Low-head hyperfiltration through Jurassic-Cretaceous metamorphic Darrington Phyllite discs (from the Northwest Cascades of Washington State, USA)

Low-head hyperfiltration through Jurassic-Cretaceous metamorphicDarrington Phyllite discs (from the Northwest Cascades of WashingtonState, USA)

Megan Hart

Abstract Membrane behavior in naturally occurring andengineering systems refers to the restriction of solutemigration through a membrane relative to the migration ofthe solvent. Hyperfiltration is the net flux that results whentwo solutions of different concentration are separated by amembrane and an external force is applied in excess of theosmotic pressure. Clay membranes containing layeredfabric have higher efficiencies than membranes withrandom fabrics. Therefore, low-permeability rocks with afoliated fabric might exhibit membrane properties. Fourhyperfiltration experiments were conducted on samples ofDarrington Phyllite from the Easton Metamorphic Suite ofthe Northwest Cascades, Washington (USA). Chloridesolutions were passed through thin, intact discs atrelatively low heads. At the end of the experiments,dissolved chloride concentrations had increased 110–140% and calculated reflection coefficients ranged from0.87 to 0.88; this was attributed to partial solute rejectionby the phyllite. Natural scenarios in which phyllite mightexhibit membrane properties include (1) shallow perchedaquifers bounded by phyllite, (2) overpressured aquifersbounded by phyllite, (3) phyllite-bounded aquifers withsignificant vertical groundwater flows, and (4) ultrafiltra-tion during metamorphic devolatilization at depth. Mem-brane processes exhibited by phyllite may also contributeto the formation of some low-temperature ore bodies.

Keywords Diffusion . Membrane . Hyperfiltration .Laboratory experiments/measurements . USA

Introduction

There are two major membrane processes that are believedto occur in the subsurface: osmosis and hyperfiltration(Fritz 1986). Osmosis occurs when two solutions ofdiffering concentrations are separated by a semipermeablemembrane (Fig. 1). The flux of water and solute are then afunction of the respective concentration gradients. Previ-ous work published in the literature assumes that the resultproduces a net flux of solute from the more concentratedsolution across the membrane into the less concentratedsolution. The net flux of water molecules opposes thisflow since the water is less concentrated in the solutionwhich contains the highest concentration of dissolvedsolids. Flux across the membrane induces a chemicalpotential termed the osmotic pressure, which is defined asthe pressure necessary to stop the flow of solute from highto low concentration across the membrane (Martin 1964).

Hyperfiltration, often called solute-sieving, is also anaturally occurring geologic phenomenon in which asolute is partially rejected when groundwater passesthrough a membrane-functioning lithology (Benzel andGraf 1984). When hydraulic head in excess of osmoticpressure exists across a membrane-functioning lithology,hyperfiltration can occur (Fig. 2). Rejected solutes concen-trate adjacent to the high-pressure face of the membrane,forming a zone of increased concentration called a concen-tration polarization layer, or CPL (Fritz 1986).

Membrane filtration efficiency is defined as the abilityof a membrane to selectively pass one substance relativeto another. This value ranges from 0, no membraneeffects, to 100, which is considered a perfect membranecompletely inhibiting the progress of a solute relative to asolvent. Previous experiments have shown that layeredfabrics in naturally occurring remolded clays increasemembrane efficiency (Benzel and Graf 1984), withefficiency almost doubling for oriented fabrics versusnon-oriented fabrics. It would stand to reason that lowpermeability rocks with layered fabric such as some low-grade metamorphic rocks, might also exhibit measurablemembrane properties. Phyllite is a representative, low-grade foliated low permeability rock exhibiting bothrandom and orientated fabrics and lacks the high cationexchange capacity that is typically associated with

Received: 22 August 2011 /Accepted: 17 August 2012Published online: 10 October 2012

* Springer-Verlag 2012

Electronic supplementary material The online version of this article(doi:10.1007/s10040-012-0905-8) contains supplementary material,which is available to authorized users.

M. Hart ())Department of Civil Engineering,Saint Louis University,3450 Lindell Blvd., Room 1033, Saint Louis, MO 63103, USAe-mail: [email protected]

Hydrogeology Journal (2013) 21: 481–489 DOI 10.1007/s10040-012-0905-8

http://dx.doi.org/10.1007/s10040-012-0905-8

experimentally determined high membrane filtration effi-ciencies. Therefore, thin, intact phyllite discs exhibitingrandom metamorphic fabric were tested for membraneproperties and the membrane filtration efficiency wasdetermined. The experiments reported here were con-ducted on intact Darrington Phyllite cores with randomfabric orientations, at room temperature, under relativelylow differential pressures of 0.5 and 1.0 m using chloridesolutions of 0.005 and 0.01 M.

