Moore modeling framework design of collector wells GW 2011

A Modeling Framework for the Designof Collector Wellsby Rhett Moore1, Vic Kelson2, Jack Wittman2, and Vern Rash3

AbstractWe present results of a design study performed for the Saylorville Wellfield in Iowa, which is owned and

operated by the Des Moines Water Works. The purpose of this study was to estimate wellfield capacity and providea preliminary design for two radial collector wells to be constructed in the outwash aquifer along the Des MoinesRiver near Saylorville, Iowa. After a field investigation to characterize the aquifer, regional two-dimensional andlocal three-dimensional, steady-state groundwater flow modeling was performed to locate and design the wells. Thismodeling was the foundation for design recommendations based on the relative performance of 12 collector welldesigns with varying lateral numbers, elevations, screen lengths, and orientations. For each site, alternate designswere evaluated based on model estimates of the capacity, the percent of surface water captured, and the productionper unit length of screen. Many of our results are consistent with current design practices based on experienceand intuition, but our methods allow for a quantitative approach for comparing alternate designs. Although theresults are site-specific, the framework for evaluating the hydraulic design of the Saylorville radial collector wellsis broadly applicable and could be used at other riverbank filtration sites. In addition, many of the conclusionsfrom this design study may apply at other sites where construction of radial collector wells is being considered.

IntroductionRadial collector wells have been constructed in the

United States for more than 70 years, beginning withthe 1936 Ranney Well in Canton, Ohio. A radial collectorwell (collector well) is comprised of a central caisson withhorizontal lateral well screens projected radially out intothe aquifer. Groundwater infiltrates into the lateral armsand then flows into the caisson, where it is extracted byone or more pumps. Typically installed in alluvial aquifersclose to a dependable surface water source, these wells

1Corresponding author: Layne Christensen, 320 West 8th St.,Suite 201, Bloomington, IN 47404; (812) 333-9399; [email protected]

2Layne Christensen, 320 West 8th St., Suite 201, Bloomington,IN 47404.

3Des Moines Water Works, 2201 George Flagg Parkway, DesMoines, IA 50321-1190.

Received January 2011, accepted July 2011.© 2011, The Author(s)Ground Water © 2011, National Ground Water Association.doi: 10.1111/j.1745-6584.2011.00850.x

take advantage of a natural filtering process referred toas riverbank filtration (RBF). This natural filtration cansimplify treatment by reducing particulates and turbidity,removing pathogens such as Cryptosporidium and Gia-rdia, and reducing organic compounds and disinfectionby-product precursors. In the right setting, collector wellsare ideal for RBF applications because the screens canbe located close to or beneath surface waters, maximizinginduced recharge and enhancing well capacity.

Although collector wells have been constructed formany years, formal guidance for their hydraulic designis limited. Hantush and Papadopulos (1962) provideanalytical solutions for the drawdown distribution nearcollector wells in a variety of settings including confinedaquifers, unconfined aquifers, and aquifers bounded ontop by a constant head boundary such as a riverbed.For many years, these analytical solutions have beenused to estimate well yield during design. In practice,these equations are used in conjunction with extensiveexperience in constructing, testing, and operating collectorwell systems in similar aquifer settings.

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In many cases, collector wells are ideal raw watercollection devices in RBF systems because the engineerhas many design options when determining the locationand orientation of the lateral well screens. However, theperformance of any design is determined by the ability ofthe well to induce recharge from surface waters. In manyalluvial settings, this is limited by the vertical hydraulicresistance of riverbed sediments (Hubbs et al. 2006). Theeffects of low-permeability riverbed sediments on wellyield are not addressed by the equations of Hantushand Papadopulos (1962); however, an ad hoc methoddescribed by Kazmann (1946, 1948) and Rorabaugh(1956) have been used to account for partially-penetratingand partially-clogged riverbeds. This approach accountsfor the presence of a leaky riverbed indirectly by definingan effective distance to a hypothetical line of recharge (thea-distance) based on the aquifer’s response to a pumpingtest (Hantush 1959). The effective distance technique iscommonly used as a tool for aquifer test analysis but haslittle theoretical basis.

In RBF facilities, water quality and treatment advan-tages are often as important as pumping capacity. RBFnaturally combines ambient groundwater with inducedrecharge from surface waters near the well. As a result,predicting the character of the mixture requires insightinto regional groundwater flow and the local hydraulics ofthe well-aquifer-river system. Surface waters (and inducedrecharge) often have a very different chemical compo-sition than ambient groundwater. As induced rechargemoves toward the well and interacts with riverbed andaquifer sediments, its chemical composition changes,especially if the water becomes anoxic. As a result, waterquality in a collector well is determined by the mix-ing ratios, residence times, and flow trajectories (particlepaths) of groundwater that enters the well.

