this describes how easily gases are able to flow through soils. Dry, sandysoils are much more permeable than wet, clay soils. Gases tend to move through highlypermeable soils as opposed to low permeability soils. As landfill caps are usuallyconstructed of less permeable materials to inhibit infiltration of water, gases in thelandfill tend to migrate laterally.

Because of these physical factors, the movement of landfill gases through soil is also influencedto varying degrees by external conditions such as the type of cover materials, the presence ofnatural or man-made permeable zones or conduits, wind speed and direction, moisture content ofthe soil (moisture lowers permeability), groundwater level, temperature (which affectsdiffusion), and barometric and soil gas pressures. Low atmospheric barometric pressure isusually associated with increased migration of landfill gases. Strong seasonal variations mayalso be observed, in part due to the breakdown of landfill gases, such as methane, by naturalprocesses (Christopherson and Kjeldsen 2001). The migration of landfill gases is a verydynamic process, and can change quickly as a result of changes in environmental conditions.

Like methane, VOCs present in landfill gas are capable of migrating for some distance. Theirmovement is affected by similar sub-surface processes such as adsorption, breakdown,oxidation, and condensation. Different VOCs will be affected by different processes. In a studyof a landfill in England, it was found that while the sub-surface concentration of VOCs known ascycloalkanes decreased with distance from the landfill, the concentration of chlorinated VOCs(such as methylene chloride and vinyl chloride) actually peaked approximately 40 meters fromthe landfill (Ward et al 1996). This study also showed that the most mobile compounds arefreons (often used in the past as aerosol propellants), such as dichlorodifluoromethane (alsoknown as Freon 12).

Because of all of the above factors, it is difficult to estimate how far landfill gases will migrate atany particular site. ATSDR (2001) described a study of 38 New York landfills. Gas migrationdistance was measured at up to 1,000 feet at one landfill, 500 feet at 4 landfills, and 250 feet atthe remaining 33 landfills. Travel distances as far as 1,500 feet have also been reported. It isalso possible that VOCs will migrate further than methane gas, because methane is easilyoxidized in the subsurface under certain conditions, while some VOCs are very resistant todegradation in the environment.

Landfill or soil gases (including methane) are capable of entering structures through minutecracks in the foundation, pipe or utility penetrations through the concrete floor slab or walls, orthrough foundation drainage systems. Gases may also actually move through porous concreteblock walls. Soil gas entry into structures is usually the result of pressure differentials, whichare mainly caused by indoor-outdoor thermal differences, wind loading on structures and soil,and unbalanced ventilation systems that can result in the depressurization of a building (Hodgsonet al 1992). Increased soil moisture, which often occurs in the spring after the ground thaws andsnow melts, can also drive soil gases from the surrounding soil into the relatively dry soilbeneath structures, increasing the potential for infiltration. Many factors therefore influence theamount of soil gas that may enter a building at any given time.

For many of the above reasons, winter conditions tend to promote the infiltration of landfill orsoil gases into structures. Frozen ground may limit the vertical migration of subsurface gases. Under winter heating conditions, building basements can be greatly underpressurized relative tothe surrounding soil (Hodgson et al 1992). This is sometimes referred to as the "stack effect."

Methane gas continues to be found at levels near or above the LEL in methane monitoring points MMP-6 and MMP-8, even after the most recent reconstruction of the methane extraction trench. While some of the methane gas may be due to natural sources (i.e. the degradation of natural plant material in the wetland adjacent to the trench), the continued detection of toluene in MMP-6 suggests that it is due in part at least to landfill gas from the Consolidated Waste Area. While it appears from the July 2002 push probe data that methane gas was not migrating at that time through the utility trench beneath Greenhaven Drive, it was detected at low levels in other borings in the development in October and November, 2002. The July samples were collected during the summer when conditions typically do not favor the subsurface migration of landfill gas and the natural generation of methane gas is presumed to be high. Additional samples during the winter and spring months are needed to determine if landfill gases are migrating beneath or south of Greenhaven Drive.