Geologic background

The Darrington Phyllite is a micaeous Jurassic-Cretaceousmetamorphic rock found in the Easton Metamorphic Suiteof the Northwest Cascades in Washington State, USA(Figs. 3, 4, and 5). Originally the Easton MetamorphicSuite was comprised of oceanic basalt with overlyingdeep-ocean mud and sand, forming approximately 150million years ago during the Jurassic. Metamorphism ofthe Easton Terrane occurred some 30 million yearsfollowing deposition, during the Cretaceous Period. Thebasalt became what has long been called the ShuksanGreenschist and is now a well-recrystallized, fine-grained,epidote-chlorite-amphibole quartz albite schist containingrare occurrences of blue amphiboles. Groundwater flowthrough the Shuksan is primarily fracture driven. The

overlying oceanic shale and sandstone protolith are nowthe Darrington Phyllite. Most of the Darrington isgraphitic in nature with quartz-albite-sericite phyllitepredominant throughout the unit. Some localized well-recrystallized fine-grained muscovite schist is foundcommonly in association with albite porphyroblasts andwell-developed lawsonsite (Brown 1987). DarringtonPhyllite is variable in its hardness and resistance toweathering. Exposure of the phyllite terrain yields asmooth topographic expression with some scatteredevidence for mass-wasting.

The Eocene Chuckanut Sandstone unconformablyoverlays the Darrington Phyllite and consists of thin-to-medium bedded sandstone. The Chuckanut Formation is afluvial deposit of mostly feldspathic arenites that devel-oped in a faulted, down-dropped basin post-terrainemplacement. Subsequent displacement from faulting,uplift of the lowland basins, and changes in regionaltectonics led to intense folding and faulting of theChuckanut. Fine-scale stratigraphic sequences that arefining-upwards are significant throughout the entireChuckanut aquifer, and reduce the overall hydraulicconductivity of this member. The Chuckanut formationis bounded by glacial drift and Darrington Phyllite,forming a confined, low head aquifer (US EPA2004).Deformation within the Chuckanut is related to subductionof the San Juans under the North Olympics. Jointing,slickensides, heavy metal deposits adjacent to faults andphyllite boundaries, and coal are all present in thisformation.

Low-temperature, sulfide-bearing gold, pyrite, andoxide mines exist locally in the Darrington Phyllite andChuckanut Sandstone contact area, including the areawhere the samples for these experiments were collected(Brown 1987; Derkey et al. 1990). Several mechanismshave been postulated for formation of these sulfide-bearing ores including low-temperature epithermal alter-ation along shear zones (Derkey et al. 1990) andpercolation of heated waters during metamorphosis(Brown 1987). The ores are not regional in extent andactually form closer to more foliated zones of phyllite(Derkey, et al. 1990). Intact hand samples of theDarrington Phyllite were collected near a shear contactzone at 48°54′28″N, 121°37′48″W, west of the SilvertipMine, and close to Goat Mountain, Washington State.

Relatively few published laboratory studies haveexamined the ability of intact rock to act as geologicmembranes (Young and Low 1965; Walter 1982; Baroneet al. 1990; Whitworth and DeRosa 1998; Bader and Kooi2005; Oduor et al. 2009). Young and Low (1965) testedsiltstones and claystones to determine if osmosis waspossible at natural compaction pressures under a very highsodium chloride concentration gradient. Very low calcu-lated osmotic efficiencies were determined from theseexperiments. Walter (1982) found that fractured tuffsdemonstrated osmotic effects while studying matrixdiffusion of solutes through pores. Whitworth and DeRosa(1998) and Oduor et al. (2009) performed experiments todetermine the osmotic potential of shales and siltstones. In

Fig. 2 Diagram showing flows during reverse osmosis or hyper-filtration where C1 and C2 are solute concentrations, Js is soluteflux, and Jw is water flux across the membrane. Since C1 > C2,solute flux is from the high concentration to the low concentrationof solute; while the water flux is from the high pressure side of themembrane to the low pressure side. Diagram shows a static flowconfiguration for simplicity

