be taken on the basis of sound scientific data that include a statistically valid groundwater sampling program and verifiable chemical analysis procedures to determine the existence, quantity, and persistence of pesticides’, (1). It would be impossible to cover all relevant aspects of the relationship between crop protection chemicals (herbicides, insecticides, fungicides, fertilizers, growth regulators) and groundwater contamination issues in this REPORT. Instead, I will highlight the areas that require the most immediate attention. An excellent summary is available for those desiring background information on agriculture and groundwater quality (2).
Jeffrey A. Graham Monsanto Company T4K 800 North Lindbergh Boulevard St. Louis, MO 63167
The s t a t u s of t h e “pesticides i n groundwater” question was recently put into perspective by former US. Environmental Protection Agency 0003-2700/91/0363-613A/$02.50/0 0 1991 American Chemical Society
(EPA) official Richard M. Dowd, who wrote: “That pesticides exist i n groundwater in agricultural areas cannot be disputed. However, the source of the pesticides, their transfer route to groundwater, their persistence, and level of contamination are subject to considerable controversy. Effective regulatory decisions and subsequent remedial actions can best
Study design As with many other environmental studies, groundwater and well water monitoring studies require diverse designs because of differing objec tives and constraints such as time, economics, and available facilities. Regardless of design, validity of data from any study always depends, in part, on the validity of the model and the sample (3, 4). According to Taylor (5),the model represents the conceptualization of t h e problem to be solved and describes the samples that should be analyzed, the data required, and the way the model will be used. Obviously, even flawless measurement data will be of little value if the basic concepts are faulty. The necessity for a sound sampling program in any study of pesticide distribution in the environment is generally recognized. Yet programs are often designed such that they are statistically unsound, and a valid, well-designed plan may be compromised by expediency or carelessness (6). The effort expended on evaluation of sampling designs for pesticide monitoring is usually exceedingly small compared with that expended on analytical measurements. The first step in designing any study is to identify the target population to which the results will be applied (6). Once the target population is defined, a sample of it is taken. It is this process that appears to be the weakest link in the chain for most groundwater and well water monitoring studies. Taylor (7) has proposed three kinds of samples that can be used in any measurement process: intuitive, statistical, and protocol. Selection of intuitive samples is based only on the judgment of the person(s) choosing the samples or devising some process
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REPORT to do so. As such, only judgmental conclusions can be drawn about the results and, when there is controversy, decisions may be based on the perceived expertise of those responsible for sampling. Collection of statistical samples is based on random sampling of the population and usually provides the opportunity to draw probabilistic conclusions. Predictions can be made and inferences can be drawn from such a sample. Although the conclusions may appear to be noncontrover sial, the population represented by the sample may be a matter of controversy. The third kind of sample, a protocol sample, is specified in the protocol for a given study. Liggett (8) has proposed that any study design should contain three parts: a basis for separating the contamination of interest from the background, an independent method for assessing errors i n sampling and measurement, and a protocol for making trace chemical measure ments and reporting results. The challenge is to integrate these three parts into a design that efficiently addresses the objective of the study. To accomplish this integration, experienced analytical chemists and statisticians must be involved a t the earliest point in the design stage. Types of studies Studies t h a t assess occurrence of crop protection chemicals (CPCs) in groundwater and well water can generally be categorized as either prospective or retrospective. Prospective studies. The primary objective of a prospective study is characterization of the subsurface environmental fate of a particular CPC or combination of CPCs. It is a controlled study, as opposed to a retrospective study, which is observa-