The screening for methane gas in completed townhome units was conducted after sealing the units for up to 62 hours with minimal ventilation. The heating system was not operating during this time. Only low levels of methane gas (a maximum of 0.025% of the LEL of 5.5%) were found in the townhome units screened. Data has not been presented for all completed units, however. The units tested were sealed prior to the screening under the assumption that this would maximize the infiltration and accumulation of soil gases. As described above, however, the use of forced air heating systems may underpressurize a structure relative to the surrounding soil, increasing the potential infiltration of soil gas. It is important to re-screen the completed townhomes during the winter and spring, while their heating systems are in operation, to determine if there is an effect on soil gas infiltration.

The installation of radon gas extraction systems in the townhomes appears to minimize theinfiltration of soil gases into the townhomes by reducing the pressure under the slab, andproviding a preferential pathway for soil gases to escape to the air. These types of systems havebeen in successfully in use for some time for radon gas mitigation. The vents should beperiodically inspected and maintained, however to ensure they do not become blocked by debris,leaves, animals, etc. The townhomes are also equipped with advanced ventilation systemsdesigned to increase the amount of fresh air exchange, which would dilute any soil gases thatwould infiltrate the structure.

Indoor Air

The use of Summa canisters has been shown to be an effective method for the collection ofambient air samples for analysis of low levels of VOCs. The stability of collected mixtures ofambient gases can be affected by physical adsorption or absorption processes with the collectionvessel, reactions with the chemicals in the collected sample, or instability of the compounds. The stainless steel construction of the Summa canisters minimizes physical adsorption andabsorption processes. A study of the accuracy, precision, and storage stability of 194 VOCscollected in Summa canisters demonstrated percent mean recovery rates of 93.7% to 123.8% foreight carcinogenic VOCs of potential concern in indoor air (Brymer et al 1996). The eightcarcinogenic VOCs were benzene, chloroform, chloromethane, 1,2-dichloroethane, methylenechloride, PCE, trichloroethylene (TCE) and vinyl chloride. Other studies have confirmed theseresults, but do indicate that some VOCs may breakdown or be adsorbed to the walls of theSumma canister if the sample is stored for long periods of time (Sin et al 2001). This was notthe case for the air samples collected at the site, which were analyzed within a few days ofcollection.

The samples were collected instantaneously from the indoor spaces (typically the mechanicalroom in the basement) and sump baskets, as opposed to over several hours. Long-term samples(8 to 24 hours) are preferable for indoor air to minimize temporal and spatial variations. Samplescollected in the sump baskets are more likely to contain soil gas than samples of indoor air dueto the connection of the sump to perforated drainage pipes located under the foundation slab. However, because the sump is sealed from the rest of the structure and vented to the outside, theconcentrations found in the sump are not reflective of potential human exposure, but may beindicative of soil gas conditions beneath the structure. The air samples collected from themechanical room air or other indoor spaces represent potential human exposures.

Comparison between samples collected at different units is difficult because the samples werecollected at various times of the year and at various stages of construction. One VOC (toluene)was detected in every air sample collected from the townhome units tested. Relatively highconcentrations of toluene, well in excess of the HRV were found in some samples on indoor air. The only other VOCs found in indoor air samples that exceeded their respective HRV or ISCwere 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, and possibly benzene. The HRV forbenzene is actually a range, representing the range of potential cancer slope factors reported forbenzene by EPA. Benzene is a known human carcinogen. Exposure to high levels of toluene inair may be associated with adverse effects on the central nervous system, respiratory system,kidneys, and liver (ATSDR 1998). Exposure to high levels of trimethylbenzenes, which are usedin solvents, paint thinners and fuels, may be associated with adverse effects on the liver andkidneys (EPA 1999).

Many of the VOCs detected in the indoor air samples (including toluene) are commonly found inbuilding materials such as adhesives, paints, sealants, etc, and can be found in vehicle emissionsand tobacco smoke. Major sources in new construction include plywood flooring, latex paint,and sheet vinyl flooring (Hodgson et al 2000). They can also be absorbed by building materialsduring construction and slowly emitted for long periods thereafter (Brown 2002). In a study ofhomes in the Chicago area, the highest calculated emission rates from building materials andother indoor sources were for xylenes, toluene, 1,1,1-trichloroethane, propylene, andethylbenzene (Van Winkle and Scheff 2001). Clearly, the detections of VOCs in indoor airsamples collected in the townhomes are not surprising, but it is difficult to determine if theconcentrations are elevated over what might be expected in any new dwelling. The Brown(2002) study indicated that VOC concentrations decay slowly over a long period of time afterconstruction. Additional samples over time would help to determine if the VOC concentrationsobserved, particularly toluene, persist. It may also help determine whether their source lieswithin the building.

