Volatile Organic Compounds in the Nation's Ground Water and Drinking-Water Supply Wells: Supporting Information
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The THM chloroform was the most frequently detected VOC in aquifers (fig. 8), in 7 percent of all samples at an assessment level of 0.2 µg/L. The other three THMs—bromodichloromethane, dibromochloromethane, and bromoform—were each detected in about 1 percent of aquifer samples. Concentrations of chloroform and other THMs generally were less than 1 µg/L. Of samples with detections, the median concentration was 0.08 µg/L for chloroform, and the median total trihalomethane (TTHM) concentration was 0.09 µg/L (Appendix 6).
Chloroform was the most frequently detected VOC in the Nation’s aquifers; however, concentrations of chloroform generally were less than 1 µg/L.
Chloroform has a long history of use, research, and regulation (sidebar 25). In 1999, 98 percent of chloroform produced by industry (fig. 20) was used in the production of the refrigerant hydrochlorofluorocarbon-22 (HCFC-22) to replace dichlorodifluoromethane (CFC-12) for home air conditioners and commercial freezers. The remaining 2 percent of chloroform was used for other purposes, including reagents and extraction solvents,(59, 60) fumigants, insecticides, and a precursor for dyes and pesticides.(61, 62) In 1998, the most recent year with production data, about 46 million gallons of chloroform were produced by industry.
In contrast to chloroform, the production of the other three THMs has been relatively small; for example, only about 46,000 gallons of bromoform were produced in 1977, the most recent year for which production data are available.(63) Furthermore, there is no information on the U.S. production of bromodichloromethane and chlorodibromomethane, presumedly because of the limited or non-existent use of these compounds. Examples of limited current or prior use(s) of these three THMs are as solvents for fats, waxes and resins, heavy liquid for geological assays, fire-extinguisher agents, and as an intermediate in chemical synthesis.
Chloroform and other THMs are commonly produced during the chlorination of water and wastewater (sidebar 26). Chlorination of water and wastewater produces an estimated 0.37 million gallons of chloroform in the United States annually,(64) which includes the treatment of drinking water, wastewater from municipalities and the bleaching process of pulp and paper, and rinse water from domestic and commercial cleaning and laundry operations. In addition, some THMs occur naturally in the environment (sidebar 27).
Figure 20.Industrial production of chloroform continues to increase in the United States.
In studies of the formation of multiple THMs during water chlorination, it has been shown that the relative concentrations (and their relative detection frequencies) usually decrease with increasing bromine content (chloroform > bromodichloromethane > dibromochloromethane > bromoform).(65,66) This occurrence pattern also was noted in the NAWQA-collected data,(11, 13, 36, 67, 68, 69, 70) and indicates that chlorinated water is an important source of chloroform and other THMs to ground water.(71) Sources of chlorinated water to ground water may include irrigation of lawns, gardens, golf courses, and parks; leaking drinking-water distribution and sewer pipes; artificial recharge of wastewater; regulated discharges of cooling process blowdown water from electric power-generating plants; combined sewer overflows;(62, 72) and unintended backflow of chlorinated water to supply wells.
Trihalomethanes (THMs) are produced during water and wastewater treatment when chlorine is added for disinfection.
Chloroform has been detected in urban air(73) and in rainwater.(74) In studies of chloroform occurrence in shallow ground water in Indiana(75) and New Jersey,(76) however, the concentrations of chloroform in the atmosphere and rainwater were not high enough to account for all the mass of chloro-form measured in shallow ground water. The sources of the chloroform frequently detected in ground water in the New Jersey study were attributed to use of chlorinated drinking water and the associated recharge from septic tanks or from the irrigation of crops and lawns.(76)
Chloroform has been detected in ground water beneath a variety of land-use settings.(11, 68) In NAWQA’s assessments, THMs were detected more frequently beneath urban residential areas than beneath agricultural areas (fig. 21). Furthermore, the percentage of urban land, the number of RCRA hazardous-waste facilities, and septic system density were shown to be statistically associated with chloroform in aquifers, whereas sewer systems were associated with the occurrence of bromodichloromethane (table 4).
