Project Summary Determination of Capillary Pressure-Saturation Curves Involving TCE, Water and Air for a Sand and a Sandy Clay Loam

Transcription

1 United States Environmental Protetion Ageny Researh and Development Robert S. Kerr Environmental Researh Laboratory Ada, OK 70 EPA/00/SR-9/005 Marh 99 Projet Summary Determination of Capillary Pressure-Saturation Curves Involving TCE, Water and Air for a Sand and a J. H. Dane, M. Oostrom, and B. C. Missildine The ontamination of aquifers and other groundwater by Non-Aqueous Phase Liquids (NAPLs) suh as hlorinated solvents, has beome a major onern in many areas of the U.S. Charaterization and modeling of these ontaminants require aurate and realisti data for the fluids and media involved. Most apillary pressure (P ) - saturation (S) urves are determined with a pressure or tension apparatus that often ontains a porous medium sample of more than 5 m in height. If the porous medium sample onsists of a rather oarse material and the interfaial tension between the wetting and non-wetting fluid is suffiiently low, it is not inoneivable that large hanges in S our over the height of the sample. Using the standard proedure of measuring the outflow volume of one of the fluids, from whih average values of S are alulated, an therefore result in substantial errors. In this study, a method is proposed to measure P -S drainage and imbibition relationships for TCE/air and TCE/water systems at points along a 0.9-m long sand olumn and a 0.9-m long sandy lay loam olumn with the help of a gamma radiation system and from knowledge of the fluid pressure distributions in the porous media at hydrauli equilibrium. The results showed that the S-values of the fluids present in the sand, either TCE and air or TCE and water, hanged from omplete saturation to their residual values, and vie versa, over P hanges ranging from.5 to 0 m of water pressure. For the sandy lay loam the hanges in S of the fluids were less dramati with hanges in P, making the use of a pressure ell more aeptable, although the P -S urves will still not be as aurate as for the method used in this study. This Projet Summary was developed by EPA's Robert S. Kerr Environmental Researh Laboratory, Ada, OK, to announe key findings of the researh projet that is fully doumented in a separate report of the same title (see Projet Report ordering information at bak). Introdution Dense non-aqueous phase liquids (DNAPLs), suh as trihloroethylene (TCE) are the ause of many urrent ground-water ontamination problems. An essential omponent in understanding and simulating multiphase fluid flow is the aurate determination of the hydrauli properties of the different fluids involved. It has been standard proedure to use pressure ells to determine apillary pressure (P )-saturation (S) urves, where P = P nw - P w = σ/r (P nw = pressure of the non-wetting fluid; P w = pressure of the wetting fluid; σ is the interfaial tension; and r is the radius of urvature at the interfae of the two fluids). The interfaial tension at 0 o C between TCE and air is only 0 mn/m, and mn/m between TCE and water, as ompared to 7 mn/m

2 between water and air. The height of the pressure ell may therefore be ritial during the displaement of one fluid by another if S hanges rapidly with small hanges in P, as is the ase for oarser materials (the density of TCE is.5 g/m and its visosity ratio with respet to water 0.5, both at 0 o C). The main objetive of this study was therefore to explore an improved way to determine P -S urves for TCE/air and TCE/water during wetting and drainage of the wetting fluid (hysteresis loops) in a sand and a sandy lay loam. Two additional fators of importane for simulation and leanup purposes are the P -values at whih the non-wetting fluid starts to displae the wetting fluid and the values for the residual saturation of TCE. These values were determined as well. Materials and Methods A.0-m long glass olumn experiment was designed to allow aurate P -S information to be olleted at any given Elevation, m Figure. P =P H O w H gz O gz TCE H O/TCE - System point along the olumn. The olumn (i.d. = 7.5 m), with Teflon end aps, was first filled uniformly to a height of 0.9 m with Flintshot. Ottawa (medium) sand and later with a sandy lay loam (5% nonswelling lay, 0% silt and 55% sand). The outlet at the bottom of the olumn was onneted to a TCE supply (or drainage) bottle by Teflon tubing. This bottle was also used to adjust the fluid pressures in the olumn by lowering or raising it. For the TCE/air ombination, the initially air-saturated olumn was subjeted to the following yles: Saturation from the bottom, by slowly raising the bottle, until TCE was ponded on the surfae. It was not possible to displae all of the air in this manner, so a ertain amount of air should be onsidered trapped. TCE displaed by air by stepwise lowering the bottle. Air displaed by TCE by stepwise raising the bottle. P TCE =P nw Pressure, Dynes m - (x000) P* H P* O TCE Graphial display of fluid pressures in a TCE/water system when the TCE level in the supply/drainage bottle is 5.7 m above the referene level. At 5.5 m above the referene level the pressure in the TCE is P TCE = P nw = ( ) x.5 x 000 = 9,00 dynes/m, while the water pressure P HO = P w = (9-5.5) x x 000 =,500 dynes/m. Therefore, P = P nw - P w = 5,00 dynes/m or 5. m of equivalent water pressure. Upon reahing equilibrium after eah step hange (no more flow from or into the supply bottle), dual-energy gamma radiation measurements were taken at the desired loations to determine the volumetri TCE ontent, θ TCE. P -values were obtained from knowledge of the height of the TCE-level in the supply/ drainage bottle. Corresponding S-values were alulated from S = θ TCE /porosity. By mathing the orresponding P and S- values, P -S data points were obtained. Upon ompletion of the TCE/air experiments, a -m layer of water (top of water at 9 m above referene level) was maintained on the soil surfae to displae the TCE by water, or vie versa, and P -S urves were determined for TCE/water in a similar manner as desribed for TCE/air. The dual-energy gamma radiation system now determined both θ TCE and the volumetri water ontent, θ w. An example of the P determination for a TCE/water system is shown in Figure. The TCE-level in the supply/drainage bottle for this example was at 5.7 m. The P -S data were fitted with the van Genuhten urve fitting proedure. The P entry value for the non-wetting fluid was taken as /α. Results and Disussion The full explanation of this researh is reported in the full report. It ontains extensive data tables and parameter values for the van Genuhten funtion obtained for many different imbibition and drainage yles, plus graphial representations of both the aquired and fitted data. Average values for the bulk density, porosity, θ TCE, and S as measured at nine loations along the TCE-saturated olumn are listed in Table for both the sand and sandy lay loam. Flintshot. Ottawa Sand An example set of TCE saturation data during the displaement of TCE by air (TCE drainage) is given in Table and graphially displayed in Figure. An example of similar results, obtained during the displaement of air by TCE (TCE wetting), is illustrated in Figure. The solid lines in Figures and represent the urves as fitted by the van Genuhten proedure. It should be noted that the fitted urves look somewhat awkward at times, beause they only onnet points alulated from measured P -values. To appreiate the amount of hysteresis, the fitted van Genuhten funtions are shown without values. Based on atual measurements, these values were

