II. Evaluation of Environmental Contamination and Human Exposure
The following section discusses how people might come into contact with contamination and potential health effects that may result.
For exposure to occur, all elements of an exposure pathway must be present. A completed pathway consists of five elements: source, environmental media/transport, point of exposure, route of exposure, and receptor population. If one of these five elements is missing, no exposure will occur, but the potential for exposure may still exist. The ATSDR exposure evaluation process is presented in Figure 5.
Pathways are evaluated to determine whether people have been exposed to site-related contaminants in the past, are currently being exposed, or might be exposed in the future. When an exposure pathway is considered to be complete, a determination is made whether the exposure may pose a health hazard. ATSDR uses comparison values in selecting contaminants for further evaluation within an exposure pathway. These values are derived for specific environmental media (air, water, soils, etc.), and reflect the concentration for a given chemical that is not likely to cause adverse health effects from long-term exposure, given standard assumptions about body weight, ingestion, and contact rates. Since comparison values do not represent thresholds for toxicity, exposure to concentrations above comparison values will not necessarily produce adverse health effects. Comparison values used in this Public Health Assessment include USEPA’s Maximum Contaminant Levels (MCLs) and ATSDR’s Environmental Media Evaluation Guides (emEGs), Reference Dose Media Evaluation Guides (RMEGs), and Cancer Risk Evaluation Guides (CREGs). Additionally, NHDES’s and USEPA’s risk based concentration tables were used when no ATSDR comparison value was available (NHDES 1998a, USEPA 1998). MCLs are enforceable drinking water regulations developed to protect public health, but also consider economic and technological factors in setting the standard. CREGs, emEGs, and RMEGs are strictly health-based values and are not enforceable.
Chemicals disposed or released into the environment have the potential to come into contact with people, resulting in exposures that may cause adverse health effects. However, chemical releases do not always result in exposures. People can only become exposed if they come into contact with the chemical by ingestion (eating or drinking a substance containing the chemical), inhalation (breathing air containing the chemical), or by dermal absorption (skin contact with a substance containing the chemical).
Many factors are involved that determine whether an exposure will result in health effects. The type and severity of health effects that occur in an individual from contact with a chemical depend on the exposure concentration (how much), the frequency and duration of exposure (how often and how long), the route that the chemical enters the body (such as breathing, eating, and skin contact), toxic properties of the chemical, and interactions between other chemicals in the body. Once exposure occurs, many characteristics such as age, genetics, health, and nutritional status influence how the chemical behaves in the body. Together these factors affect the type, severity, and likelihood of health effects that may occur from exposure to hazardous substances.
NHDHHS evaluated available information and site conditions at Pease AFB to determine whether people could be coming into contact with chemical contaminants. If exposure pathways were completed, levels of exposure were evaluated to determine the likelihood of adverse health effects. Two completed exposure pathways were identified: (1) past consumption of contaminated groundwater; and (2) past recreational use of Peverly and Bass ponds (Table 1a). However, these pathways are categorized as no apparent public health hazard because the levels of exposure are not expected to result in adverse health effects.
(a) Hydrogeology and Groundwater Use
Groundwater typically occurs 5 to 25 feet below ground surface on Pease AFB. Water depth varies as a result of natural and human factors such as precipitation and pumping rates. Overburden (shallow) groundwater generally flows east to southeast, while bedrock (deep) predominantly moves southeast. The principal overburden aquifers on the base are the Upper Sand and Lower Sand deposits, which merge in the center of the base under the flight line to form a 40-60 foot thick section of saturated, permeable sand (USAF 1990). This aquifer is the principal base water supply. The aquifer is susceptible to water quality impacts from contamination originating on or near ground surface.
Water for Pease AFB was supplied by three major wells located on base: the Haven well, the Smith well, the Harrison well, and three smaller wells now located within an area operated by the U.S. Department of the Interior as a wildlife refuge (Figure 6). The Haven well is the primary production well with a pumping capacity of 740 gallons per minute. The Smith and Harrison wells have pumping capacities of 420 and 225 gallons per minute, respectively. Prior to 1981, the wells all fed into a common distribution system. After 1981, a treatment plant was constructed and the supply wells were piped into a common point for blending, treatment, and distribution (CDM 1994). Currently, only the Haven and Smith wells supply water to the base. Since 1996, the Smith well has also served the golf course. The Harrison well has been off-line since 1987 due to poor condition of the well casing (CDM 1996).