At this time it is difficult to determine if the VOC detections in air samples collected from thesump baskets represent emissions from construction materials (such as ABS plastic cement) inthe sump and drain system, landfill gas that has migrated from the Consolidated Waste Area, orsome other residual source nearer the townhome sites. It is possible that a minor source ofVOCs remains in the subsurface in the former South Disposal Area, where barrels of paints andsolvents were at one time disposed. While many of the non-chlorinated VOCs detected in thesump basket air samples are likely due to construction materials (such as glues or solvents), insome samples, chlorinated VOCs that are not typical of construction materials (such as carbontetrachloride and tetrachloroethene) were found. The sample with the highest number ofchlorinated VOCs detected was collected from the sump basket of unit 13 (4651 Weston WoodsWay) in December of 2001. This unit is located in the area of the former South Disposal Area. Dichlorodifluoromethane, toluene, and methylene chloride were detected in this air sample,along with several other chlorinated VOCs. Dichlorodifluoromethane was also detected in a soilgas sample collected from a soil boring drilled at lot 13/14, the future location of 4651 WestonWoods Way. Toluene and methylene chloride were detected in a soil gas sample collected at thefuture location of units 15/16 (which have not been constructed), which is adjacent to units13/14. It should be noted that the chlorinated VOCs were not found in an air sample collectedfrom the sump at 4651 Weston Woods Way in October of 2001, indicating the variabilityinherent in this kind of sampling. These VOCs were also not found in the indoor air samplescollected at the same times in that unit, however, indicating that the sump basket seal and vent iseffectively blocking entry of the chlorinated VOCs into the structure.

Children's Health Considerations

ATSDR's Child Health Initiative recognizes that the unique vulnerabilities of infants andchildren make them of special concern to communities faced with contamination of their water,soil, air, or food. Children are at greater risk than adults from certain kinds of exposures tohazardous substances at waste disposal sites. They are more likely to be exposed because theyplay outdoors and they often bring food into contaminated areas. They are shorter than an adult,which means they breathe dust, soil, and heavy vapors close to the ground. Children also weighless, resulting in higher doses of chemical exposure per body weight. The developing bodysystems of children can sustain permanent damage if toxic exposures occur during criticalgrowth stages. Most importantly, children depend completely on adults for risk identification andmanagement decisions, housing decisions, and access to medical care.

At this time it does not appear that children are being exposed to contaminants at the site through contact with contaminated soil or groundwater. There is also no indication that the VOCs detected in soil gas in the subsurface and sump basket air samples are entering living spaces, but further monitoring may be needed.


III. CONCLUSIONS

The Weston Woods Development is being constructed adjacent to the former Highway 96Dump, a state Superfund site. Soil and groundwater contamination associated with the formerHighway 96 Dump are contained by cover materials and a groundwater pumpout system,respectively, and do not represent a public health threat to residents of the development orpassers-by. Landfill gases generated within the dump are vented through passive vents drilledinto the dump, and a soil gas interceptor trench that has been constructed along GreenhavenDrive to prevent the migration of landfill gases (including methane gas) towards thedevelopment. The gas interceptor trench (which has been reconstructed) has not fully preventedmethane gas from migrating past the trench, and while the extent of methane gas migration hasnot been definitively defined, it appears to be limited. Only very low levels of methane gas, wellbelow explosive concentrations, have been detected in completed townhome units. While VOCshave been detected in samples of indoor air collected from completed townhome units,sometimes at concentrations in excess of health-based screening levels, they are likely the resultof the construction process. Low levels of VOCs that have been detected in soil gas samples andin the sump baskets of at least one townhome unit may be the result of residual soilcontamination from the former South Disposal Area, or from the migration of VOCs in soil gasfrom the former Highway 96 Dump. These VOCs have not been detected in indoor air, and donot represent a human exposure issue at the present time.