Fig. 1 Diagram showing osmotic flows where C1 and C2 are soluteconcentrations, Js is solute flux, and Jw is water flux across themembrane. Since C1 > C2, solute flux is from the high concentration tothe low concentration of solute, while the water flux is from the highconcentration of water (low solute concentration) to the lowconcentration of water (high solute concentration)

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these experiments, shale separated solutions of differingchemical concentration. For all experiments, the osmoticpressure was measured and recorded over time bymeasuring the change in water level in a litho-osmometer.These experiments found that shales displayed relativelysignificant osmotic efficiencies (∼50–70 %), even at lowchemical concentration differences. While minimal workhas been completed on intact rock cores, this reportfocuses on the ability of a randomly oriented, low-grademetamorphic phyllite to act as a membrane. To theauthor’s best knowledge, phyllite has not previously beentested for membrane properties. Experimental testing ofthe hand samples was completed following previouslyestablished procedures.

Methods

Multiple samples, weighing between 4.5 to 13.6 kg (10–30 lbs) each were taken near an abandoned mine. Thesehand samples appeared competent and strongly foliated.The Darrington Phyllite samples used in this study hadcation exchange capacities (CEC) of 35–46 meq/100 g,which were similar to those found in the literature forother phyllites (Cook and Rich 1962), but is relatively lowcompared to most charged membranes. These CEC valuesare on the higher spectrum of kaolinite clay (Bergaya andVayer 1997). This is attributed to the decomposition ofmicaeous materials within the phyllite (Bergaya and Vayer1997). The hand samples were cored perpendicular to the

Fig. 3 Locational map for collection of hand samples overlain on the 2002 Geologic Map of the Darrington 7.5-min Quadrangle, Skagitand Snohomish counties, Washington Geologic Map (Adapted from Dragovich et al. 2007)

Fig. 4 Thin section photo of Darrington Phyllite sample at 40×magnification, showing mineralization

Microfoliation

Fig. 5 Thin section photo of Darrington Phyllite sample used inExperiment 4 exhibiting microfoliation at 40× magnification

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fabric to obtain 6.35 cm (2.5 in.) diameter discs. Thesediscs were then ground down using silica carbide paste onglass until each was approximately 1.5 mm thick. Thindiscs were used instead of thicker columns of core in orderto shorten the time needed to perform the experiments.The resultant thin discs and 0.5–1.0 m pressure heads usedto expedite the experiments gave rise to relatively highhydraulic gradients. ,

The discs were placed into a custom-made hyper-filtration cell consisting of a transparent acrylic cylinderwith an internal area of 15 cm2 and wall thickness of0.64 cm similar to those used by Hart and Whitworth(2005) (Figs. 6 and 7). The acrylic cylinders were fitted totwo O-ringed, 3.80-cm thick Garlite™ caps. The capswere held in place by eight threaded rods that passthrough both caps parallel to the cylinder, which measured2.6 cm in length. The hyperfiltration cell components werethoroughly washed and then rinsed multiple times withdeionized water before each experiment.

A Mariotte flask, similar to those used in soil mechanicsand chromatography, was suspended at 0.5 and 1.0 m ofheight (Fig. 6). The Mariotte flask is a sealed reservoir withan air inlet and a siphon and functions using basic principlesof the ideal gas law to supply a constant pressure. The siphonconnects to the experimental apparatus, providing a constantsupply of solution under a pressure equivalent to the distancebetween the top of the membrane and the bottom of the airinlet tube (bubble tube in Fig. 7). Initially, deionized waterwas passed through the phyllite in order to determine thewater permeation coefficient (Lp; Fritz andWhitworth 1994).The deionized water was then removed from the experimen-tal cell and replaced with 0.005 and 0.01 M solutions ofchloride (Table 1). The experiments began immediately afterinsertion of stock solution into the cell. Effluent sampleswere collected at intervals during the experiment—Fig. 8shows a representative graph of concentration over time for asingle experiment; graphs for the other experiments are

given in the electronic supplementarymaterial (ESM). At theend of the experiment the cell solution was collected and theCl– concentration within the cell was measured. Reagentgrade chemicals were used to make the solutions. Sodiumand chloride concentrations were measured with a DionexDX-120 ion chromatograph and compared against stand-ards. Chemical analysis precision was calculated by findingthe standard deviation of triplicate testing of effluent samples(Table 1).

