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4 Assessment of Exposure to Organophosphorus Compounds, Carba mates, and Volatile Organic Chemicals This chapter reviews methods for assessing exposure to organophosphorus compounds, carbamates, and volatile organic chemicals. It also provides orientation on the concentrations that could be encountered in water and on the resulting human exposures to them, and it indicates the importance of considering not only ingestion but other routes of exposure, such as skin contact and inhalation. Later chapters use these compounds to illustrate some toxicologic considerations of mixtures in drinking water. Analytic methods for organophosphorus compounds and carbamates in water are well developed and standardized. An EPA method (EPA, 1984) for the determination of carbamates uses direct-injection high-performance liquid chromatography (HPLC). Less than 1 ml of a sample of filtered water is directly injected onto a reversed-phase HPLC column, and separation is achieved by gradient elusion chromatography. The eluted compounds are hydrolyzed and then react with o-phthalaldehyde to form a fluorescent de- rivative, which is analyzed with a fluorescence detector. Although the de- tection limits for specific compounds in water vary, they are typically about 1 ~g/liter for aldicarb, propoxur, carbaryl, carbofuran, and methomyl. Fail- ure to detect a pesticide of the organophosphate or carbamate class known to be a potential contaminant of a specific water supply might not signify its absence; the pesticide might have hydrolyzed or undergone other chemical changes to other toxic substances, or the detection limit used in the analysis might have been too high. An EPA method (EPA, 1986) for the assay of organophosphorus insec- ticides in water involves collection of a water sample, extraction of the sample with 1 liter of methylene chloride, concentration of the extract, and analysis 133

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134 DRINKING WATER AND HEALTH by gas chromatography with a capillary column and a nitrogen-phosphorus detector. For the typical organophosphorus insecticides diazinon, disulfoton, fonofos, and terbufos, the detection limit is about 1 mg/liter. The organo- phosphorus and carbamate insecticides hydrolyze at various rates in water, and in some cases their hydrolysis products are of toxicologic concern. Maximum contaminant levels (MCLs) ranging from 0.002 to 0.75 ma/ liter, depending on the compound, have been promulgated by EPA for the following volatile organic chemicals (VOCs) and became effective January 9, 1989: benzene, vinyl chloride, carbon tetrachloride, 1,2-dichloroethane, trichloroethylene, p-dichlorobenzene, 1,1-dichloroethylene, and 1,1,1-trich- loroethane. There was already an MCL of 0.1 mg/liter for total trihalome- thanes (THMs), a class of VOCs that includes chloroform, bromoform, and the mixed chlorobromomethanes. The THMs and VOCs are measured by purge-and-trap gas chromatography with estimated method detection limits (MDLs) of about 0.2-1.9 ~g/liter (EPA, 1987a). OCCURRENCE OF ORGANOPHOSPHORUS COMPOUNDS AND CAR BAMATES I N OR I N Kl NG WATER Several surveys have compiled data on concentrations of organophosphorus compounds and carbamates in surface waters, groundwaters, and, in some cases, completely treated ("finished'') or well waters. Typically, their con- centrations are 1 ~g/liter or less (often undetectable), although substantially higher concentrations have been found in isolated instances. Kelley et al. (1986) showed that many commonly used pesticides, including carbamates and organophosphorus compounds, leach into groundwater. Typical concen- trations in ~groundwater in Iowa were 0.5-2.0 ~g/liter, although some wells had total pesticide concentrations of 20 ~g/liter. Data collected by various regulatory agencies are entered into the EPA surface-water and groundwater data base, including data on several pesti- cides. Such data have been summarized in health advisories prepared by EPA (1987b) for some of these pesticides. These and other data on various carbamate and organophosphorus insecticides are presented here to provide perspective on the concentrations of pesticides encountered in drinking water in several studies. Aldicarb In 17 of 106 wells sampled in California. aldicarb was detected at up to 14 parts per billion (ppb, equivalent to ~g/1) (NRC, 19861. In the 15 states where aldicarb was found in groundwater~ it was found typically at 1-50 ~g/liter (Cohen at al., 19861.