Indoor air in the townhomes at the site currently represents no apparent public health hazard;however, methane gas migration from the Consolidated Waste Area represents an indeterminatepublic health hazard that bears additional monitoring to ensure that no public health hazard develops.


IV. RECOMMENDATIONS

  1. Monitoring of the methane monitoring probes and soil gas vents should be continued asproposed (biweekly) in order to determine trends in methane migration, and to document theperformance of the interceptor trench. If methane continues to be detected in the monitoringpoints lying beyond the trench, modifications to the interceptor trench should be made toimprove its performance.

  2. The extent of methane gas migration beyond the interceptor trench should be determinedunder worst-case (i.e. winter and spring) conditions. Additional methane monitoring points may be needed south of Greenhaven Drive to document this.

  3. VOC samples should be collected from the gas vents in the Consolidated Waste Area andfrom the methane monitoring probes on at least an annual basis to determine the composition of the landfill gases, which will change over time.

  4. Methane gas readings (instantaneous) from inside the townhomes nearest the ConsolidatedWaste Area, and air samples (for VOCs) from the sump baskets (instantaneous) and mechanicalrooms (preferably long-term) should be collected under worst-case (i.e. winter and spring)conditions to verify that soil gases are not infiltrating through the foundation, and that theradon/soil gas venting system is functioning properly. Periodic monitoring should be conducted into the future to ensure this continues to be the case.

  5. Construction of townhome units on the lots near the Consolidated Waste Area that arecurrently vacant (especially those lots near MMP-6 and MMP-8) should be postponed until itcan be verified that the methane gas interceptor trench is operating as intended.

  6. The information generated from the above recommendations, as well as from past activities, should be consolidated into clear report(s) and made available to any interested parties.

  7. Updates on the environmental issues should be provided to residents of the development on a regular basis, at least annually, and information should continue to be provided to prospective buyers prior to purchase.

  8. A plan for funding and implementing the long-term maintenance of the sump basket ventingsystems in each townhome near the Consolidated Waste Area, and for the monitoring,maintenance, and operation of the methane gas interceptor trench should be developed.

  9. Because of the presence of multiple monitoring wells and gas vents, access to theConsolidated Waste Area should be restricted and the area should not be designated as a "park".

V. PUBLIC HEALTH ACTION PLAN

MDH's Public Health Action Plan for the site consists of consultation with MPCA staff and thedeveloper on site monitoring and investigation activities, preparation of an information sheetbased on this document, answering questions from the public regarding the site, and participation in any planned public outreach activities.


VI. REFERENCES

ATSDR 1998. Toxicological Profile for Toluene. Agency for Toxic Substances and DiseaseRegistry, Atlanta, GA. August 1998.

ATSDR 2001. Landfill Gas Primer. Agency for Toxic Substances and Disease Registry,Atlanta, GA. November 2001.

Bay West 2001a. Site Construction Contingency Plan for Weston Woods of White Bear. BayWest, Inc., May 2001.

Bay West 2001b. Electronic correspondence from Allan Bowles, Bay West, to Karen Kromar,MPCA dated July 17, 2001.

Bay West 2001c. Modified Site Construction Contingency Plan for Weston Woods. Bay West,Inc., July 20, 2001.

Bay West 2001d. Site Methane Monitoring Plan for Weston Woods of White Bear. Bay West,Inc., June 2001.

Bay West 2002a. Correspondence from Allan Bowles, Bay West, to Hans Neve, MPCA datedMay 3, 2002.

Bay West 2002b. Correspondence from Allan Bowles, Bay West, to Hans Neve, MPCA datedJune 17, 2002.

Bay West 2002c. Weston Woods Air Sampling Results. December 26, 2002.

Brown, S.K. 2002. Volatile organic pollutants in new and established buildings in Melbourne,Australia. Indoor Air 12: 55-63.

Brymer, D.A., Ogle, L.D., Jones, C.J., Lewis, D.L. 1996. Viability of using SUMMA polishedcanisters for the collection and storage of parts per billion by volume level volatile organics. Environmental Science & Technology 30: 188-195.