Membrane efficiencies were calculated using thereflection coefficient σ, a unitless measure of osmoticefficiency (Staverman 1952). If σ=1.0, the membrane isperfect and rejects all dissolved solute. If σ=0, there is nosolute rejection. For intermediate values, rejection ispartial and proportional to the value of σ. The steady-state reflection coefficient can be calculated from theexperimental data without prior knowledge of the concen-tration at the membrane interface via Eqs. (4)–(5)(Whitworth et al. 1999).

LP ¼ JvDI$P

ð1Þ

co ¼ 12LPvRT Jv$x�2vDð Þ � �2LPvRTJvci$xz � 4LPv2RTDcið

þ$xzJvLP$P � $x1=2z1=2 �8z$xciJv3TRvLP þ 8z$xciJv2TRvLP2�

þz$xJv4 � 2z$x$PLPJv3 þ z$x$P2LP2Jv2 � 16Jv2ciDTRv2LP2�1=2�

ð2ÞFig. 6 Experimental apparatus schematic

Fig. 7 Experimental testing schematic

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σ ¼ LP$P � Jv=LPvRT co � cið Þ ð3Þwhere Lp=water permeation coefficient (m/Pa/s), JVDI

=deionized water flux through (m/s), ΔP=pressure differ-ence across the membrane (Pa), co=concentration at thehigh-pressure membrane face (M), v is a factor thatcorrects for the number of particles due to ion formation,R is the gas constant (8.314 J /K mole), T is thetemperature in K, Jv=experimental solution flux (m/s)through the membrane, Δx is membrane thickness (m),D=diffusion coefficient for free solution (1.89×10–9m2/s), ζis the tortuosity and is defined here as the ratio of the actualpath length through the membrane to the membranethickness and was equal to 1.79 (Dullien 1979), and ci=theinput solute concentration (M). Tabulated results forindividual experiments can be found in Table 1.

Results

Experimental results indicate that the volumetric solutionflux decreases over the time of the experiment where Jv,

the steady-state volumetric solution flux was calculated byusing

Jv ¼ V

A � T ð4Þ

Where, V is the volume of effluent, A is the area of themembrane, and T is the total time elapsed during a specificsample. Jv decreases as the osmotic pressure increaseswithin the cell until a steady-state flux is established. Jvreported in Table 1 for each experiment is the average ofthe last three samples taken once steady state wasestablished.

A mass-balance approach was used to predict theexpected concentration within the cell at the end of theexperiment. The total moles input into the cell wascalculated using empirical data. This value was thencompared to the total moles that were collected during theexperiment within the cell. Equation (5) yields anestimated accumulation at the end of the experimentwhich has been previously determined in the literature tobe equivalent to the ending concentration predicted(cpredicted in Table 1). This value was then compared to

Table 1 Experimental parameters and calculated results

Parameter Exp. 1 Exp. 2 Exp. 3 Exp. 4

Head (m) 1.00 0.50 1.00 0.50Membrane thickness (mm) 1.51 1.56 .59 1.28Membrane area (m2) 0.0015 0.0015 0.0015 0.0015Solution flux, Jv (m/s) 9.290×10−8 3.391×10−8 7.405×10−8 3.791×10−8

Error (%) ±2.01 ±2.12 ±2.11 ±2.25ci (M Cl−) 0.0100 0.0100 0.0050 0.0050cfinal (M Cl−) 0.0220 0.0210 0.0120 0.0110cpredicted (M Cl−) 0.0210 0.0200 0.0110 0.0110Change in cell concentration (%) 120 110 140 120Permeability, Lp (m/Pa/s) 1.917×10−12 1.229×10−12 1.010×10−12 1.017×10−12

co (M Cl−) 0.0110 0.0102 0.0057 0.0055Calculated steady state σ 0.87 0.88 0.87 0.88Charge balance error −1.20 0.95 0.11 0.02Time to steady state (days) 115 112 117 118

Measurements recorded at 21 °C

0.00E+00

5.00E-07

1.00E-06

1.50E-06

2.00E-06

0 20 40 60 80 100 120

Time Elapsed (days)

So

luti

on

Flu

x (c

m/s

)

0

50

100

150

200

250

300

350

400

Eff

luen

t C

on

cen

trat

ion

of

Cl-

(p

pm

)

Effluent Flux

Solution Concentration

Fig. 8 Effluent concentration and flux versus time for experiment. Error bars are defined by the charge balance error noted in Table 1

485


the measured cell concentration within the cell at the endof the experiment or cfinal (Table 1).