tional. Prospective studies attempt to follow CPCs from the time they are applied to the land surface (or plant) until they have degraded and/or migrated through the subsurface (the unsaturated zone) and possibly to groundwater (the saturated zone). Three different environments can be monitored to track CPC movement: soil, soil-pore water in the unsaturated zone, and groundwater in the saturated zone (9). Soil samples are typically collected in depth increments by using a coring tool or by manual or mechanical digging. Collection of representative deep soil cores immediately following application of CPCs can be difficult when surface soil falls into the core hole after removal of shallower cores. This operational aspect presents an especially difficult problem with CPCs applied to the soil. Soil-pore water, which can be difficult to collect on a routine basis, is usually collected with suction lysim eters. Because the “zone of influence’’ of these lysimeters is not clear, their number and distribution in a given site are topics of debate. Groundwater samples are general ly collected from specially installed monitoring wells rather than nearby water supply wells. The design and installation of monitoring wells can have a direct effect on the composition of the samples collected from them. Swanson (IO)recently pointed out that if one does not have a hydraulic seal in the annulus (the well bore hole surrounding the well casing), one might have migration of contaminants down the well bore, which would either cause groundwater contamination where it did not exist or make the problem look worse than it actually is. Some researchers, however, question whether a water sample collect -
Accumulation of sma disposal, and/or equipment cleaninc Accidents or spills of mixed/diluted equipment cleaning Rack-siphoning of CPCs frnm cnr: lirectly into wells ry of CPC-containing runoff from treated ito well bore holes, wells, abandoned we1 Entry of CPC-containing runoff from treated fi rectly into natural or man-made conduits to groun tural drainage wells or sinkholes) nrharnn nf rnntaminatd
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ed from even a properly constructed monitoring well can ever be representative of the water in the aquifer it is attempting to represent. For example, Pennino (11)has commented that extensive laboratory quality control procedures and painfully accurate analytical methods hardly seem justified when the groundwater sample is more representative of the sampling conditions than it is of the true groundwater quality. Characteristics of the site where the prospective study is being conducted, properties of the CPCs of interest, and the required accuracy and precision of the results determine the number, distribution, type, configuration, and frequency of sampling of monitoring wells. The number, size, distribution, depth, collection mode, and frequency of soil sample collection also are influenced by site characteristics. The location of such studies is almost entirely influenced by subsequent use of the results. Registrants of new CPCs will generally conduct such studies in a perceived “worst case” situation because favorable results from a worst case site imply that CPCs will exhibit acceptable environmental fate in less vulnerable situations. This conclusion, however, is only as reliable as the ability of the researcher or study designer to determine what constitutes worst case sites. If results obtained from worst case studies are unacceptable, additional prospective studies at “typical” sites are often necessary to document acceptable environmental fate. Retrospective studies. The primary objective of retrospective studies is assessment of the extent of occurrence of a particular CPC o r combination of CPCs in well water or groundwater in a specifically defined population or location. These studies are conducted after the CPCs of interest have been in use for several years. Many such studies suffer from lack of planning, as described by Kratochvil (6),and it is difficult, if not impossible, to ascertain how frequently detectable levels of CPCs occur in drinking water obtained from public and private wells. According to Cohen et al. (IZ),the extent of the problem of pesticides in groundwater is not known: “What we have is a collection of monitoring studies, conceived with different objectives and using different design strategies.” The key concern during the design stages of any study is the target population of interest. In retrospective studies of groundwater and well water, groundwater cannot be assumed
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to be representative of well water, and vice versa. If one is interested in groundwater quality, the study population is entirely different than if one is interested in the quality of drinking water obtained from wells. Many, if not most, large-scale retrospective studies have been conducted using existing water wells, such as drinking water supply wells and irrigation wells. If the objective of the study is exposure assessment, use of existing water supply wells is certainly justified. The challenge t h e n becomes one of ascribing a mechanism to the observed contamination so that measures can be taken to mitigate future contamination. The numerous ways i n which groundwater and well water may become contaminated agrichemicals are shown in the box on p. 614. Determining t h e mechanism or process of Contamination is further complicated by the fact that many wells used for private water supplies-particularly in rural areasare known to suffer from construction, siting, and maintenance defects (13).These defects are directly attributable to contamination from agrichemicals as well as other chemicals and organisms (13-18).Existing wells have been and will continue to be used in retrospective studies primarily because the installation of monitoring wells, especially in the numbers necessary to make meaningful observations on a large scale, is a n expensive proposition. Water collected and analyzed from these wells may be representative of that consumed by persons using the wells, but it will not necessarily reflect the composition of the water in the aquifer from which it was obtained (11). Installation of large numbers of monitoring wells, however, is not out of the question. The North Carolina Pesticide Board proposed a retrospective study design in which 200 monitoring wells would be installed and sampled to assess groundwater quality in selected areas of the state (19).Although this study will provide information on the specific areas selected, it will not yield results that can be accurately extrapolated to all areas of the state, nor will it indicate the quality of water provided by private rural wells, which are so often the major cause of contamination. Retrospective well water monitoring studies often focus on those wells that are perceived to be worst case: They are either in extremely poor condition or are sited in or near fields where CPCs are known to have been used and where the soil and geologi616 A