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Organophosphorus Compounds, Carbamates, and VOCs 135 Carbary' Carbaryl has been found in 61 of 522 surface water samples and 28 of 1,125 groundwater samples in eight states (EPA, 1987b). In samples with detectable concentrations, the 85th-percentile concentrations were 260 fig/ liter in surface water and 10 ~g/liter in groundwater. The highest concen- trations were 180,000 ~g/liter in surface water and 10 ~g/liter in ground- water. Carbofuran Carbofuran has been found in groundwater in three states, typically at 1- 50 ~g/liter (Cohen et al., 19861. Dazlnon Diazinon has been found in 13 wells (total number sampled not available) in California at up to 9 ~g/liter (NRC, 1986, p. 201. It has been found in 7,230 of 23,227 surface-water samples and 115 of 3,339 groundwater sam- ples in 46 states (EPA, 1987c). The 85th percentile of detectable concentra- tions was 0.2 mg/liter in surface water and 0.25 ~g/liter in groundwater. The highest concentrations were 33,400 ~g/liter in surface water and 84 fig/ liter in groundwater. Fonofos In Iowa, fonofos was detected in about 2% of the samples at a typical concentration of 0.4 ~g/liter; the maximum was 0.9 ~g/liter (Kelley et al., 1986). It was detected in groundwater in California at 0.01-0.03 ~g/liter (EPA, 1987d). Malathion Malathion has been detected in five wells (total number sampled not avail- able) in California at up to 23 ~g/liter (NRC, 1986, p. 201. Methy' parathion Methyl parathion has been found in 1,402 of 29,002 surface-water samples and 25 of 2,878 groundwater samples in 22 states (EPA, 1987e). The 85th percentile of detectable concentrations was 1.2 ~g/liter in surface water and

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136 DRINKING WATER AND HEALTH 1 ~g/liter in groundwater. The highest concentrations were 13 ~g/liter in surface water and 1 6 ~g/liter in groundwater. Terbufos In one study in Iowa, terbufos was found in 5% of the samples at a typical concentration of 5.4 ~g/liter; the maximum was 12 ~g/liter (Kelley et al., 19861. It has been found in 444 of 2,106 surface-water samples and 9 of 283 groundwater samples in five states (EPA, 1987f). The 85th percentile of detectable concentrations was 0.1 ~g/liter in surface water and 3 ~g/liter in groundwater. The highest concentrations were 2.3 ~g/liter in surface water and 3 ~g/liter in groundwater. OCCURRENCE OF VOLATILE ORGANIC COMPOUNDS IN DRINKING WATER There have been several surveys of THMs and VOCs in surface waters, groundwater, and finished water supplies. The straight lines fitted to results of an early EPA survey for THMs in finished water supplies of 80 U.S. cities are shown in Figure 4-1 (EPA, 19751. The median total concentration of THMs was about 20 ~g/liter, and chloroform usually dominated the other THMs. Results of several surveys of VOCs in surface waters and groundwater are summarized in Tables 4-1, 4-2, and 4-3. Although Table 4-1 shows very high concentrations of some specific organic chemicals (such as trichloroe- thylene at 35,000 ~g/liter and 1, 1,1-trichloroethane at more than 400,000 ~g/liter), "more commonly, contamination is found at less than 10 ~g/1 with smaller percentages in the 10-100 ~g/1 and in the 100-1,000 ~g/1 range'' (EPA, 19821. In one national survey, the VOCs most frequently found in finished groundwater supplies (other than the THMs) were trichloroethylene, 1,1,1-trichloroethane, tetrachloroethylene, cis- and trans-1,2-dichloroethy- lene, and 1,1-dichloroethane (Westrick et al., 19841. Table 4-2 summarizes the occurrences of the compounds detected at 186 randomly sampled sites serving more than 10,000 people each. Differences in median concentrations shown in Figure 4-1 and Table 4-2 arise largely out of the differences in sampled supplies. Figure 4-1 includes surface-water supplies, some of which are likely to be heavily chlorinated, whereas the data in Table 4-2 derive from groundwater sources, which are less likely to be chlorinated. The dis- tribution of the summed concentrations of these VOCs, shown in Table 4 3, demonstrates that large systems were likely to exceed a summed concen- tration of 5.0 ~g/liter slightly more frequently than small systems, as might be expected from purely statistical considerations. In both their random and nonrandom samplings, the median concentrations of specific compounds in