Christopherson, M. and Kjeldsen, P. 2001. Lateral gas transport in soil adjacent to an oldlandfill: factors governing gas migration. Waste Management and Research 19: 579-594.

CRA 2001. 2000 Annual Monitoring Report - Highway 96 Site. Conestoga-Rovers &Associates, May 2001.

EPA 1999. Risk Assessment Issue Paper for: Derivation of a Provisional RfD for 1,2,4-Trimethylbenzene and 1,3,5-Trimethylbenzene. National Center for Environmental Assessment(NCEA), Washington, DC. June 30, 1999.

Foth & Van Dyke 2002. Interim Report on Landfill Gas System Interceptor TrenchPerformance. Foth & Van Dyke, July 2, 2002.

Foth & Van Dyke 2003. Correspondence from Curtis L. Hartog, Foth & Van Dyke, to BarbaraJackson, MPCA dated January 10, 2003.

Hodgson, A.T., Garbesi, K., Sextro, R.G., and Daisey, J. 1992. Soil-gas contamination and entryof volatile organic compounds into a house near a landfill. Journal of the Air and WasteManagement Association 42: 277-283.

Hodgson, A.T., Rudd, A.F., and Chandra, S. 2000. Volatile organic compound concentrationsand emission rates in new manufactured and site-built houses. Indoor Air 10: 178-192.

MDH 1991. Health Assessment, Highway 96 Dumpsite. Minnesota Department of Health, St.Paul, MN. January 1991.

MDH 1993. Health Consultation, Highway 96 Dump and North Oaks GroundwaterContamination. Minnesota Department of Health, St. Paul, MN. June 1993.

MPCA 2001. No Association Determination. Correspondence from Barbara Jackson, MPCA toMark Smith of Mark of Excellence Homes, Inc. Minnesota Pollution Control Agency, St. Paul,MN. April 12, 2001.

MPCA 2002. Environmental Conditions in 2002 at the Highway 96 Dump Superfund Site. Information Sheet, Minnesota Pollution Control Agency, St. Paul, MN. January 2002.

Sin, D.W., Wong, Y., Sham, W., and Wang, D. 2001. Development of an analytical techniqueand stability evaluation of 143 C3-C12 volatile organic compounds in Summa canisters by gaschromatography - mass spectrometry. Analyst 126: 310-321.

Van Winkle, M.R., and Scheff, P.A. 2001. Volatile organic compounds, polycyclic aromatichydrocarbons, and elements in the air of ten urban homes. Indoor Air 11: 49-64.

Ward, R.S., Williams, G.M., and Hills, C.C. 1996. Changes in major and trace components of landfill gas during subsurface migration. Waste Management and Research 14: 243-261.


PREPARER OF REPORT

James Kelly, M.S.
Health Assessor
Site Assessment and Consultation Unit
Minnesota Department of Health
tel: (651) 215-0913
james.kelly@health.state.mn.us


CERTIFICATION

This Weston Woods Development Site Health Consultation was prepared by the MinnesotaDepartment of Health under a cooperative agreement with the Agency for Toxic Substances and Disease Registry (ATSDR). It is in accordance with approved methodology and procedures existing at the time the health consultation was begun.

Alan W. Yarbrough
Technical Project Officer, SPS, SSAB, DHAC
ATSDR


The Division of Health Assessment and Consultation, ATSDR, has reviewed this public health consultation and concurs with the findings.

Lisa C. Hayes
for Roberta Erlwein
Chief, State Program Section, SSAB, DHAC, ATSDR


Table 1:

Townhome VOC Sample Results
Parameter Sample Location and Date MDH
Chronic
HRV
MDH
Interim
Screening
Conc.
Block 2, Unit 5
10/7/2002
Block 2, Unit 9
8/23/2002
Block 2, Unit 10
8/23/2002
Block 2, Unit 11
10/18/2002
Block 2, Unit 12,
11/11/2002
Block 2, Unit 13
10/7/2002
Unit 13 - Resample
11/11/2002
Block 2, Unit 14
11/11/2002
Block 1, Unit 10
12/24/2001
Block 1, Unit 9
12/24 /2001
Sump Mech. Rm Sump Mech. Rm Sump Mech. Rm Sump Mech. Rm Sump Mech. Rm Sump Mech. Rm Sump Mech. Rm Sump Townhouse Townhouse
Dichlorodifluoromethane -- -- -- -- -- -- 7.9 -- 3.4 -- -- -- 14.9 -- -- -- -- NA 200
Trichlorofluoromethane -- -- -- -- -- -- 5.1 -- -- -- -- -- 12.4 -- -- -- -- NA 700
1,1,1-Trichloroethane 14.2 -- -- -- -- -- -- -- -- -- -- -- 11.5 -- -- -- -- NA 2200
1,2-Dichlorobenzene -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 12.2 NA 200
1,3-Dichlorobenzene -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 24.4 NA 110
1,4-Dichlorobenzene -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 36.6 NA 800
Methylene chloride -- -- -- -- -- -- -- -- -- -- -- -- 23.9 -- -- -- -- 20 53.2
Carbon tetrachloride -- -- -- -- -- -- -- -- -- -- -- -- 39.0 -- -- -- -- NA 1.62
1,1-Dichloroethane 5.7 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- NA 500
1,2-Dichloroethane 6.9 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- NA 0.94
Benzene -- -- -- -- -- -- 3.5 -- -- 1.9 1.9 -- 14.7 -- -- -- -- 1.3-4.5 3.12
Toluene 45.2 >4524 1809.6 565.5 1055.6 67.9 105.6 37.7 49.0 1432.6 1319.5 373.2 124.4 263.9 245.1 30.6 61.3 400 NA
Tetrachlorethene 9.5 -- -- -- -- -- 29.8 -- -- -- -- -- 8.8 -- -- -- -- NA 8.11
Ethylbenzene 4.3 -- -- 34.7 -- -- 24.7 -- -- 15.6 15.2 34.3 27.8 33.9 33.9 -- -- NA 1000
m&p-xylene 20.0 130.2 -- 112.8 -- 7.4 82.5 5.6 8.7 60.8 56.4 112.8 91.1 104.2 104.2 -- -- NA 700
o-xylene 9.1 -- -- 26.0 -- 2.5 27.3 -- 3.1 27.8 28.2 47.7 28.2 34.3 34.3 -- -- NA 700
Styrene -- -- -- 34.1 34.1 7.7 -- 4.0 4.3 -- -- 19.6 4.7 3.6 3.8 -- 26.0 1000 NA
1,2,4-Trichlorobenzene -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 163.2 NA 200
1,3,5-trimethylbenzene 18.7 -- -- -- -- -- -- 3.3 5.9 167.3 182.0 73.8 16.2 42.8 42.8 -- 19.7 NA 6
1,3,2-trimethylbenzene -- -- -- 9.8 -- -- -- -- -- -- -- -- -- -- -- -- -- NA NA
1,2,4-trimethylbenzene 36.4 -- -- 39.4 -- 6.9 24.6 7.4 11.8 260.8 265.7 137.8 38.4 73.8 73.8 -- 59.0 NA 6

Notes:
Results in µg/m3
-- results are below laboratory reporting limit
MDH- Minnesota Department of Health
Chronic HRVs - Chronic Health Risk Values (January 2002).
Interim Screening Concentration (ISC) for long-term residential exposure developed by MDH.
NA - Not applicable - value not established.
Bold - Indicates value exceeds chronic HRV and/or ISC.

Source: Bay West 2002c


FIGURES

Site Location
Figure 1. Site Location

Historical Site Features Highway 96 Dump
Figure 2. Historical Site Features Highway 96 Dump

Townhome Locations
Figure 3. Townhome Locations

Methane Monitoring Points and Int. Trench
Figure 4. Methane Monitoring Points and Int. Trench

Methane Gas Concentration vs. Time: MMP-5,-6,-7
Figure 5. Methane Gas Concentration vs. Time: MMP-5,-6,-7

Methane Gas Concentration vs. Time: MMP-8,-9,-10
Figure 6. Methane Gas Concentration vs. Time: MMP-8,-9,-10


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