CREOSOTE MOBILITY IN SHALLOW SAND AQUIFERS Richard E. Jackson, INTERA Inc., Niwot, Colorado, USA ABSTRACT Creosote NAPLs behave in shallow sand aquifers in a manner significantly different from less viscous NAPLs such as gasoline and chlorinated solvents. Their unique behavior is due to the interaction of their physical-chemical parameters: a density often greater than water, a viscosity significantly greater than water, and an interfacial tension that yields a positive initial spreading coefficient at air/water/NAPL interfaces. This results in slow, creeping flow that causes long-term contamination at wood-preserving sites and of their adjacent surface waters. Multiphase simulations (UTCHEM and Buckley-Leverett) of this creeping flow are shown for sites along the lower Fraser River in British Columbia and in Florida. The long term consequences of the migration of viscous NAPLs in alluvium are discussed from the perspective of site characterization and brownfields redevelopment. RÉSUMÉ La créosote et autres liquides visqueux (NAPL en anglais) se comportent dans les sols alluvionnaires de façon bien différente de ceux moins visqueux, par exemple, l'essence et les dissolvants chlorés. Leur comportement unique se doit à l'interaction de leurs parameters chimio-physiques: une densité souvent plus importante que celle de l'eau, une viscosité bien plus élevée que l'eau et une tension interfaciale qui produit un coefficient initial d'extension positif aux interfaces de l'air, de l'eau et de l'huile. Il s'ensuit que ces huiles visqueuses se répandent lentement et entraînent la contamination à longue terme de certains sites qui ont utilisé ces liquides immiscibles, et des cours d'eau adjacents. Des simulations numériques (UTCHEM et Buckley-Leverett) de la migration de ces huiles visqueuses présentent en sol alluvionnaire dans des sites en Colombie Britannique et en Floride. Nos résultats indiquent que cette huile visqueuse peut se répandre pendant plusieurs années avant de déboucher dans des fleuves à des niveaux plus bas. 1. INTRODUCTION Engineers and geoscientists tend to assume that the mobility of non-aqueous phase liquids (NAPLs) in shallow sand aquifers is largely a matter of their density and of the permeability of the aquifer and aquitard materials. What is often forgotten is the strong dependence of NAPL mobility upon the viscosity of the NAPL. The low viscosity of chlorinated solvents and fuel hydrocarbons allows them to migrate rapidly in the subsurface. By contrast creosote and other viscous NAPLs are constrained by their high viscosities such that their migration may occur over decades of years (Jackson et al. 2006). Table 1 presents the physical-chemical properties of four creosote NAPLs from sites across North America. The densities and viscosities appear to vary based upon the presence of other NAPLs, e.g., diesel fuel leading to lower densities, and to the weathering of the NAPL leading to higher viscosities, i.e., > 30 dynes/cm. Two cases of creosote migration are considered. The first in Burnaby B.C. where creosote was discovered seeping into the Fraser River ten years after the wood-preserving site was closed (Patrick and Anthony 1998). Subsequent investigation indicated that migration had been widespread and continuous over decades. The second is in Gainesville, Florida, where an active wood-preserving site has been discovered to have caused deep contamination through a supposedly impermeable clay formation and extensive lateral contamination in a shallow sand aquifer. Approximations of creosote mobility are obtained through the application of the Buckley-Leverett theory of two-phase flow which has been widely used in

petroleum reservoir engineering to predict the simultaneous flow of oil and groundwater. Table 1. Physical-chemical properties of field creosotes

NAPL Density [kg/L]

Viscosity [cP]

Interfacial Tension

[dynes/cm]

Burnaby BC

1.06-1.07

20–21 (10ºC)

3-6

Cape Fear NC

1.11-1.12

41-67 (13ºC)