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Organophosphorus Compounds, Carbamates, and VOCs 137 300 100 50 - z o Hi 10 LL is o is I IL o 6 1.0 I tar 0.5 0.1 - / / / ~1 o l l 1 1 1 1 ~ I I 1 1 1 1 1 1 2 5 10 20 40 60 80 90 95 98 99 PERCENT EQUAL TO OR LESS THAN GIVEN CONCENTRATION FIGURE 4-1 Frequency dis- tnbution of trihalomethane con- centrations found in the National Organics Reconnaissance Sur- vey (NORS) of halogenated or- ganic compounds in drinking water in 80 U.S. cities. From EPA, 1975. the positive samples ranged from about 0.2 to 9 ~g/liter. One can conclude that VOCs other than THMs are normally found at concentrations of less than 10 ~g/liter- and often less than 1 ~g/liter in finished groundwater supplies. ROUTES OF HUMAN EXPOSURE TO CHEMICALS IN DRINKING WATER The usual estimates of exposure to contaminants in drinking water are based on ingestion and are calculated from the standard of 2-liters/day inges- tion of water by a 70-kg man. Ingestion of 2 liters/day is used to develop MCLs when the dose-response relationships are known. There has been some

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138 DRINKING WATER AND HEALTH TABLE 4-1 Occurrence of Volatile Organic Chemicals in Finished Drinking Watera No. No. Concentration Compound SurveybSamples Positive Range, /liter Trichloroethylene State data''2,894 810 Trace-35,000 NOMSd113 28 0.2-49.0 NSP~142 36e Trace-53.0 CWSS~452 15 0.5-210 Tetrachloroethylene State data''1,652 231 Trace-3.000 NOMSd113 48 0.2-3.1 ASPS142 24e Trace-3.2 CWSS~452 22 0.5-30 Carbon tetrachloride State data''1~659 166 Trace-170 NOMSd113 14 0.2-29 NSP"142 37'' Trace-30 cwsstl452 9 0.5-2.8 I.l~l-Trichloroethane State data''1.611 370 Trace-401.300 NOMS~113 19 0.2-1.3 NSP~142 32'' Trace- 21 CWSS'i452 19 0.5-650 1,2-lDichloroethane State data''1.212 85 Trace-400 NOMS113 2 0.1 - 1.8 NSP'~142 2e Trace-4. CWSS~451 4 0.5-1.8 Vinyl chloride State data''1.033 73 Trace-380 NOMSd113 2 0.1 -0.18 NSPtl142 7" Trace-76 cwss~f I 1 UFrom EPA. 1982. hNOMS. National Or~anics Monitoring Survey. 1976-1977: NSP. National Screening Program. 1977- 1981; CWSS. Community Water Supply Survey. 1978 (EPA. 1987~). ''All Round water sources; aggregated from various state reports on local contamination problems. '~Surface-water and groundwater sources. ''Tentative identification by single-column gas chromatography. fCompound not surveyed. effort to determine the effects of variability in the quantities of water ingested (Gillies and Paulin, 19831. Results of studies in Canada, Great Britain, The Netherlands? and New Zealand indicate that mean daily intakes, including those of beverages made with tap water, range from 0.96 to 1.34 liters/day. In one of the New Zealand studies, the mean intake of 960 ml/day had a standard deviation of 570 ml/day. A more recent study in the United States indicated higher water intake (Ershow and Cantor, 19861. Thus, in estimating the toxic effects of ingestion of water that contains pesticides, one should consider both the mean and the variability in intake in the population of interest. Most attention should be paid to persons with the highest intake. Attention has recently shifted toward exposure to chemical contaminants

