Review and Analysis of Chlorinated Solvent Dense Nonaqueous …web.cecs.pdx.edu/~gjohnson/Parker et al 03.pdf · 2011. 2. 23. · Review and Analysis of Chlorinated Solvent Dense

Review and Analysis of Chlorinated Solvent Dense Nonaqueous Phase LiquidDistributions in Five Sandy Aquifers

B. L. Parker,* J. A. Cherry, S. W. Chapman, and M. A. Guilbeault

ABSTRACT zone. In recent years, several in situ technologies havebeen proposed for remedial restoration of chlorinatedTo select and design effective remedial measures for dense, non-solvent DNAPL source zones (Interstate Technologyaqueous phase liquid (DNAPL) source zones, better understanding

of the architecture of these zones is needed. In this study, a suite and Regulatory Council DNAPL Team, 2002), and nu-of investigative techniques was applied to perform detailed vertical merous site trials have been conducted, but no completedelineation of chlorinated-solvent source zones in sand aquifers at successes have been documented to date.five contaminated industrial sites (two in Connecticut, and one each One of the limiting factors in the design and applica-in Florida, New Hampshire, and Ontario). The DNAPL occurs in the tion of these in situ technologies is the paucity of infor-middle of the aquifers at three of the sites and at or near the bottom mation about the architecture of DNAPL source zones.at the other two. The DNAPL entered the subsurface at these sites dec-

Dense nonaqueous phase liquid source zones at indus-ades ago, and therefore the DNAPL zones have aged due to ground-trial sites typically formed decades ago, when free-prod-water dissolution. The suite of investigative techniques was used touct DNAPL was spilled or leaked, causing infiltrationperform profile sampling using direct-push methods, in which depth-of free product into the groundwater zone. Nearly alldiscrete soil and groundwater samples were taken with extremely

close vertical spacing. The sampling included methods to distinguish published information about the nature of DNAPL sourcebetween free-product and residual DNAPL at two of the sites. At zones in sand deposits comes from laboratory and fieldeach location where DNAPL was found, the DNAPL occurred in experiments and simulations using numerical models.one or a few thin layers, generally between 1 and 30 cm thick. These Field experiments conducted in the sand aquifer at thelayers were positioned within distinct grain-size zones, or at contacts Borden site showed that infiltration of free-product tetra-between sedimentological layers. In some cases, the DNAPL layers chloroethylene (PCE) resulted in complex source-zonehave no apparent textural association. For any particular sampling

architecture (Kueper et al., 1993; Brewster et al., 1995).hole to have a high probability of finding such layers, continuousShortly after the DNAPL marked with red dye was re-cores must be collected and sampling of these cores must be doneleased in these experiments, the DNAPL achieved aat very close vertical spacing (5 cm or less). Free-product DNAPLstable distribution comprised of two types of DNAPLoccurrences in conventional wells at three of the sites indicated, mis-

leadingly, much greater DNAPL layer thicknesses than actual, and subzones: (i) horizontal layers or thin pools contain-in one case, the conventional well may have caused short-circuiting ing most of the DNAPL mass and (ii) vertical residualof DNAPL from the middle to the bottom of the aquifer. Although DNAPL pathways connecting the layers. The layers,all of the DNAPL source zones are comprised of only sporadic, thin which represent discontinuous patches of DNAPL, com-DNAPL layers representing little total mass, these source zones are prise both residual and free-product DNAPL. The lay-the cause of high-concentration dissolved plumes down gradient. ers typically form on top of interfaces between small

sedimentological units exhibiting different texture. Kue-per et al. (1989) and Illangasekare et al. (1995) used

Persistent chlorinated solvent contamination is laboratory experiments to show that layers form in sandcommon in unconfined sandy aquifers in industrial deposits even when the grain-size contrasts between

areas, normally as a result of organic solvents present beds in the sand are small, such as a change from coarseas DNAPLs residing below the water table in zones to somewhat finer sand. In the Borden experiments citedknown as source zones (U.S. EPA, 1992; Feenstra et al., above, extremely small-scale sampling of cores and vis-1996). Natural groundwater flow through the source zones ual inspection for small-scale core features were neededcauses the formation of plumes of aqueous-phase con- to locate the DNAPL layers. The DNAPL layers formedtamination, which typically evolve to occupy much larger preferentially in the coarser-grained horizons, which wasaquifer volumes than the source zones, and pose much expected given the nonwetting nature of the DNAPL.more risk to receptors and the environment than the Although laboratory and field experiments, as wellsource zones. The common remedial action taken to as mathematical modeling, have provided considerablereduce these risks is to control the plume or the mass insight into the nature of DNAPL source zones in sandyflux from the source zone by pump-and-treat. However, aquifers at a few research sites, little is known about thethis approach requires pumping for many decades or nature of DNAPL source zones at actual contaminatedeven longer, and therefore it is not a permanent solution industrial sites where the DNAPL entered the subsur-to the problem. A permanent solution requires removal face decades ago. The DNAPL source zones at indus-or destruction of the DNAPL mass from the source trial sites are commonly between two and five decades

old. Old DNAPL source zones in sandy aquifers mayB.L. Parker, J.A. Cherry, S.W. Chapman, and M.A. Guilbeault, De- have DNAPL volumes and distributions that are muchpartment of Earth Sciences, University of Waterloo, 200 UniversityAvenue West, Waterloo, ON, Canada N2L 3G1. Received 14 Nov.

Abbreviations: bgs, below ground surface; DCT, drainable core tech-2002. Special Submissions—Contaminant Characterization, Trans-nique; DNAPL, dense nonaqueous phase liquid; MIP, membraneport, and Remediation in Complex Multiphase Systems. *Correspond-interface probe; PCE, tetrachloroethylene; PITT, partitioning inter-ing author ([email protected]).well tracer test; TCE, trichloroethylene; VOA, volatile organic analy-sis; VOC, volatile organic compound.Published in Vadose Zone Journal 2:116–137 (2003).

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www.vadosezonejournal.org 117

Fig. 1. Schematic representations of four scenarios for dense nonaqueous phase liquid (DNAPL) source zones in sandy aquifers: (a) DNAPLpenetrates to bottom of homogeneous sand aquifer to form a bottom pool, (b) DNAPL penetrates through homogeneous sand and accumulatesin a layered transition zone, (c) DNAPL forms layers of residual and free-product DNAPL suspended in the sand aquifer, (d) DNAPL formsmultiple layers distributed throughout the aquifer thickness. Note that all scenarios have residual trails because the source zones are representedat early time.

different from what existed at the time of formation. did not reach the top of the aquitard, because ofThe dissolution and mass export of DNAPLs from the a stratified transition between the aquifer andsource zones for decades would have removed a portion aquitard. The DNAPL penetrated into the coarser-of the DNAPL mass, and this may have caused consider- grained layers in this transition zone, where hori-able change in some aspects of the source-zone archi- zontal DNAPL accumulations formed. In some sit-tecture. At most DNAPL sites, the actual DNAPL has uations, the DNAPL would not reach the aquitardnever been found (Feenstra and Cherry, 1996), and the surface because of large retention capacity in thefield evidence used to designate a site as a DNAPL site transition zone compared with the volume of re-is typically indirect (U.S. EPA, 1992). There is a need for lease, or a lack of stratigraphic discontinuities ordetailed information about the occurrence and distribu- pathways downward through the transition zone.tion of DNAPL at selected industrial sites for improve- 3. In another case (Fig. 1c) the DNAPL descends intoment of conceptual models of DNAPL source zones.

the sand aquifer, where it forms multiple DNAPLThis review presents results of field investigations oflayers due to effects of subtle permeability con-the nature of DNAPL source zones at five industrialtrasts in the sand or presence of finer-grained siltysites on sandy aquifers, where trichloroethylene (TCE)or clayey layers. This DNAPL layering, comprisedor PCE DNAPL has caused groundwater contamination.of both residual and free-product DNAPL, pro-One of the sites is located in the Province of Ontario,vides greatly enhanced retention capacity in thetwo are in Connecticut (Connecticut A and B), and oneaquifer, which limits DNAPL penetration.each in New Hampshire and Florida. Each of the source

