Numerical Simulations to Assess the Monitoring Zone Achieved during Low-Flow Purging and Sampling

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1 Numerical Simulations to Assess the Monitoring Zone Achieve uring Low-Flow Purging an Sampling by Mark D. Varljen, Michael J. Barcelona, James Obereiner, an Davi Kaminski Abstract A etaile three-imensional numerical simulation of groun water flow into the screen of a monitoring well uring low-flow purging an sampling provies a means to investigate the actual monitoring zone (i.e., where the water comes from ) achieve uring these sampling operations. Such a simulation allows for examination of the influence of several variables such as pump placement, well screen an san-pack configuration, an aquifer heterogeneities on the actual part of the aquifer that is interrogate. In these simulations, groun water was sample from the entire length of the well screen, although as coul be expecte, flux was higher into the well screen from zones of higher permeability, if present. Furthermore, at low flow rates, pump position within the screen ha little influence on the vertical istribution of flux into the well, an in all cases where a monitoring well partially penetrates an aquifer, the zone monitore extene above an below the well screen. At the low flow rates examine (250 an 500 ml/min), there was no increase in flux (from the aquifer into the well screen) ajacent to the pump intake. The major implications of these finings are that a etection monitoring approach preicate on sampling from the entire well screen is vali when purging at low flow rates (i.e., contaminants are not misse ), or conversely, that iscrete vertical sampling may only be accomplishe with very short well screens ue to potential concentration averaging that woul occur along the entire length of a screen. Finally, the simulation results emphasize that aquifer heterogeneities must always be recognize an consiere in groun water monitoring esign. Aquifer heterogeneities have the most significant influence on the actual monitoring zone (an therefore the potential to hit or miss contaminants in a etection program). Pump placement an purging rate have little influence on the vertical interval that is sample (as compare to screen length an placement) an therefore shoul be selecte only so as to minimize sampling artifacts an promote sampling efficiency. Introuction The benefits of low-flow purging an sampling in terms of ata quality (repeatability, control of false positive results) have been ocumente in the literature for both volatile organic compouns (Barcelona et al. 1994) an metals (Puls et al. 1992). Furthermore, the operational efficiency an cost-effectiveness have been well establishe (Schilling 1995; Shanklin et al. 1995). Acceptance an actual implementation of low-flow sampling methos in some situations, however, have been slow or ifficult. An often-cite concern is the assumption that a sample collecte at a low flow rate only originates from a very narrow vertical interval near the pump intake an that contaminants coul be misse. Because of this concern, the issue of where to place a pump in the well screen has become a topic of interest, especially with relatively long-screene wells (i.e., 10 to 20 feet). Copyright ª 2006 The Author(s) Journal compilation ª 2006 National Groun Water Association. In general, these questions result from a limite unerstaning of the hyraulic performance of wells uner lowflow pumping conitions. Elci et al. (2001) performe a etaile simulation of a monitoring well; however, the simulation was only performe uner ambient (nonpumping) conitions. The publishe stuies of well performance uring pumping have generally focuse on concentration changes over time (Reilly an Gibs 1993; Reilly an LeBlanc 1998; Martin-Hayen 2000a, 2000b; Gibs et al. 2000) rather than where the water that is entering the pump intake comes from. Furthermore, publishe stuies on well performance have generally been conucte assuming much higher flow rates than those use for low-flow purging or i not evaluate the specific scenario encountere uring low-flow sampling (i.e., the pump locate within the screene interval). For example, Cohen an Rabol (1988) conucte a simulation of hyraulic performance uring sampling. The focus of that work was to assess well esign. More 44 Groun Water Monitoring & Remeiation 26, no. 1/ Winter 2006/pages 44 52