(b) Opportunities for Exposure to Trichloroethylene in Groundwater
(i) Nature and extent of groundwater contamination near the Haven well
In 1977, in response to complaints about fuel odors in the drinking water, water from the base wells was tested and found to contain trichloroethylene (TCE), a volatile organic solvent widely used for cleaning and degreasing operations on the base. When first discovered in the spring of 1977, the maximum concentration detected at the Haven wellhead was 391 micrograms per liter (μg/L), and 28.5 μg/L at the Harrison well (Bradley 1982; Weston 1990). No standards for TCE in drinking water existed at that time, but this exceeded the current drinking water standard of 5 μg/L. By 1978, further sampling did not detect TCE in the Harrison or Smith wells (Bradley 1982).
Samples were only collected at the wellheads, not at the taps that supplied drinking water. Since the three wells fed into a common distribution system, blending of water from the three wells likely would reduce the actual levels at the tap.
There are many uncertainties about well operations that might have affected contamination levels at the tap. Since the wells fed into the distribution system at different locations, it is feasible that water in areas of the distribution system closest to the Haven well may have contained higher concentrations of TCE than other areas of the system closer to the Smith and Harrison wells. Another area of uncertainty is the operational schedules for individual wells. Past pumping schedules are unknown, and it is not clear whether the wells pumped in combination or were cycled one at a time. In the absence of more information about the well operational schedules, it is assumed that the wells were all on line and pumping into the distribution system simultaneously.
According to the water supply engineer for the City of Portsmouth, following discovery of the contamination, the wells were shut off and clean water was supplied to the base by the City of Portsmouth during the period of 1977-1978 (Craven 1998). During that time, the U.S. Geological Survey (USGS) investigated the contamination and identified a likely source to the north of the well (Bradley 1982).
During the investigation, the Haven well was heavily pumped, thus reducing the contaminant levels as clean groundwater entered the Haven well area. In Fall 1978, the wells went back on line. At the time, the Surgeon General established a TCE concentration limit of 280 μg/L in drinking water (USAF 1990). The concentrations of TCE in the Haven well had dropped below this level, but there was still concern regarding the safety of the drinking water. In 1981, the Air Force agreed to construct a water treatment plant. The treatment plant was finished in 1984 but never went on-line due to operational problems. Since January 1986, Haven well water samples indicate that TCE levels remain consistently below the current drinking water standard of 5 μg/L (Weston 1990).
The Air Force later determined that the likely source of TCE contamination was a leaking storm sewer line that passed in the vicinity of the Haven well (Weston 1995). This line carried TCE-contaminated wastewater that discharged into the storm sewer system from floor drains in building 227. TCE leaked from joints in the storm sewer line into groundwater in the well vicinity, where it was drawn into the well as groundwater was pumped into the water distribution system. A conceptual model of the Haven well contamination is in Figure 7. Remedial actions controlled the contamination source, and during 1985, TCE concentrations had dropped below the drinking water standard. Figure 8 and Table 2 show the contamination trends from 1977 until 1993. The declining trend in TCE concentrations indicates that the contamination has been lessened by natural breakdown, cessation of contributing sources of contamination, and infiltration of clean water into the Haven well area. Also, it is thought that reduced water usage following discovery of the contamination allowed the water table to rise above the storm
sewer system. This impeded leaks of TCE contaminated wastewater from the pipes (Weston 1993). Currently, TCE concentrations are below the drinking water standard and remain stable at low levels. The pumping rate of the Haven well is limited to 300 gallons per minute so that clean-up operations in nearby Zone 3 are not affected (CDM 1994).
Groundwater contaminant plumes exceeding drinking water standards are located in several areas around the base (Figure 9). No exposure is occurring to these contaminant plumes, and the sources and plumes are under remediation, institutional controls, and long-term monitoring.
Although a plume of contaminated groundwater from site 8 has moved off-base into Newington, the plume underlies the Newington Town Forest, and no drinking wells are located in the area. An off-base well inventory indicated that no drinking water wells were located in areas of groundwater contamination (Weston 1992).
(ii) Current Exposure
Currently, plumes of groundwater contamination at Pease AFB are not impacting any drinking water wells. All public drinking water on base meets state and federal regulations and is routinely tested according to Safe Drinking Water Act requirements.