4. On the other hand, if sufficient DNAPL was re-zones is the cause of a distinct and persistent dissolved-leased to exceed the bulk retention capacity of thephase plume that has been subjected to intensive plume

monitoring. At the time of our studies, no subsurface aquifer, DNAPL may also reach and accumulateremedial actions had been undertaken in the study ar- at the aquifer bottom (Fig. 1d).eas, except for source zone containment and minimal

Most or all of the DNAPL zones in these cases consistfree-product DNAPL recovery at the Connecticut A site.of thin, vertically discontinuous layers (i.e., lenses) ofGiven what was known about the geology of eachDNAPL. Therefore, a suite of techniques was assem-site and historical information on solvent use, four con-

ceptual cases for DNAPL source-zone architecture bled for sampling, in vertical holes, the sediment and(Fig. 1) were considered as reasonable paradigms for groundwater with exceptionally small vertical spacingthe formation of the source zones decades ago. between samples. At three of the five field sites, DNAPL

had been previously found in monitoring wells, and1. In the first type of source zone (Fig. 1a), thetherefore the new data allowed comparisons among theDNAPL penetrated to the bottom of the relativelynature of the source zones conceptualized based onhomogeneous sand aquifer, where it accumulateddifferent monitoring techniques and scale of sampling.in a free-product layer or pool on top of the aquit-The variability of the conditions found at the five studyard. As the DNAPL traveled downward, it left asites makes the results relevant to many other chlori-trail of residual DNAPL.

2. In the second case (Fig. 1b), much of the DNAPL nated solvent contamination sites in sandy aquifers.

118 VADOSE ZONE J., VOL. 2, MAY 2003

Table 1. Summary of characteristics at five field sites investigated.

Connecticut A Connecticut B Ontario Florida New Hampshire1996–1997 1999 1998–2000 1998–2000 1997–1998

Aquifer depositional valley train glacial fluvial glacial spillway coastal, beach glaciofluvial depo-environment glacial outwash deposits (lacu- environment, sition in bedrock

strine sands, shallow marine valleykames, outwashsands)

Depth to water 4 3 2.5 4.5 3table, m bgs

Saturated aquifer 6 12 3 �25 �15thickness, m

Aquifer bottom varved silt aquitard layered silt and clay clayey silt till at clay and silty clay at silty till at 20 m bgsat 9 m bgs at 15 m bgs 5.5 m bgs; inter- 20 m bgs

bedded transitionzone over bottom1 m of aquifer

Type of facility manufacturing manufacturing various including metals cleaning and tool and dye factoryprecision furniture and fabricatingmachined parts electrical equip-

ment manufac-turing

Years of operation 1952–2001 industrial site c1900; late 1920s–1990 1964–present late 1940s–early 1980scurrent site opera- (now abandoned)tions 1946–present

Organic solvent TCE PCE TCE TCE PCEContaminant leaking underground multiple releases unknown 2–55 gal drums 1966, septic discharges,

releases storage tanks and from storage hose burst 1977, spills aroundproduct distribu- tanks, degreasers routine spillages loading dock,tion lines in tank from cleaning routine spillages,farm area 1964 to 1977 aboveground

storage tank leakinto floor drain

Estimated release late 1950s to early 1950 to late 1980s suspected 1960s, late 1960s–1970s throughout opera-time(s) 1970s possibly earlier tions period

DNAPL distribution bottom suspended and transition at bottom suspended suspendedtype transition at

bottom

Figure 2 shows the general geologic setting of each of theSITE USES AND GENERAL FEATURESfive DNAPL source zones. The sand aquifers are unconfined in

The five sites have several characteristics in common, but the sense that the sand deposits forming the aquifers extendalso show considerable variety in geologic origin and site use to ground surface. Therefore, DNAPL released to the ground(Table 1). The features in common include: single-component had to infiltrate through only a few meters or less of permeableDNAPL (either TCE or PCE) that has been in the ground for sand before entering the groundwater zone. Groundwater flowdecades, shallow water table (�5 m below ground surface [bgs]), is horizontal through the DNAPL source zones, which createdshallow maximum depth of DNAPL occurrence (�20 m) and the down gradient contaminant plumes. The aquifer sands aregeological conditions suitable for effective use of direct-push unconsolidated and cohesionless and, therefore, acquisitiondrilling equipment (e.g., lack of obstructions such as cobbles of core samples from below the water table requires use of cor-or boulders). The DNAPL at each site is the result of routine ing methods that prevent the sand and pore water from fallingsolvent use and storage at facilities engaged in production of out of the core barrel.metal products. Each of the sites has several locations whereDNAPL may have entered the subsurface, which contributes

APPROACH AND METHODSto considerable spatial variability of the source zones. Informa-tion on site use (Table 1) indicates that DNAPL probably first The approach taken for the field investigations involvedentered the subsurface at these sites in the 1950s or 1960s, acquisition of detailed concentration data from vertical coresand the releases ceased by the early 1970s to 1980s. Althoughhistorical information on the use of chlorinated solvents isavailable for all but the Ontario site, the volume of DNAPLlost to the subsurface and the exact locations of the releasepoints are generally unknown. The prospect for source zoneremediation using in situ technologies is an issue at all of thesites. At each of the sites, groundwater monitoring was con-ducted by consultants for the site owners for at least severalyears before the initiation of our studies in 1996. At three of thesites, free-product DNAPL was encountered during previousinvestigations in conventional monitoring wells (ConnecticutA and B, Ontario). At the Connecticut A site, the area inwhich wells showed free-product DNAPL was isolated in 1994with a steel sheet piling enclosure keyed into the underlyingaquitard. At the other sites, no subsurface remedial measures Fig. 2. Columns representing the geology at each of the five studywere deemed needed or implemented until after our source sites. The depth zone of DNAPL source zone investigations at

each site are indicated by the dashed lines.zone studies were completed.


Table 2. Physical properties of DNAPL at three sites where DNAPL occurred in conventional wells.

Organic Measurement InterfacialSite solvent Well ID Date analyzed Composition temperature§ Density§ Viscosity§ tension§ Solubility¶

g mL�1 cP mN m�1 mg L�1

Connecticut A TCE MW-61S1 July 1996 �99.5% TCE, 23�C 1.45 0.55 21.1 NAminor PCE,CT, TCA

Connecticut A TCE MW-68S1 July 1996 ″ 23�C 1.45 0.54 23.5 NAConnecticut B PCE MW-7M Jan. 2003 �99.9% PCE, 20�C 1.62 1.11 34.2 160

minor TCEConnecticut B PCE MW-18M Jan. 2003 ″ 20�C 1.62 1.11 23.6 170Ontario TCE BH401 Jan. 2003 �98% TCE, 20�C 1.44 0.79 17.4 1400

minor PCELiterature† TCE – – – 25�C 1.46 0.57 – 1100 (1385)#Literature‡ TCE – – – 20�C 1.46 0.57 34.5 1100Literature† PCE – – – 25�C 1.63 0.90 – 200 (237)#Literature‡ PCE – – – 20�C 1.62 0.89 44.4 150

† Table A1, Pankow and Cherry (1996) for DNAPL at 25�C.‡ Table A.1, Cohen and Mercer (1993) for DNAPL at 20�C.§ Density, viscosity, and IFT analyses by Department of Civil Engineering, Queen’s University, Kingston, ON, Canada by ASTM methods D1217

(density), D445 and D446 (viscosity), and D971 (IFT) (American Society for Testing and Materials, West Conshohocken, PA).¶ Solubility measurements by Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada.# Literature and calculated (in brackets) solubility values reported.