2 specifically, Cohen an Rabol (1988) investigate whether or not extening a san pack across the entire thickness of a confine aquifer (fully penetrating san pack) can allow a short-screene well to collect water from the entire aquifer. While this work i aress where the water was coming from, the simulations were conucte at pumping rates (1 gpm or 3785 ml/min) up to 15 times higher than rates typical of low-flow sampling (250 to 500 ml/min), an in all simulations, the pump intake was locate above the screen. The simulations escribe in this publication examine the effect of pump placement within the screen as well as lower flow rates. Objectives The objective of this work was to assess the vertical interval sample uring low-flow purging an sampling. In aition to showing flowpaths (on a cross section) entering the well screen uring low-flow purging an sampling, the work aime to eluciate the effect of iffering well geometry, pump placement, an subsurface heterogeneities on the vertical istribution of flux into the well along the length of the screen. Specifically, the following are investigate in terms of the vertical flux istribution into the well screen: Screen length Pumping rate Submerge screen vs. screen open across water table Pump position in screen Formation anisotropy an heterogeneity San-pack configuration Full vs. partial penetration. This work is not intene to evaluate wellbore processes that may alter chemical concentrations as the water moves from the screen entrance to the pump intake (e.g., mixing within the well) but rather iscusses the origin of the water sample an the vertical istribution of water flowing into a well screen uring sampling. Base on observations by Puls an Paul (1998), it appears that mixing processes insie the screen are not a significant factor in terms of sampling results. The ata presente by Puls an Paul (1998) emonstrate that for all practical purposes complete mixing occurs even uner low-flow pumping (there may be zones insie the wellbore that o not mix; however, these zones o not affect chemical concentrations in a sample). Therefore, the vertical istribution of water flowing into a well screen uring sampling is consiere to be the most significant factor controlling chemical concentrations in the groun water sample. Approach Simulations were implemente with MODFLOW (McDonal an Harbaugh 1998) using a finite-ifference gri spacing of 2 inches. Input of the moel structure, population of the finite-ifference gri with require parameters, an evelopment of the ata files necessary for MODFLOW was complete using the program Processing MODFLOW for Winows Ò (Chiang an Kinzelbach 2001). Processing MODFLOW fully implements the many features of MODFLOW, incluing thir-party aitions to the original MODFLOW coe, which inclues horizontal flow barriers, preconitione conjugate graient solver, etc. Steay-state flow in the formation near the well, san pack, an insie of the well uring low-flow sampling was simulate. It shoul be emphasize that the simulations illustrate steay-state conitions. That is, stabilization of the pumping water level in the well is complete. This is analogous to fiel-observe stabilization of the pumping water level (an water quality inicator parameters) that must be achieve prior to sample collection uner stanar low-flow sampling protocols (Puls an Barcelona 1995). Therefore, the results are irectly applicable to true fiel conitions. Following the steay-state flow simulations, flowpaths were plotte in a cross-sectional view. Flowpaths were compute using PMPATH for Winows (Chiang an Kinzelbach 2001). PMPATH is a particle-tracking program that irectly uses the result files from MODFLOW. PMPATH uses the semianalytical particle-tracking scheme escribe by Pollock (1988) an use by the program MODPATH (Pollock 1989) to calculate the groun water paths an travel times. While the first version of MODPATH is limite to simple forwar tracking in steay-state flow fiels, PMPATH oes forwar an backwar particle tracking in a steay-state or transient, two- or three-imensional flow fiel. After plotting flowpaths, the cell-by-cell flux terms generate by MODFLOW representing groun water flow into the well screen were tabulate an plotte to obtain a graphic representation of the flux istribution along the length of the screen. Finally, the proceure was repeate with ifferent configurations to illustrate the effects of pumping rate, screen length an configuration, permeability, pump position, iffering san packs, subsurface heterogeneities (horizontal:vertical anisotropy an stratification), an bounary conitions (i.e., fully penetrating vs. partially penetrating screens). Moel Setup Figure 1 illustrates the horizontal configuration of the