(iii) Past Exposure
It is not known when exposure to TCE in the Haven well began. No well sampling data were available prior to 1977. Upon discovery of the contamination in 1977, the supply wells were shut off and the City of Portsmouth provided water to the base. This action stopped exposure to TCE at concentrations as high as 391 μg/L. When the Haven well was placed back on line in the fall of 1978, the TCE levels had dropped to below 115 μg/L (Figure 8). After 1985, levels of TCE in the Haven well had dropped below the drinking water standard of 5 μg/L. Therefore, exposures to TCE above current drinking water standards were possible from 1978 until 1985. Table 2 and Figure 8 show the trend of TCE contamination in the Haven well over time.
Since there are no data on TCE concentrations in the base water supply before 1977, exposures to TCE earlier than this date are unknown. To account for this uncertainty, NHDHHS used very conservative assumptions about TCE concentrations and duration in its evaluation of past exposures to TCE in the base water supply:
- Base residents were assumed to have been exposed to TCE at 122 μg/L between 1978 and 1985, even though the average concentration of TCE in the Haven well from 1977 through 1985 was 58 μg/L. This average includes data from a period in 1977-1978 when base residents were being supplied water from the City of Portsmouth and the TCE concentrations in the Haven well were near their maximum.
- The actual concentrations of TCE in water consumed by base residents was likely to be lower than the well head concentrations used in the exposure assessment because water from Haven well was diluted with water from the Smith and Harrison wells before being distributed to residential taps.
- In the 1970s and 1980s, the population on Pease Air Force Base was primarily composed of military personnel and their dependents who were stationed on base. NHDHHS assumed that base residents lived there for nine years (the median time in one residence for U.S. citizens from USEPA 1997), which is longer than the duration of time that exposures to TCE above the current drinking water standard were known to be possible (1978-1985).
While there are uncertainties about exposures before 1977, NHDHHS chose to be protective of public health by making conservative assumptions about the ways people may have been exposed in the past that likely overestimated actual exposures to TCE from using base drinking water.
NHDHHS compared the estimated exposure levels with available health guidelines, comparison values and information from the scientific literature regarding the health effects from exposure to TCE to assess the likelihood of adverse health effects.
NHDHHS methodology is consistent with the approach used by other public health agencies in its estimation of exposures to hazardous substances. To be more concise for the general public, a detailed explanation of the assumptions and calculations used to estimate exposures and determine the likelihood of adverse health effects is not presented in this document, but is available upon request.
(iv) Public Health Implications from Past Exposure
Based upon an evaluation of exposures to TCE and information about the toxicity of this chemical from the scientific literature, adverse health effects to adults, children, and unborn infants are unlikely. Our evaluation of exposure levels in comparison to health guidelines and information on the toxicity of TCE, showed that no adverse health effects are likely in adult and child residents and adult workers from exposure to TCE-contaminated groundwater. For children, exposure levels slightly exceeded a provisional oral reference dose (RfD) for TCE. The RfD is an estimate of the daily exposure to a substance that is likely to be without risk of adverse non-cancer health effects for a lifetime. Exposure levels for TCE in Haven well water above the RfD will not necessarily produce adverse health effects. The RfD does not represent a threshold for toxicity, but rather establishes a dose that, if exceeded, increases the possibility that adverse health effects may occur as exposure levels and duration increase. Furthermore, based on review of the toxicological literature for TCE, the levels present in the Haven well have not been shown to cause adverse health effects.
Additional information on the toxicity of TCE that was considered in our evaluation is presented in the following three subsections: (1) cancer effects from exposure; (2) systemic (non-cancer) effects from exposure; and (3) child health considerations.
Liver and lung tumors were seen in rats and mice following high doses of TCE administered in experimental studies. The significance to humans of the results seen in animal studies is unclear, as the mechanisms of toxicity may differ between rodents and humans (ATSDR 1997). The exposure doses of TCE causing cancer in animal studies were 4 million times higher than the estimated doses from consuming Haven well water.
The link between exposure to TCE and cancer in humans is controversial, and insufficient evidence exists to define TCE as a human carcinogen. Studies of human populations exposed to TCE in well water are contradictory. A study in New Jersey showed an association between TCE exposure and development of leukemia and non-Hodgkin’s lymphoma, but a Finnish study demonstrated no association (ATSDR 1997). Both studies had several limitations, including simultaneous exposures to other chemicals and difficulties in classifying exposure levels.
In 1997, the Massachusetts Department of Public Health (MDPH) completed an epidemiologic study of childhood leukemia in Woburn, Massachusetts (MDPH 1997). In this study, MDPH observed an association between exposure to water drawn from Woburn’s water supply wells G&H and the development of childhood leukemia. This association was strongest for exposures to the water that occurred in utero.