Several numerical models of collector wells haveappeared in the literature. Ophori and Farvolden (1985)present a finite-element model of a collector well inOntario, Canada. Eberts and Bair (1990) describe acoarse MODFLOW model of four collector wells inColumbus, Ohio; this model was subsequently improvedby Cunningham et al. (1995) with finer horizontal andvertical discretization. Finite-difference and finite-elementcodes that require horizontal discretization of the flowdomain present difficulties for accurate representationof lateral arms. Rectangular finite-difference grids areinappropriate for defining the radial orientation of thelaterals; finite-element grids are more flexible, but requireseparate meshes for alternate well geometries.

More recently, Bakker (2001), Bakker and Strack(2003), and Bakker et al. (2005) have presented an ana-lytic element approach for modeling three-dimensional(3D) flow to partially-penetrating wells, horizontal wells,and radial collector wells. This method does not requirehorizontal discretization of the flow domain. Bakker et al.(2005) identified the benefits of modeling collector wellsusing their approach compared to using the more com-mon MODFLOW finite-difference model. Haitjema et al.(2010) offer a methodology for simulating converging

flow to collector well laterals by using an appropriate entryresistance in a two-dimensional (2D) model.

We present an improved modeling methodology forthe design of collector wells. Our approach uses regional-and local-scale modeling, and allows the consideration ofnumerous design alternatives for lateral layout and lateralelevation at a particular site. This design methodologywas developed and tested during a collector well designproject for Des Moines Water Works (DMWW) in DesMoines, Iowa.

Improved Modeling MethodologyFew tools are available for simulating collector wells.

This is due to the challenges associated with represent-ing the radial geometry of the wells, accounting for theeffects of converging 3D flow to the lateral screens, andaccounting for the interactions between the well screensand nearby boundaries. The horizontal discretization ofthe flow domain required by finite-difference and finite-element codes is inconvenient when simulating the radialorientation of collector well laterals. In plan view, a ver-tical well appears as a single cell in a numerical model;a horizontal screen has a linear geometry, and will likelyextend over several neighboring cells. If the well screensdo not align with rows or columns in a numerical model,it is unclear how the lateral screens should be represented,including for example the number of cells that are needed,what the cell sizes should be, and what (if any) con-ductance parameter values are needed. Furthermore, asthe lateral diameter is small compared to the saturatedthickness of the aquifer, it is necessary to include theeffects of converging 3D flow to the lateral well screens.Finally, horizontal collector wells typically have 4 to 20lateral arms installed around a central caisson and oftenare located near surface water features. Thus, the modelmust account for hydraulic interference between the indi-vidual lateral screens and interactions with surface waters.

We present a new, efficient framework for thehydraulic design of collector wells that utilizes groundwa-ter models based on the analytic element method (AEM).A detailed description of the AEM is provided by Strack(1989). The main advantage the AEM holds for simu-lating collector wells is that 2D and 3D line elements areavailable that can explicitly represent the extent and geom-etry of the lateral screens. Furthermore, the AEM requiresno grid, and interference between the laterals and surfacewater-groundwater interactions are explicitly modeled.

Our collector well design framework addresses thechallenges associated with designing collector wells andallows the designer to account for the 3D interactionsbetween individual laterals and between the laterals andnearby surface-water boundaries. We made use of aregional 2D model that predicts the effects of regionalflow and a local 3D model that simulates the detailed flowfield near the well. The AEM code ModAEM (Kelson2004) was used for the regional 2D model and the 3DAEM code TimML (Bakker 2010), enhanced by additionalcode developed specifically for collector wells (Bakker

2 R. Moore et al. GROUND WATER NGWA.org

Figure 1. Sites 1 and 2 at the Saylorville Wellfield along the Des Moines River.

et al. 2005), was used to develop the local 3D flow model.The regional ModAEM model was linked to the local 3DTimML model in a fashion similar to the telescoping meshrefinement commonly applied to MODFLOW models(Ward et al. 1987). Flux data were extracted from theModAEM results and used as input along the boundariesof each of the local TimML models. The regional modelwas calibrated to a variety of data using the parameterestimation software PEST (Doherty 2004).

SettingThe Saylorville Wellfield lies in the Des Moines River

Valley of Polk County, Iowa (Figure 1). Source water forthe Saylorville collector wells will be a mixture of upgra-dient groundwater and induced recharge from the DesMoines River. The natural filtration that occurs in RBFsystems provides effective pretreatment for pathogensand other undesirable surface water constituents (Gollnitzet al. 2003, 2004, 2005). Compared to groundwater, sur-face water is generally lower in hardness, total dissolvedsolids, alkalinity, dissolved iron, and dissolved manganeseconcentrations.

In the study area, the surficial aquifer systemis comprised of alluvial deposits and buried channelglacial outwash deposits that consist of permeable sandand gravel. This setting is typical of many productiveaquifers in the Midwestern United States. A typical,simplified geologic cross section at the wellfield isshown in Figure 2. Low-permeable shale, limestone, anddolomite underlie the unconsolidated surficial deposits.Fine-grained terrace deposits overlie the coarser outwashdeposits that form the aquifer throughout the wellfieldarea. Laterally, the outwash deposits are bounded by low-permeability glacial till.