and also from adjacent groundwater sampling holes. The goal of the aquifer deposits. Therefore, at four of the five sites,the general depth intervals in which the coring techniqueswas to determine the vertical thickness and characteristics of

all DNAPL zones occurring at each sampling location. Each were applied most intensively were those identified first usingthe Waterloo Profiler, in combination with rapid on-site VOChole was drilled using efficient direct-push techniques that

minimized disturbance to the sampling zones. Groundwater analyses so sampling decisions could be made in the field as theinvestigations proceeded. Each of the investigative techniquessamples were collected at larger vertical spacing (15–60 cm)

than the collection of soil samples from cores (typically 2–5 cm) applied at these sites is summarized below.because spacing of groundwater samples smaller than about15 cm would produce overlapping of sampling zones because Waterloo Groundwater Profilerof purge and sample volume requirements. Thus, the coredata are more depth-specific than the groundwater data. At The Waterloo Profiler (Pitkin et al., 1999) is a direct-pushthree of the sites (Ontario, Connecticut A and B), where free- device for collecting depth-discrete groundwater samples inproduct DNAPL had been found previously in conventional unconsolidated granular deposits. This tool allows rapid col-monitoring wells, two or more of the sampling holes were po- lection of samples at multiple depths in the same hole withoutsitioned very near these wells, and DNAPL was found in all retrieving, decontaminating, and redriving the tool betweenof these holes. The DNAPL samples from these sites were samples, with no drill cuttings and only minimal purge watercollected from the conventional wells and submitted for mea- generated. The device comprises a profiler head, consisting of

a 4.4-cm-diam. stainless-steel drive-point with open ports fit-surement of DNAPL physical properties (Table 2). At the twoother sites, where DNAPL had not been found in the previous ted with stainless-steel screens. The ports convey water into

a common internal fitting, which is then connected to 3-mm-site investigations, several core holes had to be drilled to findone or two locations with DNAPL. o.d. stainless-steel tubing. The head screws into conventional

AW drill rods of the same diameter, with the sample tubeThe sampling devices applied at the sites included the Wa-terloo Profiler (Pitkin et al., 1999) for groundwater sampling running inside the rods to convey groundwater from the head

to ground surface, using a peristaltic pump placed downstreamand the piston core barrel (Zapico et al., 1987) for collectionof continuous cores. These two devices take samples from from a stainless-steel sampling manifold, containing 25- or

40-mL volatile organic analysis (VOA) sample vials. This setupholes that are subsequently sealed or allowed to cave once thesampling is done. Permanent multilevel groundwater sampling avoids contact between the groundwater sample and the pump

tubing and exposure to air.systems, modified from the design of Cherry et al. (1983), wereused at two of the sites in the vicinity of the source zones. The In summary, to collect a groundwater sample, the Profiler

tip is advanced using direct-push equipment, while contami-cores were used for visual inspection of geologic features, andfor extraction of many small soil samples for volatile organic nant-free water (e.g., distilled water) is pumped down the

sample tube and out the ports to prevent ports from cloggingcompound (VOC) analysis and determination of immisciblephase presence or absence. At the Connecticut A and B sites, with fine sediment, and to purge formation water from the

previous sampling depth. When the desired sampling depthcores were also used for depth-discrete tests to determine pres-ence or absence of drainable (free-product) DNAPL. is reached, the pump is reversed to begin pumping water to

the surface, and sufficient volume purged (typically 200 mL)Each of the investigative techniques filled a particular role.Whenever a hole is sampled, DNAPL depth and thickness to remove any distilled water remaining in the tubing before

collecting the sample. If necessary, additional water can beare unknown until DNAPL is encountered. For each hole,there was the expectation that the DNAPL would occur in thin purged to “develop” the zone around the sampling ports if

turbidity is a problem. After collection of the sample, the tiplayers, and therefore it was necessary that at least a portion ofthe hole be sampled at very close vertical spacing. Therefore, is advanced to the next depth, again injecting clean water,

and the process repeated until the maximum desired depth isto focus this intensive core sampling on zones with the highestprobability of DNAPL occurrence, the Waterloo Profiler was reached. Samples can be collected at vertical spacings as close

as 15 cm without causing overlap of sample zones in the aqui-used first at all but the Ontario site, where the flow rate intothe sampling tool was too slow due to the fine-grained nature fer. In finer-grained silty or clayey zones, the time required


to obtain sufficient sample volume may be large, so that sam- The standard version of the DCT is mostly applicable tocoarser-grained sand aquifers where fluids drain readily underpling in such zones is not practical. Purge times can be used

to provide a relative indication of permeability for each sample gravity (e.g., Connecticut A Site). The DCT requires corescollected using the piston core barrel, and it is applied whendepth. By monitoring pressures with a gauge positioned before

the sampling manifold, the depths of low permeability zones the cores are still contained in the aluminum sample tube. Atlocations where DNAPL accumulations are at the bottom of(e.g., aquitard interface or lower permeability layers) can be

accurately determined. An adaptation of the profiler head is aquifers, the core run is selected to extend a short distanceinto the underlying aquitard to allow the aquitard material tothe use of a disposable tip, so that the hole can be grouted

during removal in cases where collapse is not expected and form a plug in the bottom of the core; this ensures containmentof DNAPL in the core while the core is raised to the surface.contaminant cross-connection is of concern (e.g., when sam-

pling within DNAPL source zones). During and after removal from the core barrel, the core tubeis maintained in a vertical position throughout the drainingprocedure. The core is clamped to a vertical board or wall,Direct-Push Piston Sampler and the upper core cap removed. The core is then sequentiallydrained from top to bottom at closely spaced vertical intervalsCores in the aquifers, and in some cases from the upper(typically 2.5–5.0 cm) by drilling small holes (3 mm) into thepart of the underlying aquitards, were collected using thecore tube, and collecting fluids that drain from each depthpiston core barrel described by Zapico et al. (1987). This coringinto a 25- or 40-mL glass vial. Where DNAPL drained frommethod provides excellent recovery of relatively undisturbedan interval, it typically flowed first followed by water. Aftersamples of cohesionless sandy deposits from below the waterfluid drainage ceased from an interval, usually after about 10table. The major components of the system consist of an alumi-min, the volumes of fluids (water/DNAPL) that drained fromnum sample tube (5.1-cm o.d., 1.52 m long, 1.3-mm wall thick-the interval were recorded and draining of the next intervalness) inside a steel casing that forms the core barrel and awas started. Draining of the core continued until the aquitardpiston inside the sample tube. The sample tube contains theinterface was reached and no fluids would drain from the core.sample, and a new tube is used for each core run. Steel casingThe core was then subsampled in the manner described below.is connected to the top of the core barrel and a drive head isApplication of the DCT typically requires several hours forattached to the top of the steel casing. The core barrel waseach core, although several cores can be drained concurrentlydriven into the ground using the air-hammer and scaffoldingto increase efficiency. For sites with finer-grained sands wheremethod of Starr and Ingleton (1992) or the direct-push rigpore fluids do not drain readily by gravity, such as the Connect-described by Einarson (1995). The bottom of the piston ex-icut B site, the technique can be modified. In this case, fluidtends beyond the sample tube and is pointed to facilitateextraction was enhanced by applying pressure to sections ofdriving the sampler through the soil and to prevent materialthe core to expel the pore fluids. The modified DCT was ap-from entering the sample tube before the sampling depth isplied to specific sections of cores collected adjacent to an ini-reached. The sampler is driven to the desired core start depthtial continuous core that was screened for DNAPL using thewith the piston fixed in place. Then the piston is tied off andSudan IV method described below to target DNAPL zonesthe core barrel advanced through the sampling interval. Theidentified by the rapid screening technique.moving piston creates suction that prevents the sand or gravel

from falling out of the core barrel as the sampler is broughtto the surface and also helps retain the original pore fluids in Core Subsampling Procedurethe core. In DNAPL zones, the core was handled with extracare to minimize the potential for fluids (water and DNAPL) Core samples contained within the aluminum core tubes

were subsampled using both a screening technique using a hy-to redistribute within the core before subsampling. The abilityto collect relatively undisturbed cores using the piston corer, drophobic dye (Sudan IV) for visual identification of DNAPL

and for quantitative laboratory analysis of VOCs. In this pro-which provided excellent recovery (typically �95%), was es-sential for the subsequent core examination and subsampling cedure, the core tube is placed horizontally on a firm surface

in a wooden holder, and split from end to end, first using aprocedures described below.circular saw to cut through the aluminum, and then using awire to separate the soil material. The half of the core intendedDrainable Core Technique for subsampling is immediately covered with aluminum foilto minimize volatilization of VOCs and moisture loss, andAt two of the field sites (Connecticut A and B), cores werethe other half used for core photographs, detailed geologicsubjected to a draining procedure in the field before soil sub-logging, selection of depths for subsampling, and for subse-sampling, termed the drainable core technique (DCT), to de-quent physical parameter tests (e.g., foc, grain size analysis,termine the depths where high DNAPL phase saturations

were present and free product would drain from depth-specific permeameter tests). Numerous small cylindrical soil samplesare collected along the core using stainless-steel sampler andzones in the core. The DCT is used in combination with the