The water from wells G&H in Woburn was tested once for toxic substances before the wells were shut down. Contaminants detected in this sample were: trichloroethylene (TCE), tetrachloroethylene, chloroform, methyl chloroform, trichlorotrifluoroethane, 1,2-dichloroethylene, and arsenic. Other chemicals (i.e., trans-1-dichloroethyene, lead, chlordane, 1,1,1-trichloroethylene, and vinyl chloride) were also detected in the groundwater on properties presumed responsible for contaminating the wells G&H water. Since the wells were contaminated by multiple chemicals whose relative concentrations over time were unknown, MDPH could not conclude that exposure to TCE or any other specific chemical in particular was the cause of the elevated childhood leukemia incidence. Exposures to one, some, or all of the chemicals in the wells G&H water could have played a role.
As part of this Public Health Assessment for Pease AFB, NHDHHS reviewed available information on cancer incidence for the surrounding towns of Newington, Portsmouth, and New Castle (see Appendix D for details). Cancer incidence within Portsmouth and New Castle for the period between 1987 and 1991 (all the data that were available) were not statistically different from the number of cases seen in the State of New Hampshire as a whole during this same period. However, there were two cancer types that were elevated in Portsmouth: non-Hodgkin’s lymphoma among males and cervical cancer among females. Environmental exposures are not thought to be the primary risk factors for the development of these cancer types. There were very few cases observed in Newington during this period which prevented further analysis.
Much of what is known about non-cancer effects from TCE exposure comes from animal studies and studies in humans who breathed or drank high levels of TCE. Dizziness, headache and a feeling of facial numbness have occurred in workers breathing TCE or people who have used TCE in unventilated areas. In the past, TCE was used as an anesthetic because of its effects on the central nervous system. More severe effects on the central nervous system, such as unconsciousness and death, were found to occur at high levels of exposure (ATSDR 1997).
Some health effects may occur from long-term exposure to TCE. This information is primarily from animal studies, which have shown that exposure to TCE can produce liver and kidney damage; effects on the blood; and tumors of the liver, kidney, and possibly tissues responsible for forming white blood cells (leukemia). Drinking alcohol can make people more susceptible to liver and kidney injury from exposure to TCE (ATSDR 1997). The lowest dose of TCE at which no adverse systemic health effects were seen in long-term animal studies was at least 5,000 times greater than the estimated doses from consumption of contaminated water from the Haven well (Maltoni 1986). Therefore, adverse health effects from exposures to TCE in the drinking water at Pease AFB are unlikely.
Infants, older children, and developing fetuses require special attention when evaluating the likelihood of adverse health effects from exposure to hazardous substances. These populations differ in their sensitivity and response to chemical exposures in relation to adults, and are often the most sensitive populations of concern for chemical injury.
This is due to many reasons, including behaviors that can lead to increased contact with substances containing chemical contaminants, smaller body weights that result in increased exposure levels, sensitivity of developing organ systems, and differences in the way children’s bodies respond to chemical exposures, such as how the body breaks down and eliminates chemicals (ATSDR 1998a).
The developing fetus can experience adverse health and developmental effects at levels below those of concern for health effects in the mother. The mother’s body protects the developing fetus to some extent, but chemical injuries during critical periods in fetal development can increase the risk of birth defects, low birth weight, or miscarriage (Guzelian 1992).
Trichloroethylene rapidly crosses the placenta, with subsequent exposure to the fetus. In three out of ten pregnancies, concentrations of TCE in umbilical venous blood (indicative of fetal blood concentrations) exceeded concentrations in maternal blood after 10-15 minutes of exposure to TCE (Trilene ®) and nitrous oxide anesthesia (Laham 1970).
Important reproductive and developmental effects due to TCE exposure have not been clearly identified in humans. A few human studies have demonstrated an association with TCE exposure and developmental effects (Bove 1995, Goldberg 1990, Lagakos 1986, Schendel 1996, Sonnenfeld 1997), but methodological problems and other limitations in the studies make it difficult to clarify a causal link between exposure to TCE and adverse reproductive and birth outcomes.
Animal studies indicate that TCE can act as a developmental toxicant, though often at doses that cause maternal toxicity as well. In a study involving pregnant rats exposed to TCE in drinking water, the lowest maternal exposure dose at which developmental toxicity was seen was more than 42 times higher than the estimated exposure levels to mothers consuming TCE-contaminated water from the Haven well (Dawson 1993).