cal conditions make t h e surficial aquifer extremely vulnerable to contamination from the land surface. Sometimes, documented cases of CPC contamination already exist. AIthough selection of wells in these studies has not generally followed any valid statistical design, a distinct lack of contamination of the water collected from these worst case wells might be interpreted to indicate that wells perceived to be less vulnerable probably exhibit at least comparable, if not better, water quality. When worst case studies reveal certain levels of contamination, extreme care must be taken to present the results in proper perspective. For example, in discussing well water monitoring data collected in conjunction with a program conducted in Ontario, Canada (ZO),Ripley et al. (18)reported that the database was biased because the researchers selected shallow wells near areas of heavy CPC use, sandy soil, and susceptibility to surface runoff. Most CPC contamination, they indicated, occurred because of spills and carelessness around well heads, surface runoff from nearby fields, and, in some cases, faulty well construction. A retrospective study conducted to assess exposure may be most relevant for determining how many people are drinking water that contains detectable levels of CPCs. The answer to this question can be reliably estimated only by using statistical survey methodology (21).A sample of wells is valid only when each well in the target population has a known probability of selection. Results from the study can then be extrapolated to
ize of the sample to be collected Procedure for addition of stabilizin for preparation of f for preparation of f servational data to for sample preserv ures for sample shipping entation of chain of custc
ANALYTICAL CHEMISTRYl VOL. 63, NO. 11 JUNE 1, 1991
other members of the population, including the majority of wells that were not sampled. An example of a survey that was incorrectly conducted was recently reported by Steichen et al. (22). Although the authors had the laudable objective of “a random survey to determine the degree and extent of contamination of drinking water from farmstead wells (in Kansas) by pesticides, volatile organics, heavy met als, and other inorganic constituents,” they failed to implement the study properly. This is evident in the description of the well selection pro cedure which, because of high analytical costs, limited the number of wells to be tested to about 100. The authors then restricted well selection by establishing criteria to identify possible wells for sampling: The well had to be at a farmstead where f a r m operations a r e p e r formed, and residents had to be willing to cooperate, had to use the well water in their homes, and had to be familiar with the activities near the well for the past 10 years. Well selection was further restrict ed because the authors lacked a list of farmstead well owners in each county. County extension agriculture and home economics agents in the selected counties were requested by letter to provide the names of five individuals whom they thought met the criteria. Unfortunately, this biased selection of wells does not allow the results obtained from the 100 selected wells to be reliably extrapolated to the estimated 40,000 farmstead wells in Kansas. Two examples of large-scale retro-
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REPOR7 spective studies conducted for exposure assessment are t h e National Pesticide Survey, conducted by EPA in 1988-90 (231, and the National Alachlor Well Water Survey, conducted by the Monsanto Agricultural Company in 1988-89 (24). The current understanding of the occurrence of CPCs in wells is that they are distributed in a “segregated” way. That is, wells with certain characteristics located in certain geological formations would be expected to exhibit a higher rate of CPC occurrence, as the result of nonpoint source contamination, than in other areas. These areas typically contain shallow wells i n vulnerable soils with relatively high levels of CPC usage. In this kind of situation, stratified random s a m pling is more efficient than simple random sampling (6). In fact, if strat a are chosen (or domains assigned) appropriately, errors in overall population estimates will generally be smaller than with simple random sampling (24,25). In both of these surveys, every well in the target population had a known probability of being sampled. Therefore, results obtained from sampled wells could be reliably extrapolated to the other members of the target population. The precision of any estimates made is, of course, dependent on the number of wells for which reliable analyses are obtained.