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Organophosphorus Compounds, Carbamates, and VOCs 139 of tap water by routes other than ingestion, including skin contact with and inhalation of chemicals that volatilize indoors and can be inhaled at the point of water use or elsewhere or in moves from room to room. Very few data are available on cutaneous exposure, although Brown et al. (1984) estimated that bathing can cause exposures in the same range as those caused by the daily ingestion of 2 liters of the same water. Field measurements, experimental studies, and models have measured the volatilization of chemicals found in water used for various purposes, in- cluding showering and bathing (Andelman, 1985; McKone. 1987~. Estimates of the resulting inhalation exposures vary, but suggest that showering can cause about as much exposure as ingestion and that exposure due to all water uses can be substantially higher than that due to direct ingestion, in part because the intake of air (about 20,000 liters/day) is about 104 times that of water. How quickly and how completely a pesticide or other contaminant in TABLE 4-2 Summary of Occurrences of Volatile Organic Chemicals at 186 Randomly Sampled Groundwater Sites Serving More Than 10,000 Persons Eacha Quantitation Occurrences Median of Limit, Positives. Maximum. ~Doter /liter Vinyl chloride 1.0 1 0.5 1.1 1.1 1.1-Dichloroethylene 0.2 5 2.7 0.28 2.2 1,1-Dichloroethane 0.2 8 4.3 0.54 1.2 cis- and trans 1 ,2-Dichloroethylene 0.2 1 3 7.0 1.1 2.0 1,2-Dichloroethane 0.5 3 1.6 0.57 0.95 1, 1,1-Trichloroethane 0.2 15 8.1 1.0 3.1 Carbon tetrachloride 0.2 10 5.4 0.32 2.S 1,2-Dichloropropane 0.2 5 2.7 0.96 21 Trichloroethylene 0.2 2 1 11.3 1 .0 78 Tetrachloroethylene 0.2 21 11.3 0.52 5.9 Benzene 0.5 2 1 . I 9.0 1 5 Toluene 0.5 2 1.1 2.6 2.9 Ethylbenzene 0.5 1 0.5 0.74 0.74 Bromobenzene 0.5 1 0.5 1.7 1.7 m-Xylene 0.2 2 1.1 0.46 0.61 o + p-Xylene 0.2 2 1.1 0.59 0.91 p-Dichlorobenzene 0.5 3 1.6 0.66 1.3 Chloroform 0.2 106 57.0 1.6 300 Bromodichloromethane 0.2 101 54.3 1 .6 7 1 Dibromochloromethane 0.5 96 51.6 2.9 59 Dichloroiodomethane 1.0 3 1.6 1. ~4.1 Bromoform 1.0 57 30.6 3.8 50 aAdapted from Westrick et al., ]984. with permission.

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140 DRINKING WATER AND HEALTH TABLE 4-3 VOC Concentrations in Random Samples of Finished Groundwater~ Water Supplies with Summed Concentrations of VOCs Exceeding Value Shown at Left Summed Systems Serving Systems Serving Up to 10 000 Persons More Than 10 000 Persons Concentrations of VOCs, ~g/liter No. % No. % Quantitation limitb 47 16.8 52 28.0 1.0 20 7.1 26 14.0 5.0 8 2.9 12 6.5 10 5 1.8 7 3.8 50 1 0.4 1 0.5 100 0 0 0 0 Adapted from Westrick et al.. 1984. with permission. hQuantitation limits not same for all compounds. In most cases. quantitation limit is either 0.2 ~/1 or 0.5 ~/1. This difference in quantitation limits can confuse interpretation of data somewhat so results of survey should be viewed with differing quantitation limits in mind. Occurrence is any specific finding at, or in excess of. the quantitation limit. drinking water will volatilize depends on its physical and chemical properties, including its solubility in water, its vapor pressure, its Henry's law constant (H), and its coefficient of diffusion in water at the water-air interface (An- delman, 1985), as well as physical characteristics of the water, such as temperature, agitation, and spraying. The constant H is equal to the ratio of the equilibrium concentration in air to the concentration in aqueous solution. The vapor pressures of carbamates and organophosphorus compounds are low, as are their H values. Other aqueous insecticides, such as dieldrin and aldrin, can readily volatilize from water-air interfaces, although probably at lower rates than compounds (such as benzene and toluene) that have higher H values (Mackay and Leinonen, 19751. Polychlorinated biphenyls (such