Two sites investigated for placement of collectorwells are identified in Figure 1, which shows the proposed

wellfield adjacent to the Des Moines River. Borings weredrilled at both Site 1 and Site 2 and pumping tests wereconducted to estimate aquifer properties. Locally, thebedrock surface has been observed to have at least 12.2 mof relief and is highly variable. The aquifer was observedto be unconfined to semi-confined based on water-levelmeasurements in observation wells. The thickness ofaquifer materials at Site 1 is approximately 15 m. AtSite 2, the aquifer is thinner and about 11 m thick. Theaquifer at both sites is relatively thin, which restricts thevertical placement of the lateral arms of any collector wellconstructed there.

Design AlternativesThe geometry of collector well screens combined

with the influence of a nearby surface-water bodygenerates a more complex groundwater flow field thana simple vertical well screen. Consequently, in additionto the location of the central caisson, the designerhas choices about the number of laterals, the elevationof the laterals, their lengths, and their orientation relativeto the nearby surface water body. Each design choice maybe constrained by various factors at the site, but will havean effect on groundwater flow patterns near the well andwater quality in the caisson.

Lateral Layout OptionsCollector well laterals may be installed in many

configurations, and the choice of the lateral configurationwill influence well performance and water quality. Fivelateral design alternatives were evaluated, as shown inFigure 3. With the exception of the Mousetrap, theillustration shows the laterals in an orientation that isintended to maximize infiltration of river water. We namedeach lateral design according to its appearance; thesenames are not common in the industry. The names weassigned for three designs that are found in many of

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Figure 2. Typical geologic cross section at Sites 1 and 2 showing the outwash aquifer.

Figure 3. Geometric designs of radial collector wells.

the existing wells in the United States are the Star, theFan, and the Deadman (Figure 3). In addition, we haveevaluated two designs that are not commonly used: theMousetrap and the Pecten. The Mousetrap was includedbecause DMWW has several collector wells with thisdesign at its Maffitt Wellfield. The designs are describedin more detail below.

The Star is a common 6-arm or 8-arm design withsymmetrically spaced laterals of equal length.

The Fan also has equally spaced laterals of equallength, but only those laterals nearest the river areincluded. The Fan design is typically chosen to increasethe percentage of surface water captured (as comparedto the Star).

The Deadman design is similar to the Fan, but asingle upgradient lateral perpendicular to the river (theDeadman) is included. The Deadman lateral is intendedto produce primarily upgradient groundwater.

The Pecten design is similar to the Fan, but theupgradient laterals are reduced in length, giving thisdesign the appearance of a pecten shell. The short lateralsare intended to prevent the long laterals from pumpingexcessive amounts of upgradient groundwater.

The Mousetrap has three laterals with two nearlyparallel arms. The Mousetrap is not a common design,but six collector wells at DMWW’s Maffitt Wellfield have

this lateral layout. The design reflects trade-offs madeduring construction owing to site constraints. This layoutwas evaluated to understand the consequences of designchanges on well performance during construction.

Depending on the local setting and desired well yield,the Star, the Fan, the Pecten, and the Deadman may beconstructed with varying numbers and lengths of laterals.The hydraulics of each of these designs was compared forthe Saylorville Wellfield.

Length of LateralsBased on DMWW’s experience at the Maffitt Well-

field, laterals 70 m long can be installed routinely. For thepurposes of this design, we assumed this length would beachievable at the Saylorville Wellfield, and used it as thebasis for designing the lateral arms at both sites. Lateralsas long as 88.4 m have been constructed in other locations.The advantage of longer laterals is that they minimizeinterference between adjacent arms due to the diverg-ing radial configuration. Designs with laterals as long as88.4 m were tested to evaluate the effects of increasedarm length on well performance.

Distance of Well from RiverThe locations of the well caissons in the wellfield are

fixed by site constraints. At Sites 1 and 2, the distance tothe river bank is 82.3 and 30.5 m, respectively. Designswith the collector wells closer to the river were tested todemonstrate the effect of distance from the river on wellperformance.

Groundwater Flow ModelingAlthough the modeling performed for the study of

the Saylorville Wellfield is the basis of our design results,we focus in this article on the results themselves. Only afew details about the extensive modeling performed areprovided here to provide a general background and docu-ment the methods that were employed. Full details of theregional model development, calibration, and predictiveanalysis are described in the final project report (WHPA2007).


Figure 4. Extent of the regional groundwater flow modeling boundary conditions applied at the perimeter of the model,transmissivity zones, and recharge zones.

Regional ModelWe developed a regional 2D model of groundwater

flow to integrate regional and local hydrologic andgeologic data, assess variation in the regional aquifer’sproperties, and to assess the regional interaction ofsurface water and groundwater. After calibration, themodel was used to evaluate the regional impacts ofthe proposed wellfield, including interference between thetwo proposed wells. Finally, the regional model providedperimeter boundary conditions for the local 3D flowmodels of each collector well.

The areal extent of the regional ModAEM model andthe conditions specified along the perimeter of the modelare shown in Figure 4. The areal extent of the outwashaquifer model is much greater than the extent of thewellfield. The regional model includes impermeable wallsat the outwash perimeter and spatial variations in arealrecharge rates and aquifer transmissivity. A large polygonsimulates areal recharge over the entire model region. Twosub-domains where the Late Wisconsin Terrace Depositsare 6 to 9 m thick were assigned a relatively low rechargerate (Figure 4). No recharge is simulated on the bedrockupland downstream of the wellfield (Figure 4).

