€SmFEATURES
I
I Revealing the Location, Depth, and Concentration process of 3-D digital imaging Before sites contaminated below of Subsu$ace Pollutants is The ground can be cleaned up, the locademonstrated by an example that tion, depth, and concentration of the pollutant mass must be characterized. The interpretation of data collected during characterization activities at such sites requires the synthesis of diverse and sometimes voluminous data sets. The information collected typically includes chemical, geologic, and hydrologic measurements at scattered positions. After collection, the fundamentally interrelated data often are processed and presented with minimal coordination. As a result, understanding and communicating information about the site can be difficult (1-3). Three-dimensional digital imaging (TDDI), however, makes it possible to obtain and handle these disp a r a t e data i n a c o o r d i n a t e d , systematic fashion. A technically robust and cost-effective process, TDDI generates graphic images that show how the hydrogeology and geometry of a subsurface system control the flow of groundwater and the transport of contaminants. TDDI produces familiar “color photograph” images that portray underground environmental conditions in an easily understandable form. One image can show information that otherwise would be presented in hundreds of pages of data tables and many two-dimensional contour m a m of contaminant d u m e s in aquifers. The consistencv and loeic of a TDDI representation lend credence to the process of deciding on site 642 Environ. Sci. Technol.. VoI. 26,No. 4, 1992
Ralph L. Nichols Brian B. Looney JonathanE. Huddleston Westinghouse Savannah River Company Savannah River Laboratory Aiken, SC 29808 management and remediation. The consolidation of all of the data at a specific site can help characterize the site and define the types of data most needed by decision makers. In some cases, the quantity of data needed may be reduced. In one example presented below, 20 proposed monitoring wells were eliminated on the basis of TDDI results. TDDI systems can be run on modest hardware such as personal workstations. One feature of TDDI is that cbemical analyses of groundwater samples taken at different depths and depth ranges can be combined with information about the lithology and hydrology of the aquifer. The result is an understandable image of contaminant behavior. This image demonstrates the 3-D nature of groundwater and contaminant flow and can be viewed from any position in 3-D space. The information conveyed by a TDDI image can be used to focus other site characterization efforts on areas where they will be effective for planning cleanup. The steps are shown in the box “Development of the TNX TDDI Model” and discussed in more detail below.
uses data from the Savannah River Site (SRS) in South Carolina. The site is a research and testing facility at SRS, hereafter called the TNX site. Chlorinated solvents, especially trichloroethylene (TCE),shown in the form of a plume, contaminate the groundwater that underlies this site. A range of data, including contaminant data collected from multilevel well screens, bas been collected at the site. A complete site description and a description of the steps in the digital imaging process are provided in the sections that follow. Images from the site and from other areas at SRS are presented to demonstrate the utility of TDDI. Site description SRS, a 300-miz facility near Aiken, SC (Figure 11, operated for the Department of Energy, is located on the upper Atlantic Coastal Plain and is underlain by an approximately 1000-foot thick wedge of unconsolidated interbedded sands, silts, and clays. On a sitewide scale, groundwater flow is controlled by different local discharges. Flow at and immediately below the water table is toward the nearest local tributary: flow in the lower water-bearing zones is toward the Savannah River or its major tributaries. The TNX facility is adjacent to the Savannah River in the southwestern portion of the laboratory area (Figure 1).The facility itself is a semiworks-a plant where pilot-scale testing and evaluation of compo-
0013-936W9210926-642$03.00/00 1992 American Chemical Society
What is 3-D digital imaging? iree-dimensional digital imaging (TDDI) is a special type of computing and display used to generate realistic pictures for a variety of applications. An imaginary light source reflects off the objects that the computer renders. Thus the images have surface texture and shadows just like those in a photograph. The process can manipulate the picture to remove objects, add objects, and rotate and slice the volume. Recent advances in hardware and software make it possible to perform TDDl on inexpensive systems such as personal graphs workstations. Such modest hardware requirements will make it possible for ivironmental scientists to use this promising technology. TDDl has been most widely used in the movie industry, for example in Abyss and Terminator2. The medical and chemical industries use TDDl to in,rpret information from CAT scans and to model molecular structures. In medical and chemical applications, the positions and density of the data can be carefully controlled to ensure that an accurate picture is produced. A unique problem for the environmental scientist and geologist is the expense of data collection and limited access to the underground system. Often data must be collected from wells that are spaced relatively far apart. This leads to what is called a "scattered data set" because each measurement is associated with an x-, y-. and z-coordinate: these coordinates are distributed irregularly in space. Important advances in computer programming, many developed for oil exploration companies, have resulted in improved methods of gridding the :attered data so that it can be imaged. These advances in TDDl will help enronmental professionals ''see underground."