moel. The vertical configuration is shown in Figure 2. The omain was 37 rows by 37 columns an was 78 layers (106,782 total cells). The screen zone spanne 30 layers, proucing a true three-imensional simulation, rather than a conventional layere or pseuo three imensional simulation where the well screen is moele as only one layer. If the well was moele as a single layer, vertical variations of hyraulic flux into the well screen coul not have been evaluate. Each cell was two inches square (for a total omain size of ~6 by 6 feet). The thickness of each layer was uniform in each simulation; however, it varie epening on the screen length specifie in each scenario. Layers were 2, 4, or 8 inch thick for 5-, 10-, an 20-foot screen simulations, respectively. Constant hea cells were specifie along two borers to set up an ambient graient of 02 ft/ft. Before exhaustive moel runs were complete, the possible influence of these bounaries was evaluate by completing a baseline simulation an a moifie simulation M.D. Varljen et al./ Groun Water Monitoring & Remeiation 26, no. 1:

3 Constant Hea = 13ft Constant Hea = ft Horizontal Flow Barrier (4 cell faces) to Simulate Well Casing or Screen (epening on conuctance) High Hyraulic Conuctivity To Simulate Insie Of Well Casing or Pumping Cell at Pump Intake Elevation Grout Seal or Sanpack (epening on hyraulic conuctivity) Graient=02 Figure 1. Horizontal layout of moel omain. with an irregularly space gri that allowe the constant hea cells to be move farther away from the pumping well. Results of the baseline simulation (hyraulic hea contours an configuration of groun water flowpaths) were compare to the moifie simulation an no ifference was note. This sensitivity analysis inicate that the bounaries were locate sufficiently far enough away from the pumping well to prevent any influence. The monitoring well was not represente as a single pumping cell, rather a collection of features esigne to simulate the complexities of monitoring well esign. The only pumping cell esignate represente the actual intake of the pump. Horizontal flow barriers were specifie as shown in Figure 1 to represent the well casing. Barriers were not use for the well screen, inicating very little resistance to flow, which is consiere vali for the graients an intake velocities achieve uring low-flow sampling. A san pack was represente by a zone of high hyraulic conuctivity (10 ÿ2 cm/s) immeiately surrouning the well screen an extening 1 foot above the well screen. The annular space above the well screen was simulate to be a grout seal by specifying a low (10 ÿ7 cm/s) hyraulic conuctivity in the borehole outsie the well casing above the san pack. The insie of the wellbore was simulate with a very high hyraulic conuctivity (1000 cm/s or 10 6 times greater than the formation hyraulic conuctivity). While pipe flow an mixing were not explicitly simulate with this approach, a contrast in resistance to flow between the outsie of the screen an the insie was represente. Because it was not known exactly what the contrast shoul have been to represent a real worl monitoring well, the sensitivity of the simulations to this assumption was teste. It was etermine that the simulations were not sensitive to the hyraulic conuctivity value selecte for the insie of the well screen, provie the hyraulic conuctivity was higher than the surrouning formation (which will always be true). This simplification was therefore of no consequence for the assessment of hyraulic performance uner steay-state conitions. However, ue to this simplification, the moel was not capable of simulating transient mixing. The results of these simulations are therefore only applicable to groun water sampling if sample collection is conucte after hyraulic steay-state conitions have been achieve. As state earlier, hyraulic steay state is a requirement prior to sampling to ensure water representative of the formation (an not the stagnant water in the blank casing above the well screen) is sample. In the fiel, the steay-state conition simulate by the moel is inicate by the stabilization of the pumping water level (i.e., when the yiel of the well catches up with the pumping rate). Results Baseline Scenario The baseline scenario consiste of a monitoring well partially penetrating a homogeneous isotropic formation with a hyraulic conuctivity of 10 ÿ3 cm/s. The pump intake was positione in the center of a 10-foot-long well screen. The pumping rate was 250 ml/min. To initiate a flowpath analysis, particles were place at regular vertical intervals along the upgraient sie of the omain an forwar tracke. Figure 3 illustrates the configuration of flowpaths entering the well screen (paths not capture by the well are not shown). Only flowpaths that go into the well are epicte to allow for a convenient illustration of the capture area. All flowpaths above an below those shown 46 M.D. Varljen et al./ Groun Water Monitoring & Remeiation 26, no. 1: 44 52