Sudan IV screening test, described below, which identifies in plunger devices. The potential for cross-contamination is mini-mized by scraping off the upper few millimeters of soil andthe field the presence of any DNAPL, but does not distinguish

between free-product (i.e., potentially mobile) and residual taking care not to advance the sampler to the outside wall ofthe core tube. Subsamples are taken at close vertical spacings,DNAPL. Therefore, the DCT can be used to determine depths

at which free-product DNAPL occurs, and by default the typically 2.5 to 10 cm, so that detailed concentration profilesare obtained. The samplers and plungers are decontaminateddepths of residual product are inferred where the Sudan IV

screening test indicates a positive result for nonaqueous phase between each sample depth using a three-part wash and rinsesequence with soapy water, methanol, and deionized water.liquid (NAPL) presence, but free-product DNAPL was ab-

sent during the draining. Because this procedure is time-con- Two subsamples are collected from each depth, one for visualdetection of NAPL phase using the hydrophobic dye (Sudansuming, it was not conducted during initial investigations of

DNAPL accumulation zones, but only after the depths of IV) test, and the other for later laboratory VOC analyses.Cohen et al. (1992) described the use of Sudan IV in DNAPLDNAPL zones were first investigated by detailed core subsam-

pling (described below). investigations. The sample for DNAPL screening was extruded


Table 3. Summary of retardation factor and DNAPL indicative concentration estimates for each study site.

Organic Number of Estimated Soil concentrationSite contaminant Stratigraphic zone Average foc foc samples R factor † indicative of DNAPL‡

% �g g�1 wet soilConnecticut A TCE medium to coarse sand 0.038 7 1.2 225Connecticut B PCE all aquifer layers 0.038§ 0§ 1.7 60Ontario TCE silty sand 0.040 9 1.3 245Ontario TCE silt/clay layers in 0.090 4 1.5 330

transition zoneFlorida TCE upper aquifer sands 0.010 4 1.1 210Florida TCE silt/clay layers 0.670 22 4.4 970New Hampshire PCE all aquifer layers 0.038§ 0§ 1.7 60

† Calculated using Koc � 126 mL g�1 for TCE, 364 mL g�1 for PCE (from Table A.1, Pankow and Cherry, 1996). �b � 1.70 g cm�3, φ � 0.35 forsand; �b � 1.60 g cm�3, φ � 0.40 for silt/clay.

‡ Calculated using literature solubility values for TCE � 1100 mg L�1, PCE � 200 mg L�1 (from Table A.1, Pankow and Cherry, 1996).§ Assuming same foc value for aquifer sediments as Connecticut A site.

into a 25-mL glass vial containing a small amount of Sudan Feenstra et al. (1991) described such partitioning calculations,including more general cases involving multicomponent DNAPLIV powder (Fisher Scientific, Hampton, NH), a hydrophobic

dye that solubilizes into immiscible-phase chlorinated organic and unsaturated conditions. The retardation factor is esti-mated using the relation:liquids, causing a bright red color, and a few milliliters of de-

ionized water. The sample for VOC analysis was extrudedinto a preweighed 25-mL glass VOA bottle, containing a known R � 1 � ��b

�� Kd [2]volume of HPLC-grade methanol (approximately 15 mL) forpreservation and VOC extraction. Small variations in analyti-

where Kd is the distribution coefficient and �b is the dry bulkcal procedures were followed for the different sites; however,soil density. Where possible, site-specific parameter valuesthe following is a general summary of the procedure. Soilwere applied, while in some cases estimated values were used.samples preserved in methanol were shaken on a vortex mixerThe distribution coefficient can be obtained from laboratoryafter arriving at the lab to break up the soil. Samples werebatch or column experiments, or estimated using the correla-then stored at 4�C for at least 2 wk to allow adequate timetion Kd � Kocfoc, using literature values for Koc (e.g., Table A.1for extraction. The samples were then centrifuged to separatein Pankow and Cherry, 1996) and measured foc values (whenthe soil and methanol in the sample vial, and a small aliquotavailable). Equation [1] can also be used to estimate the ex-of methanol extract from the sample vial diluted into pentanepected minimum soil concentration in a sample containing(capillary GC grade) containing an internal standard. FurtherDNAPL, by setting Cw to the solubility limit and calculating thedilutions of samples into methanol were performed as neces-expected total soil concentration. Thus the presence of NAPLsary based on initial analysis of the undiluted sample. Stan-can be inferred where the measured total soil concentrationdards were laboratory-prepared mixtures of the target ana-exceeds this minimum calculated value, providing a check onlytes (typically PCE, TCE, DCE isomers) in methanol, whichthe Sudan IV screening test for NAPL presence. Site-specificwere spiked into pentane as described above for the samples.foc measurements were performed on sediments from the Con-The pentane was then analyzed by direct injection on a gasnecticut A, Florida, and Ontario sites using the method de-chromatograph equipped with a micro-electron capture detec-scribed by Churcher and Dickhout (1987), and were subsequentlytor and a liquid autosampler. This technique provides the totalused for estimates of retardation factors and concentrationsTCE content per mass of wet soil, and therefore does notindicative of DNAPL presence (Table 3). For the Connecticutdistinguish between TCE present in the aqueous, sorbed, andB and New Hampshire sites, foc measurements were not per-NAPL phases.formed, so for these sites the aquifer foc was assumed to bethe same as the Connecticut A site, given the similarity inCalculation Procedure Applied to Volatile Organic depositional environments.

Compound Analyses In zones where DNAPL occurs, the NAPL saturation (Snw),which is the ratio of the NAPL volume to the total pore vol-The total soil concentration (Ct) obtained directly from theume, can be estimated usinglab analysis represents the total analyte (e.g., TCE) mass per

unit mass of bulk wet soil sample, and therefore includes dis-solved mass in the pore water and sorbed mass on the solids, Snw �

�bwetCt

�nw�(106)[3]

as well as any DNAPL phase in the sample. This total analyteconcentration can be converted to a pore water concentration where Snw is the nonwetting phase (NAPL) saturation (unit-(Cw) assuming equilibrium chemical partitioning between the less), �b wet is the soil wet bulk density (g cm�3), Ct is the totalsolid and water phases, and no DNAPL phase present using soil concentration (g VOC g�1 wet soil), �nw is the NAPL

density (g cm�3), and φ is the aquifer porosity (unitless). ThisCw � Ct

�bwet

R�[1] calculation assumes that all of the contaminant mass occurs

in the DNAPL phase and thus neglects mass in the sorbedand aqueous phases. This assumption is reasonable because,where Ct is the total analyte concentration in the bulk sample

from the lab analyses (g g�1 wet soil), �bwet is the wet bulk soil in the types of soil materials involved in this study, the massin the sorbed and aqueous phases is small compared with thedensity (g cm�3), φ is the soil porosity, and R is the estimated

retardation factor due to sorption that assumes rapid, linear, DNAPL mass. For example, using the minimum Ct valuesrepresentative of DNAPL presence (i.e., maximum aqueousand reversible partitioning between the VOC and solid-phase

organic C in the aquifer sediments. Presence of DNAPL is in- and sorbed mass present), the NAPL saturation values wouldbe less than about 0.1 and 0.05% for TCE and PCE, respec-ferred for samples where the estimated porewater concen-

tration exceeds the aqueous solubility of the contaminant. tively, using typical site parameters. In most cases such low


values are well below estimated DNAPL saturations, and core indicated a total DNAPL thickness of 13 cm, withtherefore neglecting the sorbed and aqueous mass is reason- a free-product thickness of 8 cm at the base of theable unless the DNAPL saturation is very low. At the Connect- aquifer and overlying residual of 5 cm. At this location,icut A site, cores subjected to the DCT were subsequently there was a discrepancy in the elevation of the aquitardsubsampled for VOC analyses. For depths where DNAPL

interface reported at the conventional well location anddrained, this calculation of NAPL saturation based on VOCthat determined at the core location. The elevation ofanalyses alone could greatly underestimate the percentage ofthe DNAPL column in the well should correspond withNAPL saturation. Therefore, the DNAPL volume drained

from free-phase zones before core subsampling was also in- the elevation of the free-product DNAPL layer in thecluded in the NAPL saturation estimates. core. In this case, free-phase DNAPL from the thin layer

at the base of the aquifer presumably flowed into thewell, where the screen extended below the aquitard inter-RESULTSface, causing the large apparent DNAPL product col-