17

Laramie WY 1.03 54 n/r Gainesville FL

1.07-1.10

10-50 (20ºC)

n/r

n/r = not reported While the mobility of free-phase chlorinated solvent or gasoline can be readily estimated by bale testing and other pumping schemes, it is much more difficult to estimate the mobility of viscous NAPLs, such as creosote, because of creosote’s relatively high viscosity and the manner in which the Darcy flux of a viscous fluid is affected by the relative permeability effect in two-phase flow. In a two-phase geosystem of groundwater (w) and DNAPL or oil (o), the Darcy flux of water and oil flow, qw and qo, is controlled by the relative permeability functions,

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krw and kro, and the viscosity difference between DNAPL and water:

Lp

Akk

qw

w

rww

∆

∆

µ−= [1]

Lp

Akk

qo

o

roo

∆

∆

µ−= [2]

where k is the intrinsic permeability of the sand (cm2), µw and µo are the absolute viscosities of groundwater and oil (cp), A is the cross sectional area of flow through the porous medium and ∆p/∆L is the pressure gradient for each of the phases. Because k and A are common to both equations and assuming ∆pw/∆L = ∆po/∆L, the two fluxes, qw and qo, are a function of their relative permeabilities (kr) and, for creosote, µo ~ 10-50 µw. The feedback between the viscosity and relative permeability of creosote (kro) results in ever decreasing creosote flow to a well and a higher water-to-creosote production, i.e., krw increases relative to kro, and qw >> qo. Consequently creosote can still be mobile but its viscosity and relative permeability will slow its flow relative to ground water. Long-term and low-flow rate pumping is recommended to detect the presence of mobile creosote (Jackson et al. 2006). 2. BUCKLEY-LEVERETT EQUATION The Buckley-Leverett displacement theory is based upon assumptions that the flow of oil and groundwater is one-dimensional, that pressure is maintained throughout the displacement, and that the oil and water are immiscible (Willhite 1986; Dake 1994; Lake 1989). Its general use in contaminant hydrogeology has been questioned by McWhorter and Kueper (1996); the principal simplification that it implicitly introduces is an assumption concerning the gradient of capillary pressure, i.e., dPc/dSw = 0, where Pc is the capillary pressure and Sw is the saturation of water in the pore spaces (McWhorter and Sunada, 1990). Thus, the implication is that the capillary pressure vs. saturation function is flat over the range of Sw, which is a simplification that would be reproduced by a well-sorted sand in which the pore-throat sizes are all similar (Vavra et al., 1992). The errors would arise at very low and very high values of Sw and SD, because the DNAPL saturation, SD = 1 - Sw. Therefore, the simplification implicit in the use of the Buckley-Leverett equation is that of a homogenous porous medium in which dPc/dSw = 0. The assumption of immiscibility of the two fluids indicates that there is no physical or chemical mixing of the two fluids. The average frontal velocity of the oil or creosote as it displaces the groundwater is given by the following:

o

oo

dSdf

AQ

vθ

= [3]

Where Qo is the constant injection rate of oil into the geosystem, A is the cross-sectional area of flow, θ is its effective porosity, and dfo/dSo is the fractional flow of oil, or creosote DNAPL, with respect to the oil-phase saturation, So.

That is,

0.1ff

qqq

qq

f

qqq

wo

ow

o

t

0o

wot

=+

+==

+=

[4]

In equation [4], qt is the total flow of oil and water through the aquifer, which sums to 1.0. It is worth noting that the pressure gradients for oil and water (i.e., the ∆p/∆L terms in equations 1 and 2) are assumed to be the same or similar because capillary pressure is being neglected. The concept of fractional flow of oil developed in the oil industry (Dake 1978) and provides a simple conceptual model that is adoptable for approximating NAPL migration given that knowledge of the capillary pressure function for a particular site is seldom known. Thus, we will assume that the creosote migration is being facilitated by the recharge of groundwater into the NAPL zone such that, as the creosote moves downgradient due to its gravity potential, it is replaced by groundwater. 3. FRASER VALLEY SITE The site is typical of those along the Fraser River Delta (Patrick and Anthony, 1998). It is underlain by a thick sequence of fluvial sands and overbank silts that are shown in Figure 1.

