Skip directly to search Skip directly to A to Z list Skip directly to site content




A. Methods

The following sections contain an analysis of the environmental data available for the Dry Bridge Road Landfill. In preparing this analysis, ATSDR staff members have used established methodologies for determining how people may be exposed to hazardous substances and what harmful effects, if any, may result from such exposure. For a complete discussion of quality assurance considerations, human exposure pathways analyses, ATSDR's health comparison values, and the methods of selecting contaminants of concern, please refer to Appendix C.

B. Extent of Contamination


Potentially contaminated airborne dust may have settled in off-site areas where residents could come into contact with the accumulated dust particles. According to the RIDEM Office of Waste Management, there are no on-site soil characterization data available for the Dry Bridge Road Landfill. However, the most likely point of exposure through this pathway is a single house on the southern perimeter of the landfill. To address this potential exposure pathway, ATSDR requested off-site soil samples from this residence to be collected and analyzed. RIDEM recently fulfilled this request. ATSDR will evaluate the soil sampling results for possible human health implications from potential soil exposure in an additional health consultation in the future.


ATSDR reviewed data from on-site and offsite monitoring wells and from off-site municipal water wells to determine if the Dry Bridge Road Landfill has had an impact on groundwater in the North Kingstown area. A private well located 2500 feet upgradient and west of the site is unaffected by the presence of the landfill. No other private wells were located in the vicinity of the site. The groundwater flow across the site area is from west to east. At present, no completed human exposure pathways, either on-site or off-site, exist for groundwater.

Municipal Wells Data

Residents of North Kingstown receive drinking water from nine municipal wells. Four of these municipal wells (wells 1, 2, 4, and 5) are located east of the Dry Bridge Road Landfill and lie approximately 6,000 feet downgradient of the site and the groundwater flow path (which runs from west to east). See Figure 2 in Appendix A for a map of the area. The remaining municipal wells are located within a different watershed. The municipal wells are tested quarterly for metals, volatile organic compounds, and synthetic organic compounds.

North Kingstown municipal drinking water sampling data indicate that municipal wells have not been adversely impacted by contamination from the landfill. Beryllium, a metal occurring naturally in the environment, was detected in three of the nine municipal wells. These three municipal wells are in a wellfield separate from that of wells 1, 2, 4, and 5. The highest concentration of beryllium detected was 0.7 micrograms per liter (µg/L). The presence of beryllium in the municipal wells is unrelated to the landfill. RIDEM reports that beryllium can be found at naturally-occurring levels in Rhode Island's groundwater as high as 48 µg/L (9). The 0.7 µg/L of beryllium detected in the municipal well exceeded ATSDR's Cancer Risk Evaluation Guide (CREG) of 0.008 µg/L but did not exceed the Environmental Protection Agency's (EPA) Maximum Contaminant Level (MCL) of 4 µg/L. The ATSDR CREG is an inappropriate comparison value to use in this situation because it is based on studies in which toxic effects were observed after workers inhaled beryllium metal fumes. However, when animals or humans ingest low to moderate amounts of beryllium, these same toxic effects are not observed. Little of beryllium that is ingested ever gets into the bloodstream because beryllium is poorly absorbed from the intestine [9]. The levels of beryllium detected in the North Kingstown municipal wells are below levels known to cause adverse health effects following ingestion of beryllium.

The groundwater is estimated to flow in the direction of North Kingstown water supply well 5. Currently, well 5 has not been affected by the Dry Bridge Road Landfill or by any other potential source of contamination.

Monitoring Well Data

RIDEM's Division of Waste Management requires the installation of monitoring wells both upgradient and downgradient of the solid waste disposal area. These wells must sample groundwater quarterly, and the samples must be analyzed by a certified laboratory. ATSDR staff members reviewed the quarterly monitoring well data for the Dry Bridge Road Landfill.

According to analytical results from the landfill's on-site monitoring wells, the only contaminant consistently detected above the Maximum Contaminant Level (MCL) is di-2-ethyl-hexylphthalate (DEHP) (10). The groundwater monitoring water data do not show that the landfill is the sole source of DEHP. The results from August 1997, however, indicate that DEHP has been contained on the site and has not been detected in the off-site monitoring well that is located 50 feet downgradient of the landfill or in the well that supplies North Kingstown's municipal water supply (11).