TDDl model quence of sctlvltles Drill wells-hydrogeological characterization Analyze samples, organ able data Interpret cross-sections * Digitize cross-sections * Create 3-D image * Disseminate data
Data needed for TNX exsmpl Historical waste disposal data ly lmations lnal hydrostratigrap ies
Site physiography and phology Soil and vegetative classification studies Aerial photographs River sta e and channel Topogra$icmaps Waste site characterization (e.g., soil gas surveys) Water uality data (grou and su ace water) Potentiometric data Geophysical and lithologic records Aquifer tests
4
.Siave analyses
idwater modeling, cross secand hydrologic interpretation
nents and processes are done for research and development and for testing large equipment before its use with radioactive materials. In 1984 the first series of groundwater wells was installed at the TNX site. Groundwater sampling from these wells indicated seepage from unlined basins, leaks from process sewers, and leachate from
other waste disposal activities in the area. These leaks contaminated groundwater beneath and downgradient [i.e., in the direction of subsurface flow) of the TNX facility. This facility is about 400 m east of the Savannah River [on a terrace between Upper Three Runs Creek to the north and Four Mile Creek to the south, at an elevation of approximately 46 m above mean sea level [MSL)].Immediately west of the facility is a portion of the Savannah River floodplain at 27-29 m above sea level. A small terrace divides the floodplain and serves as the hank of the river during high-water stages. The floodplain is covered by bottomland hardwoods, cypress, and tupelo trees typical of Savannah River swamps. At the site, flow in all groundwater zones is toward the Savannah River, first through shallow coastal plain and then through fluvial (stream-deposited) sediments. Additional images from the vadose zone (the unsaturated zone above the water table where pollutants may be trapped) and groundwater at the metallurgical fuel and target fabrication area (M Area) also are presented. The M Area is located near the northwestern boundary of SRS. Similar to TNX, the suhsurface in M Area consists of interhedded coastal plain sediments with approximately 40 m of vadose zone that overlies two relatively disconnected aquifer systems. The use of degreasing solvents in the M Area process plant and at the adjacent research laboratory resulted in releas-
es of contaminants to the subsurface between 1955 and 1980. Full-scale action to remediate this contamination was under way by 1985. More than 350 monitoring wells, completed at various depths, have been installed to monitor the quality of the groundwater underlying the M Area. Description of process Data collection. The data collected at the TNX site range from general facility history (procedures and records) to detailed technical site characterization. Historical data at the TNX facility consist of files and reports that describe the period of operation, the type and location of waste disposal, and operating procedures, as well as lists of chemicals used. This information determines the type of data to he collected, including well locations, suspect constituents, and data collection points. Historical data combined with monitoring, water level, and flow data, as well as modeling results, are used to define the upgradient [i.e., direction from which the groundwater flows) margin of contamination for the 3-D imaging process. Various site maps and other data provide information on the shape of the site (physiography] and the affected environment. These data include topography, classification of soil, identification of wetlands, and description of ecological setting. The topography data were digitized for direct use in the imaging process. Much of the other data in this category provides evidence that the imaging process is properly representing the site. For example, in and near wetlands plant species and soil series are indicators of recent and historical positions of the water table. Such data, combined with that on topography and surface water hydrology, are used to define the hydrogeologic boundaries of the system. The hydrogeologic boundaries can he viewed as the locations where water enters and leaves the system. The rate of water movement is controlled by the characteristics of the subsurface materials and the subsurface hydrologic gradients. Characterization of the hydrologic properties and variability of the types and nature of materials that make u p the subsurface is performed using surface geophysics and data from pump tests and monitoring wells. Data from monitoring wells at the TNX site include field and laboratory logs that identify materials in a core, sieve analysis of Environ. Sci. Technol., VoI. 26,No. 4, 1992 643