as Arochlor 1242) and chlordane volatilized from water surfaces at about 20%- 30% of the rate of oxygen in reaeration studies, but dieldrin, which has a substantially lower H value, volatilized at only 1%-5% of the oxygen reaer- ation rate (Atlas et al., 19821. The H values of specific organophosphorus and carbamate compounds vary, but many are low. On the basis of their water solubilities and vapor pressures, one can calculate H values at room temperature of 2 x 10-6 and 8 x 10-9 atm m3/mol for aldicarb and carbofuran, respectively. Comparable values for dieldrin, aldrin, and Ar- ochlor 1242 at room temperature are 2 x 10-7, 1 X 10-5, and 6 x 10-4 atm m3/mol (Mackay and Leinonen, 19751. In contrast to the organophosphorus and carbamate compounds, the typical VOC or THM has a relatively large H value at room temperature, generally

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Organophosphorus Compounds, Carbamates, and VOCs 141 in the range of 10-2-10-3 aim m3/mol (Roberts and Dandliker, 19831. The mass-transfer rate constants for the VOCs and THMs are also substan- tially higher than those for the organophosphorus and carbamate compounds, typically about 60% of that for oxygen reaeration. Thus, one would expect that substantial fractions of these components would volatilize during typical indoor water uses and thereby contribute to inhalation exposures, especially of the person at the point of use, but perhaps of others if the volatile con- stituents are disseminated by air movement. As has been shown for trichlo- roethylene and chloroform, volatilization from both showers and baths is substantial, usually greater than 50% and sometimes as high as 90%, de- pending on temperature, air flow, and the geometry of the water system (Andelman, 1985; Andelman et al., 1986, 19871. For the VOCs and THMs with higher H values, Henry's law equilibrium is generally not attained, so mass transfer at the water-air interface often limits the rate and extent of volatilization. A recent attempt to detains whether a surrogate chemical, sulfur hexafluoride, could be used to estimate the volatilization of such constituents associated with indoor water uses (Giardino et al., 1988) was encouraging, but additional research is required. CONSI DERATIONS OF TOTAL EXPOSURE The principal focus of this report is the assessment of toxicity associated with exposure to mixtures of chemicals in drinking water. But other media, such as food, are also potential sources of exposure. Exposures related to nonwater sources, such as exposure to polycyclic aromatic hydrocarbons in ambient air, can be much greater than those related to water. When human toxicity associated with exposure through water is assessed, combined ex- posures through other media have the potential for raising an apparently low exposure through water to the point where a toxic threshold is exceeded or, in the case of a carcinogen, a risk is increased. An early multimedia-exposure analysis for some of the chemicals consid- ered here addressed the multiple routes and variability of uptake of chloroform and carbon tetrachloride (NRC, 19781. The exposure data on air, water, and food in that analysis were often meager and not precise, but the analysis did use what was known about variability in absorption after ingestion or in- halation. Table 4-4 shows three hypothetical scenarios for uptake (based on exposure and absorption) of carbon tetrachloride and chloroform from water, food, the atmosphere, and the three together. It appears that most exposure to chloroform at typical levels is by water, whereas air is typically more important for carbon tetrachloride. However, in any given instance, almost any route can dominate, so it is essential to consider all sources when one is assessing individual exposure to a specific chemical and the associated risk.