The most recent sampling data from the on-site monitoring wells and the off-site monitoring wells 50 feet downgradient of the site occasionally show that cadmium was detected. The maximum concentration of cadmium detected was 0.008 mg/L in August 1997, slightly above ATSDR's Chronic Child Environmental Media Evaluation Guide comparison value of 0.007 mg/L. Low concentrations of lead (0.002 mg/L) were also sporadically detected in both on-site and off-site monitoring wells. The Maximum Contaminant Level Goal (MCLG) for lead in drinking water is 0 mg/L.

These sampling data are from monitoring wells only; they are not indicative of potable water systems in North Kingstown. All other substances monitored in these wells meet federal drinking water standards, and substances in the groundwater are below ATSDR's comparison values. Future projections are that this landfill will have very little impact, if any, on North Kingstown's municipal water quality. This projection is based on the frequency of groundwater monitoring, groundwater flow rate, and the landfill's monitoring well data that show that levels of substances in the groundwater are consistently below federal guidelines.

Ambient Air

Since December 1995, Dry Bridge Road Landfill has been required to sample for a variety of air contaminants on a quarterly basis, including volatile organic compounds (VOCs), hydrogen sulfide, sulfur dioxide, dust, and asbestos. Beginning in January 1997, additional air sampling was conducted on and off the landfill property by EPA and by RIDEM's Office of Air Resources. The sampling locations were based on the varying prevailing wind direction on the sampling day and the location of the work day's disposal area (called the "working face" of the landfill). Upwind and downwind ambient air samples were taken just beyond the landfill fence (at the perimeter), along Exeter and Dry Bridge roads. There was heavy truck traffic on the days the air sampling was conducted. It should be noted that although these sampling locations are classified as "off-site," the samples were not taken from residential neighborhoods. The ambient air samples collected near the single house that exists along the landfill boundary were taken just beyond the landfill fence, but still on Hometown Properties' land. However, these sampling locations are also considered "off-site."

VOC samples were collected with sorbent tubes and analyzed with a gas chromatograph fitted with a mass selective detector. Hydrogen sulfide was measured with a gas detector, and sulfur dioxide was measured by a sulfur dioxide analyzer. Dust samples were analyzed using the filter weight method, and asbestos was measured by the phase contrast microscopy method.

On-site chemical concentrations were measured at the working face of the landfill, a location at which landfill employees are the only potentially exposed population. The concentrations of the on-site chemicals detected above ATSDR's comparison values were below the exposure limits of both the Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health, which are listed in Table 3 of Appendix D. Therefore, the chemicals listed in Table 3 of Appendix D were not selected as contaminants of concern because occupational exposure was limited to site workers in the past and trespassers are restricted from the site.

Table A: Ambient Air Contaminants
Selected for Additional Data Review

Carbon tetrachloride
Methylene Chloride

**Not selected as a contaminant of concern

Table A presents the contaminants of concern for ambient air. Tables 1 and 2 of Appendix D present a summary table for the chemicals detected at levels above ATSDR's health guidelines. The results for hydrogen sulfide, sulfur dioxide, and VOCs are reported in parts per billion (ppb).

ATSDR reviewed data for each on-site and off-site monitoring station. At the completion of the data review and analysis, ATSDR selected the following chemicals as contaminants of concern: benzene, carbon tetrachloride, and methylene chloride. Additional environmental and toxicological evaluation was required for each of these chemicals.

The following sections discuss how ATSDR eliminated chloroform and 1,1-dichloroethylene as COCs.