Connecticut A Site umn. The elevation of the aquitard interface may belower than that reported at the conventional well, con-The TCE DNAPL source zone at the Connecticut Asistent with variations in the interface over small lateralsite occurs in a former tank farm area at the east sidedistances observed elsewhere in the source area, and thisof a large manufacturing facility. The geology in thiswould explain the difference in free-product thicknessarea (Fig. 2) consists of a 9-m-thick medium- to coarse-observed. However, it is also possible that the aquitardgrained sand aquifer with exceptional uniformity over-interface was inaccurately determined during installa-lying a thick clayey silt aquitard, with an abrupt contacttion of the conventional well, so that the well screen ac-separating the aquifer and aquitard. The elevation oftually penetrates deeper into the aquitard. In any case,the aquitard interface varies by �1 m within the source

area. Three conventional wells in the source zone indi- the interpretation of the free-product thickness in the aqui-cated an apparent DNAPL thickness at the base of the fer from the DNAPL column height in the well causedaquifer ranging from about 0.3 to 0.7 m. Therefore, the a significant overestimation of the volume of DNAPLinitial conceptual model for the DNAPL distribution was present in the aquifer. At many of the other core loca-a continuous pool of free-product DNAPL at the base of tions, only residual DNAPL was present, such that wellsthe very permeable sand aquifer. The majority of the positioned at these locations would not have detectedsource zone area was isolated by a steel sheet-pile enclo- the DNAPL occurrences since residual DNAPL wouldsure installed in late 1994, which was keyed into the not be expected to flow into the well.underlying aquitard. In cores where the DNAPL accumulation zones were

In this study, the nature of the DNAPL accumulation thicker, typically within depressions on the aquitard sur-zone was investigated using the detailed core subsam- face, the DNAPL was stratified vertically in discontinu-pling procedures and DCT described above during field ous layers of residual or free-product DNAPL (Fig. 3bepisodes from 1996 to 1997, where a total of 36 cores and 3c). This vertical stratification suggests that the tex-were collected throughout the source area within and tural variations controlling DNAPL distribution are sooutside the enclosure, in an approximate area of 30 by subtle that they are not evident during visual core exami-40 m. Cores were typically collected over a 1.52-m inter- nation. A conventional well next to one of these loca-val at the base of the aquifer, extending a short distance tions (Fig. 3b) suggests a thick continuous DNAPL col-into the underlying aquitard. Dense nonaqueous phase umn with the top of the column coinciding with theliquid was present at 23 of the 36 locations, typically uppermost DNAPL layer in the core. No conventionaloccurring within the bottom 0.3 m of the aquifer, with 13 well was positioned next to the other core locationof the locations indicating both free-phase and residual

(Fig. 3c). However, it’s likely that the DNAPL level inDNAPL and 10 locations indicating residual DNAPLsuch a well would be controlled by the upper free-phaseonly. At three locations where the aquitard surface waslayer and therefore would indicate a thick continuousfound at a lower elevation, DNAPL was found in thezone of free-phase DNAPL in the aquifer. In both casesbottom 1 m of the aquifer. Where DNAPL was present,where cores were collected adjacent to conventionalit was distributed in thin layers typically ranging fromwells, the wells provided a misleading interpretation oftwo to several centimeters thick. Figures 3a to 3c showactual DNAPL conditions, giving the impression of athree profiles collected from this site. Site-specific mea-large free-product pool in the bottom of the aquifer.surements of foc of the aquifer sediments were used

Using the measured total soil concentrations, esti-for estimates of the TCE retardation factor and soilmates of the DNAPL phase saturations were performedconcentration indicative of DNAPL (Table 3), shownusing Eq. [3]. The DNAPL volumes that drained fromon the example profiles.the cores during application of the DCT were also in-Different scenarios for DNAPL residual and free-cluded in these estimates. The DNAPL saturations (Snw)product distribution were observed in cores, with theranged from about 10% or less in residual zones to moremost common being a single thin layer of free-productthan 50% in free-phase zones at the base of the aqui-and/or residual at the aquitard interface (Fig. 3a) withfer. These values indicating residual and free-producta total DNAPL thickness of 5 to 15 cm. In this case, aare consistent with the ranges reported by Feenstranearby conventional well indicated an apparent DNAPL

product thickness of about 40 cm (1996), whereas the et al. (1996).


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Connecticut B Site by the Sudan IV method coincided closely with DNAPLpresence indicated by the total PCE concentrations,The Connecticut B facility is situated on a layer ofsuggesting that the parameters used to estimate thefill with a thickness of 1.5 to 3.0 m, which overlies a DNAPL indicative concentrations were appropriate. The6-m-thick medium to fine sand aquifer. The aquifer sedi- product levels in the nearby conventional well at thisments become finer and change from brown to gray at location (MW-18M) were misleading and provided noa depth of 10 to 12 m bgs, marking the top of a transition useful measure of the actual DNAPL distribution in thezone consisting primarily of layered silt and fine sand. subsurface. At this location, it is quite possible thatBelow the transition zone is a layered silt and clay aquit- DNAPL entered the well from the shallow layers (possi-ard (Fig. 2). At this site, two DNAPL source areas were bly Zone 6, which intersects the top of the well screen,

investigated in 1999 using the Waterloo Profiler and and contained free-phase DNAPL) to cause accumula-detailed core subsampling and modified DCT proce- tion at the bottom of the well. This cross-connectiondures described above. In each source area, a monitor- may have caused the deeper DNAPL zones observeding well indicated the presence of free-product PCE. In in the formation near the bottom of the well (Zone 7).the first stage of investigation at this site, the Profiler It is conceivable, however, that the DNAPL migratedwas used to collect depth-discrete groundwater samples downward from the upper DNAPL layers throughfrom a hole at each location. Then, at each location, stratigraphic discontinuities, and then laterally at depthstwo continuous cores were collected, with one core sub- where the deepest DNAPL layers were observed injected to subsampling and the other subjected to the the cores. Such vertical pathways would be difficult tomodified DCT with screening for DNAPL using Sudan observe in a limited number of vertical cores.IV in both the drained fluids and soil core subsamples. The second area investigated at the Connecticut BSite-specific measurements of foc were not performed. site (BT) was in the area of a former PCE storage tank,Therefore the average value for aquifer sediments at where a conventional well (MW-7M) indicated the pres-the Connecticut A site was assumed for estimating the ence of free-product DNAPL, with a thickness rangingPCE retardation factor and DNAPL indicative concen- from 0.4 to 0.5 m during 1997 to 1999 (Fig. 4b). Intration (Table 3). These estimates are subject to more contrast to the first location (AT), DNAPL was onlyuncertainty, particularly given the degree of geological found below 11.0 m bgs in this area. The upper threelayering at this site, but compare well to Sudan IV DNAPL zones (Zones 1, 2, and 3) were observed in thetest results. depth range of 11.0 to 11.6 m bgs near the bottom of