0 10 100Scales in feet

JI

K

Silt

GW Table

Sand

Fraser River bankMean level of Fraser River

Sand

Horizontal scale in meters

10 meters vertical scale

Bottom of Numerical Model

Seepage face in River

0 10 100Scales in feet

JI

K

JI

K

Silt

GW Table

Sand

Fraser River bankMean level of Fraser River

Sand

Horizontal scale in meters

10 meters vertical scale

Bottom of Numerical Model

Seepage face in River

Figure 1. Hydrostratigraphy at the Burnaby BC site used in simulating creosote migration. Wood-preserving operations at the KICL site in Burnaby B.C ended in the early 1980s and the site was re-developed as a warehouse. It was unfortunate that the various site investigations that were undertaken in the early 1980s failed to reveal the extent and mobility of the creosote contamination present in-situ before the

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construction of the warehouse proceeded on the site. Continued creosote migration into the Fraser River for 15 years following site de-commissioning lead to regulatory actions initiating ground-water extraction, subsequent building settlement, and finally litigation over the recovery of costs for the $35 million remediation program now completed (Bryant and Kullmann 2004). Jackson et al (2006) simulated the migration of creosote from this site using the multi-phase simulator UTCHEM (Delshad et al. 1994) and demonstrated that mobile creosote might take 40 years to reach the seepage face in the Fraser River bottom sediments. Thus the long-term migration of creosote and other viscous NAPLs may be described as ‘creeping flow’. This migration can be readily simulated with the Buckley-Leverett equations, for which the assumptions are: • The hydraulic conductivity of the sand aquifer is 2.1 x

10-4 m/s (21 darcies). • The creosote DNAPL has a specific gravity of 1.06

and a viscosity of 20 cp. • Assume the injection rate of creosote into the

surficial aquifer is 17 L/d for 15,000 days (~ 40 years) and results in a creosote plume 100 m wide moving eastwards towards the Fraser River.

• Assume the relative permeability curves of both the creosote and the groundwater phases have residual saturations = 0.2, exponents of 2 for both creosote and groundwater and each with endpoint relative permeabilities of 0.8.

The slope of the aquifer-aquitard contact is ~0.05 (3°) and the initial saturation of the sand aquifer is set at 15% creosote, i.e., earlier discharges have caused the geosystem to be everywhere at residual saturation and further discharge causes creosote to become mobile. The rate of spillage of creosote at the site that this initial saturation implies is 39 drums per year or 17 L/day over the period 1931 to 1982 (Jackson et al. 2006). This spill rate is used as the injection rate for the purpose of the Buckley-Leverett calculations, which indicate that creosote migration over a 100 meter cross sectional area for a distance of 100 metes to the seepage face requires a travel time of 38 years. This is in good agreement with the UTCHEM simulations that incorporate capillary pressure relations (Jackson et al., 2006). 4. GAINESVILLE, FLORIDA SITE

The karstic Floridan Aquifer beneath Gainesville (population 175,000) produces approximately 30 million gallons per day (1.3 m3/s) at the Murphree well-field and treatment plant on the northern edge of the City. The Floridan Aquifer was long thought to be protected by 30-40 meters of silts and clays of the Hawthorn Group formation above which is the Surficial Aquifer that is heavily contaminated with creosote DNAPL from 80 years of wood treatment operations.