Chloroform was not selected as a contaminant of concern (COC) because the validity of the ambient air sampling results was questionable. The concentration of chloroform at off-site perimeter monitoring stations ranged from 0.10 to 0.65 ppb with an average concentration of 0.38 ppb. The levels of chloroform detected during the ambient air sampling program appear to be due to contamination, poor duplication of results, and possible laboratory error. The questionable results were also reviewed and verified by the Rhode Island Department of Environmental Management Office of Air Resources (12). Canister samples of the landfill gas were collected at a port in the landfill gas collection system before the gas flare section. Analysis of these landfill gas samples showed that chloroform was not detected in the landfill gas prior to flaring; this indicated that there were other possible sources of chloroform in the ambient air (13).


1,1-dichloroethylene was not selected as a COC due to the questionable validity of the ambient air sampling results. The concentration of 1,1-dichloroethylene at off-site perimeter monitoring stations ranged from 0.10 to 0.39 ppb, with an average concentration of 0.20 ppb. The levels of 1,1-dichloroethylene detected during the ambient air sampling appear to be due to contamination during the two sampling dates that included 1,1-dichloroethylene analysis. The questionable results were also reviewed and verified by the RIDEM Office of Air Resources.

Canister samples of the landfill gas were collected at a venting port in the landfill gas collection system before the gas flare section. Analysis of these landfill gas samples showed the maximum concentration of 1,1-dichloroethylene was 6.5 parts per billion (ppb) in the landfill gas prior to flaring [13]. ATSDR has derived a Minimal Risk Level (MRL) of 20 ppb for intermediate-duration inhalation exposure based on a No Observed Adverse Effect Level (NOAEL) of 5000 ppb for liver effects in guinea pigs (14).

C. Public Health Implications of Contaminants of Concern

The following sections provide public health evaluations for each of the chemicals selected as a Contaminant of Concern (COC).


ATSDR concludes that human exposure to concentrations of benzene detected in the ambient air monitoring program are not expected to result in adverse health effects. The levels of benzene reported in the Dry Bridge Road Landfill ambient air data are similar to levels detected in most low density residential areas (0.47 ppb) (15). Because the levels are similar to those detected in most residential areas and the levels are much less than those known to cause adverse health effects, long-term exposure to these levels of benzene in North Kingstown is not expected to result in increased adverse health effects.

Benzene is widely distributed in the environment. Benzene is released from manmade and natural sources. Natural sources include crude oil seeps, forest fires, and decaying vegetation. Sources from human activities include automobile exhaust, automobile refueling, cigarette smoke, and industrial emissions. The most significant source for release of benzene to the environment is from combustion of gasoline [15].

A few human studies indicate that chronic exposure to low levels of benzene (which has been determined to be 10,000 parts per billion or less) may be associated with leukemia (16). However, these studies are not conclusive and cannot rule out other exposure factors. Most studies indicate that carcinogenic effects are the result of exposures to concentrations of benzene greater than 10,000 ppb [16]. Levels of benzene in the vicinity of Dry Bridge Road Landfill are significantly lower than benzene levels potentially associated with leukemia.

Benzene levels detected at Dry Bridge Road Landfill ranged from 0.05-1.51 ppb, with an average concentration of 0.35 ppb. The maximum level (1.51 ppb) was detected at a location on the perimeter of the landfill close to heavy truck traffic along Dry Bridge and Exeter roads (17). This sampling location would affect the readings because benzene is a constituent of combusted gasoline. Although the maximum detected value of 1.51 ppb is greater than the CREG (0.031 ppb), it is lower than ATSDR's intermediate-inhalation minimal risk level (MRL) of 4 ppb [15]. The only residential benzene readings are from two off-site ambient air samples taken at a single location at which the maximum level detected was 0.23 ppb. Although the levels of benzene are above the cancer risk evaluation guideline of 0.03 ppb, the data indicate that the benzene levels detected in the monitoring program may be related to general air quality in North Kingstown.

Carbon Tetrachloride

ATSDR concludes adverse health effects are not expected from exposures to the levels of carbon tetrachloride detected in the Dry Bridge Road Landfill monitoring program. The concentrations of carbon tetrachloride detected at the monitoring stations ranged from less than 0.05 to 0.33 ppb with an average level of 0.12 ppb. These levels are similar to national ambient air concentrations.