Initial values specified for the model parameterswere based on independent estimates from sieve tests

and the results of pumping tests at Site 1 and Site2 (Table 1). The thickest outwash zones were assigneda high transmissivity and the relatively thin sand andgravel layers near the valley margin were assigned a lowtransmissivity (Figure 4). A bedrock upland area south ofthe wellfield where the outwash aquifer is not present issimulated as a zone of very low transmissivity (1.08 ×10−6 m2/s). Nearby abandoned gravel pits are simulatedas zones of very high transmissivity (1.08 × 10−2 m2/s).

Surface water features are modeled with resistanceand head-specified line-sink elements. The line sinks wereassigned head values based on water-level observations atseven U.S. Geological Survey stream gauges located onthe Des Moines River, Beaver Creek, and the RaccoonRiver. The line-sink resistance represents the hydraulicresistance of the riverbed. Initial values specified forthe model parameters were based on aquifer test data(Table 1) and observations of the bank deposits.

The steady-state model was calibrated to severaldifferent sets of potentiometric head observations:

• Local and regional water levels measured throughoutthe aquifer on November 22, 2004

• Local and regional water levels measured on June12, 2004


Table 1Observed Aquifer Properties at Site 1 and Site 2

Site Aquifer Thickness (m) Hydraulic Conductivity (m/s) Transmissivity (m2/s) Riverbed Resistance (s)

1 15 5.79 × 10−4 to 7.83 × 10−4 8.68 × 10−3 to 1.18 × 10−2 1.16 × 105 to 3.71 × 105

2 11 8.22 × 10−4 to 10.10 × 10−4 9.04 × 10−3 to 1.11 × 10−2 1.18 × 105 to 3.39 × 105

Table 2Model Parameters for Calibrated Regional Model

Transmissivity Zones (m2/s) Recharge Zones (cm/year) Riverbed Resistance (s)

Global 8.13 × 10−3 Global 7.48 × 10−9 Site 1 3.16 × 105

Very high 1.08 × 10−1 Low 3.74 × 10−9 Site 2 1.28 × 105

High 1.48 × 10−2

Low 5.65 × 10−3

Very Low 1.08 × 10−6

• Horizontal hydraulic gradients in the aquifer and ver-tical hydraulic gradients between the aquifer and rivermeasured during November and December 2004 at thewell sites

• Projected steady-state drawdowns in response to pump-ing tests conducted at the two sites

The inverse model code PEST (Doherty 2004) was usedfor automated calibration. The calibrated parameters forthe regional model are shown in Table 2. A plot of theresiduals after model calibration is shown in Figure 5.The calibrated model is able to simultaneously simulateeach type of measured data from the site. A statisticalpredictive analysis was also performed using PEST toevaluate the effects of uncertainty.

Local ModelThe methods employed to model 3D flow near

collector wells are described by Bakker et al. (2005). Tosimulate 3D groundwater flow to the collector well lateralsand to evaluate the effects of lateral arm placement withinthe aquifer, the domain of Site 1 was discretized into 19layers and Site 2 into 12 layers.

To ensure consistent results with the regional model,it was necessary to configure each local-scale model suchthat the effects of vertical flow were negligible at thelocal model perimeter. Based on Strack (1989), the effectsof 3D flow are negligible at a distance of three orfour times the representative leakage length (λ) froma partially-penetrating feature (e.g., a collector well). In ahomogeneous aquifer, the leakage length is:

λ =√

T c (1)

where T is the aquifer transmissivity and c is the resis-tance of the riverbed. Using observed parameter valuesfrom each site and accounting for the length of the lat-erals, the minimum radius of the local model of Site 1

Figure 5. Results of the model calibration.

was 259 m, whereas the minimum radius at Site 2 was218 m. Beyond this distance, groundwater flow may beconsidered to be 2D. Local-scale 3D collector well modelswere constructed for Sites 1 and 2 as sub-domains of theregional model. Flux-specified conditions were imposed atthe perimeter of each local-scale model using simulatedgroundwater fluxes from the regional model; this ensuredcompatibility with the regional model. The distance fromthe collector well to the perimeter was large enough toensure that Dupuit-Forchheimer conditions were met atthe perimeter of the local-scale model. The fluxes speci-fied at the perimeter of the local models were defined bythe results from the regional model when simulating thesame collector well geometry and discharge as the localmodel.


For the local models, local-scale boundary conditionswere added to represent the river and the collector wellin three dimensions. The line sinks that represent theriver and the collector well in the 2D regional modelwere replaced by equivalent 3D line sinks in the localmodel. For the river, the 2D entry resistance was used inthe local model. For the collector well, no resistance wasused. The layer containing the laterals in the local modelhad the same height as the lateral diameter; thus, 3D flowto the laterals was explicitly included.