However in 2004 napthalene, 2,4-dimethyl phenol and other components of creosote were discovered in the Floridan Aquifer at concentrations indicating that DNAPL had penetrated the Aquifer. Not only does it appear that creosote DNAPL has penetrated to the Floridan, it also appears that creosote has migrated along the base of the Surficial Aquifer, i.e., along the top surface of the Hawthorn Group clays, to beyond the perimeter extraction well network that was intended to prevent off-site dissolved contaminant migration (Cleary et al. 2006). The Surficial Aquifer is a 6-10 m thick fine- to medium-grained quartz sand with an average percentage of sedimentary organic carbon present of approximately 0.16% (std. deviation = 0.20%). The upper surface of the Upper Hawthorn Group (UHG) clay (i.e., that at the base of the Surficial aquifer) is shown in Figure 2. The northeasterly dip of the UHG Clay shown in this figure, which forms a potential DNAPL migration pathway, is 0.009.

DNAPL Source

Area

DNAPL Source

Area

Figure 2. Aerial photograph of the Gainesville site shows the DNAPL source area and the direction of creosote migration towards off-site well ITW-21. ‘NAP’ indicates that naphthalene has been detected since year 2000 in the wells indicated at concentrations > 1,000 µg/L. ITW-21 increased in naphthalene contamination from 1,320 µg/L in 1985 to 2,700 µg/L in 1988, and then doubled again to 5,570 µg/L by February 2004, when it was plugged and abandoned (‘P&A’) for fear that it was a short circuit to deeper formations. Fourteen extraction wells (EW series) surround the down-dip portion of the site but appear to be ineffective in capturing creosote. In Figure 2, the hypothesized direction of creosote migration towards monitoring well ITW-21 is shown by the arrow along the dip of the contact between the sand and

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clay. Figure 3 shows the topography of the UHG clay, which has a shallower dip than that of the Burnaby site. Again the Buckley-Leverett algorithm can be used to estimate the migration distance of creosote in the surficial aquifer following its release in the DNAPL sources zone identified in Figure 2. The DNAPL source zone – a former lagoon -- shown in Figure 2 operated from the 1930s to the 1970s and the volume of DNAPL-contaminated soils is approximately 19,000 m3 (350 by 160 by 12 ft3). Assuming a porosity of 0.3 and an average DNAPL saturation of 0.05, the DNAPL volume is ~ 300 m3 (i.e., 75,000 gallons). This volume of 300 m3 would be accounted for by daily release to the former lagoon of 22 L/day (i.e., 6 gallons/day) over 35 years and it is this value that is used for computations of DNAPL migration.

Figure 3. Topography of the Upper Hawthorn Clay dips northeastwards at an angle of ½°. The DNAPL source area of Figure 2 is the triangular-shaped area in the center of figure. ITW-21 is shown to the north-east. Because an average DNAPL saturation of 0.05 would not in itself cause DNAPL to be mobile, the estimate of the 300 m3 of DNAPL present beneath the FNL—and the

derived rate of release of 22 L/d—are both very conservative and greatly reduce the possibility of overestimating the degree of migration of the creosote. Because of the capillary pressure vs. depth relationship for DNAPL, a 5% average saturation could actually represent a thin zone at the base of the aquifer in which the DNAPL saturation might be as much as 40% with much decreased saturations above. This then is the conceptual model of creosote migration along the surface of the Upper Hawthorn Group clay, one which is similar to that shown in Figure 4 but with mobile rather than ponded DNAPL. Therefore, the assumptions of the simulations are: • The hydraulic conductivity of the Surficial Aquifer is K

= 7 x 10-5 m/s (7 darcies). • The creosote DNAPL has a specific gravity of 1.08

and a viscosity of 25 cp. • Assume the injection rate of creosote into the

surficial aquifer is 22 L/d for 10,000 days (~27 years) and results in a creosote plume 10 m wide moving eastwards towards ITW-21.

• Assume the relative permeability curves of both the creosote and the groundwater phases have residual saturations = 0.2, exponents of 2 for both creosote and groundwater and each with endpoint relative permeabilities of 0.8.