The first source area investigated (AT) as part of the sandy aquifer in thin layers (10, 5, and 8 cm, respec-this study site was the location of a former plating-shop tively). The fourth and fifth layers of DNAPL (Zones 4degreaser, where a conventional well (MW-18M) showed and 5) were observed in the finer-grained layered siltyfree-product DNAPL with a thickness ranging from deposits below the aquifer contact (transition), in the0.5 to 0.7 m during 1997 to 1999 (Fig. 4a). The detailed interval between 11.9 to 12.1 m bgs. These layers coin-core sampling done at this location found seven separ- cide with the bottom of the backfill material below theate thin DNAPL layers. Six of these DNAPL layers conventional well (MW-7M); therefore, they may beoccurred in the depth range of 4.9 to 7.0 m bgs, and the result of short-circuiting from shallower zones (e.g.,one at 10.7 m bgs. Seven DNAPL layers were also ob- Zone 1) where DNAPL saturations indicate free prod-served in the cores subjected to the modified DCT at uct. The positions of the DNAPL zones indicated bylocation AT-2 collected about 1 m away. The shallowest the Sudan IV method coincide closely with measuredDNAPL zone (Zones 1 and 2) were located from 4.9 total PCE concentrations in the BT-1 cores. The productto 5.5 m bgs and were about 0.25 and 0.45 m thick, levels in the conventional well again were misleadingrespectively, with only a 2.5-cm-thick layer separating and provided no useful measure of the actual DNAPLthem. Free-phase DNAPL was not observed in the distribution in the subsurface.drained fluids in AT-2 from this depth, indicating resid- Deeper cores were collected at an adjacent locationual DNAPL only. Three thinner DNAPL layers (Zones (BT-2), located about 0.9 m away, from 12.8 to 14.9 m3, 4, and 5) were observed in AT-1 between 5.8 to bgs, with core subsampling to determine if DNAPL6.1 m bgs, with a thickness of about 10, 5, and 5 cm, had penetrated into the layered aquitard. Total PCErespectively. These were not observed in the adjacent concentrations from these deeper cores were added toAT-2 core, suggesting the DNAPL layers may be quite the graph shown in Fig. 4b. Four thin zones of DNAPLdiscontinuous laterally. The sixth DNAPL zone (Zone (Zones C1–C4) were observed in the depth range of6) was observed from 6.5 to 7.0 m, in both AT-1 and 12.8 to 13.7 m bgs, with a layer thickness ranging fromAT-2, with DNAPL also observed in the drained fluids about 5 to 15 cm, indicating DNAPL penetration intofrom AT-2, suggesting the presence of free-phase the upper 2 m of the finer-grained transition zone. TheDNAPL. The seventh DNAPL zone (Zone 7) was a deeper DNAPL zones observed in BT-2 cores suggestthin layer (approximately 5 cm) at a depth of 10.7 m that the DNAPL Zones 4 and 5 in BT-1 cores may notbgs observed in AT-1 and in the soil at AT-2. A final be a result of short-circuiting in the well backfill, asand eighth DNAPL zone was observed from 11.0 to suggested above, but instead suggest deeper DNAPL11.3 m bgs in AT-2 drained fluids and soil, but this zone migration through natural pathways. In the bottomwas not observed in the AT-1 cores and is not shown 1.5 m of the subsampled cores at this location, PCE

concentrations decline by more than two orders of mag-in Fig. 4a. The positions of the DNAPL zones indicated


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nitude below the calculated concentration indicating in the underlying aquitard was determined to nondetectconcentrations at one location (Fig. 5), with the shapeDNAPL phase, to values at or close to detection limits,

which strongly suggests that DNAPL did not penetrate of the concentration profile below the aquitard interfaceshowing diffusion-dominated transport in the aquitard,any deeper into the aquitard at this location.confirming the maximum depth of DNAPL penetrationand lack of DNAPL entry into the aquitard. The abruptOntario Siteshift in the total soil TCE concentration across the aqui-

At the Ontario Site, a former manufacturing facility tard interface (Fig. 5) can be attributed to the differencesituated within a primarily residential area, the DNAPL in sorption for these lithologies. Based on an averagezone comprised of TCE occurs in a small area near the foc of the aquitard of 0.24% (average from four samples),northeast corner of the property with a portion of the the TCE retardation factor is estimated to be 2.2 forsource occurring beneath the building and an adjacent the aquitard, compared with about 1.2 for the aquiferroadway. No records of TCE use at the site were found sediments. The difference in retardation factors, alongin previous studies. It is surmised that releases occurred with differences in porosity and bulk density, adequatelyfrom a storage tank in this area, or by disposal of spent accounts for the observed shift at this depth.solvents just outside the building. The general geology In the profile shown in Fig. 5, several samples withinin the source area (Fig. 2) consists of an upper perched the transition zone have total soil TCE concentrationsaquifer comprised of about 4 m of fine sand, overlying close to or exceeding the limit where DNAPL is ex-a 1-m-thick finer-grained and layered transition zone pected in sandy zones (Table 3); however, no DNAPLthat abruptly changes to a uniform clayey silt till aquit- was detected at these depths using Sudan IV screening.ard. The transition zone over the bottom meter or so In most cases, these samples were from silty or clayeyof the aquifer exhibits much more textural variability, layers within the transition zone, where the retardationwith thin layers or lenses of silty and clayey material factor and DNAPL indicative concentrations are pre-separating distinct sandy layers. Four conventional mon- sumably higher than for the sandy layers (Table 3), sup-itoring wells in the source zone showed free-product porting the lack of DNAPL in these layers. Ground-thickness ranging from 20 to 100 cm in previous investi- water flushing in the bottom of the aquifer within thegations. Contours of the surface of the underlying clayey transition zone is very slow, so that most of this zonetill aquitard suggested that the DNAPL was trapped in has concentrations at or close to TCE solubility. Fora stratigraphic depression in the aquitard surface. the identification of DNAPL based only on total soil

Source zone investigations, conducted between 1998 concentrations, it would be necessary to characterize theto 2000, included collection of continuous cores from variability of the parameters used to estimate DNAPL19 locations, spanning the zone from the water table (ap- indicative concentrations, which illustrates the uncer-proximately 2.5 m bgs) to slightly into the underlying tainty of DNAPL identification based solely on totalaquitard. In a few cases, cores were advanced deeper into mass concentrations. Therefore, this method is less reli-the aquitard, so that the full TCE distribution in the able than the Sudan IV technique. It is also possibleaquitard could be investigated. Given the fine-grained that the Sudan IV method, being a visual technique,and layered nature of this aquifer, the DCT was not may not have the sensitivity to detect extremely smallapplied. Table 3 provides estimated TCE retardation amounts of DNAPL in samples, but this uncertaintyfactors and soil concentrations indicative of DNAPL, seems much less than the uncertainty of estimating theusing separate average foc values for the silty sand aqui- amount of sorption in sediments. Estimates of DNAPLfer and sandy layers in the transition zone, vs. the silt– saturation for all samples identified as containingclay layers in the transition zone. DNAPL by the Sudan IV screening test, from all of the

Figure 5 shows a representative profile at one core lo- cores collected during the source investigations, indi-cation adjacent to a conventional well containing about cated a range from �1 to 35%, with a mean of 7%.100 cm of DNAPL. At this location, the detailed coring Therefore, the Sudan IV method indicated good sensi-and subsampling indicated the DNAPL occurs entirely tivity for DNAPL detection, even for sample depthswithin the transition zone, within two thin vertically where the DNAPL saturation was very low.discrete sandy layers separated by silty or clayey beds. The difference in the actual DNAPL distribution vs.These results are consistent with results from other loca- the initial impression based on large product thicknesstions, where DNAPL also occurs in one or two thin layers in wells has important implications. First, the actual

volume of DNAPL present within the thin layers iswith a maximum layer thickness of 10 cm. No DNAPLmuch lower than expected if DNAPL formed a continu-was found in the aquifer overlying the transition zone,ous free-product pool. Second, because the DNAPL issuggesting there has been sufficient time for any DNAPLsuspended within the transition zone and not presentwithin the aquifer to dissolve away. The conventionalat the aquitard interface, there is no driving force forwells indicated much thicker DNAPL accumulations.DNAPL entry into and migration through the aquitard,Where cores were collected adjacent to wells withwhich is relatively thin, and may have fractures or otherDNAPL, the top of the DNAPL zone in each well co-preferential pathways for DNAPL flow. This is a favor-incided with the highest DNAPL level in the cores,able condition because the aquitard is underlain by ansuggesting that DNAPL from the shallow thin layersaquifer used for municipal water supply. Finally, as theflowed into the wells to provide the apparent large prod-profiles indicate, considerable TCE mass occurs withinuct column. The full extent of the TCE contamination