4 Grout San Pack Screen Zone Pump Intake En Cap Figure 2. Vertical configuration of moel omain. are not capture an continue on in the ambient flow fiel. It is noticeable that the actual monitoring zone (the area encompasse by the flowpaths) extene above an below the screen an that the actual monitoring zone extene further above the top of the screen than it extene below the bottom of the screen. This was ue to the presence of the san pack. The san pack extening above the top of the screen essentially extene the length of the screen. It shoul also be note that the flowpaths originating from above an below the screen bunche up where they entere the screen. This is consistent with the work Cohen an Rabol (1988) conucte at higher pumping rates with the pump intake above the screen. The Cohen an Rabol (1988) simulation inclue a confining layer below the screen; however, the san pack extene above the screen, which resulte in a greater component of upper formation water entering the screen. Similar to the Cohen an Rabol (1988) simulation, the san pack acte as an extension of the screen, an the flowpaths collecte by the san pack were focuse into the top portion of the actual screen. In the baseline scenario, of the 12 particles that were tracke, 5 entere at the top 1-foot interval compare to 1 to 2 entering in the center 1-foot interval. Four particles entere at the bottom 1-foot interval compare to the one or two that entere at the center 1-foot interval. Where there is no confinement above or below the screen, the vertical imension of the the actual monitoring zone (as efine by the area inscribe by the flowpaths that are capture by the well) was relate to the screen zone length by a factor of approximately two (e.g., for a 10-foot screen, a vertical area of ~20 feet was capture). Note that this specific fining may only apply to this baseline scenario. Many site-specific factors (heterogeneity, anisotropy, etc.) can affect the specific vertical extent of the area capture. To further evaluate the movement of water into the screen, cell-by-cell flow terms were tabulate along the length of the well screen. These results are shown in Figure 4 for the baseline case, as well as a simulation conucte at a pumping rate of 500 ml/min. As a reliability check for each simulation, all the cell-by-cell fluxes into the well screen were summe an compare to the expecte withrawal from the well. In all cases, the summe fluxes were within 10% of the withrawal rate. Also, shown in this figure is a graphical representation of the scenario, incluing pump intake location. The graphical representation of tabulate flux terms is positione ajacent to the M.D. Varljen et al./ Groun Water Monitoring & Remeiation 26, no. 1:

5 Well Casing (Horizontal Flow Barrier) Grout 1 1 Homogeneous; Intake at Mipoint Pump Intake at 10 ft 2 Flux (ml/min) per 8-inch of Screen Figure 5. Twenty-foot screen. Screen Zone (10 ft) Pump Intake () San Pack (k=.01 cm/s) flux istribution calculate for the 20-foot simulation. The vertical flux istribution was essentially the same as that etermine for the 10-foot screen. To allow for a more etaile, higher resolution evaluation of other scenarios, a 5-foot screen was consiere. Figure 6 shows the flux istribution for the 5-foot simulation, which was essentially the same as that shown with the baseline 10-foot an 20-foot simulations. Because the flux istributions were similar for the three ifferent screen sizes, it was etermine that further simulations woul be conucte assuming a 5-foot screen in orer to achieve more etaile resolution. En Cap Figure 3. Illustration of flowpaths Uner baseline scenario. screen illustration, so it can be seen where the zones of highest flux relate to the screen. Screen Length Recognizing that in practice many legacy well screens are 20 feet in length, the baseline simulation was re-create using a 20-foot-long screen. Figure 5 shows the Anisotropy The baseline scenario isplaye significant vertical groun water movement near the well; therefore, the effect of ifferent horizontal an vertical hyraulic conuctivities was explore. A ratio of 10:1 (horizontal:vertical) was specifie. All other parameters an