Particle tracking within the local models was used toevaluate the percentage of the total well discharge inducedfrom the Des Moines River. This was done by forwardtracing particle trajectories that originate at the bottom ofthe river. One or more particles were released from eachriver line-sink element, and each particle was assigned avolumetric rate that reflected the infiltration rate of the linesink. The quantity of river water in the well was computedby accumulating the volumetric rates of all particles thatterminate at the well.

Vertical Placement of Lateral ArmsVertically converging flow to a lateral well screen

must be accounted for as part of the design of acollector well. In a model, this may be done by explicitlyincluding vertical flow in a fully 3D local model (e.g.,MODFLOW or the AEM code TimML). This approachis useful when localized aquifer layering is known orwhen detailed vertical flow paths are needed to makeregulatory or treatments decisions. Our regional modelapplied the method described by Haitjema et al. (2010) tosimulate effects of the collector wells. We represented thelateral screens of the collector wells in the regional 2DModAEM model using the equivalent resistance providedin Haitjema et al. (2010). This model bounded the 3DTimML models for local well analyses.

Each lateral screen is divided into one or more seg-ments, each of which is represented by a 2D line-sinkelement. Although a single line sink may be used, con-figuring the lateral using a collection of shorter segmentsallows the model to simulate a varying rate of inflow perunit length, or “sink density” along the lateral. At thecenter of each segment, the sink density σ is given as

σ = w

cls(ha − hlateral)(2)

where ha is the Dupuit-Forchheimer head in the aquiferand hlateral is the head inside the lateral at the center ofthe segment, w is width of aquifer along the lateral wherewater flows into the segment, and cls is a resistance thataccounts for converging 3D flow into the lateral. Haitjemaet al. (2010) provide a formulation for the resistance cls

in a confined aquifer:

cls = − w

2πKln

{4 sin

(π

2

γw

H

)sin

(π

h + γw

2

H

)}(3)

where K is the hydraulic conductivity, rw is the lateralradius, H is the aquifer thickness, and h is the height of

Figure 6. Equivalent 2D entry resistance for a 0.3 m diame-ter collector well lateral as a function of its elevation withinthe aquifer at Site 1.

the lateral centerline above the aquifer bottom. The widthw cancels algebraically when Equation 3 is substitutedinto Equation 2. Thus the value of w may be selectedarbitrarily; by convention we have chosen to set w = 1 inall model simulations. Although Equation 2 was derivedfor a horizontal well in a confined aquifer, Haitjema et al.(2010) demonstrate that it can also be used in an uncon-fined setting. The equivalent entry resistance for a lateralat Site 1 varies with its elevation within the aquifer asshown in Figure 6. The resistance is minimized if thelateral is placed at the center of the aquifer. At otherelevations, the effects of hydraulic interference betweenthe lateral and the aquifer top or bottom increase theresistance.

Figure 6 implies that it is preferable to place thelateral at the center of the vertical section. Operationalconstraints complicate the analysis because the capacityof any well is also limited by the available drawdown.In the case of a collector well, the available drawdown islimited by the geometry and hydraulics of the laterals andthe pump in the central caisson. The water level in thecaisson must be sufficiently above the pump inlet to satisfysuction-head requirements, and the laterals must lie belowthe caisson water level. If the laterals are placed relativelyhigh in the aquifer to reduce interference with the aquiferbottom, the resulting limitations on available drawdownmay reduce well capacity.

The local model was used to evaluate the optimal ele-vation for placing well laterals. The Star design with sixlateral arms was used for the analysis of the vertical place-ment of the arms. At each site, the desired well capacitywas specified in the local model, and the resulting head inthe caisson was evaluated for scenarios with lateral eleva-tions varying from the aquifer bottom to the highest pointallowed by pumping. In addition to lateral placement, the


Figure 7. Site 1—Effects of lateral elevation on the head inthe caisson.

effects of aquifer anisotropy on the head in the caissonwere also assessed. Results for anisotropy ratios (Kv :Kh)of 0.2 and 0.5 were evaluated. For the design calculations,an anisotropy ratio of 0.2 was assumed for both sites.

Figure 7 shows results for the Site 1 analysis; resultsfrom Site 2 were similar. The results are intuitive—as thelateral elevations are raised from the bottom of the aquiferto the streambed, the head in the caisson rises, indicatingthat the higher the lateral, the greater the specific yield.Lateral elevation has a moderate effect on the modeledhead in the caisson. At Site 1, for a production rate of0.324 m3/s, and assuming an anisotropy ratio of 0.2, rais-ing the laterals 3.5 m from a centerline elevation of 226.5to 229.8 m above mean sea level increases the head in thecaisson by less than 0.5 m. Similar results were obtainedat Site 2.

Given that the specific capacity of these wells isexpected to be large, vertical placement of the lateralscould be based entirely on available drawdown and logis-tical considerations of the well construction and opera-tion without having a significant impact on well yield.As a practical matter, the elevation of the laterals mustallow sufficient suction head at the pump, while ensuringthat construction can be efficiently completed. Based onsite borings and construction constraints, the preliminarydesign of the Saylorville Wellfield recommended lateralelevations of 3.7 m above the bedrock surface at Site 1,and 2.1 m above the bedrock at Site 2.