The maximum detected value of 0.33 ppb was detected at the perimeter of the house that lies adjacent to the landfill. On the date this maximum value was detected (December 1995), the location of the sampling station at the perimeter of the house was upwind from the landfill. The source of this peak carbon tetrachloride concentration is difficult to determine. The house perimeter had an average carbon tetrachloride concentration of 0.14 ppb throughout the ambient air sampling program. At the other sampling station locations, the maximum concentration of carbon tetrachloride detected during the monitoring program was 0.15 ppb. Carbon tetrachloride concentrations of 0.07-0.15 ppb appear to be the general air quality of the North Kingstown area.

Carbon tetrachloride is ubiquitous in ambient air. The average concentration reported in the National Ambient Volatile Organic Compounds Database updated in 1988 was 0.168 ppb (18). Average values reported in four U.S. cities ranged from 0.144 to 0.291 ppb [18]. Average concentrations reported from five coastal monitoring stations around the world were 0.1 to 0.13 ppb. Similar concentrations of carbon tetrachloride in air were also reported at five hazardous waste sites and one landfill in New Jersey. Average values ranged from 0.02 to 0.12 ppb. Carbon tetrachloride is also commonly detected in indoor air. For 2,120 indoor air samples in the United States, the average concentration was 0.4 ppb. A typical level in homes in several U.S. cities was 0.16 ppb [18]. ATSDR calculated an acute inhalation MRL of 200 ppb based on a LOAEL of 50,000 ppb for liver effects in rats in an acute 4-day inhalation study [18]. Levels of carbon tetrachloride detected in the vicinity of Dry Bridge Landfill are below ATSDR's MRL.

Methylene Chloride

ATSDR concludes that adverse health effects are not expected from exposures to the levels of methylene chloride detected in the Dry Bridge Road Landfill Monitoring Program. The sampling results during December 1996 and March 1997 were not considered valid by ATSDR and the RIDEM Rhode Office of Air Resources, because there was inaccurate sampling duplication and contamination of the samples during the sampling trip and during laboratory analysis. After further review of the remaining ambient air sampling results, the concentrations of methylene chloride detected at the monitoring stations ranged from less than 0.09 to 0.59 ppb with an average level of 0.24 ppb. This average methylene chloride concentration is similar to the national background average for rural ambient air, which is 0.18-0.21 ppb (19). ATSDR calculated an acute inhalation MRL of 400 ppb based on a LOAEL of 300,000 ppb in an acute study in humans which evaluated the effects of methylene chloride on the central nervous system [19]. ATSDR has also calculated an intermediate inhalation MRL of 30 ppb based on a LOAEL of 25,000 ppb for liver effects in an intermediate-duration (100 day) inhalation study in rats [19]. The levels detected off-site at the perimeter of the landfill were well below ATSDR's MRL.

ATSDR Child Health Initiative

As part of ATSDR's Child Health Initiative, ATSDR is adding a section in all of its health based documents regarding potential exposures of children to contaminants. During the evaluation of contaminant levels in the vicinity of Dry Bridge Road Landfill, ATSDR used the Environmental Media Evaluation Guidelines (EMEG) for children who are considered the most sensitive segment of the population. ATSDR did not identify any chemical contaminants at levels of health concern.

D. Community Concerns

According to the complaint log provided by RIDEM's Office of Compliance and Inspection, the majority of the citizens' complaints about the Dry Bridge Road Landfill are about hydrogen sulfide and sulfur dioxide odors.

Hydrogen sulfide:

Due to the concerns about hydrogen sulfide levels and odors, RIDEM collected several off-site hydrogen sulfide samples during 1997. There is no public health hazard associated with the current concentrations of hydrogen sulfide; however, there is evidence that previous levels of hydrogen sulfide presented nuisance odors at this site.

Hydrogen sulfide odors are detectable at very low concentrations. An odor threshold of 0.005 ppm (5 ppb) or less has been reported. Many years ago, Rhode Island promulgated a secondary ambient air standard for hydrogen sulfide of 0.01 ppm (10 ppb), averaged over a 1-hour period (20). These standards are not health based, but are based on aesthetics, such as odor.