Protocol development Once a general study design has been identified, one or more protocols must be developed to conduct t h e various phases. With respect to chemical measurements, a water sampling and chemical analysis protocol must be developed. Although sampling and analysis can be incorporated into a single protocol, it is generally more straightforward to develop protocols for each separately. A quick summary (26) and a n excellent detailed discussion (27) of the information required to develop sampling a n d analysis protocols a r e available. Sampling protocols. Development of an adequate water-sampling protocol for collection of groundwater or well water samples is not a trivial undertaking. Whether the constituents are organic or inorganic, the protocol must contain all necessary information and directions to ensure that sampling personnel collect a water sample that is representative of the population member (the specific well selected for sampling). An excellent discussion of water sampling is available (28). 618 A
The first question to be answered is where to collect the water sample. Will the water sample be collected at the wellhead, if a tap is located there, or a t a tap inside the house? Will the sample be collected before water treatment equipment-such as water softeners, granulated activated carbon filters, and iron and sulfur removal cartridges? If the study is designed to assess exposure, which tap do household members use to obtain most of their drinking water? Although t h e s e questions may seem trivial, it is important to remember that the concentrations of analytes, typically parts per billion or less, are of interest in an exposure assessment. Differences in the concentration of a trace organic compound in water samples collected before and after a water softener can be immense. Ion-exchange resins used in water softeners are based on a styrene-divinylbenzene polymer resin similar to XAD-2, which is used to concentrate hydrophobic trace organ ic compounds from water samples prior to chemical analysis. Likewise, granular activated carbon filters, which a r e commonly installed to eliminate taste and odor, affect the level of trace organic compounds in water pumped from the well. If, however, the study is designed to assess exposure, it may be realistic to collect samples following installation of household treatment systems. Sampling personnel must be aware of these potential problems and how to deal with them and, a t a minimum, record observational data on any situation that may affect the representativeness of the water sample. For well monitoring, where to obtain the water is straightforward. The more complex question is how to assure that the water sample collected from the monitoring well is representative of the groundwater in the water-bearing formation surrounding the well. Composition of the water removed from monitoring wells is influenced by the materials used to construct the wells and the manner in which they were installed (29, 30). Assuming that the well will provide a representative sample of the water in the surrounding aquifer, it is necessary to specify how the groundwater samples can be removed so that their composition is not altered during water sample collection (11, 29, 30). The water sample collection protocol must, at a minimum, specify each of the items identified in the box on p. 616 (31).One subject often missing from protocols and ignored in study designs is the variability as a func-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 11, JUNE 1,1991
tion of time of trace constituents of water obtained from a well, be it a monitoring or a n existing supply well. I have observed concentration variations of a factor of > 20 in water samples collected from a n existing supply well only seconds apart (32). There are no simple rules for developing well water and groundwater sample collection protocols. The researcher must be aware of the nuances of groundwater and well water sampling, as well as the objectives of the study and the particulars of the analytes of interest, to develop valid and practical protocols. Analysis protocols. Analytical chemists agree that valid monitoring results also depend on reliable and accurate chemical analyses. The preponderance of user-friendly instrumentation appears to have led to freq u e n t reporting of questionable results in many other fields. In response to this trend, William Glaze, Editor of Environmental Science & Technology, recently published an editorial reminding authors and reviewers of the need for “confirmation of structure” (33). So-called second GC column confirmation has been used in groundwater and well water monitoring programs, but its utility appears to be to reduce the number of samples requiring further investigation by GCfMS or some other more sophisticated and costly technique. GC/MS has been and will continue to be the most commonly used method for both determination and confirmation of trace organic compounds in aqueous environmental samples. But even for experienced scientists, GC/ MS is not a panacea. An example of the difficulty in obtaining accurate and precise analyti cal results for trace-level organic compounds in aqueous samples using GCMS has been reported by AlfordStevens et al. (34).They describe results of a study in which six laboratories used t h e same analytical procedures to analyze two (distilled and granular activated carbon filtered) water samples fortified with polychlorinated biphenyls (PCBs) at two levels, 148 mg/L and 37 mg/L (expressed as total PCBs), and 11 chlorinated hydrocarbon CPCs at two levels, in the ranges of 9-30 mg/L and 3-10 mg/L. Five of the six laboratories were qualified to perform special analytical services for EPA’s Contract Laboratory Program. The results of the PCB and CPC analyses were disappointing, and the authors concluded that although fortified rea g e n t w a t e r s w e r e expected to present no challenge to analytical
Figure 1. Peak atrazine levels in water samples collected from Big Spring, Clayton County, from 1982 to 1984. (Adapted from Reference 38.)