The Savannah River site In 1950, international tensions prompted the US. Atomic Energy Commission (AEC, now the Department of Energy) to build the Savannah River Site (SRS) to produce nuclear materials for national defense. The atomic age had begun about 10 years earlier. On Sept. 23,1949, President Harry Truman announced the successful testing by the Soviet Union of its first atomic weapon. As a result, Truman directed the AEC to expand the development of U.S. nuclear weapons. In 1950, the AEC and President Truman requested Du Pont to design, construct, and manage a nuciear material production facility. The search for land began in July of that year. One hundred fourteen potential sites in 18 states were examined before the western South Carolina location along the Savannah River was chosen. Westinghouse succeeded Du Pont as the operating contractor in 1989. To carry out its mission, SRS has operated several major facilities that COI er about 10% of the 310-mi2 area. These include nuclear reactors, chemical separations areas, a heavy water extraction plant. a metallurgical fabrication facility, waste management areas such as the Defense Waste Processing Facility, and research laboratories. The remainder of the land is pine forest that is protected and managed by the U.S. Forest Service. In 1969, DOE designated SRS as a National Environmental Research Park to encourage the use of protected land as an outdoor ecological laboratory. SRS currently dedicates $800 million per year to environmental restoration. The two TDDI example sites discussed in this paper are the metallurgical fabrication facility (M Area) and a small research and semiworks facility known as TNX. Both sites ha\been contaminated by chlorinated solvents over the years.
the core, borehole geophysical logs, and water elevation and bulk water quality data. Information about the depositional setting [Le., under what circumstances the sediments were laid down) is incorporated into the portrayal of hydrogeologic and geologic cross-sections. Calibration of groundwater flow models provides insight about the characteristics of the subsurface system. Much of the data described above is used to define the site and to generate a preliminary or conceptual model of the system and, eventually, a digital image. First, 3-D contaminant concentration data from monitoring wells and soil gas surveys are collected. Next, a series of transformations and calculations is performed to develop an image that is consistent with the concentration measurements and the conceptual model of the subsurface system. Listed in the box “Development of the TNX TDDI Model” are 15 kinds of data used for creating an image of the plume of volatile organic carbon [VOC) contaminants in the aquifer beneath and downgradient of the TNX site. Some of the data are used mainly to generate the conceptual model of the site: other data are used explicitly [e.g., to define tops of zone); and still other data represent a consolidation of information that is performed before inputting into the imaging process. Hydrogeologic interpretation. The facility is underlain by sediments composed of interbedded 644 Environ. Sci. Technol.. Vol. 26, NO. 4, 1992
sands, silts, and clays that were deposited on hard (crystalline) bedrock. Water-bearing sediments at the TNX site can be divided into two main aquifer systems: an upper or shallow system and a lower or deeper system of sediments, which are separated by a confining zone [a zone through which little or no groundwater can pass). The relatively higher hydraulic head in the lower system limits the flow of water released to the subsurface from TNX to the shallow aquifer system. This shallow system consists of a water table zone and a semiconfined zone beneath TNX. Water enters the shallow aquifer system through recharge at the ground surface and then discharges to the Savannah River and surface water in the floodplain. This volume of discharge, combined with the characteristics of the subsurface layers, controls the rate of horizontal and vertical groundwater movement. Water in the water table flows downward and toward the Savannah River in the eastern portion of TNX, which is located at a topographic high near a groundwater divide [a high point or ridge that separates the directions of water flow). As the groundwater approaches the floodplain and discharge area, the vertical gradient of flow rate shifts as the upper and lower zones join and their water flows upward into the seeps [i.e., spots where water oozes to the surface) and river. The resulting curved path [downward