Figure 21. Detection frequencies of chloroform in shallow ground water were larger beneath urban areas than beneath agricultural areas.
Chloroform was the most frequently detected VOC in samples from domestic and public wells. Detection frequencies of chloroform were about 5 percent in domestic well samples and about 11 percent in public well samples at an assessment level of 0.2 µg/L (fig. 22). As previously mentioned, this difference may be a consequence of the higher pumping rates of public wells and their proximity to developed areas, compared to domestic wells (p. 40 and 41). A pattern of detection frequencies (chloroform > bromodichloromethane > dibromochloromethane > bromoform) is apparent for domestic well samples (fig. 22). This pattern indicates that chlorinated water is an important source of THMs in domestic well water. The frequent occurrence of brominated THMs and THM mixtures in public well samples supports the importance of chlorinated water as a source of THMs to these wells.
Chlorinated water is an important source of chloroform to ground water.
Although chloroform was detected frequently, most TTHM concentrations were two orders of magnitude lower than the USEPA MCL of 80 µg/L. No TTHM concentrations in samples from domestic or public wells were greater than the MCL (Appendixes 9 and 11). However, each of the four THMs had concentrations less than but within a factor of 10 of the TTHM MCL.
Figure 22. Detection frequencies of trihalomethanes (THMs), especially the brominated species, were much greater in public well samples than in domestic well samples.
THMs have been detected in septic tank effluent,(85, 86) and in improperly designed, maintained, or operated septic systems, resulting in shallow ground-water contamination in the vicinity of the septic system.(82) Water from a domestic well may be contaminated if the well is drawing contaminated water from the vicinity of an improperly operating septic system. Other potential sources of chloroform and other THMs to domestic wells are laundry wastewater containing bleach and the well disinfection practice of shock chlorination. In both cases, chloroform and other THMs are produced through the haloform reaction of the chlorine in bleach with organic matter.
Detection frequencies of THMs were larger in public well samples than in domestic well samples; however, no concentrations were greater than the USEPA drinking-water standard.
Chloroform and other THMs are persistent under oxic conditions and have relatively low tendencies to sorb to soil and aquifer organic carbon. Because of these properties, THMs are expected to persist and may migrate substantial distances through the subsurface, especially in aquifers with detectable concentrations of dissolved oxygen and low amounts of organic carbon.(71, 87) These properties are important considerations in the western United States where drinking-water sources are sparse, and the injection of chlorinated wastewater into aquifers for future supply and to reduce land subsidence may become a more common practice (sidebar 29).
Monitoring chloroform and other THMs in samples from domestic and public wells is important because these compounds likely are indicators of sources of contamination. It also is important to evaluate trends in the occurrence of these compounds and to better understand the origin and human-health relevance of the low concentrations in water supplies.
29.THMs Persist in an Aquifer in the Western United States Chlorinated surface water was injected into an oxic, unconsolidated sand and gravel aquifer in Antelope Valley, California, as part of a program to assess the long-term feasibility of using injection, storage, and recovery as a water-supply method and to reduce water-level declines and land subsidence in Antelope Valley.(88) In the sand and gravel aquifer, THM formation continued after the treated water was injected. Once all the residual chlorine reacted, the concentrations of THMs were controlled primarily by mixing and dilution with native ground water. Potential for natural THM attenuation in the aquifer by biodegradation and sorption was low because the aquifer has oxic conditions and low organic matter content. A model used to forecast the effects of repeated cycles of injection, storage, and recovery indicated that the cycles increased concentrations of THMs in the aquifer. These repeated cycles could yield aquifer THM concentrations approaching 100 percent of the injection-water THM concentration within 10 annual cycles, provided mixing within the aquifer does not lower concentrations markedly.