XMolesin �

XMolesout ¼ Accumulation ð5Þ

This approach predicted Cl– accumulations between0.0107 and 0.0117 M Cl– for the 0.01 M chloride initialstarting concentration conditions and 0.0062–0.0066 MCl– solutions for the 0.005 M initial starting conditions.Comparison of the predicted (cpredicted) and measured(cfinal) concentration values resulted in a less than 5 %difference for all experiments (individual experimentalcomparisons are presented in Table 1) which is within therange expected (Fritz 1986).

All experiments resulted in significant increases inchloride concentrations within the experimental cells.Final measured cell concentrations ranged from 0.0118to 0.0220 M Cl– (Table 1). The values of σ calculatedfrom Eq. (3) for the experiments reported herein rangedbetween 0.87 and 0.88 for chloride (Table 1). Thesevalues indicate that the phyllite discs exhibited significantmembrane properties. Values of Lp, calculated fromEq. (1), ranged from 1.59×10−11 to 9.95×10−11 (m/Pa/s).The steady-state values of maximum concentrations in thecells, located at the membrane face (co), were calculatedfrom Eq. (2) to be between 0.0055 and 0.0108 M Cl–

indicating concentration increases at the membrane be-tween 115 and 154 % greater than the initial concentra-tion. For a rigorous derivation of semipermeablemembrane mathematics please see Fritz and Whitworth(1994) or Hart et al. (2008).

Discussion

The purpose of this report was to experimentally determine ifphyllite could behave as a semipermeable geologic mem-brane under relatively low head conditions using a conser-vative solute. Calculated reflection coefficients (σ) rangedbetween 0.87 and 0.88. When compared to other lithologies,these reflection coefficients are substantial. This discussionis broken down into a comparison of reported values fromthis study to those of charged clays in the literature, the rolesthat a phyllite could take as a semipermeable membrane andthe subsequent effects these roles may play on the geologicand hydrogeologic regimes, and the general implications ofthese reported values on the work reported in the literature onsemipermeability.

Comparison to literatureReflection coefficients of 0.19 have been reported at35 ppm NaCl for lightly compacted, charged smectitemembranes (Saindon and Whitworth 2005; Saindon andWhitworth 2006). Milne et al. (1964) found reflectioncoefficients of 0.14 –0.60 for mixtures of chargedbentonites and silica silt-sized particles using 0.1 Nsodium chloride solutions. Additionally, Fritz and Marine(1983) found reflection coefficients of 0.04–0.89 for

bentonites under static head conditions using 0.01,0.096, and 0.094 M sodium chloride solutions. Thereflection coefficients reported herein for phyllite are onthe higher end of those reported from hyperfiltrationtesting of remolded and compacted smectite membraneswith chloride solutions. Thus, some phyllites may havethe potential to act as highly efficient membranes. Thismay be in part due to the foliated fabric noted within thesamples (Fig. 4) causing dead-end pore-throat constric-tions and additional frictional drag experienced within themicro-fracture flow pathways (Bresler 1973). It is inter-esting to note that the cation exchange capacity resultingfrom decomposing micas and transitional minerals withinthe phyllite itself may not play a substantial role in thereflection coefficients found in this study. It has beennoted in the literature that charged membranes experi-encing high applied pressure are the most efficient(Shackelford and Malusis 2002; Kang and Shackelford2009). However, this phyllite demonstrates a departurefrom this commonly accepted theory as this membraneexhibits minor CEC and has a high reflection coeffi-cient; higher than would be expected from a “solute-sieving” membrane which functions on pore restric-tions alone. Flow through this low permeabilitymembrane appears to be primarily fracture based insitu, and therefore geologic scenarios in which theflow is fracture-based are discussed in the following.

Role of a phyllite membrane in ore depositionOne potential role of phyllite membranes may be in theformation of some low temperature ore deposits. Mackay(1946) postulated membrane processes might play a rolein hydrothermal and low temperature ore deposits byconcentrating metals to supersaturation and inducingprecipitation of ore minerals. Several areas which corre-spond to this low temperature ore deposition are the lead-zinc fields in Mezica, Slovenia, the lead-zinc field inRaihbl, Italy, the quicksilver deposits in Idria, Italy, acopper deposit in Cyprus, the iron, copper, gold, and zincdeposits of Noranda, Quebec, and the gold deposits ofTanganyika and Nigera (Mackay 1946). To test thefeasibility of Mackay’s (1946) ideas, Whitworth andDerosa (1998) experimentally demonstrated that com-pacted smectite membranes can concentrate some heavymetal solutions from below saturation to above saturationand produce heavy-metal precipitates including leadchloride, cobalt chloride, and copper chloride at lowtemperature.