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142 DRINKING WATER AND HEAl TH TABLE 4-4 Adult Human Male Uptake (Based on Exposure and Intake) of Carbon Tetrachloride (CC14) and Chloroform (CHC13) from Environmental Sources Uptake, mg/year Source CCI ~CHEW At minimum exposure levelsh Water and water-based drinks 0.73 0.037 Atmosphere 3.60 0.41 Food 0.21 0.21 Total 4.54 0.66 At typical exposure levels'' Water and water-based drinks 1.78 14.90 Atmosphere 4.80 5 . 90 Food 1.12 9.17 Total 7.70 92.3 At maximum exposure levels" Water and water-based drinks 4.05 494 Atmosphere 618 474 Food 7.33 16.4 Total 629 9X4 "Adapted from NRC, 1978. pp. 180-181. Minimum exposure and minimum intake for all sources. ''Typical conditions assumed. For CC1~: water and water-based drinks, exposure at 0.0095 my/ liter and reference-man intake; atmosphere. average of typical minimum and maximum absorption; Mods average exposure and intake. For CHAIN water and water-based drinks. median exposure and reference-man intake; atmosphere, average of typical minimum and maximum absorption; foods average exposure and intake. 'iMaximum exposure and maximum intake for all sources. CONCLUSIONS AND RECOMMENDATIONS If joint exposure to THMs or to all VOCs with roughly equivalent potency could be considered to have additive toxic effects, it would be useful to have an analytic method for monitoring purposes that could be used as a measure of the total concentration of members of the group. For example, the sum of the volatile organohalide concentrations could be measured with a single instrument, even though it would measure a group of compounds such as vinyl chloride, carbon tetrachloride, 1,2-dichloroethane, trichloroethylene, p-dichlorobenzene, 1,1-dichloroethylene, and 1, 1,1-trichloroethane with widely varied toxic effects. The potential deficiency of such a method is that other, possibly harmless volatile organohalide compounds in the water sample would also be detected. In the case of the organophosphorus compounds, it

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Organophosphorus Compounds, Carbamates, and VOCs 143 is unlikely that a single simple analytic measurement with a gas chromato- graphic phosphorus detector can be developed that would depend only on the presence of organophosphorus insecticides, because other, unidentified phosphorus compounds could be present. The HPLC method described earlier might be more successful in this regard, but it would have to be shown that the derivatization procedure is specific to the hydrolysis products of the carbamates of interest and that other naturally occurring chemicals and their hydrolysis products would not introduce serious inaccuracies. If a simple analytic process could be developed to provide a summary measure of the concentrations of an entire class of toxicologically similar constituents in drinking water, it is likely that it would also detect other, potentially confounding constituents in the water. In assessing the toxic impacts of individual or mixed constituents of drink- ing water, it is essential to consider all forms of exposure to the constituents, including exposure through water, soil, air, and food. For contaminated drinking water, exposure by inhalation, skin contact, and ingestion should be assessed. Whether organophosphorus and carbamate compounds volatilize to a substantial extent during domestic indoor water uses should be deter- mined, so that total exposures to these chemicals can be assessed. For the THMs and VOCs, exposure due to volatilization can be substantial and should be considered in assessing human toxic impact and risk. REFERENCES Andelman, J. B. 1985. Inhalation exposure in the home to volatile organic contaminants of drinking water. Sci. Total Environ. 47:443-460. Andelman, J. B., S. M. Meyers, and L. C. Wilder. 1986. Volatilization of organic chemicals from indoor water uses. Pp. 323-330 in Chemicals in the Environment J. N. Lester. R. Perry, and R. M. Sterritt, eds. London: Selper. Andelman, J. B., L. C. Wilder, and S. M. Meyers. 1987. Indoor air pollution from volatile chemicals in water. Pp. 37-41 in Proceedings of the 4th international Conference on Indoor Air Quality and Climate, Vol. 1. West Berlin, August 1987. Atlas, E., R. Foster, and C. S. Giam. 1982. Air-sea exchange of high molecular weight organic pollutants: Laboratory studies. Environ. Sci. Technol. 16:283-286. Brown, H. S., D. R. Bishop, and C. A. Rowan. 1984. The role of skin absorption as a route of exposure for volatile organic compounds (VOCs) in drinking water. Am. J. Public Health 74:479-484. Cohen, S. Z., C. Eiden, and M. N. Lorber. 1986. Monitoring ground water for pesticides. Pp. 170-196 in Evaluation of Pesticides in Ground Water. ACS Symposium Series Vol. 315. Washington, D.C.: American Chemical Society. EPA (U.S. Environmental Protection Agency). 1975. Preliminary Assessment of Suspected Carcinogens in Drinking Water, and Appendices. A Report to Congress. Washington' D.C.: U.S. Environmental Protection Agency. EPA (U.S. Environmental Protection Agency). 1982. National revised primary drinking water regulations, volatile synthetic organic chemicals in drinking water; advanced notice of pro- posed rulemaking (March 4, 1982). Fed. Regist. 47(43):9350-9358.