and then up into the receiving surface water) often is taught in hydrogeology classes, but is seldom seen in waste site study reports. The TDDI process is an ideal tool to show this behavior. Data integration The background information and groundwater characterization data collected for the TNX model were integrated into interpreted cross sections (Figure 2 ) . These cross-sections represent the model area in a series of two-dimensional planes that run parallel and perpendicular to the direction of groundwater flow, and are used to generate the 3-D image. During the preparation of cross sections, care was taken to ensure that the contours that represent concentrations of a VOC-in this case, TCE-agree with actual data from the monitoring wells: and to ensure that the model shows that the migration of the contaminant plume follows the horizontal and vertical gradients. For the TNX site, where the plume is small relative to the size of the monitoring network and thin relative to the width of the w e l l s c r e e n s [see box “ W e l l Screens”), the assembly of a few carefully interpreted cross sections leads to a TDDI image that integrates all of the types of data listed above. Water drawn through each well screen makes up a sample that provides a composite concentration of contaminants. Because the plume is thinner than the width of the well screen, the cross-section process allows the vertical distribution of TCE to be depicted as a contour image that accurately represents actual subsurface conditions (Figure 21. Also, use of a cross-section allows the plume to be shown in a way that conforms with data on hydrologic gradients and groundwater divides. Internal consistency between crosssections is preserved by making sure that there are no mismatches in TCE contours where cross sections meet. Data in the cross-sections were converted to digital form to construct a computer-compatible data set that contained 3-D information on the TCE distribution in groundwater at TNX. A local rectangular coordinate system coincident with the cross-sections was used to facilitate the process. Digitizing crosssections that run parallel to groundwater flow, this process generated data sets with constant north coordinates, varying east coordinates
t
and elevations (depths) for each TCE concentration value. Cross-sections oriented perpendicularly to groundwater flow yielded data sets with constant east coordinates, varying north coordinates, and elevations for each TCE concentration value. The data from all crosssections were merged to generate a scattered data set that the digital imaging software could process. The final scattered data set took the form of x,y,z, p; where x is the east coordinate, y is the north coordinate, z is the elevation, and p is the property to be modeled (TCE concentration).
Data modeling The 3-Dinformation on TCE concentrations was modeled with the aid of the Interactive Volume Mod-
eling (IVM) software package developed by Dynamic Graphics (3) and runs on a personal graphics workstation (Silicon Graphics). IVM uses the scattered data set and a minimum tension algorithm to calculate a uniform 3-D grid that represents the distribution of the TCE concentration within the model domain. A uniform grid containing 102,400interpolated points was generated. The top and bottom surfaces of the gridded 3-D domain can be constrained by irregular two-dimensional surfaces that represent the water table, surface topography, or aquifer boundaries as appropriate. Once an image file is prepared the model can be viewed as a multicolored shaded solid in which each color represents a different 3-D con-
tour or shell of TCE concentration. The Silicon Graphics workstation that was used for this project is capable of rotating, cutting, and slicing the image in real time. At first, the data were modeled using the actual TCE concentrations that were digitized. Despite an approximate grid spacing of 50 feet x 50 feet x 2 feet, examination of the results indicated that the scheme of interpolating values to construct shells between data points did not adequately represent the steep concentration gradients (e.& 5000 Fg/L to < 5 pgIL in 100 feet in one area). To properly depict the steep changes in concentration that are typical of plume fronts in an aquifer, the log,, of each concentration was obtained before the 3-D grid was deEnviron. Sci. Technol.. Vol. 26,No. 4, 1992 645