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low permeability layers within the transition zone and water Profiler sampling from an adjacent location lessthan 30 cm away both showed evidence of DNAPL atin the underlying aquitard. Removal or in situ destruc-

tion of such mass will be difficult due to diffusion con- the 10.45-m depth, and indicated no deeper DNAPLoccurrences. The occurrence of DNAPL below the thin,trol, which has implications for selection and design of

source zone remedies. but apparently widespread, clayey layer is consistent withthe configuration of the down gradient plume (Guil-beault, 1999), which by back projection indicates thatFlorida SiteDNAPL got through this clay layer. No DNAPL was

The Florida site is an operating metal fabricating and observed at any of the other core locations, illustratingcleaning facility situated on an aquifer formed of beach the difficulty in locating DNAPL at some sites in areassand and bioclasts, with a thin (5–15 cm) continuous of known releases.clayey layer at a depth of approximately 8 to 10 m bgs,and a few discontinuous silt and clay lenses at greater New Hampshire Sitedepths (Fig. 2). Accidental TCE releases may have oc-curred in three general areas on the property between The New Hampshire site is located in a valley where a

tool and dye factory used PCE for degreasing operations1964 and 1976, when records show TCE was used atthe site. Trichloroethylene contamination in groundwa- between 1957 and 1983. The site lies on an unconfined

sandy aquifer of valley fill of glaciofluvial origin com-ter was discovered in 1966, which prompted samplingof shallow domestic wells in the adjacent residential posed mainly of stratified sand and gravel on top of a

sandy glacial till deposited directly on bedrock (Fig. 2).area in 1968. Several monitoring wells were installed in1984, and an extensive network of monitoring wells was A boulder and cobble layer extends 3 to 6 m below the

ground surface, and bedrock occurs at about 30 m bgs.installed between 1992 and 1994, with wells located onand off the property. Industrial activities at the site ceased in 1983, after test-

ing of a municipal well located approximately 1 kmIn 1996, detailed sampling along a vertical sectionacross the plume was conducted at a location immedi- down gradient from the factory showed contamination.

At this time, inspection of site infrastructure and map-ately down gradient of areas of the suspected DNAPLrelease (Guilbeault, 1999). The TCE concentration dis- ping of the contaminant plume using conventional moni-

toring wells, combined with soil sampling, indicated thattribution along this cross section indicated, by projectionup gradient, the presence of three areas where DNAPL part of the plume was caused by leaks through a floor

drain from an aboveground PCE storage tank in themust reside in the sand aquifer near and beneath themain industrial building. A total of 64 cores, each 1.52 m building.

A small area centered on the location of the formerlong, were collected from 14 holes during a search forDNAPL in these source areas. The Waterloo Profiler PCE storage tank was selected for detailed investigation

as part of this study after the demolition of the buildingwas also used in this search. One of these locations(B207), within the aquifer of a documented DNAPL in 1998. In an area with a diameter of 5 m, groundwater

sampling at five locations was done using the Waterloorelease in 1971, showed TCE concentrations at aqueoussolubility (about 1100 mg L�1, Table 2) at one sampling Profiler. Target zones for coring to search for DNAPL

zones were then selected based on these groundwaterdepth. Based on this indirect evidence of DNAPL, acore hole (PM-18) was drilled within 0.3 m of this loca- PCE concentration profiles, with collection of eight cores,

each 1.52 m long, using the piston core barrel fromtion. Figure 6 shows the results of the detailed samplingat PM-18, along with groundwater sampling at B207. six locations.

Using the Sudan IV method at 5-cm spacing, DNAPLThe Sudan IV method showed DNAPL presence in onethin zone where the sample spacing was 2.5 cm. In this was found at two of these core holes (SM-2 and SM-5),

with two layers in each hole (Fig. 7a and 7b). In onedepth zone, two adjacent core subsamples clearly showedDNAPL presence, and therefore the Sudan IV results in- core, SM-5 (Fig. 7a), the two layers were barely distinct

within a 30-cm interval, with only a 2.5-cm layer withdicate a DNAPL zone less than 5 cm thick. The measuredTCE concentrations from soil VOC analyses showed a no DNAPL (based on the Sudan IV test) separating

them. However, the total soil PCE concentration at thispeak in one sample at this same depth, but at a valueslightly less than the estimated TCE DNAPL indicative depth falls above the DNAPL indicative value, sug-

gesting a single DNAPL layer, as opposed to two distinctvalue for the aquifer. This thin DNAPL zone at theshallower peak occurs in sand lying on top of a distinct layers. Site-specific measurements of foc were not per-

formed on these cores, so the same average value deter-clayey layer. Additional high VOC concentrations occurin samples below where the Sudan IV results were posi- mined for sediments of the Connecticut A site was ap-

plied for estimation of the PCE retardation factor andtive. However, these occur within the fine-grained clayeylayer that has a higher affinity for TCE sorption, and DNAPL indicative concentration for these aquifer sedi-

ments (Table 3). In the second core, SM-2 (Fig. 7b), thetherefore do not necessarily indicate the presence ofDNAPL. The DNAPL indicative value for the clayey two DNAPL layers were separated by a 70-cm-thick

zone. Soil PCE concentrations in the upper 15 cm of thelayers is much higher (Table 3) than the total TCEconcentrations, which supports the lack of DNAPL de- coarser-grained sandy layer, below the upper DNAPL

zone, are also above the DNAPL indicative value, indi-termined from the Sudan IV tests in the clayey zone.Sudan IV sampling in the same core hole and ground- cating DNAPL may also occur in this layer, but at ex-


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Fig. 7a. Profile from New Hampshire Site SM-5 where DNAPL was found.

tremely low residual saturations that may not have been nesses at each of the five sites. The DNAPL was not visi-observed by the Sudan IV screening technique. How- ble in the cores without using the Sudan IV dye test. Theever, the lack of site-specific parameters for estimating core samples analyzed for VOCs, which were generallysorption leaves this technique unreliable compared with taken at the same vertical spacing as the Sudan IVthe Sudan IV test. The top of the upper DNAPL layer samples, corroborated the Sudan IV results. However,and bottom of the lower DNAPL layer were not deter- if the VOC results had been used alone, the DNAPLmined at this location. identifications would have been uncertain because the

In all cases, the DNAPL zones were present in the calculation procedure for DNAPL identification de-finer-grained sandy sediments in the aquifer. Laboratory pends on estimates of porosity and sorption (i.e., masspermeameter measurements on 12 samples from three partitioning). This uncertainty is particularly relevant toof the cores from the site (Guilbeault, 1999) indicated those DNAPL layers where the percentage of DNAPLa less than one order of magnitude range in hydraulic saturation (Snw) is very low. The depth-discrete ground-conductivity, from 2.2 10�3 to 1.5 10�2 cm s�1 (5.5

water sampling done using the Waterloo Profiler served,10�3 cm s�1 mean), with all but one higher value fallingfor screening purposes, to identify high-concentrationwithin a narrower range of just over a factor of three.zones where DNAPL occurrence was most probable.These results indicate that DNAPL persists after severalThis sampling only rarely identified actual DNAPL lay-decades of natural gradient groundwater flushing in aers, because the sample spacing was too large, and be-few distinct layers of moderately lower permeability.cause of dilution caused by the depth integration of eachThe total mass of DNAPL remaining in the source zonessample, given the thinness of the DNAPL layers typicalat the present time is probably very small, given the lowof these source zones.DNAPL saturations (�1–15%), with DNAPL present

The drainable core technique is the most time-con-in thin layers. Even though this study shows very littlesuming of the field methods, and it was particularly use-remaining DNAPL mass, very high concentrations are

present in conventional wells down gradient, and an ex- ful at only one of the five sites. This technique did notpensive source zone containment system (slurry cut-off always yield DNAPL from finer sand layers, even whenwall) was recently installed to contain the contamination we were certain that free product existed in these layers.in the area of the subsurface DNAPL sources. Even when cores do produce DNAPL, the conditions

within the core allowing DNAPL drainage are not con-DISCUSSION AND IMPLICATIONS trolled or monitored. Therefore, this method is not widely

applicable. Of the three methods for finding DNAPLContinuous cores subjected to Sudan IV sampling at(DCT, soil VOC analyses and partitioning calculations,extremely close spacing, 5 cm and sometimes closer,

were required to find DNAPL and define layer thick- and the Sudan IV hydrophobic dye method), the Sudan


Fig. 7b. Profile from New Hampshire Site SM-2 where DNAPL was found.