The ambient air in residential areas had hydrogen sulfide readings ranging from 0.001- 0.005 ppm (1-5 ppb). An ongoing indoor air monitoring program was implemented in 1996 to detect landfill gas migration to nearby residential and commercial buildings on a monthly basis. Hydrogen sulfide has not been detected in the basements of residences and commercial buildings along the Exeter and Dry Bridge roads since the indoor air monitoring program began [4]. A maximum concentration of 0.4 ppm (400 ppb) was detected once in the past (December 1995) around the landfill's immediate boundary before the flares were implemented. The hydrogen sulfide readings were measured utilizing portable gas detectors over a 2-minute period. A 24- hour sampling regiment was not available before the flare system was implemented. Since the hydrogen sulfide samples were collected as 2-minute readings, ATSDR recognizes that previous residential samples may have missed other peak concentrations of hydrogen sulfide.

An acute-duration inhalation MRL for hydrogen sulfide of 0.5 ppm (500 ppb) was derived by ATSDR (21). The MRL is based on a NOAEL value of 5 ppm (5000 ppb) hydrogen sulfide, a concentration at which no respiratory effects were noted in individuals exposed for 30 minutes during exercise [21]. In deriving the MRL, the exposure concentration of 5 ppm was divided by an uncertainty factor of 10 for human variability. An intermediate-duration inhalation MRL for hydrogen sulfide of 0.09 ppm (90 ppb) was derived. The MRL is based on a NOAEL of 8.5 ppm (8500 ppb) for respiratory effects in pigs exposed to hydrogen sulfide for 17 days [21].

Sulfur Dioxide:

When combusted in the flares, hydrogen sulfide is converted to sulfur dioxide, resulting in a burnt sulfur odor that has been reported since the flares began operating (22). Before the SulfaTreat® pretreatment system was installed in the collections system, sulfur dioxide concentrations in the ambient air ranged from 0.075 to 0.322 ppm (75-322 ppb) along Dry Bridge Road, just beyond the landfill boundary. EPA's primary 24-hour average National Ambient Air Quality Standard for sulfur dioxide is 0.139 ppm (139 ppb). After the SulfaTreat® pretreatment system was implemented in August 1997, there was a onetime peak of sulfur dioxide with a concentration of 0.030 ppm (30 ppb) at the landfill's boundary. However, sulfur dioxide concentrations predominantly measured between 0 ppm to 0.003 ppm (0-3 ppb). Based on these readings, there does not appear to be a current or future public health hazard from sulfur dioxide.


Gas monitoring wells containing the landfill gas (as part of the extraction system that feeds into the landfill flares) at Dry Bridge Road Landfill have been sampled by Hoffman Engineering and EPA and evaluated by RIDEM. Over the course of the implementation and operation of the collection and flare system in March 1997, areas of the site have been found to contain varying gas compositions. Methane concentrations in the landfill gas before it is flared have been found to range between 0% to as high as 44% in different areas at the site, depending on atmospheric conditions (23). Methane was detected in the gas monitoring wells at the perimeter of the site at concentrations exceeding 25% of the lower explosive limit (LEL). To date, methane has not been detected in the residential and commercial buildings that are being monitored for landfill gas migration [23]. Additional landfill gas collection trenches have been built on the perimeter to control the lateral migration toward residential and commercial buildings near Dry Bridge Road Landfill. These collection trenches are connected to the landfill gas extraction system to actively draw the landfill gas to the flaring system. Since the implementation of this active landfill gas collection system, methane has not exceeded the 25% LEL in the perimeter collection trenches or gas monitoring wells.

Table of Contents      Next Section The U.S. Government's Official Web PortalDepartment of Health and Human Services
Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta, GA 30341
Contact CDC: 800-232-4636 / TTY: 888-232-6348

A-Z Index

  1. A
  2. B
  3. C
  4. D
  5. E
  6. F
  7. G
  8. H
  9. I
  10. J
  11. K
  12. L
  13. M
  14. N
  15. O
  16. P
  17. Q
  18. R
  19. S
  20. T
  21. U
  22. V
  23. W
  24. X
  25. Y
  26. Z
  27. #