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144 DRINKING WATER AND HEALTH EPA (U.S. Environmental Protection Agency). 1984. Method 531. Measurement of N-Methyl Carboxylamines and N-Methylcarbamates in Drinking Water by Direct Aqueous Injection HPLC with Post Column Derivatization. Cincinnati, Ohio: Environmental Monitoring and Support Laboratory, U.S. Environmental Protection Agency. EPA (U.S. Environmental Protection Agency). 1986. Method 1. Determination of Nitrogen- and Phosphorus-Containing Pesticides in Ground Water by GC/NPD. January 1986 Draft Report. Cincinnati, Ohio: Environmental Monitoring and Support Laboratory, U.S. Envi- ronmental Protection Agency. EPA (U.S. Environmental Protection Agency). 1987a. Volatile Organic Chemicals: Methods and Monitoring Document. Cincinnati, Ohio: Technical Support Division, U.S. Environ- mental Protection Agency. EPA (U.S. Environmental Protection Agency). 1987b. Carbaryl. August, 1987. Health Ad- visory. Draft. Washington, D.C.: Office of Drinking Water, U.S. Environmental Protection Agency. 20 pp. EPA (U.S. Environmental Protection Agency). 1987c. Diazinon. August, 1987. Health Ad- visory. Draft. Washington, D.C.: Office of Drinking Water, U.S. Environmental Protection Agency. 19 pp. EPA (U.S. Environmental Protection Agency). 1987d. Fonofos, August 1987. Health Advi- sory. Draft. Washington, D.C.: Office of Drinking Water, U.S. Environmental Protection Agency. 16 pp. EPA (U.S. Environmental Protection Agency). 1987e. Methyl Parathion' August 1987. Health Advisory. Draft. Washington D.C.: Office of Drinking Water? U.S. Environmental Pro- tection Agency. 24 pp. EPA (U.S. Environmental Protection Agency). 1987f. Terbufos, August, 1987. Health Ad- visory. Draft. Washington, D.C.: Office of Drinking Water, U.S. Environmental Protection Agency. 15 pp. EPA (U.S. Environmental Protection Agency). 1987g. Drinking water; substitution of con- taminants and priority list of additional substances which may require regulation under the Safe Drinking Water Act. Fed. Regist. 52(130):25720-25734. Ershow, A., and K. P. Cantor. 1986. Population-based estimates of water intake. Fed. Proc. 45:706. Giardino, N.~ J. B. Andelman~ J. E. Borrazzo. and C. I. Davidson. 1988. Sulfurhexafluoride as a surrogate for volatilization of organics from indoor water uses. J. Air Pollut. Control Assoc. 3:278-280. Gillies, M. E.. and H. V. Paulin. 1983. Variability of mineral intakes from drinking water: A possible explanation for the controversy over the relationship of water quality to cardio- vascular diseases. Int. J. Epidemiol. 12:45-50. Kelley. R.. G. R. Hallberg, L. G. Johnson, R. D. Libra, C. A. Thompson, R. G. Splinter. and M. G. DeTroy. 1986. Pesticides in ground water in Iowa. J. Natl. Well Water Assoc. August:622-647. Mackay, D., and P. J. Leinonen. 1975. Rate of evaporation of low-solubility contaminants from water bodies to atmosphere. Environ. Sci. Technol. 9: 1178- 1180. McKone. T. E. 1987. Human exposure to volatile organic compounds in household tap water: The indoor inhalation pathway. Environ. Sci. Technol. 21: 1194- 1201. NRC (National Research Council). 1978. Chloroform. Carbon Tetrachloride and Other Hal- omethanes: An Environmental Assessment. Washington, D.C.: National Academy of Sci- ences. 294 pp. NRC (National Research Council). 1986. Pesticides and Groundwater Quality: Issues and

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Organophosphorus Compounds, Carbamates, and VOCs 145 Problems in Four States. Written by Patrick W. Holden. Washington, D.C.: National Acad- emy Press. 124 pp. Roberts, P. V., and P. Dandliker. 1983. Mass transfer of volatile organic contaminants from aqueous solution to the atmosphere during surface aeration. Environ. Sci. Technol. 17:484- 489. Westrick, J. J., J. W. Mello, and R. F. Thomas. 1984. The groundwater supply survey. Am. Water Works Assoc. 76(5):52-59.