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The Site Assessment Section (SAS), Ohio Department of Health, was requested by the Southwest District Office of the Ohio Environmental Protection Agency (SWDO-OEPA) on-site coordinator to evaluate the health hazards associated with the off-site migration of methane gas at the North Sanitary (aka. Valleycrest) Landfill Superfund site in Dayton, Ohio. Specifically, SAS was asked to evaluate: 1) the Explosive Gas Monitoring Plan currently in-place at the site; 2) the Landfill Gas Abatement Plan proposed for the site; and 3) the physical hazards associated with the off-site migration of methane from the North Sanitary Landfill (NSL) site.

The North Sanitary Landfill (NSL) site was listed as a US EPA Superfund site May 31, 1994. The site is in a mixed residential and commercial area in the northern portion of Dayton, Montgomery County, Ohio (Figure 1). The 100-acre site consists of five former sand and gravel quarries used for the disposal of both municipal and industrial wastes from 1966 to 1989 (US EPA, 1993). The site is bounded by mixed residential and commercial portions of Valley Street on the south, Avondale and Pompano Court residential neighborhoods to the northeast and east, open fields and a small residential area at the north terminus of Valleycrest Drive to the north, and a mixed commercial and residential part of Brandt Pike to the west. Valleycrest Drive bisects the site property into east and west portions. High-density residential areas abut the site on the southwest between Valley Street and Brandt Pike, along Pompano Court to the east (Dayton Metropolitan Housing Authority housing project), and at the northeast corner of site in Avondale (City of Riverside). A large cluster of gasoline storage tanks and transfer terminals are located on the west side of Brandt Pike, immediately to the east of the site.

Surficial soils underlying the NSL site consist of 15-50 feet of porous and permeable sand and gravel. The primary areas of concern at the NSL site, Disposal Areas 1 and 2, both were excavated down to the water table, to depths of 20-30 feet below the ground surface within the upper sand and gravel unit (Conestoga-Rover & Associates, 1996). Between 1967 and 1975, waste was backfilled into previously excavated sand and gravel pits, laid down in layers with little intervening cover and often in contact with standing water. Material disposed in both landfill areas included a variety of household, commercial, and industrial wastes (Ecology & Environment, 1991). These wastes included large volumes of putrescible garbage, solid household and commercial wastes (paper, cardboard, rubber, plastic, wood), plus hundreds of 55-gallon drums containing liquid and semi-solid industrial wastes (inks, waste paint, paint thinners, waste oil, and other volatile chemicals). Both disposal areas were closed in 1975 with closure allegedly consisting of grading of the surface and covering with a thin layer of native soil. Neither landfill was covered with an engineered landfill cap (Ecology & Environment, 1991).

The NSL site, topographically, is more or less level. The underlying porous and permeable sand and gravel layers are continuous with and extend under adjacent residential and commercial areas (Ohio Department of Natural Resources, well logs). These sand and gravel deposits, 5 to 50 feet thick, are underlain by clay-rich, nearly impermeable till of variable thickness (Conestoga-Rovers & Associates, 1996). The water table in the vicinity of the NSL site occurs approximately 25 to 30 feet below the ground surface. Groundwater flow in the upper sand and gravel unit at the site is from the east to the west with localized "mounding" of the groundwater in the central portion of site. The City of Dayton Miami North municipal well field is located less than one mile northwest of the site and may exert an influence on groundwater levels in the vicinity of the NSL site, as well as the pumping of groundwater recovery wells at the Brandt Pike petroleum terminals and the Van Dyne Crotty site, both across Brandt Pike immediately west of the site. Local water table levels may be a factor affecting the vertical movement and horizontal distribution of soil gases in landfills (Peteranecz & Kear, 1992).