laboratories, analysts should not be complacent about preparing and analyzing water extracts. Groundwater and well water samples with u n known compounds a t levels e 10 pg/L ( a n d usually e 1 pg/L) certainly present a n equivalent challenge, if not a significantly greater one. Data interpretation Once substantial amounts of time and effort have been invested in a study, the dividends are paid when the resulting data a r e objectively evaluated and interpreted. All environmental monitoring data are notorious for complexity and difficulty in interpretation. Likewise, the time scale by which many natural and weather-dependent phenomena are affected is such that they may be adequately evaluated only by long- term monitoring over years or decades. A classic example of the pitfalls of data interpretation can be appreciat ed by reviewing the Clayton County Big Spring Basin studies, a multiyear project on t h e groundwater quality of a karst basin in northeast Iowa (35,36).These were the first reported groundwater monitoring studies of high-volume crop protection chemicals. Two significant claims were made. First, most pesticides are delivered to the groundwater system in the Big Spring Basin via leaching following normal application of pesticides. Second, groundwater concentrations of pesticides may increase over time in a manner analogous to the rise in nitrate levels seen between 1950 and 1980. These studies have been widely distributed and routinely reported. They serve as the original source of many references to t h e nonpoint mechanism of contamination from CPCs such as alachlor, metolachlor, cyanazine, and metribuzin.
What do the results actually show? Water samples were taken from Big Spring, which serves as the discharge point for virtually all water in the basin, and from a series of private wells in the basin. Atrazine was detected in spring water and sometimes throughout the entire calendar year. Occurrence of CPCs such as alachlor, metolachlor, metribuzin, and cyanazine in the spring water was always associated with storms t h a t caused immediate and direct surface water run-in to the groundwater system via sinkholes common to the basin. Interestingly, few CPCs were detected in private wells during the sampling period. The total flux of atrazine through the groundwater system in the 100sq.-mi. Big Spring Basin has been estimated to be e 10 lbs. per year (37), a testimony to the remarkable power of modern analytical chemistry. The total amount of atrazine applied in the basin on a yearly basis was estimated to be about 50,000 lbs. The agronomic practices in this karst environment are also noteworthy. Because of poorly drained soils and sloping topography, the basin is extensively terraced and tile drained. To prevent ponds from forming, stand-pipes are placed in the terraces to collect runoff water and route it to the tile drain system. Because virtually no surface water leaves the basin, all tile drain water eventually reaches the shallow aquifer. The situation is further complicated by the presence of hundreds of sinkholes throughout t h e basin, which in the past have been used for disposal of pesticide containers as well as other refuse, such as obsolete trucks and farm equipment, The first reports from these studies covered the years 1982 through 1984 (35, 36, 38). Figure 1 shows the
Figure 2. Peak atrazine levels in water samples collected from Big Spring, Clayton County, from 1982 to 1988. (Adapted from Reference 39.)