IV method is the most direct and provides results rapidly nearly all DNAPL layers occur within finer-grainedsand units, as layers sandwiched between silty or clayeyin the field as sampling proceeds.

Free-product thicknesses measured in conventional strata in transition zones or at aquitard interfaces. Inthis study, the field techniques used to find the DNAPLmonitoring wells at three of the sites indicated major

DNAPL pools, but these measurements do not repre- layers performed well; however, these techniques are notthe only ones that have been advocated for determiningsent what is actually in the ground, because the wells do

not indicate the layered (i.e., vertically discontinuous) the nature of chlorinated-solvent DNAPL source zones.Two prominent alternative techniques are the partition-nature or position of the DNAPL distribution. There-

fore, estimates of the DNAPL volume in these source ing interwell tracer test (PITT) method described byJin et al. (1995) and direct-push probes that can sensezones based on product thickness in conventional wells

were grossly exaggerated relative to estimates based chlorinated solvents, such as the membrane interfaceprobe (MIP) described by Christy (1996) and Griffinon the measured thicknesses of the individual layers.

Another implication of the finding that the DNAPL and Watson (2002). The occurrence of DNAPL layers inless permeable aquifer zones, or in geologically layeredzones consist of thin layers rather than thick pools is

that the DNAPL has much less potential for downward transition zones, is disadvantageous for PITT becauseof the propensity for the tracer solution to bypass suchremobilization, except in situations where wells cause

short circuiting of free-product layers. The DNAPL oc- zones. Probes such as MIP offer potential to locate high-concentration zones of chlorinated solvent contamina-currences at the five sites displayed a wide range in

DNAPL saturations (Snw). Some of the layers had free tion, but quantification to the degree necessary to differ-entiate highly sorptive zones from DNAPL, residualproduct, indicated by high Snw values and by results from

the drainable-core tests. Installation of a monitoring from free-product DNAPL, or the vertical separation ofthin layers of DNAPL is unlikely. Both PITT and MIPswell through such layers can worsen the site contamina-

tion if the well screen or sand pack allows DNAPL offer possibilities for contributing insight to the nature ofDNAPL zones; however, they should be applied in con-drainage from a layer positioned near the top of the

screen or sand pack down to the bottom of the well. junction with the techniques based on continuous cores,with detailed subsampling at the scale commensurate withThe Connecticut B site provided a plausible example

of this type of DNAPL cross connection. subtle textural variability, such as those described here.The history of industrial operations at each of theIn the conceptual model for aged DNAPL source

zones in sandy aquifers supported by this study, all or five sites indicates that these DNAPL source zones have


Fig. 8. Illustration of the evolution of a layered DNAPL source zone showing complete dissolution of residual trails, shrinkage of some layers,and complete removal of others due to decades of groundwater flushing.

had a long period of aging, caused by three to five decades cause of reduced permeability, with most contact andmass transfer occurring at the top of the DNAPL layersof flushing by natural groundwater flow in the horizontal

direction. The Darcy flux through the source zones at residing on the aquitard interface. It is also evident atthis site, where local depressions in the aquifer–aquitardmost of the sites is in the range of 5 to 10 cm d�1, except

for the Ontario site where it is lower. It is reasonable to interface are present and initial DNAPL accumulationswere thicker (Fig. 3c), that variable rates of groundwaterexpect that the vertical pipes or fingers of residual

DNAPL with initially low DNAPL saturation, expected flushing over a few decades has created suspended lay-ers of free product separated by zones completely de-to range from about 2 to 15% in granular saturated

media (Feenstra et al., 1996), that initially connected the pleted of immiscible phase, even though contrasts inaquifer permeability are subtle. Although this discussionDNAPL layers (Fig. 1), have disappeared due to dissolu-

tion. Therefore, the contaminant discharge from such focuses only on the DNAPL source zones at the fivestudy sites, the plumes emanating from these sourceaged source zones is much lower today than in previous

decades. For hypothetical cases, Anderson et al. (1992) zones were also studied in spatial detail using depth-discrete monitoring along transects perpendicular toand Sale and McWhorter (2001) showed, using mathe-

matical models, the rapid relative rate of finger and pipe groundwater flow, and consistency was found betweenthe source zone characteristics and the down gradientremoval due to natural groundwater flow. However, at

the five study sites, it is also likely that the decades of plumes.The major finding of this study is that present-daygroundwater flow have removed many DNAPL layers,

in addition to the pipes and fingers, and significantly DNAPL in sandy aquifer source zones resides in spo-radic thin horizontal layers as a result of a few decades ofreduced the DNAPL thickness and saturation of other

layers. This expectation is based on the occurrence of groundwater flushing. Where DNAPL persists suggestsslower rates of dissolution due to the position of theDNAPL layers only in finer-grained aquifer units, or

sandwiched between silty or clayey layers in the transi- DNAPL in the less permeable layers or in layered transi-tion zones, at the bottom of the aquifers at the aquitardtion zones at some of the sites. Initially, most DNAPL

probably occurred in coarser-grained zones, as was contact, or just where groundwater flushing is reduceddue to high NAPL phase saturations in a relatively uni-observed in the PCE DNAPL release experiments in

the Borden sand aquifer (Kueper et al., 1993; Brewster form sand aquifer. This has important implications forremediation. First, there is a lot less DNAPL mass pres-et al., 1995). The more rapid groundwater flow in these

coarser-grained zones causes preferential removal of ent in sandy aquifer source zones than previously thought.Second, the DNAPL distribution is more heterogeneousDNAPL from these zones, as illustrated in Fig. 8. There-

fore, the present-day DNAPL occurrences represent only than the sensitivity and measurement scale of the con-ventional tools being used during site characterizationthe less flushable remnants of the original DNAPL zones.

In the case of the Connecticut A site, where DNAPL and relied on for remediation system designs. Nearlyall of the technologies proposed for in situ remediationgenerally resides at the bottom of a uniform and very

permeable aquifer, contact with flowing groundwater of DNAPL in sandy aquifers involve flushing of treat-ment chemicals into or through the DNAPL source zone.would have been limited within the DNAPL zones be-


and coring equipment. Additional field assistance with sam-For these technologies to make efficient use of the treat-pling was provided by Hester Groenevelt, Colin Meldrum,ment chemicals, the delivery of these chemicals mustDave Thomson, Jennifer Hurley, and Shannon Turcotte. On-be focused on the DNAPL layers, which enhances pros-site groundwater VOC analyses were conducted by Darylpects for destroying or removing the DNAPL in practi-Bassett and Hester Groenevelt, and the VOC analyses donecal time frames. Focused delivery is a major challenge, in the laboratory were conducted by Maria Gorecka. We thank

but it can be facilitated by acquisition of detailed infor- the site owners for site access and representatives of the sitemation on the hydrogeologic structure of the source owners or their consultants for logistical and other assistance,zone and the style of the DNAPL distribution within specifically Lauren Levine and Erin Sullivan at Connecticutthis structure. site A; Andrew Dasinger, Stephen Walbridge, and Michael

Loundsbury at Connecticut site B; R. Goelhert and R. Wileyof the U.S. EPA and R. Bush from Aries Engineering at theSUMMARY OF CONCLUSIONS New Hampshire site; and Andre J. Shye and Siva Thotapalliat the Florida site. General funding for the project was pro-The field investigations conducted at the five sitesvided by the University Consortium Solvents-in-Groundwateron sandy aquifers showed DNAPL occurrences in thinResearch Program, and site-specific funding was obtainedlayers, generally in the range of 1 to 30 cm thick. Atfrom the owners of the Connecticut A and B sites, the U.S.the three sites where free product existed as thick col-DOE for the New Hampshire site and Precision Fabricatingumns in monitoring wells, the actual DNAPL in situ oc- and Cleaning for the Florida site.

curred only in thin layers. At these sites, the new resultsbased on core sampling lowered the estimates of DNAPL

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of dense chlorinated solvents into ground water: 1. Dissolution fromgrained layers within a sequence of sand layers. It isa well-defined residual source. Environ. Sci. Technol. 26:250–256.likely that DNAPL previously also existed in coarser- Brewster, M.L., A.P. Annan, J.P. Greenhouse, B.H. Kueper, G.R.

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