The solvent group is one of the two most frequently detected VOC groups in the Nation’s aquifers (the other is the THM group; fig. 5). Seven solvents were among the 15 most frequently detected VOCs (fig. 8). PCE was the second most frequently detected VOC, and TCE, TCA, and methylene chloride were the 4th, 7th, and 14th most frequently detected VOCs, respectively.
Seven of the 15 most frequently detected VOCs in aquifers were solvents.
Chlorinated solvents are organic compounds that are used in a variety of industrial, commercial, and domestic applications (sidebar 30). Four of the most commonly used solvents are given focus here—methylene chloride, PCE, TCA, and TCE. Although relatively large quantities of these solvents are still used today, their production began declining as early as the 1970s (fig. 23) in response to human-health and environmental concerns.(3) Specifically, all four solvents have known human-health concerns, and methylene chloride and TCE are classified as probable human carcinogens (sidebar 31).
One or more of the four commonly used chlorinated solvents given focus here were detected in about 6 percent of aquifer samples at an assessment level of 0.2 µg/L. Among the four solvents, detection frequencies were about 4 percent for PCE, 3 percent for TCE, 2 percent for TCA, and 1 percent for methylene chloride. The detection frequencies of these solvents, except for methylene chloride, were higher in public well samples than in domestic well samples (fig. 24). Solvents also were detected frequently in drinking water supplied by CWSs (sidebar 32).
Figure 23. Production of most solvents in the United States began declining as early as the 1970s.(3, 89, 90)
Figure 24. The detection frequencies of most solvents were higher in public well samples than in domestic well samples.
The median concentrations of detections of all four solvents in aquifer samples were less than 0.2 µg/L (fig. 25). TCE had the highest median concentration of detections, followed by PCE, TCA, and methylene chloride. If all laboratory analyses (including non-detections) are considered, the ranking of solvents by concentration is the same as the ranking by detection frequency.
In samples from domestic and public wells, solvents were among the VOCs that most frequently had concentrations greater than USEPA MCLs.
In samples from domestic and public wells, the concentrations of 8 VOCs were greater than their MCL, and 3 of these were the solvents PCE, TCE, and methylene chloride. In domestic well samples, TCE and PCE were ranked second and third among VOCs with concentrations that were greater than their MCLs. In public well samples, PCE and TCE were both ranked first, and methylene chloride was ranked third among VOCs with concentrations that were greater than their MCLs. In addition, PCE and TCE had, relative to other VOCs, a large number of concentrations less than but within a factor of 10 of their MCLs in samples from public and domestic wells (Appendixes 9 and 11).
Figure 25.Concentrations of solvents in aquifers typically were less than 1 microgram per liter.
The four solvents occurred together in any mixture in 5 percent of aquifer samples at an assessment level of 0.2 µg/L. The most frequently occurring two-solvent mixture, PCE–TCE, probably represents the degradation of PCE to TCE, which occurs through a process called “reductive dechlorination.”(94) In reductive dechlorination, a chlorine atom in PCE is replaced by a hydrogen atom producing TCE and is mediated by microbes (fig. 26). Several lines of evidence indicate that at least some PCE is being degraded in aquifers with anoxic conditions (sidebar 33).
PCE degrades to TCE in aquifers with anoxic conditions.
A statistical model of solvents in aquifers and shallow ground water indicated that the occurrence of one or more of the four solvents increased with dissolved-oxygen concentrations (table 7). The probability of occurrence of the four solvents also increased with increasing percentage of urban land use, population density, number of RCRA hazardous-waste facilities, and septic system density. Conversely, the probability of occurrence of the four solvents decreased with increasing sand content of soil and depths to the top of the screen/open interval in the well.