Comparing Design AlternativesTwelve collector well geometries, each a variation

of one of the five types illustrated in Figure 3, weretested with the local groundwater flow model. Table 3lists the details for each alternate design, identified asCases A through L. The alternate designs are labeled andshown graphically at the bottom of Figure 8. Each design

includes a 4.0 m diameter caisson and a 3.0 m blank(unscreened section of lateral) adjacent to the caisson wallon each lateral. The lateral arms are placed in the modelsat elevations of 3.7 and 2.1 m above the aquifer base,respectively, at Sites 1 and 2. The rationales for the casesinvestigated are explained with reference to Table 3.

Cases A through E allow for comparison of the Star,the Deadman, the Fan, and the Pecten geometric designswhile maintaining a similar number of laterals and totalscreen length. For Cases A through C, the length per lat-eral is fixed at 70.1 m. As the number of laterals andlengths are the same, the construction costs for thesedesigns will be similar. The Pecten design in cases D andE include two shorter upgradient laterals with a length of33.5 m each. To maintain the total screen length of about402 m, the remaining laterals were extended. In the caseof the 6-arm Pecten (Case D), the length of the long later-als is 88.4 m; the 7-arm Pecten (Case E) has 70.7 m longlaterals. These five cases allow us to evaluate the effects ofgeometric design on yield and raw water quality withoutsignificant variation in construction costs. Cases D and Ealso facilitate evaluation of the effects of increased armlength on well performance.

Cases D through G represent variations of the Pectendesign with successive reductions in the number of later-als, total lateral length, and, therefore, construction costs.As discussed earlier, both the arm number and length varyfor cases D and E with a constant total screen length ofabout 402 m. Cases F and G have increased individual lat-eral lengths of 88.4 m, and reduced total screen lengths of232 and 317 m, respectively. Similarly, cases H through Jallow for comparing varying total screen length of the Fandesign. The length of each lateral remains at 70.1 m, butlaterals are successively removed for total screen lengthsof 335, 268, and 201 m, respectively. These cases werecompared to evaluate the minimum number of lateralsneeded to meet capacity, thereby minimizing the construc-tion costs.

Case K is the same geometric design as Case C, butthe distance from the caisson to the river bank is decreasedfrom 30.5 to 15.2 m at Site 1 and from 76.2 to 30.5 mat Site 2. Case K was not a feasible option at Site 1 orSite 2 due to practical site constraints. It was not pos-sible to locate the wells this close to the river due to abicycle path that runs along this reach of the Des MoinesRiver. Nevertheless, we included this design to demon-strate the effect of river distance on well performance.Finally, the Mousetrap design (Case L) was included toinvestigate the potential reduction in well performanceresulting from placing two laterals close together toaccommodate problems encountered during construction.

ResultsThe following parameters were calculated for each

case identified in Table 3 and used to compare andevaluate the performance of the alternate designs:

1. The total production from the wells at the two sites


Table 3Collector Well Design Alternatives

Case GeometryNo. of

LateralsLateral

Lengths (m)Total ScreenLength (m)

Site 1 Distance toRiverbank (m)

Site 2 Distance toRiverbank (m)

A Star 6 70.1 402 30.5 76.2B Deadman 6 70.1 402 30.5 76.2C Fan 6 70.1 402 30.5 76.2D Pecten 6 4 × 88.4, 2 × 33.5 402 30.5 76.2E Pecten 7 5 × 70.7, 2 × 33.5 402 30.5 76.2F Pecten 4 2 × 88.4, 2 × 33.5 232 30.5 76.2G Pecten 5 3 × 88.4, 2 × 33.5 317 30.5 76.2H Fan 5 70.1 335 30.5 76.2I Fan 4 70.1 268 30.5 76.2J Fan 3 70.1 201 30.5 76.2K Fan 6 70.1 402 15.2 30.5L Mousetrap 3 70.1 201 30.5 76.2

2. The percentage of surface water captured by the twowells

3. The pumping rate (in m3/s) per meter of lateral screen

The first parameter, wellfield capacity, is often aprimary objective when designing a any well systembecause suitable designs must meet the desired productioncapacity. The second parameter, percentage surface watercaptured, is used to inform the treatment plant design andis based on the assumption that bank-filtered surface wateris less expensive to treat than groundwater. In the caseof the Saylorville Wellfield, increasing the percentage ofsurface water captured decreases treatment costs primarilybecause the upgradient groundwater is harder and hashigher concentrations of iron and manganese. The thirdparameter allows for a comparison of construction costs;determining the capacity per total screen length helps toevaluate the relative costs of construction.