assumptions employe for the base case were kept constant. Results are shown in Figure 7. As can be seen in relation to Figure 6, the flux istribution into the well uner anisotropic conitions was essentially the same as that simulate in the base case. Layering To illustrate the effect of a monitoring well screen intersecting a layer of higher permeability, the base case was moifie to inclue a thin (1 foot) layer of increase hyraulic conuctivity positione 1 foot below the top of the screen. The resulting flux istribution is shown in Homogeneous; Intake at Mipoint Q=500 ml/min Pump Intake at 5 ft Flux (ml/min) per 4-inch of Screen Homogeneous; Intake at Mipoint Figure 4. Ten-foot screen. Figure 6. Five-foot screen. 48 M.D. Varljen et al./ Groun Water Monitoring & Remeiation 26, no. 1: 44 52

6 Anisotropic (Kh:Kv=10:1); Intake at Mipoint Layere Inhomogeneous; Intake at Top Figure 7. Effect of anisotropy. K=10-2 cm/s Pump Intake at 1 ft Figure 9. Results of moving the pump intake ajacent to the zone of high permeability. Figure 8. As can be expecte, a very large increase in flow into the well occurre ajacent to the high-permeability layer. These results are consistent with those obtaine by Cohen an Rabol (1988) at a much higher flow rate an with the pump intake locate above the screen. Pump Intake Location The potential benefit of locating the pump intake ajacent to the zone of highest permeability was investigate. Figure 9 shows the results of moving the pump intake ajacent to the zone of high permeability. Compare to Figure 8 (intake in the mile of the screen), no ifference was observe in where the water comes from. The same conclusion may be rawn for position of the pump intake for the homogeneous base case. Figure 10 shows the base case simulation with the intake positione at the top, bottom, an above the screen. Compare to each other (top, bottom, an above the screen) an compare to the base case simulation with the intake in the mile of the screen (Figure 6), no ifference in the location of water entering the well was seen. Further Investigations of Nonuniform Flux Distribution Even though the results showing higher flux per unit screen length at the top an bottom of the screen compare to the center of the screen is consistent with previous work (Cohen an Rabol 1988), further investigation was conucte to help characterize this phenomenon an emonstrate the reasons for this nonuniform istribution. In the baseline simulation, it appeare that the san pack extening above the screen collecte water an irecte it into the Layere Inhomogeneous; Intake at Mipoint K=10-2 cm/s Figure 8. Effect of layering. screen (potentially proviing a partial explanation of the higher flux observe at the top of the screen compare to that observe at the center). To investigate this observation, a simulation was conucte without the san pack extening above the screen (i.e., the grout seal extens all the way to immeiately above the screen). As shown in Figure 11, this limite the isproportional amount of flux at the very top of the screen; however, it was not eliminate. Reucing the vertical hyraulic conuctivity (anisotropy ration of 10:1) further ecrease the isproportionate flux (Figure 12). Sealing the borehole below the screen (Figure 13) similarly reuce the isproportionate flux from below the screen. Finally, a fully penetrating simulation (Figure 14) emonstrate uniform flux along the length of the screen. This group of simulations emonstrate that the isproportionate flux was ue to vertical movement of water into the screen from above an below the screen in aition to the horizontal movement, which is all that was observe to enter the mile of the screen. Wells Complete across the Water Table Because many monitoring wells are complete with the screen open across the water table, a final simulation was conucte to illustrate the monitoring zone (configuration of flowpaths) an vertical istribution of flux achieve Homogeneous; Effect of Intake Location Intake Above Screen Intake at Top (0.83 ft) 2.5 Intake at Bottom (4.17 ft) Intake at Bottom Intake at Top Intake Above Screen Figure 10. The base case simulation with the intake positione at the top, bottom an above the screen. M.D. Varljen et al./ Groun Water Monitoring & Remeiation 26, no. 1:

7 No San Pack Above Screen; Homogeneous; Intake at Mipoint Figure 11. A simulation without the sanpack extening above the screen (i.e., the grout seal extens all the way to immeiately above the screen) limite the isproportional amount of flux at the very top of the screen; however, it was not eliminate. with this type of well configuration uring low-flow purging an sampling. All parameters an conitions of the baseline 10-foot screen simulation (Figures 2 an 3) were maintaine for this simulation; however, the static water level was set 5 feet below the top of the screen an the pump intake was set in the mile of the water column (7.5 feet below the top of the screen). Figure 15 illustrates the configuration of flowpaths entering the well. Figure 16 shows the istribution of flux along the well screen. Similar to the baseline case, the flux increases at the bottom of the well, likely ue to vertical movement of water from below the well in aition to ajacent to the well. There is no increase in flux at the top of the saturate portion of the screen because the water table surface acts as a no-flow bounary (creating a conition similar to the confine or fully penetrating example shown in Figure 16). Conclusions This work illustrates the following points for purging at low flow rates (250 to 500 ml/min) an sampling after steay-state conitions have been achieve (as evience by stabilization of the pumping water level an water quality inicator parameters): No San Pack Above Screen; Borehole Below Screen Seale; Anisotropic Figure 13. Sealing the borehole below the screen reuce the isproportionate flux from below the screen. The entire well screen is sample, although preferential flux into the screen occurs if the screen intersects a zone of higher permeability. This result was consistent for screens that were 5, 10, an 20 feet in length. The location of the pump intake within the screen oes not influence the actual zone sample. If the monitoring well partially penetrates an aquifer, some flowpaths that enter the well screen originate from above an below the screen, so the actual zone monitore is longer than the length of the screen (provie there are no overriing vertical heterogeneities in permeability). The fining that wellbore flow woul occur (resulting in the entire screen zone being sample) in response to low volume/rate pumping is consistent with the work of Reilly et al. (1989) an Hutchins an Acree (2000) who foun that significant intrawellbore flow can occur in response to very small hea changes uner ambient conitions. This fining is also consistent with the empirical finings of Puls an Paul (1998). Puls an Paul (1998) emonstrate in a fiel experiment that a sample collecte using low-low purging was reflective of the average of iscrete irect-push samples collecte ajacent to an along the entire length of the screen. Because the iscrete samples showe consierable vertical variation, this supports the notion that the entire well screen is sample uring No San Pack Above Screen; Anisotropic; Intake at Mipoint Figure 12. Reucing the vertical hyraulic conuctivity (anisotropy ration of 10:1) further ecrease the isproportionate flux. Homogeneous; Intake at Mipointoint; Fully Penetrating Figure 14. A fully penetrating simulation emonstrate uniform flux along the length of the screen. 50 M.D. Varljen et al./ Groun Water Monitoring & Remeiation 26, no. 1: 44 52

8 Screen Zone (10 ft) Well Casing (Horizontal Flow Barrier) Grout Pump Intake () San Pack (k=.01 cm/s) the flow rate). This fining supports previous qualitative recommenations to specify screen lengths an positions accoring to known heterogeneities (Keely an Boateng 1987; Gibs et al. 1993; Barcelona et al. 1994; Puls an Barcelona 1995; Martin-Hayen an Robbins 1997). Changing pump position within a screen, however, oes not provie an opportunity to collect iscrete samples. Recommenations for pump intake location are simply to keep it within the well screen instea of above it to minimize mixing with stagnant water an the require purge volume an to keep the intake off the bottom of the well where accumulate seiment may be rawn into the sample. It was emonstrate that it is not necessary to locate the pump intake above the well screen to obtain water from the entire screen. Locating the intake above the screen means more stagnant water must be purge (reucing the efficiency of the sampling operation), an because wellbore mixing processes are not perfectly unerstoo, the possibility of obtaining representative samples (i.e., not mixe with the stagnant water) may be more ifficult to ensure. Locating the pump intake at any location within the well screen provies the best opportunity to collect samples representative of water across the entire well screen, an because the entire well screen is sample, the shorter the well screen, the less concentration