The sources of solvents to ground water are numerous and include releases associated with the production, transport, or use of these chemicals. Urban land-use practices, regulated hazardous-waste facilities, and dry-cleaning businesses are likely sources of solvents. Conditions such as recharge, vertical soil permeability, and dissolved oxygen control the transport and fate of the solvents. Although solvents occur in air in urban areas, concentrations are not large enough for atmospheric deposition to account for all the mass of solvents detected in ground water.(76)
Figure 26. Perchloroethene (PCE) degrades to trichloroethene (TCE) under anoxic conditions through reductive dechlorination.
|Table 7. Explanatory factors associated with the occurrence in ground water of one or more of the four solvents-methylene chloride, perchloroethene (PCE), 1,1,1- trichloroethane (TCA), and trichloroethene (TCE).|
|[RCRA, Resource Conservation and Recovery Act]|
Positive (+) or negative (-) association
|Sand content of soil||1||-|
|Percentage of urban land use||2||+|
|Depth to screen/open interval||5||-|
|Number of RCRA facilities within 1 kilometer||6||+|
|Septic system density within 1 kilometer||7||+|
In some cases, concentrations of PCE and TCE increase with depth in an aquifer.
Because of their physical and chemical properties, different solvents move through and react in ground water in unique ways (sidebar 33). In some cases, the concentrations of PCE and TCE have been shown to increase with increasing depth of the water in the aquifer.(95) Because pure solvents are denser than water, they can penetrate the water table and collect in pools on top of less permeable layers in an aquifer. In this way, they become a source of contamination at the bottom of the aquifer. Through dissolution, diffusion, and dispersion, the solvent mass can then move upward in an aquifer and produce a concentration gradient that increases with depth (fig. 27).
Figure 27. The concentration of a chlorinated solvent may increase with depth in an aquifer.
MTBE, a gasoline oxygenate, was the second most frequently detected VOC in samples from domestic and public wells at an assessment level of 0.2 µg/L. The detection frequency of MTBE was about 3 percent in domestic well samples and about 5 percent in public well samples (fig. 28). Public wells generally have larger pumping rates than domestic wells, draw water from larger areas, and thus are more likely to be affected by releases of MTBE. Because of the large overlap in samples between studies of aquifers and domestic wells, the detection frequency of MTBE in aquifers is the same as for domestic well samples.
The NAWQA Program also analyzed samples for three other ether gasoline oxygenates—tert-amyl methyl ether (TAME), diisopropyl ether (DIPE), and ethyl tert-butyl ether (ETBE). These VOCs were detected infrequently in samples from domestic and public wells (fig. 28).
MTBE was the second most frequently detected VOC in samples from domestic and public wells despite its relatively short production and use history.
Gasoline oxygenates are compounds that contain oxygen and are added to gasoline to improve combustion and reduce harmful motor vehicle emissions. Since the 1990s, oxygenates have been used in gasoline in areas where certain air-quality standards have not been attained. In 2004, MTBE was the most commonly used gasoline oxygenate (sidebar 34); however, MTBE has been completely or partially banned in some States (sidebar 35). The Energy Policy Act of 2005 eliminated the oxygen requirement in gasoline.(98)
Figure 28. Compared to methyl tert-butyl ether, other gasoline oxygenates were detected infrequently in samples from domestic and public wells.
Potential point sources of MTBE include leaking storage tanks and associated piping, tank overflow spills, leaks from transport pipelines or bulk gasoline stations, and automobile and truck accidents. Potential nonpoint sources of MTBE include evaporative losses from tanks or pipelines, incomplete combustion in automobile engines, urban stormwater runoff, exhaust from motorized watercraft, and leaks from watercraft tanks. MTBE has been detected in used motor oil, which also could be a source of MTBE in ground water.(105) In some situations, the atmosphere can be a source of MTBE to ground water. This source of MTBE to shallow ground water was studied in a network of sites in southern New Jersey. At some sites, concentrations of MTBE in ambient air and in shallow ground water indicated that the atmosphere was a possible source of MTBE in ground water.(76) At other sites, the concentrations of MTBE in ground water indicated that other sources, such as leaking tanks or road runoff, were the dominant source of MTBE.(106) In California, artificial recharge in some areas may be a source of MTBE to aquifers (sidebar 36).