Furthermore, Lueth and Whitworth (2001) examinedlow-temperature, sandstone-hosted copper deposits inNew Mexico and found that copper sulfide concentrationsare typically highest adjacent to bounding shales anddiminish with increased distance from the shales in thesandstone. They hypothesized that hyperfiltration resultsin increased dissolved species concentration in groundwa-ter adjacent to shale membranes for both copper andbacterial nutrients under low-temperatures (Lueth andWhitworth 2001). Reduction of the sulfate by sulfate

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reducing bacteria within the CPL would produce H2S andprecipitation of copper sulfides.

The fact that at least some phyllites are membrane-functioning suggests that it is possible that phyllites mayplay a role in the origin of at least some low temperatureore deposits. Deposits of low-temperature sulfide-bearingmaterials do exist within the axis of synclines and alongfault boundaries of the Darrington Phyllite-ChuckanutSandstone (Brown 1987; Derkey et al. 1990). It ispostulated here that low-temperature hyperfiltration duringmetamorphosis may have contributed to the formation ofsome metallic sulfide deposits seen adjacent to theDarrington Phyllite. Further experiments and field workwill need to be undertaken to confirm or refute this ideaand are planned. Mackay (1946) also postulated that suchmembrane processes are also active in high temperatureore deposits but experimental evidence is lacking tosubstantiate this idea. However, further high-temperatureexperimental work is required to test the concept.

Role of phyllite membranes in diagenesisIt is also possible that the ability of the phyllite to functionas a hyperfiltration membrane might play a role inshallow, low temperature diagenesis in porous rocksadjacent to phyllite aquitards due to solute concentrationincreases in the CPL as well as exert a partial control onsolute concentration distribution in some perched aquifersbounded below by membrane-functioning phyllite. Anumber of studies have suggested that overpressured,shale-bounded aquifer systems may be affected bymembrane processes (Bredehoeft and Blyth 1963; Neuzil1986; Garavito et al. 2006; Neuzil and Provost 2009).Hyperfiltration may occur in the Milk River aquifer in thewestern Canadian sedimentary basin (Berry 1969; Phillipset al. 1986), the Illinois Basin (Bredehoeft and Blyth1963; Graf 1982), the Oxnard coastal basin, VenturaCounty, California (Greenberg et al. 1973), and theSaginaw aquifer system in the upper Grand River Basin,Michigan (Wood 1976), the Opalinus clay of Switzerland(Noy et al. 2004; Rousseau-Gueutin et al. 2008) andPierre shale and ARCO clay in China (Al-Bazali 2005),among many others. Oduor et al. (2009) showed thatshale-electrolyte systems of the Abo and Mancos shalesmay contribute to shallow ore formation.

Copper deposits in the Chupadero mine within the AboFormation occurred in localized zones near the fault zoneand immediately adjacent to the shale beds. It ishypothesized that localized formation occurred whereprobable movement of groundwater flowed parallel tobedding (Lasky 1932). Deposition of mineral materialsmay have occurred as a result of increased concentrationof the dissolved minerals adjacent to the sandstone-shaleinterface and because of the impermeability of the shale todissolved minerals. In the case of the copper deposition,pore restrictions are probably the principle cause ofsemipermeable ore formation (Oduor et al. 2009). Similarto the membrane processes occurring in the Abo Forma-tion (Oduor et al. 2009), low temperature deposition of

heavy metals may occur in localized zones perpendicularto groundwater flow or along flow focusing points. Flowfocusing may be a cause of the high reflection coefficientsnoted in the experiments. Additionally, dead-end pores,foliation, transition mineralization of mica in the phyllite,and pore throat constrictions are all possible rationale forthe high reflection coefficients noted in the experimentspresented. Based on these experiments, it is possible thatthat overpressured aquifer systems bounded by phylliteshould be investigated for membrane affects as well thepotential to affect low temperature ore mineralization.