Table 4 presents the model results for each of the12 collector well designs at the two sites. The three indexvalues are listed along with a ranking for each. These samedata are presented graphically in Figure 8. The data fromTable 4 are adjusted and normalized for ease of visualcomparison; in Figure 8, the largest normalized value foreach parameter indicates the best design in that category.The total production for each design was divided by thedesired productivity of the wellfield to normalize the wellyields; normalized yields greater than or equal to 1.0 meetor exceed production expectations. For the SaylorvilleWellfield, the desired productivity, including sufficientexcess capacity to provide dilution of treatment effluent, is0.546 m3/s. This results in a range of normalized capacityfrom 0.76 for the Mousetrap (Case L) to 1.26 for the Fanlocated nearest the river (Case K). Seven of the designswould satisfy the production objective with normalizedcapacities exceeding 1.0.

The capacity per screen length was normalized bydividing the value for each collector well by that of thebest design. The best design with respect to this parameterwas Case J, the 3-arm Fan, with a value of 2.33 × 10−3

Figure 8. Normalized results from the calibrated groundwa-ter flow models of collector well design alternatives for theSaylorville Wellfield (see Table 4).

m3/s per m of lateral. The resulting normalized range forthis parameter was 0.58 to 1.00.

Discussion of ResultsDesigns A through E, and designs G and K, which

include all basic geometric designs, meet or exceedthe required wellfield capacity, making each a feasiblealternative. The top three designs based on capacity (CasesK, D, and G) are also the top three designs based onpercentage of surface water, with values of 83, 80, and78%, respectively. These two design criteria, capacityand percentage of surface water, are related—increasing


Table 4Model Results and Ranking of Design Alternatives

Case GeometryCapacity

Site 1 (m3/s)Capacity

Site 2 (m3/s)Capacity

Total (m3/s) RankSurface

Water (%) Rank

Capacity PerScreen Length

(m3/s/m) Rank

A Star 0.326 0.222 0.548 6 62 11 1.36 × 10−3 11B Deadman 0.337 0.226 0.563 4 66 9 1.40 × 10−3 9C Fan 0.337 0.223 0.560 5 70 6 1.39 × 10−3 10D Pecten 0.397 0.245 0.642 2 80 2 1.60 × 10−3 8E Pecten 0.329 0.217 0.546 7 75 5 1.36 × 10−3 12F Pecten 0.316 0.214 0.530 9 76 4 2.29 × 10−3 2G Pecten 0.365 0.233 0.598 3 78 3 1.89 × 10−3 5H Fan 0.323 0.218 0.541 8 69 7 1.61 × 10−3 7I Fan 0.302 0.210 0.512 10 68 8 1.91 × 10−3 4J Fan 0.271 0.197 0.468 11 65 10 2.33 × 10−3 1K Fan 0.379 0.313 0.692 1 83 1 1.72 × 10−3 6L Mousetrap 0.236 0.177 0.413 12 58 12 2.05 × 10−3 3

the yield at the Saylorville Wellfield is most easilyaccomplished by increasing the amount of infiltratedsurface water. Case K is not a feasible option, but ranksthe highest in terms of yield and water quality, indicatingthat distance to the river has a significant impact on bothcapacity and raw water quality.

The Pecten designs ranked second through fifth interms of the percentage of surface water captured, indi-cating superior performance to that of the Fan with thecaisson located at the same distance from the river. Thisis due to the shorter upgradient laterals and longer laterals(88.4 m) extending toward the river. The short upgra-dient laterals of the Pecten design capture less upgradi-ent groundwater than the longer laterals of the Fan, butstill shield the longer laterals from excessive amountsof upgradient groundwater. The longer 88.4 m lateralsextending toward the river reduce the interference betweenadjacent laterals. The optimum lengths of the laterals aresite dependent because both the natural gradient from theaquifer to the stream and the hydraulic properties ofthe aquifer and riverbed have an effect on the modeloutcomes.

Ranking alternate designs based on their capacity perunit length favors the 3-arm Fan (Case J), followed by the4-arm Pecten (Case F). Cases F and J have the lowest totalscreen length with the exception of Case L. As might beexpected, including fewer laterals results in larger radialseparation, reducing the hydraulic interference betweenlaterals and increasing the capacity per unit length.However, fewer laterals generally reduce total yield;Cases F and J are ranked eleventh and ninth for capacityand tenth and fourth for percentage of surface watercaptured, respectively. As a metric, well capacity per unitof screen length must be balanced by the constraints ofmaintaining the desired yield and water quality of the well.

On the basis of the three optimization metricsquantified in this analysis, the preferred designs for theSaylorville Wellfield include the 6-arm and 5-arm Pectens(Cases D and G). These designs rank second and third,

respectively, based on both yields and the percent ofsurface water. The designs rank eighth and fifth in termsof capacity per unit length of screen. As a result, the6-arm Pecten with two laterals with 33.5 m of screenand four laterals with 88.4 m of screen was chosen asthe preliminary design for both sites at the SaylorvilleWellfield.

As expected, the Mousetrap design (Case L) per-formed poorly with respect to wellfield capacity andpercent of surface water captured, even when comparedto the other 3-arm design of Case J. Changes to thegeometric design of collector wells caused by prob-lems encountered during construction can have significantundesirable impacts on well performance. In the event thatconstruction problems occur (e.g., refusal during lateralinstallation), the model may be used to compare variouscorrective actions.