veloped. Once this grid was prepared, a n antilog transfer was performed on the grid to restore the values of the true concentrations hefore the image file was generated. This procedure led to a good match between the interpreted cross section and a slice through the model [Figures 2 and 3). Results Once the image file is generated, the model may he viewed from any angle, layers may he omitted, sections can he removed to show the core of the model, and shells of low concentration may he removed to show the fundamental shape of the plume. Figure 4a is a perspective view of the TNX area showing surface topography (shaded brown surface), the vadose zone (light green), and the water table zone. The unconfined aquifer discharges to the Savannah River and to local seeps in the floodplain east of the river. 646 Environ. Sci. Technol.. Vol. 26, No. 4, 1992
Note that the wetlands visible on the figure result entirely from the gridded water table surface intersecting the digitized topography. In addition to water, the plume of TCE can he seen cropping out into the seeps adjacent to the Savannah River. This striking visual representation shows fundamental concepts of hydrogeology a n d hydrologic boundaries, In Figure 4h, the vadose zone has been deleted. The lines across and beneath the water table surface represent the screened intervals in groundwater monitoring wells at TNX. The variation of concentrations on the surface of the water table is consistent with the expected trajectory of the plume-downward near the source area and upward as the water approaches draining boundaries. The concentrations on the surface of the water table shown on Figure 4 are more representative of the actual spatial distribution of
contaminants at TNX than those on a two-dimensional map representing the composite samples collected from fluid drawn through well screens several feet thick. It is clear from Figure 4 that new methods for depth-discrete water sampling could yield clean samples from shallow groundwater upgradient of the leading edge of the plume. In Figure 5, the shells representing low concentrations have been removed within a section of the model to demonstrate the geometry of the core of the plume. Note that the shape of the plume is well defined and that the position beneath the ground can he clearly interpreted with data from a minimum numher of monitoring wells. The adequacy of t h e monitoring well network can he evaluated using the images. Such an exercise at TNX has led to a recommendation of only four additional well clusters20 fewer wells than originally pro-
I
I
FIGURE 3
Cross section of 3-D model parallel to direction of groundwater flow at TNX"
I
-
I
FIGURE 4
(a) Perspectiveview of TNX Area model showing location of groundwater outcrops (b) Same view with vadose zone removed to reveal the distribution of groundwater contaminationat the water table'
posed. Data from these wells will confirm the plume behavior between the Savannah River and current well locations. TDDI indicates that the resulting monitoring well network will he able to provide the information necessary to determine the horizontal and vertical extent of contaminant migration and to monitor the effectiveness of future corrective actions. Preliminary evaluation of the ability of the TDDI process to estimate underground volumes (similar to estimating oil reserves) suggests that it can serve as an integral part of the design of corrective action. The volume estimation utility provides detailed estimates of volumes of fluid, soil, and rock, and inventories of contaminants throughout the model domain. For instance, the plume shown in Figures 2 through 5 contains approximately 31 kg of TCE. Figure 6a is an image of a different type of subsurface data collected in the M Area. Near a former solvent source location, duplicate core analyses for contaminants were collected on more than 385 sediment
c c
I
Annotaton show screen zones tor rnonaoring wlk, bwndary of TNX. and lacations ol digitized imerpmed c r o s &ions.
samples from 15 boreholes. This image depicts TCE concentrations in sediments above the water table. The low-concentration shells are removed to show the structure of the contamination. The TCE is localized within zones at elevations of 62 m, 96 m, and 99 m above MSL. The sediments in
these locations have a significant component of clay-sized material, suggesting that these sediments lie in the primary zones of residual vadose-zone contamination at this site. The data from the sediment analyses will be used to help design a vacuum-extraction system for vadose-zone remediation. The images Environ. Sd.Technol., Vol. 26. No. 4, 1992 647
FIGURE 5
Cut-away view of model with lower concentrations(< 50 CglL) of TCE removed to illustrate geometry of plume core Y
FIGURE 6
(a) Distributionof TCE in sediment in the vadose zone and upper aquifer zone at a research site in M Areaa (b) Oblique angle view of A/M Area groundwater contaminationb
a Contamination levels z
1 pg!g in the sediment are Shown. 'A block of the mcdel haJ been removed to reveal the intricate 3-0geometry of the groundwater contamination plume.