(c) Opportunities for Exposure to Nitrate in Groundwater
(i) Nature and Extent of Contamination
From 1994 through early 1996, nitrate levels in the Haven and Smith wells were near or exceeded the drinking water standard of 10 milligrams/liter (mg/L) (as nitrogen in nitrate), reaching peak concentrations of 11.4 mg/L in water from the Smith well. Monitoring data from prior to 1994 showed that the nitrate concentrations had increased from low levels (less than 1 mg/L) in 1990 to near 10 mg/L in 1994 (CDM 1994, CDM 1996). The likely source of the nitrate was the use of urea-based deicing agents on the runway, the application of which presumably increased during this time.
In 1995, the use of urea-based deicing agents was discontinued, and a groundwater monitoring program began (CDM 1996). A water management strategy was also adopted whereby water from the Haven and Smith wells were mixed, in proportions determined by their nitrate levels, to ensure that nitrate concentrations in the base water supply stayed below the drinking water standard (Hilton 1999). Nitrate levels in the base drinking water are currently monitored on a continuous basis via an in-line nitrate analyzer and meet all state and federal standards for drinking water. Also, the Pease Development Authority is participating in the City of Portsmouth’s Wellhead Protection Program to protect the aquifer beneath the former base.
(ii) Current Exposure
Actions taken to reduce nitrate levels have been effective, and drinking water meets current state and federal drinking water standards for nitrate contamination.
(iii) Public Health Implications from Past Exposure
It is unlikely that adverse health effects would have resulted from consumption of Haven well water contaminated with nitrates slightly exceeding the drinking water standard.
The concern for ingestion of nitrate-contaminated water is primarily due to effects on infants younger than 4 months of age who are fed formula diluted with water containing excess nitrate contamination. Nitrate consumption can result in methemoglobinemia (“blue baby syndrome”), which is a disease caused by nitrate interference with the oxygen-carrying capacity of the red blood cells. There is little evidence that breast-fed infants develop methemoglobinemia from exposure to nitrates ingested by nursing mothers (ATSDR 1991). Consuming water containing nitrates above the drinking water standard does not imply that methemoglobinemia will result, rather it indicates that with increasing concentrations the possibility of adverse health effects may also increase.
Consumption of nitrate-contaminated water has resulted in spontaneous abortion in laboratory animals and livestock (Kross 1992, Sund 1957). A report in the July 1996 issue of the Centers for Disease Control and Prevention’s “Morbidity and Mortality Weekly Report” cited a case in 1993, in which the LaGrange County Health Department in Indiana identified three women who reported a total of six spontaneous abortions between 1991 and 1993 and lived near each other. All of these women obtained water from nitrate-contaminated wells (Grant 1996). The nitrate levels identified in this report were on the order of 19-26 mg/L, higher than levels found in the base water supply in 1994 (approximately 10 mg/L at maximum levels).
In human populations, spontaneous abortions occur commonly and are directly associated with increasing maternal age (Grant 1996). Cases of spontaneous abortion may cluster by chance. The LaGrange County investigation did not establish a causal link, but demonstrated a possible association between elevated nitrate levels in water and spontaneous abortion in humans.
Therefore, based on comparisons with the LaGrange County study, it is unlikely that the nitrate levels in the Haven well were high enough to pose a risk to the developing fetus of a pregnant woman consuming the water on Pease AFB. This determination is based on the fact that the nitrate levels in the Haven well were much lower than those found in the LaGrange County wells. Furthermore, the LaGrange County wells were the primary source of drinking water for the women in the study, while Haven well water would likely have been consumed in lesser amounts during a normal work day by workers and visitors on the base.
Peverly and Bass Ponds have been used in the past for recreational activities. When people swim or wade, they may come into contact with contaminants in water and sediments. Contamination can build up in the bodies of fish to levels that can pose a health hazard to the people and animals that eat them.
(a) Nature and Extent of Contamination
Peverly and Bass Ponds were studied extensively during the Zone 2 remedial investigation. Past use of pesticides for mosquito control has resulted in detectable levels of the pesticides lindane and DDT. Products of natural DDT breakdown in the environment, DDD and DDE, were also detected. An organic form of the metal mercury, called methyl mercury, was detected in fish tissues. Other contaminants, such as PCBs, were detected in fish, sediments, and surface waters that likely were introduced into the ponds as a result of drainage via nearby Peverly Brook, which collects surface water runoff from Zone 2 IRP sites and a section of the flight line parking apron.