Utility lines generally run parallel to existing roadways in the vicinity of the NSL site. A water main extends up Valleycrest Drive between Disposal Areas 1 and 2 in the middle of the site. Water mains and gas lines parallel Brandt Pike west of the site. Water mains, sanitary sewer lines, storm sewers, and gas lines parallel roads in the residential area southwest of the site and along Valley Street south of the site. Water and sanitary lines plus a storm sewer parallel Pompano Court, with a gas main extending around the outside perimeter of the Dayton Metropolitan Housing property line, immediately adjacent to Disposal Area 1. Water and sanitary lines plus storm sewers run parallel to Rondow and Sagamore at the northeast corner of the NSL site in the Avondale residential community. A gasoline transmission line runs diagonally from the petroleum terminals along Brandt Pike west of the site to a point where Valleycrest Drive intersects Valley Street at the south end of the site. The gasoline line runs parallel to the NSL property line at the southwest corner of the site, offset from the property line by a distance of about 200 feet (Conestoga-Rovers & Associates, 1998a).

Water mains in the area are buried to depths of 4.5 feet with sand and gravel bedding up to one foot thick above and below the mains. Sanitary sewers are placed in sand bedding, but other details of their construction are unknown. Buried electric cables and gas mains around Pompano Court are placed in a common trench and are not surrounded with bedding (Conestoga-Rovers & Associates, 1998a). Landfill gases may preferentially migrate off-site following sand or gravel-filled utility trenches.

History of Agency Actions

During its active history (1966-1975), the operators of the North Sanitary Landfill were repeatedly cited by City of Dayton Health Department for a variety of problems with the landfill operation. These included odor problems, uncovered wastes, wastes in contact with standing water, the acceptance of burnable industrial wastes, the acceptance of drummed chemical wastes, and the occurrence of several large fires at the site (Ecology & Environment, 1991). The site was "discovered" by US EPA in 1981 following review of trucking manifests that indicated hazardous wastes were disposed at the site. OEPA did a preliminary assessment of the NSL site in 1985 and Ecology & Environment did an Expanded Site Inspection for US EPA in 1991. US EPA prepared an Hazard Ranking System Documentation Record for the site in 1993, and the site was placed on the NPL in 1994. With OEPA oversight, Conestoga Rovers & Associates (CRA) completed a Phase I Remedial Investigation Technical Memorandum for the NSL in March 1996.

SAS completed a Preliminary Health Assessment for the NSL site in May 1995. The site was classified as an "indeterminate public health hazard" due to the possibility of exposure to site-related chemicals and the lack of data necessary to more fully characterize the hazards presented by the site. Potential exposure pathways identified included exposure of nearby residents to soil gas emanating from the site, contaminated surface soils on-site, and contact with contaminated leachate, surface water, or sediment on-site. Identified completed exposure pathways included past exposure to contaminated drinking water by residents living adjacent to the NSL site and using private wells as their drinking water supply.

Discovery of Methane Gas Problem

As part of the Phase I Remedial Investigation, 18 landfill gas probes were installed around the perimeter of the NSL site. Two rounds of gas monitoring were conducted at these gas probes and from 16 shallow monitoring wells December 22, 1995, and January 10, 1996. Methane gas was detected at 12 out of these 34 locations. Methane levels above the lower explosive level (LEL) for methane gas (=5% methane by volume) were detected at seven locations (Table 1). These included: GP-01 at the north end of Valleycrest Drive; EPA-14 at the eastern edge of Disposal Area 1, adjacent to homes along the west side of Pompano Court; GP-08 at the southwest boundary of Disposal Area 1, north of a small cluster of homes along Valley Street; GP-09, GP-11, EPA-5 at the south end of Valleycrest Drive; and GP-14 at the southwest boundary of the site, adjacent to residences along Urbana Avenue (Figure 3).

Following review of a draft of the Phase I Remedial Investigation report, SAS staff contacted SWDO-OEPA on May 5, 1997, to express concern about the presence of explosive levels of methane gas adjacent to high density residential areas. This concern was formally expressed in a letter to the OEPA OSC dated May 23, 1997, (Appendix A) in which SAS stated that "levels of gas within the explosive range of methane, the close proximity (within 200 feet) of these sample sites to residential housing, and the probability of basements and pathways along buried utility lines, all suggest that methane migration from the NSL site poses a potentially acute public health hazard to adjacent residents." SAS recommended that "either the PRP group or OEPA pursue further investigation of this gas problem as soon as possible in order to determine the extent of the public health threat posed by these landfill gases and to determine what measures are needed to mitigate this potential hazard."