Production of MTBE has increased markedly since 1985, with large volumes of MTBE produced in the United States since about 1990 (fig. 29). The locations of ground-water samples analyzed for MTBE, as collected or compiled by the NAWQA Program, and samples where MTBE was detected are shown in figure 30. The locations include samples from studies of aquifers, domestic wells, and public wells. As is evident in this figure, many of the detections of MTBE were in the New England and Mid-Atlantic States.
MTBE has been detected in drinking water of some CWSs (sidebar 37). The current concern with MTBE in drinking water is primarily taste and odor because the human-health effects of MTBE have not yet been clearly established. Because of its chemical properties, MTBE can be smelled and tasted at low concentrations by some people. In 1997, the USEPA issued a Drinking-Water Advisory for MTBE.(107) The advisory indicates that keeping MTBE concentrations in drinking water below the range of 20 to 40 µg/L will likely avert unpleasant taste and odor effects and also be protective of human health. The USEPA currently (2005) is conducting a risk assessment on the health effects of MTBE.
Figure 29.The annual production rate of methyl tert-butyl ether (MTBE) in the United States generally has increased until 1999, but has since decreased.(89, 108, 109, 110)
Figure 30.Most detections of methyl tert-butyl ether (MTBE) are in the highly populated New England and Mid-Atlantic States.(15, 113)
Concentrations of MTBE generally were less than the Drinking-Water Advisory, with a median for samples with detections of 0.6 µg/L for samples from both domestic and public wells using an assessment level of 0.2 µg/L (fig. 31). Only one sample from a domestic well had an MTBE concentration that was greater than the lower limit of the Drinking-Water Advisory of 20 µg/L. No samples from public wells had MTBE concentrations greater than 20 µg/L.
Only one sample from a domestic well and no samples from public wells had MTBE concentrations greater than the USEPA Drinking-Water Advisory.
In the NAWQA-collected data, the single concentration of MTBE greater than 20 µg/L likely originated from some type of point source.(113) Concentrations lower than this level could originate from either point or nonpoint sources. In general, MTBE did not occur frequently with other gasoline components in ground water except when detected at high concentrations.(113) The detection of MTBE without other common gasoline hydrocarbons likely is the result of MTBE’s higher solubility, lower sorption, and greater persistence in ground water relative to common gasoline hydrocarbons.
In statistical analyses reported previously,(113) the probability of detecting MTBE in ground water was greater in areas with high population density, in areas where MTBE is used as an oxygenate in gasoline, and in areas with high ground-water recharge, compared to other areas of the Nation. Public well water and shallow ground water underlying urban land-use areas had a greater probability of containing detectable levels of MTBE than domestic well water and ground water underlying rural land-use areas.(113)
When MTBE use in gasoline was applied as a surrogate for MTBE release to the environment, the detection frequency of MTBE was significantly larger in areas of high use than in areas of low use.(113) These results indicate that the use of MTBE in large amounts in gasoline in some areas of the Nation has resulted in increased detection frequencies of MTBE in ground water.(113) MTBE was detected in some low MTBE-use areas presumedly as a result of its small volume in gasoline for octane enhancement.
Public well water and shallow ground water underlying urban land-use areas had the highest probability of MTBE contamination.
The quick arrival of MTBE in ground water relative to its production history (fig. 29) indicates that MTBE is an important concern with respect to ground-water management. Aquifers that may be vulnerable to MTBE contamination warrant evaluation with consideration given to the development of strategies to reduce the likelihood of contamination. Continued ground-water monitoring is warranted to evaluate trends in the occurrence of MTBE and to better understand the sources and transport of MTBE to ground water and to domestic and public wells.
Figure 31. Most concentrations of methyl tert-butyl ether (MTBE) in samples from domestic and public wells are much less than the U.S. Environmental Protection Agency’s Drinking-Water Advisory level.
The gasoline hydrocarbon group is defined here as aromatic hydrocarbons or alkyl benzenes whose predominant use is in motor vehicle gasoline. Eleven of the 55 VOCs analyzed by NAWQA are in this group: benzene, n-butylbenzene, ethylbenzene, isopropylbenzene, naphthalene, styrene, toluene, 1,2,4-trimethylbenzene, oxylene, mxylene, and p-xylene.