648 Environ. Sci. Technoi., Vol. 26, No. 4, 1992
also will be compared with postremediation images generated from new core samples to determine the effectiveness of the vacuum process in removing contaminants from the clay zones. Figure 6b shows the groundwater plume of TCE in M Area with the shells of low concentration removed. This plume is significantly larger than the plume at TNX, so the need to generate cross-sections is eliminated. Therefore, the x, y, z, and p data were downloaded directly from the central groundwater monitoring database at SRS for imaging. The entire imaging process for Figure 6b was completed in a few hours, and the results are more accessible than they are in the original 71 pages of data tables. Revealing conceded features In addition to concentration data, any measurable parameter such as hydraulic conductivity can be measured and illustrated. The interpolated-imaged data can be extracted at selected locations as input arrays for traditional flow or transport models. The flow path data generated from traditional groundwater models can readily be reincorporated into the image and plotted in 3-D alongside the contaminant plume. The most important advantage of 3-D displays is the way in which they enhance our ability to visualize a site and solve its problems. The images portray data from the solid subsurface system in a familiar manner that can answer questions and solve problems (21. TDDIgenerated images are satisfying because their dimensionality matches the system they represent. The ability to reveal features that are concealed from view is a powerful tool. The advantages of TDDI are predicated on proper application of the technology. Careful "reality checking" and basing decisions on technical data when setting gridding and imaging parameters are key elements in the success of any TDDI project. Carefully applied, TDDI can play a significant role in contaminant hydrogeology.
Acknowledgment The information contained in this article was developed by Westinghouse Savannah River Company (WSRC) during the course of work under contract No. DE-AC09-89SR18035 with the U.S. Department of Energy. Reference herein to any specific commercial roduct does not necessarily imply en orsement by WSRC or the U.S. government.
x
Rolph L. Nichols received a B.S. in geological engineering from the University of Missouri-Rolla and an M.S. in environmental engineering from the University of Oklahoma. Since 1988 h e has been an environmental engineer in the Environmental Sciences Section at the Savannah River Laboratory. His activities include waste site studies and development of geological and hydrological methods for these studies.
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Brian B. Looney received a B.S.in environmental sciences from Texas Christian University and a Ph.D. in environmental engineering from the University of Minnesota. Since 1983 h e has been a research environmental engineer in the Environmental Sciences Section at the Savannah River Laboratory. His interests include development of innovative methods for environmental remediation, modeling. and risk assessment.
Jonathon E. Huddleston is a graduate of the Vanderbilt University School of Engineering with a degree in civil and environmental engineering. He has participated in a DOE hazardous waste internship at the Savannah River Labomtory, Heiscurrentlya~endinglawschoolat Wake Forest University.
References (1) Mahoney.