Following extensive discussions between the Valleycrest Landfill Site Group (VLSG) and OEPA, in October 1997, a workplan was initiated to investigate the methane gas problem at the site. This plan included: 1) a utility survey; 2) installation of temporary gas probes in the vicinity of locations that exhibited exceedences of the LEL during the Phase I RI investigation and near potential landfill gas migration routes (e.g., buried utilities); 3) monitoring of these gas probes in four rounds of sampling to be carried out under varying atmospheric conditions in January and February 1998; 4) use of a PID to identify the presence of additional volatile organic compounds; and 4) evaluation of landfill gas data to determine the need for further actions.

Explosive Gas Monitoring Plan

The Explosive Gas Monitoring Plan for the NSL site was put into action with installation of 17 temporary gas probes around the landfill perimeter January 14, 1998 (Figure 3). The first round of gas probe sampling took place January 15-16, 1998. Methane was detected at levels in excess of the Lower Explosive Limit (LEL = 5% methane/volume) at probe locations GP-01, GP-06, TGP-12, TGP-23, and TGP-24 (Figure 3). Additional sampling of these probes was carried out January 16-20, 1998, confirming these elevated methane levels. With the concurrence of SAS (letter of January 26, 1998, Appendix A), SWDO-OEPA requested the VLSG contractor to start daily sampling of these probes due to the consistently elevated levels of methane detected at these five probe stations and their proximity to occupied structures. Subsequent sampling on February 2, 1998, indicated methane levels in excess of the LEL in the five previously-identified probes plus two additional probes (GP-08, GP-09) (Figure 3).

Due to the proximity of gas probes with methane levels in excess of the LEL to occupied structures surrounding the NSL site, indoor air monitoring was carried out by the VLSG contractor in 16 structures January 20 and 26, and February 4,6, and 8, 1998. As of March 23, 1998, indoor air in 25 occupied structures had been sampled at least once. There were no detects of either methane or organic vapors (Conestoga-Rovers & Associates, 1998a). As a further precaution, combustible gas indicators (CGIs) were installed in 12 structures on February 19, 1998 (Figure 4). Alarms in the CGIs are set to go off in the presence of methane or other organic vapor at levels 0.5% per volume of air. If an alarm goes off, residents are to turn off appliances, call 911 and report the incident, and then evacuate the structure. The City of Dayton Fire Department will make any subsequent rulings as to whether the structure is safe or not (SWDO-OEPA on-site coordinator, pers. comm.).

Twelve additional temporary gas probes were installed around the NSL site on February 23, 1998. These probes were placed in proximity to probes that previously indicated methane levels in excess of the LEL. Daily sampling of these new probes indicated methane levels in excess of the LEL in probe stations TGP-26, TGP-28, TGP-29, TGP-30, TGP-31, and TGP-36 (Figure 3).

Results of Gas Probe Sampling

A summary of the results of gas probe sampling at the NSL site during the months of January, February, March, and April 1998 is presented in Table 1. Gas levels in most probes remained largely constant during much of this period. Weather conditions during this period were wet and cool, although temperatures were warmer than what is normal for Ohio this time of the year. As indicated in Table 1, several areas of concern with methane gas levels consistently in excess of the LEL were identified from these sampling results. These include: 1) the north end of Valleycrest Drive at the property line with NSL and primarily west of Valleycrest (GP-01, TGP-24, TGP-30, TGP-31); 2) the area at the eastern boundary of Disposal Area 1, abutting homes along the west side of Pompano Court (GP-06); 3) the southern property line of Disposal Area 1, adjacent to the Allwaste facility on Valley Street (GP-08, TGP-23, TGP-28, TGP-29, TGP-36); and 4) the area at the south end of Valleycrest Drive,Vull, and Semler (GP-09, TGP-12, TGP-26).

Gas levels were constant in most probes with the greatest variability occurring in probes GP-01, TGP-12, and TGP-26. Methane gas levels in excess of the LEL were detected in 11 out of 50 gas monitoring probes. The highest gas concentrations are found at TGP-30 at the north end of Valleycrest Drive (averaging 65.8% methane/volume of air) and TGP-23 behind the Allwaste facility (averaging 37.2% methane/volume of air). Both of these probes appear to be sited in landfilled waste (Conestoga-Rovers & Associates, 1998a).