averaging (across ifferent vertical intervals) can be expecte to occur. En Cap Acknowlegments The authors wish to acknowlege the support of Waste Management Inc., as well thank Paulo Negrao, Clean Environment-Brazil, for assistance with the graphics. Figure 15. Illustration of flowpaths when screen intersects water table. low-flow sampling an that mixing insie the well screen is essentially thorough for all practical purposes. This quantitative illustration showe preferential flux into the screen if the screen intersects a zone of higher permeability (this increase flux woul occur regarless of Screen Intercepting Water Table Pump Intake at 7.5 ft Flux (ml/min) per 4-inch of Screen Figure 16. Screen intercepting water table. Eitor s Note: The use of bran names in peer-reviewe papers is for ientification purposes only an oes not constitute enorsement by the authors, their employers, or the Groun Water Association. References Barcelona, M.J., H.A. Wehrmann, an M.D. Varljen Reproucible well purging proceures an VOC stabilization criteria for groun water sampling. Groun Water 32, no. 1: Chiang, W.-H., an W. Kinzelbach D-grounwater moeling with PMWIN: A simulation system for moeling grounwater flow an pollution. Berlin, New York: Springer. Cohen, R.M., an R.R. Rabol Simulation of sampling an hyraulic tests to assess a hybri monitoring well esign. Groun Water Monitoring Review 3, no. 1: Elci, A., F.J. Molz III, an W.R. Walrop Implications of observe an simulate ambient flow in monitoring wells. Groun Water 39, no. 6: Gibs, J., G.A. Brown, K.S. Turner, C.L. MacLeo, J.C. Jelinski, an S.A. Koehnlein Effects of small-scale vertical variations in well-screen inflow rates an concentrations of organic compouns on the collection of representative groun water quality samples. Groun Water 31, no. 2: Gibs, J., Z. Szabo, T. Ivahnenko, an F.D. Wile Change in fiel turbiity an trace element concentrations uring well purging. Groun Water 38, no. 4: M.D. Varljen et al./ Groun Water Monitoring & Remeiation 26, no. 1:

9 Hutchins, S.R., an S.D. Acree Groun water sampling bias observe in shallow, conventional wells. Groun Water Monitoring an Remeiation 20, no. 1: Keely, J.F., an K. Boateng Monitoring well installation, purging, an sampling techniques Part 1: Conceptualizations. Groun Water 25, no. 3: Martin-Hayen, J. 2000a. Sample concentration response to laminar wellbore flow: Implications for grounwater ata variability. Groun Water 38, no. 1: Martin-Hayen, J. 2000b. Controlle laboratory investigations of wellbore concentration response to pumping. Groun Water 38, no. 1: Martin-Hayen, J., an G.A. Robbins Plume istortion an apparent attenuation ue to concentration averaging in monitoring wells. Groun Water 35, no. 2: McDonal, M.G., an A.W. Harbaugh A moular threeimensional finite-ifference groun water flow moel. USGS Open-File Report U.S. Geological Survey, Denver, Colorao. Pollock, D.W MODPATH, ocumentation of computer coes to compute an isplay pathlines using results from the U.S. Geological Survey moular three-imensional finite-ifference grounwater moel. U.S. Geological Survey Open-File Report U.S. Geological Survey, Reston, VA. Pollock, D.W Semianalytical computation of path lines for finite ifference moels. Groun Water 26, no. 6: Puls, R.W., an M.J. Barcelona Low-flow grounwater sampling proceures. Aa, Oklahoma: National Risk Management Laboratory, U.S. EPA. Puls, R.W., D.A. Clark, B. Blesoe, R.M. Powell, an C.J. Paul Metals in groun water: Sampling artifacts an reproucibility. Hazarous Waste an Hazarous Materials 9, no. 2: Puls, R.W., an C.J. Paul Discrete-level groun-water monitoring system for containment an remeial performance assessment objectives. Journal of Environmental Engineering 124, no. 6: Reilly, T.E., O.L. Franke, an G.D. Bennett Bias in grounwater samples cause by wellbore flow. Journal of Hyraulic Engineering 115, no. 2: Reilly, T.E., an J. Gibs Effects of physical an chemical heterogeneity on water quality samples obtaine from wells. Groun Water 31, no. 5: Reilly, T.E., an D.R. LeBlanc Experimental evaluation of factors affecting temporal variability of water samples obtaine from long-screene wells. Groun Water 36, no. 4: Schilling, K Low-flow purging reuces management of contaminate grounwater. Environmental Protection 6, no. 12: Shanklin, D.E., W.C. Sile, an M.E. Ferguson Micropurge low-flow sampling of uranium contaminate groun water at the Fernal Environmental Management Project. Groun Water Monitoring an Remeiation 15, no. 3: M.D. Varljen et al./ Groun Water Monitoring & Remeiation 26, no. 1: 44 52