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REPORT peak atrazine concentrations in Big Spring from those years (38).These data were promoted by the authors as a demonstration that CPC levels in the Big Spring Basin groundwater were on the increase. Sampling and analysis continued through the 1980s, and additional results have been obtained but not widely disseminated. Figure 2 shows the peak atrazine results for 19821988 (39). These data could not be used to support the increasing concentration of atrazine in the Big
Spring Basin groundwater. Figure 2 also shows the flow-weighted mean concentrations of atrazine and nitrate in water samples collected from the Big Spring. These results show a striking difference in the appearance of the CPCs and nitrate results. The Big Spring Basin is an excellent “laboratory” for studying the ef fects of agriculture on a particularly vulnerable groundwater system. The complete story, however, points to the dangers of overinterpreting lim ited results and demonstrates the
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need for a multidisciplinary a p proach to the complex problems of assessing effects of agriculture on groundwater quality. At the very least, these continuing studies show that karst regions are inappropriate locations for assessing leaching of CPCs following normal application. References (1)Dowd, R. M.; Anderson, M. P.; Johnson, M. L. Proceedangs of the Second Nation-
al Outdoor Action Conference on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods; National Water Well Association: Dublin, OH, 1988; pp. 1365-79. (2) Council for Agricultural Science & Technology; Technical Report No. 103: “Agriculture and Groundwater Quality”; Iowa State University: Ames, IA; May 1985. (3)Taylor, J. K. Anal. Chem. 1981, 53, 1588 A-1596 A. (4) Kratochvil, B.; Taylor, J. K. Anal. Chem. 1981,53, 924 A-938 A. (@Taylor, J. K. Anal. Chem. 1983, 55, 600 A-608 A. (6) Kratochvil, B. In Trace Residue Analysis, Chem om e tric Estimations of Sampling, Amount, and Error, ACS Symposium Series 284; Kurtz, D. A., Ed.; American Chemical Society: Washington, DC, 1985; pp. 5-24. (7) \. Tavlor. J. K. Trends Anal. Chem. 1986. 5, 121-23. (8) Liggett, W. S. In Quality Assurance for Environmental Measurements, ASTM STP 867; Taylor, J. K.; Stanley,’T. W., Eds.; American Society for Testing and Materials: Philadelphia, 1985; pp. 22-40. (9) Evaluation of Pesticides in Ground Water, ACS Symposium Series 315; Garner, W. Y.; Honeycutt, R. C.; Nigg, H. N., Eds.; American Chemical Society: Washington, DC, 1986; pp. 2-38, 10017, 197-255. (10) Swanson, G. J. Well WaterJ 1988, 42, 55-57. (11)Pennino, J. D. Ground Water Monit. Rev. 1988, 8(3), 4-9. (12) Cohen, S. Z.; Eiden, C.; Lorber, M. N. In Evaluation of Pesticides in Ground Water, ACS Symposium Series 315; Garner, W.Y.; Honeycutt, R. C.; Nigg, H. N., Eds.; American Chemical Society: Washington, DC, 1986; pp. 170-96. (13)Whitsell, W. J.;Hutchinson, G. D. Trans. Am. SOC.Agric. Eng. 1973,16,777-81. (14) Exner, M. E.; Spalding, R. F. Ground Water 1985,23, 26-34. (15)Frank, R. D. In Pesticide Exposure Assessment in Surface Waters, An Industry/ Agency/Un iversity Research Planning Conference; Water Quality Laboratory, Heidelberg College: Tiffin, OH, 1988; pp. 15-16. (16) Frank, R. D.; Ripley, B. D.; Braun, H. E.; Clegg, S.; Johnston, R.; O’Neil, T. Arch. Environ. Contam. Toxicol. 1987,16, 1-8. (17) Frank, R. D.; Clegg,. S.; Ripley, B. D.; Braun, H. E. Arch. Envaron. Contam. ToxiC O ~ 1987, . 16, 9-22. (18)Ripley, B. D.; Clegg, S.; Frank, R. D. Presented at the 6th International Congress of Pesticide Chemistry, Ontario, Canada, August 1986; paper 5F-07. (19) Smith, J. L., The North Carolina Pesticide Board, personal communication, June 1988. (20) Missingham, G.; Graham, H.; Burnill, B. “Alachlor Monitoring in Ontario