Figure 4. An early conceptual model of the distribution of creosote beneath the Gainesville site indicating that the creosote is not considered to be mobile, only the dissolved components of the creosote. Under these conditions, Buckley-Leverett displacement theory indicates that creosote could have readily migrated 1,200 feet from the former lagoon to well beyond ITW-21 within 10,000 days or 27 years (i.e., to the wooded area shown in Figure 2). Larger daily releases would result in longer distances of travel as would be expected if a larger average saturation value were used 5. CONCLUSIONS The Buckley-Leverett theory allows useful approximations of NAPL migration in groundwater flow systems when the capillary-pressure term may be ignored. Because the

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capillary pressure-NAPL saturation function is usually unknown in the investigation of creosote sites, the Buckley-Leverett algorithm permits the engineer or geoscientist to estimate the rate and distance of NAPL migration using readily available site information and appropriate relative permeability functions. Methods for the in-situ or field assessment of the potential for creosote mobility are discussed in Jackson et al (2006). In the case of the two creosote sites considered, it is shown that long-term migration of creosote down slope in sand aquifers results in eventual discharge off-site. Also, in both cases the regulators and the site owners and their consultants had assumed that the creosote NAPL was immobile during the 1980s and 1990s when initial investigations began. That is, the conceptual model of Figure 4, in which the creosote was at residual saturation and therefore immobile, was the conventional wisdom of the time. Because the mobility of creosote was not readily apparent, it was assumed that it must be immobile. Therefore, although much is known about NAPL migration, there still appears to be a lack of comprehension as to the long-term migration of viscous NAPLs such as creosote. 6. REFERENCES Bryant, D. and Kullman, H. (2004) Meadow Avenue

remediation: integrated contaminated site management. Innovation (Journal of the Association of Professional Engineers and Geoscientists of British Columbia), 8:22-25.

Cleary, R.W., Feenstra, S., Jackson, R.E. and Herbert, J. (2006) Review and Recommendations for the Cabot Carbon/Koppers Superfund Site, Prepared for Gainesville Regional Utilities, Gainesville, Florida, USA.

Dake, L.P. (1978) Fundamentals of Reservoir Engineering, Elsevier, New York, New York, USA.

Dake, L.P. (1994) The Practice of Reservoir Engineering. Elsevier, New York City, USA.

Delshad, M., Pope, G.A. and Sepehrnoori, K. (1996) A Compositional Simulator for Modeling Surfactant Enhanced Aquifer Remediation, Journal of Contaminant Hydrology, 23: 303- 327.

Jackson, R.E., Dwarakanath, V., Ewing, J.E. and Avis, J. (2006) The Migration of Viscous NAPLs in Alluvium, Fraser River lowlands, British Columbia, Canadian Geotechnical Journal , 43: July issue.

Lake, L.W. (1989) Enhanced Oil Recovery, Prentice-Hall Inc., Englewood Cliffs, New Jersey, USA.

McWhorter, D. B. and Kueper, B. H. (1996) Mechanics and mathematics of the movement of dense non-aqueous phase liquids (DNAPLs) in Porous Media. In: Dense Chlorinated Solvent and other DNAPLs in Groundwater. Edited by J. F. Pankow and J.A. Cherry, Waterloo Press.

McWhorter, D.B. and Sunada, D.K. (1990) Exact Integral Solution for Two-Phase Flow, Water Resources Research, 26: 399-413.

Patrick, G.C. and Anthony, T. (1998) Creosote and coal-tar DNAPL characterization in Fraser River sands. In Nonaqueous-phase Liquids: Remediation of

Chlorinated and Recalcitrant Compounds. Edited by G.B. WIckramanayake and R.E. Hinchee. Battelle Press, Columbus, Ohio, USA, pp. 149-154.

Vavra, C.L., Kaldi, J.G., and Sneider, R.M. (1992) Geological applications of capillary pressure: a review, American Association of Petroleum Geologists Bulletin, 76: 840-850.

Willhite, G.P. (1986) Waterflooding. Society of Petroleum Engineers, Richardson, Texas, USA.

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