Landfill Gas Abatement Plan

SWDO-OEPA, following indications of high levels of methane at the perimeter of the NSL site, requested that the VLSG develop a plan to reduce or eliminate the potential for methane gas to migrate from the NSL site to adjacent off-site areas with occupied structures. Conestoga-Rovers & Associates (CRA), under contract with VLSG, prepared a Landfill Gas Abatement Plan in March 1998.

CRA characterized NSL with regard to methane gas production and migration and concluded that movement of landfill gas was influenced by: 1) zones within the landfill where municipal wastes were disposed; 2) soil stratigraphy at the NSL site; 3) depth to the water table; 4) the presence of buried utilities; and 5) proximity to residential and commercial structures to the NSL site (CRA, 1998b). Methane produced within the landfill site will move from areas of higher pressure at depth within the landfill to areas of lower pressure near the surface at the periphery of the landfill or beyond, preferentially migrating along permeable paths of the least resistance. CRA noted that horizontal movement of landfill gases within the sand and gravel unit underlying the NSL site was unlimited. No utilities cross-cut disposal areas, but landfill gases from Disposal Areas 1 and 2 may preferentially move towards and along the water main underlying Valleycrest Drive (CRA, 1998b). Both Disposal Areas 1 and 2 lack engineered landfill caps and are covered with comparatively thin layers of native soil. No good data exist characterizing landfill gas pressures within former disposal areas, but pressures at the perimeter of the landfills appear to be low (CRA, 1998a).

CRA originally proposed to mitigate off-site gas migration by constructing trenches along the outer perimeter of the NSL site. This plan was abandoned after initial studies indicated that: 1) waste material occasionally extended right up to the property line in some portions of the site; and 2) on-site soils would not support the trench walls, caving readily at depth within a short period of time (CRA, 1998b). The current plan proposed by CRA is the installation of a passive vent system consisting of a series of regularly spaced gas vents. The vents, constructed of 4-inch diameter PVC piping, will be set back roughly 30 feet from the property line of the landfill in areas where gas levels at the property boundary exceed the LEL and where occupied structures exist. A vent spacing of 40 feet has been proposed by CRA (CRA, 1998b).

Public Meeting with Area Residents

SWDO-OEPA and SAS staff and a representative of the VLSG contractor (de Maximus Inc.) presented an update of activities at the NSL site to a meeting of the Old North Dayton Neighborhood Association held the evening of March 17, 1998, at the Stuart-Patterson Community Center. Approximately 25 residents were in attendance. The OEPA on-site coordinator presented a brief review of previous actions at the site and an update regarding the Remedial Investigation (RI), the surface drum removal action, and the on-going methane gas investigation. A risk-assessor from OEPA briefly described how the agency determines human health risks at Superfund sites and the role of the risk assessor in the Phase II RI. The representative from de Maximus described the Explosive Gas Monitoring Plan, the current gas sampling program, plus its results to date. The SAS staff member described the conclusions and recommendations made in the Preliminary Health Assessment completed for the NSL site in 1995 and answered questions regarding health risks posed by off-site migration of landfill gases.

Residents attending the March 17, 1998, meeting of the Old North Dayton Neighborhood Association expressed concerns about the health hazards the NSL site posed to families living adjacent to the landfill. Concerns included exposure of children to landfill gases released to ambient air in the vicinity of adjacent residences, adverse health effects from inhaling the landfill gases, and the potential for the methane to accumulate in their homes and pose an explosion hazard. A resident from the Pompano Court neighborhood indicated that no one explained to residents what the combustible gas indicators were and what they were to do if an alarm went off. Evidence of this lack of communications with Pompano Court residents is provided by an incident that occurred on March. 18, 1998. Evidently, a CGS indicator’s alarm went off on this date at a residence on Pompano Court. The resident simply unplugged the CGI to turn off the alarm and did not report the incident. It was later determined that hair spray in use in the bathroom triggered the alarm.

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