In addition to physical constraints affecting the theo-retical yield of the wells, including the aquifer transmis-sivity and the degree of hydraulic connection between theaquifer and the Des Moines River, physical constraints onthe mechanical capacity of the screen must be considered.Physical constraints affecting the mechanical capacity ofa well include the ability of the filter pack and screen topass water without clogging or degrading and the abilityof the lateral arms to conduct water to the caisson withoutdeveloping excessive velocities. The aquifer yield limitsthe ability of the aquifer to transmit water to the wells.The mechanical capacity of the filter pack and screen limita well’s ability to transmit water to the water-supply sys-tem. For the Saylorville wells, the design capacity wasdictated by the theoretical yield of the wells.

Construction and Testing of the SaylorvilleCollector Wells

The final design of collector well RCW1 at Site1 and collector well RCW2 at Site 2 were guidedby the preliminary designs based on flow modeling.


Table 5Summary of As-Built Features of the Saylorville Wellfield

RCW1 RCW2

Design As-Built Model Design As-Built Model Design

Caisson inside diameter (m) 4.0 4.0Caisson outside diameter (m) 4.9 4.9Grade elevation (m amsl) 244.9 243.8Floor elevation (m amsl) 228.4 226.8 230.9 230.1Caisson depth (m bgs) 16.5 18.3 13.0 13.7Centerline of laterals (m amsl) 230.9 229.2 231.7 231.0Minimum pumping elevation (m amsl) 232.4 229.5 233.2 231.3Lateral diameter (m) 0.3 0.3Total lateral lengths (m) 321.6 420.6 358.1 420.6Blank length per lateral (m) 3.0 3.0Total screen length (m) 303.3 402.3 339.9 402.3Distance (center caisson to river, m) 30.5 76.2

Both wells were designed as 6-arm Pectens with shortlaterals of 33.5 m and long laterals of 88.4 m. The twowells were constructed between 2007 and 2008. Table 5compares values from the model design and as-builtconditions of the two wells. As shown in Table 5, thedesigned total lateral lengths were not achieved becausecomplications during construction necessitated designchanges. At RCW1, the length of the long laterals rangefrom 44.8 to 76.2 m and the short laterals are 33.5 m each.The total lateral length at RCW1 is 321.6 m. At RCW2,the short laterals are 33.5 and 36.6 m long and the longlaterals range in length from 48.8 to 89.9 m with a totallateral length of 358.1 m.

Post-construction performance testing of the two col-lector wells was conducted in 2008 and 2009 to evaluatethe well yields and raw water quality. As part of the test-ing, a 96-h constant-rate pumping test was performed atRCW1. The results were used to estimate yields underaverage conditions at the wellfield. The anticipated yieldfor RCW1 is 0.400 m3/s. At RCW2, a 72-h constant ratetest indicates an anticipated yield of 0.220 × 10−1 m3/s.The anticipated total yield for the Saylorville Wellfield,based on performance testing, is 0.620 m3/s, exceedingthe desired yield of 0.546 m3/s by 13%. The total pre-dicted yield of both sites based on the preliminary design,0.642 m3/s, exceeded the anticipated total yield by only4%. A detailed comparison of yield based on model designand as-built conditions is inherently difficult. As shown inTable 5, the total lateral lengths were reduced, the lateralelevations were raised, and the minimum pumping lev-els in the caissons were increased at both sites. Wheneach of these changes is considered, it appears that themodel study used for preliminary design conservativelyunderestimated as-built wellfield capacity.

ConclusionsThe planning, design, and construction of a collector

well is a large investment that often benefits from a thor-ough assessment of competing design criteria, includingdemands for high yields, low treatment costs, and low

construction costs. Historically, formal design methodswere only available to directly address yield (Hantush andPapadopulos 1962); other design aspects were addressedthrough engineering judgment and experience. Modernconcerns, such as regional impacts caused by high pump-ing rates and the rising costs of water treatment, suggestthat a new design approach is needed as a compliment tothe experience of builders.

This design study suggests that lateral elevation hasonly a minor impact on well yield and should be deter-mined based on practical considerations of construction,operation, and maintenance. We have also found thatmaximizing yield and water quality at Saylorville Well-field may be achieved simultaneously—designs that max-imized the production rate also induced more recharge andmaximized the percentage of surface water captured. Con-versely, maximizing the pumping rate per unit of screenlength resulted in low capacity and low surface water out-comes. Post-construction performance testing of the twocollector wells shows that well yields are close to pre-dicted yields, even though the total screen lengths werereduced during construction.

We present a new, efficient framework for thehydraulic design of collector wells based on groundwaterflow modeling. Our method allows for a realistic repre-sentation of the effects of stream properties (geometry andbed resistance) on yield and allows for quantitative com-parisons of source water quality and construction coststo be made between alternate designs. This approach tolateral design is feasible because of the flexibility of theAEM for analyzing alternate geometric designs withoutthe constraints of a computational grid. The same designtools presented here have been used successfully for pre-liminary design of collector wells at other sites in theUnited States.

AcknowledgmentThe authors wish to acknowledge David Dahlstrom

for his work on the modeling project described in thisarticle.


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