Gasoline hydrocarbons are among the most intensively and widely used VOCs. Compared to other VOC groups, production of gasoline hydrocarbons has been extremely large. From 1980 to 2000, the median annual production of these compounds for all uses (including motor gasoline) was about 20 trillion gallons.(117) This production number was at least four orders of magnitude larger than other VOC groups (sidebar 11) and comprised 99.9 percent of the total production of all VOC groups for the time period.
Gasoline hydrocarbons were detected infrequently in aquifers at an assessment level of 0.2 µg/L.
Although the production of gasoline hydrocarbons from 1980 to 2000 was substantially larger than that of any other VOC group, it was not the most frequently detected VOC group in aquifers. Both THMs and solvents were detected more frequently than gasoline hydrocarbons at an assessment level of 0.2 µg/L (fig. 5). The smaller detection frequency of gasoline hydrocarbons relative to these other VOC groups probably is attributed to their transport and fate properties (sidebar 38).
Gasoline hydrocarbons were detected most frequently at concentrations less than 0.2 µg/L and comprised a substantial fraction (24 percent) of all low-level VOC detections. The frequent occurrence of low concentrations explains why the median concentration of samples with detections of gasoline hydrocarbons was among the lowest of any VOC group (fig. 32). In drinking water from CWSs, few concentrations of gasoline hydrocarbons were greater than MCLs (sidebar 39).
Individual gasoline hydrocarbons were detected infrequently in aquifers. Only one gasoline hydrocarbon, toluene, was detected in more than 1 percent of aquifer samples (fig. 8). No individual gasoline hydrocarbons were in the top 10 mixtures of VOCs in aquifers (sidebar 13). Furthermore, gasoline hydrocarbons occurred more frequently alone than as mixtures. Two or more gasoline hydrocarbons occurred together in only about 15 percent of samples in which gasoline hydrocarbons were detected.
The sources of most gasoline hydrocarbons in aquifers probably are releases of gasoline or other finished fuel products. However, in addition to their use in gasoline, many of the compounds identified as gasoline hydrocarbons have other uses (Appendix 4) and thus may have other sources to ground water. Gasoline hydrocarbons have been detected in used motor oil, which could be a source for some of these VOCs in ground water.(105)
Figure 32. The median concentration of gasoline hydrocarbons in aquifer samples with detections was the second lowest of all seven VOC groups.
Because of the infrequent detection of individual gasoline hydrocarbons in aquifers, few associations with anthropogenic or hydrogeologic factors that might control their occurrence could be made in this assessment. The likelihood of detecting either toluene or 1,2,4-trimethylbenzene, however, was determined to be larger in areas with higher density of gasoline storage tanks (table 4).
In a study of the occurrence of MTBE and gasoline hydrocarbons in ground water of the United States, the detection frequencies of individual gasoline hydrocarbons were smaller than the detection frequency of MTBE.(113) Although both are components of gasoline, the smaller detection frequencies of gasoline hydrocarbons were believed to be the result of differences in transport and fate properties of these compounds compared to those of MTBE.
The median concentration of gasoline hydrocarbons was among the lowest of any VOC group.
In the Reformulated Gasoline (RFG) Program, the content of benzene was limited to 1 percent by volume. Normally, gasoline contains between 1 and 1.5 percent benzene by volume.(120) RFG usually contains the oxygenate MTBE. The limitation of benzene content in RFG should have resulted in smaller detection frequencies of benzene in areas of high MTBE use compared to areas of low MTBE use. An examination of ground-water data from across the United States revealed that this was the case.(113) The smaller detection frequency of benzene, a known human carcinogen, in high MTBE-use areas could be a result of the lower benzene content in RFG and, if so, would be an important change for improving ground-water quality, especially for ground water that is used as a supply for drinking water.
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