Devolatization at depthAnother scenario in which phyllite may act as asemipermeable membrane is ultrafiltration during meta-morphic devolatilization at depth. Devolatization occurswhen there is rapid heating and pressure of the hostsediments during metamorphosis, causing devolatilizationof organic matter and hydrous minerals. The obstructionto fluid flow, formed by the rocks overlying a metamor-phic devolatilization front, causes the fluid pressuregradient in the reacting rocks to diverge from lithostatic.The positive pressure anomaly generated by the differencein confining and fluid pressure gradients drives deforma-tion and fluid flux upwards and potentially in excess ofosmotic pressure, giving rise to ultrafiltration. Whileexperiments performed in this study are not representativeof this scenario, it is important to note that the potentialexists and the role of ultrafiltration and hyperfiltration inmetamorphic devolatilization should be examined ingreater detail.

General role of a phyllite membraneThe role of membrane behavior in situ is limited in itsunderstanding, mainly due to the complexities resultingfrom coupled-flow behavior, disturbance, and dilutionof the CPL during sampling, seasonal fluctuations ingroundwater level (making long-term comparisonsdifficult), etc. Literature suggests that while semiper-meable membrane behavior is commonly seen in thelaboratory, long-term, in situ monitoring may suggestthat there is evidence to support significant potential ofnaturally occurring semipermeable membrane scenarios(Neuzil and Provost 2009).

Similarly, it is noted that investigators may not beinterpreting results correctly, or are not looking in theappropriate place to examine membrane in situ effects(Neuzil and Provost 2009). Groundwater sampling ingeneral is performed for a specific purpose with aspecific budget constraint. There is typically limitedfunding to determine which role, if any, semiperme-ability plays in abnormal overpressures, chemicalconcentrations adjacent to a low hydraulic conductivitylayer, or electric streaming potential. Focus in fieldmeasurements of semipermeability has remained onhigh pressure and high concentration anomalies; labo-ratory data for clays, shales, tuffs, geomembranes and

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now phyllite suggests that lower concentrations andlower pressures may just as feasibly be occurring,maybe even more so (Malusis and Shackelford 2002;Hart and Whitworth 2005; Derrington et al. 2006;Hart, et al. 2008; Kang and Shackelford 2009).Perhaps the search for geologic settings where natural-ly occurring semipermeable membranes are functioningshould switch to lower pressures and concentrationsand non-argillaceous materials which otherwise wouldbe overlooked by the typical engineer in the field.Based on these experiments on the Darrington Phyllite,membrane effects may need to be considered in abroader spectrum of low permeability lithologies.

Conclusions

Solutions of 0.005 and 0.01 M chloride were passedthrough thin, foliated phyllite discs (∼1.5 mm) at headsof 0.5 and 1.0 m. These are the first known hyper-filtration experiments performed on intact phyllitecores. In each experiment, ending cell concentrationssignificantly increased due to what the author charac-terizes as partial solute rejection by the phyllitemembranes. Concentration increases within the cellwere between 110 and 140 % over background.Calculated values of the reflection coefficient rangedfrom 0.87 and 0.88, suggesting that these thin phyllitediscs exhibited significant membrane effects. Experi-mental characterization of intact phyllite suggests lowchemical potential and low temperature; relatively lowhydraulic-pressure semipermeable-membrane propertiesare possible given the unique geology of the Darring-ton Phyllite. While field analysis has not yet begun, itis important to note that the low pressure, lowconcentration gradient hyperfiltration may be possiblefor foliated lithologies, previously unreported. Thepotential for foliated, relatively medium CEC intactlithologies to function as semipermeable membraneincreases the areas for field research drastically andmay shed light on the reasons behind unreportedsemipermeability in the field.

These experiments have shown that intact rockcores that exhibit foliated fabric, in particular phyllite,are capable of significant hyperfiltration effects at lowheads such as might be found in shallow regional orperched aquifers. Additionally, membrane-functioningphyllite may contribute to low-temperature metalsulfide deposition, as suggested by Mackay (1946)and Oduor et al. (2009). The potential for high-temperature membrane effects in foliated metamorphicrocks and other low permeability rocks should also beexplored relative to ore deposit formation.

Acknowledgements The author would like to thank Dr. T.M.Whitworth for his guidance, Dr. R.M. Saindon for laboratoryassistance, C. Neuzil for his comments, Dr. C. Shackelford and hisresearch group for their assistance, Dr. Ana C. Londono forgraphics, and Dr. N. Maerz for all his hard work.

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Documents

Low-head hyperfiltration through Jurassic-Cretaceous metamorphic Darrington Phyllite discs (from the Northwest Cascades of Washington State, USA)