Metals, anions, and mode (DME, HDME, SMDE, ltaneously and with low detection limits down to the ppt range, you can determine many components often without solubilization, and in the presence of inert solution partners, high acid and salt concentrations or even undissolved substances Demanding anatvses Full keyboard, screen and printer/plotter are the com ponents of the 646 VA Processor visible from the outside. The Qcility to change to any desire& analytical method
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REPORT Drinking Water, May-November 1985”; Ontario Ministry of the Environment: Ontario, Canada, Feb. 14, 1986. (21) Ferber, R.; Sheatsley, P.; Turner, A.; Waksberg, J. “What Is A Survey?” American Statistical Association: Washington, DC, 1988. (22) Steichen, J.;Koelliker,J.;Grosh, D.; Heman,A.; Yearout, R.; Fbbbins, V. Ground WaterMonit. Rev. 1988,8(3) 153-60. (23) “The National Pesticide Survey: Phase I Report, PB91- 125765”; NTIS: Springfield, VA, 1991. (24) Holden, L. R.; Whitmore, R. W.; Graham, J. A. Proceedings of the Agricultural Impacts on Ground Water Conference; National Water Well Association: Dublin, OH, 1988; pp. 237-54. (25) Cochran, W. G. Sampling Techniques, 3rd ed.; John Wiley & Sons: New York, 1977. (26) Keith, L. H.; Crummett, W.; Deegan, J.; Libby, R. A.; Taylor, J. K.; Wentler, G. Anal. Chem. 1983,55,2210-18. (27) Hunt, D.T.E.; Wilson, A. L. The Chemical Analysis of Water: General Principles and Techniques, 2nd ed.; The Royal Society of Chemistry: London, 1986; pp, 8-28. (28) Hunt, D.T.E.; Wilson, A. L. The Chemical Analysis of Water: General Principles and Techniques, 2nd ed.; The Royal Society of Chemistry: London, 1986; pp. 29-126. (29) Scalf, M. R.; McNabb, J. F.; Dunlap, W. J.; Cosby, R. L.; Fryberger, J. Manual of Ground- Water Quality Sampling Procedures; National Water Well Association: Dublin, OH, 1981; pp. 1-38. (30) Barcelona, M. J.; Gibb, J. P.; Hel-
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frich, J. A.; Garske, E. E. “Practical Guide for Ground- Water Sampling,” ISWS Contract Report No. 372; Illinois State Water Survey: Champaign, IL, 1985. (31) Taylor, J. K. Quality Assurance of Chemical Measurements; Lewis Publishers, Inc.: Chelsea, MI, 1987; pp. 55-85. (32) Graham, J. A.; Holden, L. R. Proceedings of the Agricultural Impacts on Ground Water Conference; National Water Well Association: Dublin, OH, 1988. (33) Glaze, W. H. Environ. Sci. Technol. 1990, 24, 271. (34) Alford-Stevens, A. L.; Eichelberger, J. W.; Budde, W. L. Environ. Sci. Technol. 1988,22, 304-12. (35)Hallberg, G. R. “Hydrogeology, Water Quality, and Land Management in the Big Spring Basin, Clayton County, Iowa”; Iowa Geologic Survey Open File Report No. 83-3, State of Iowa, 1983. (36) Hallberg, G. R. “Hydrogeologic and Water Quality Investigations in the Big Spring Basin, Clayton County, Iowa: 1983 Water Year”; Iowa Geologic Survey Open File Report No. 84-4, 1984. (37) Anderson, M. P. “Comments of the Iowa Geologic Survey’s Work on Pesticides in Groundwater”; prepared for Monsanto Company, St. Louis, MO; Jan. 29, 1988; available from A. J. Klein, Manager, Regulatory Affairs. (38) Hallberg, G. R.; Libra, R. D.; Hoyer, B. E. Perspectzves on Nonpoint Source Pollution; U.S. Environmental Protection Agency. U.S. Government Printing Office: Washington DC, 1985; EPA 440/ 585-001.
Jefrey A. Graham is a Senior Research Group Leader in the New Products Division of Monsanto Agricultural Company. He received his B.S. degree (1976) from Northern Illinois University and his Ph.D. (1979) from the University of Georgia, where he studied analytical chemistry and separation science under the direction of L. B. Rogers. Prior to joining Monsanto in 1980, he was a National Research Council Resident Research Associate at EPA’S Athens, GA, research laboratory, where he worked with A. W. Garrison on development of analytical methods for the determination of trace nonvolatile organic compounds in water.
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(39) “Analysis of Pesticide Data from Big Spring 1982-1988, Clayton County, Iowa”; Project Report No. 2361-0070, Eugene A. Hicock and Associates, Division of James M. Montgomery Consulting Engineers: Des Moines, IA, May 1989.
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