The origin of the contaminants is not clear, but is likely a result of past activities on the base with the exception of mercury, which is a contaminant of concern in freshwater fish throughout New Hampshire. Sediments, fish, and surface waters in Peverly and Bass ponds are under a long-term sampling program to monitor contamination levels (Bechtel 1997a). Fish tissue concentrations detected in 1996 are presented in Table 3. Contaminants in sediments and surface water that were evaluated for human health risks are presented in Tables 4 and 5.
(b) Current Exposure
No exposure is currently occurring because the ponds are within the Great Bay National Wildlife Refuge under the U.S. Fish and Wildlife Service and are off limits to swimming and fishing.
(c) Public Health Implications from Past Exposure
Based upon exposure estimates and information about the toxicity of PCBs and mercury, exposure to contaminants in surface waters, sediments, and fish of Peverly and Bass Ponds is unlikely to result in adverse health effects.
In order to reach this determination, NHDHHS developed realistic, but conservative, assumptions about exposures for recreational users (e.g., monthly-to-bimonthly visits to the ponds to swim and wade during the warm half of the year). Exposure levels were then evaluated in light of available toxicological information for the compounds of concern. Special consideration was made for the unique vulnerabilities of small children to chemical exposures as described below.
Child Health Considerations
As stated previously in this document, children are usually more sensitive to chemical exposures than adults, and health risk evaluations involving children and developing fetuses require special emphasis due to this unique vulnerability. In animal studies, exposures to high levels of PCBs and mercury have been shown to result in adverse health effects in developing fetuses. Studies in humans have also demonstrated an association between exposure and effects in children and developing fetuses (ATSDR 1998b, ATSDR 1999). The exposure levels at which effects have been seen are higher than the exposure levels estimated from use of the ponds.
Currently in New Hampshire, a fish consumption advisory is in effect for finfish caught in all inland fresh water bodies. This is based on mercury levels that have been detected in the tissues of freshwater fish throughout the state. The advisory is designed to prevent adverse health effects from long-term exposure. The advice from NHDHHS is that adults should limit consumption to four 8-ounce meals per month. Pregnant women and children should limit consumption to one 8-ounce meal per month. It is good practice to skin and remove fatty tissues from the fish before cooking and to discard cooking juices and drippings.
The 49 IRP sites, underground storage tank sites and the flight line did not, and currently do not, present public health hazards because: (1) access to site contaminants is restricted or limited (thereby limiting exposure); and/or (2) migration of contaminants to areas where exposure might occur is not expected; and/or (3) contamination has been cleaned up. Ongoing remediation and long-term sampling is designed to prevent any future exposures. A summary of the IRP sites and an evaluation of health hazards is in Appendix C.
After reviewing available environmental data, NHDHHS determined that some exposure pathways are not currently complete, but may be completed in the future (Table 1b). Therefore, these exposure situations do not pose a public health hazard at present, but should be monitored to guard against potential future hazards.
Because groundwater in some areas beneath Pease AFB is heavily-contaminated with volatile organic compounds (Pease 1999), there is the potential for indoor air contamination in some nearby buildings. The only data that are relevant to this issue are the results of a soil gas survey at site 49, where contaminated groundwater underlies an occupied building (TN&A 1998). These results do not indicate appreciable migration of contaminants from the groundwater to the soil gas at this location. First, the compounds detected in the soil gas (e.g., chloroform, tetrachloroethylene, benzene) did not match the compounds in the underlying groundwater plume (i.e., primarily TCE) (TN&A 1998, Bechtel 1997b). Second, the concentrations of chemicals in the soil gas were within either reported background ranges for indoor air or health-based screening levels for ambient air. Typically, concentrations of chemicals in the soil gas are tens to hundreds of times higher than corresponding indoor air concentrations (Fitzpatrick and Fitzgerald 1996). Therefore, based on the limited available data on soil gas, there is no evidence for current indoor air quality problems on the base.
The soil gas results from site 49 provide useful information but should be interpreted with caution. First, the analytical results have not been validated. Second, the potential exists for indoor air exposures at other locations in the future as the former base continues to be redeveloped. Developers should be cognizant of the potential for indoor air contamination when choosing sites for new buildings. A comprehensive review of groundwater data to identify potential problem locations in advance would likely prove a helpful and proactive measure. To assist in evaluating the potential for indoor air quality impacts from contaminated groundwater, guidance for evaluating potential indoor air quality issues has been developed by NHDES (NHDES 1998a, 1998b).
If new data for soil gas or indoor air quality are generated, NHDHHS remains willing to provide technical assistance on a case-by-case basis.