mas cf chemists He fo o m tne evo Ltion of :hemetry from the Stone Age wginn ngs of :eram cs ana metal Jg) m * o ~ q nt i e r se an1 def neofachemy.totnec.lm~a!onofclas. rca cnemistry n me,ate 19rh cent.!) Ciapters 1 tnro-gn 9 eac from prenistor c tecinooq. thro-gn ancent ana mecera sei. ence to t i e SLdy of cnem cas a i 0 reactions that res.ltw in m e 16111CeOLry Oicn of Sc1. enr fic cnemistry S.aseq.en1 cnap1e.s fm.8 on hey ciemets such as Sa a Bo) e. Back La voise' Dalton Berze us burert ana Arrnen ILS as tne) debeopea tne iaeas t'la! ec to c ass tal cnem st-) ana t i e concepts of mole. c ~ e schemca . reactioos. nomooq). talence. anc moec.ar form.lasara str.ct.res. among ofners. Twenr) topical 11 .strations einance the ten SIX t me ines and M Omaps he p reaaers Lnaerstana t i e nflLences of ear, h story on cnemistry About the Author hLgn W.Sazberg taLgnt cnem.st!) at the Ct L ni.ers r) of heu Vord for 35 ,ears ana of. fered coJrws n the nisroq of cnem st0 Over a per oa of 20 )ears from Caiman ro Cnem nsr reflects h s da. pass ons for cnem.stO an' histo-) ana n n profoLia adm rat on of the great m ias tnat deteopea t i e iaeas of chemst!) Huah W. Salzkra 306 pages (1961) Clothbound ISBN 0-8412-1786-6 924.95
Paperbound ISBN 0-8412-1787-4 414q5
Genetics, and Cell Biology
P
resenting the proceedings a i the Third International Meeting on Cholinesterases. this inaugural volume in the Conference 'roceedings Series offers a wealth of new in'ormation on current and future cholinester~ s eresearch. including important advances reiulting from new concepts and methodologies such as monmlonal antibodies and molecular Jenetics. The volume's 49 full oaoers and 140 ooster papers are d viaed into s x sect ons coger ng
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Po ymomniim and S t r n i r e of Cholmeneiases Co .la1 B o 09) of [nolinenerases Gene Strunwe and Espiesion 01 cnaimenerars e Ca!aly c MPtnmm 01 [no IIIOROIB*PI S1mnrre. i.nn on Re a i onrnipr of Anl#(nOlmORerm Aqenlr. Nerve Aqenli ma %liados B Pndrmxo oq [ I JI m o n 01 Anl~molmeneraw Aqmr 11o.~opalnoio~ of Cno nerqic Synemr honmolmerq Roles 01 00IIIPIIPIISPI
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Trls . ? r e E De cf great pierest to a 3roaa spect'.m of 'eaae's. nc'.c ng inow n. te'estec * tne eia .ti01 of cno 1nes:erase cata,sis researcners de.eoomg agr c.f.ra :nemtcals sc ent sts seek ng Jptodafe nformation on the treatment Of 9a.cona ana L C ~r e u o ogica a wases as A zne mer's d 5. ?ase and m,aslhenia q a v s those irerested in t i e aesign cf ar.gs to bna the enzyme t. jeif or to cno inerg c receptors. as we as Inow &no fo ow me progress towarc corn. p ere str.ctLre euciaation of chol pesterases lean Massoulie. Cenrre National de la Yechercne Scientifique. Eairor :rancis BacaL. lnrtitut National de la Yecherche Agronomique. Emor Eric Barnard Medica Research Council. Edntor Prnaud Chatonnet. Instirut National del la Rernerche Agronomique. Editor Bhupenara P. Donor. Walrer Reed Army InstitJre of Research. Editor Damet M. QJ nn Univerr ly of Iowa. Editor Conference Proceedings Series 414 pages (1991) Clothbound ISBN 0-8412-2008-5 989.95
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American Chemical Society Dlrt'lbutio? O%e. Ow: 88 I155 4aeen:l Si.. N Pi
D. P. Computer Graphics
World 1991, 1 4 ( 1 ) ,89-90,
(2)
Cholinesterases Structure, Function, Mechanism,
Van Driel. J. N . In Digital Geologic
and GeographicInformation Systems; Van Driel. J. N.;Davis, C. I., Eds.;
American Geophysical Union: Washington. DC 1989 p p . 57-62. (3) Briggs, 1. Geophysics 1974,3$1), 3 M 8 .
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Environ. Sci. Technoi.. Vol. 26. No. 4, 1992 649