3 Table. Average values for the bulk density (ρ b ), the orresponding porosity ( ) based on a partile density of.5 g/m, and the average volumetri TCE ontent (θ-tce) during TCE saturated onditions (upon TCE wetting into dry soil). N is the number of observations for eah loation, z is the distane to the referene level, and S is the degree of TCE saturation. Loation z ρ b N θ-tce N S m g/m m /m m /m Flintshot Sand Table. Volumetri TCE ontent (θ-tce) as a funtion of apillary pressure (P ), expressed in m of equivalent water pressure, during displaement of TCE with air in a -m long olumn filled with Flintshot. Ottawa sand. Loation #. θ-tce P θ-tce P θ-tce P m /m m m /m m m /m m the data points in Figure. The sets of data evaluated in the full report showed that S hanges from its maximum to its minimum value, and vie versa, over a apillary pressure differene of about 5.5 m of equivalent water pressure. The data also show that ignoring hysteresis an have a major effet on S. A similar presentation is given for the results obtained during the displaement of TCE by water (Figure 5) and for the displaement of water by TCE (Figure ). Representation of hysteresis is shown again using the fitted van Genuhten funtions in Figure 7. The hange in S with apillary pressure is again very rapid, espeially for the (water) wetting urve. The amount of hysteresis in this ase is even more pronouned than for the TCE/air system. The average value for the van Genuhten parameter α during the displaement of TCE by air was 0.55 (st. dev. = 0.00), whih means that the average air entry value into a TCE saturated system is.5 m of equivalent water pressure (st. dev. = 0.7 m). For the TCE/water system the pressure in the TCE must, on average, be at least.0 m higher (st. dev. 0. m) than in the water before it will displae the water.

4 P (m Water) Figure. p (m Water) Figure FS. Sand Loation TCE Displaed by Air TCE drainage urve at loation for a Flintshot. Ottawa sand ontaining TCE and air. 0 FS. Sand Loation Air Displaed by TCE TCE imbibition urve at loation for a Flintshot. Ottawa sand ontaining TCE and air. During the displaement of TCE by air and of TCE by water, the degree of TCE saturation had rapidly attained their residual values. Based on atual measurements, these values were 0.05 (number of measurements = 50) and (number of measurements = 50) for the TCE/air and TCE/water, respetively. Subsequent flushing of the olumn with water had no measurable impat on the residual S-value of The experimental setup and proedure for the sandy lay loam soil was the same as for the Flintshot. sand. Example graphs for the TCE saturation data during the displaement of TCE by air (TCE drainage) and for the displaement of air by TCE (TCE wetting) are shown in Figures and 9, respetively. The amount of hysteresis beomes obvious from the fitted urves (Figure 0). It is obvious that hanges in S with P are muh more gradual and that hysteresis is less profound than for the sand (Figures and 7). Example graphs of the results obtained during the displaement of TCE by water and the displaement of water by TCE are presented in Figures and, respetively. Due to the similarity in data and the limited range in S-values, several measurement loations were ombined into one graph and the P -S wetting and drainage urve for the ombined data sets was used to demonstrate the amount of hysteresis (Figure ). The average value for α during the displaement of TCE by air was (st. dev. = 0.0), whih means that the average air entry value into this TCE-saturated system was. m of equivalent water pressure (st. dev. =. m). For the TCE/water system, the pressure in the TCE was, on average, at least.0 m higher (st. dev.. m) than in the water before displaement of the water ourred. In onsidering the two porous medium systems used in this study, the data show very rapid hanges in saturation with only very small hanges in apillary pressure in the medium sand for both the TCE/air and TCE/water systems. The use of alternative proedures, suh as the use of pressure ells, should therefore be avoided for oarser materials. The hanges in saturation with apillary pressure were more gradual for the sandy lay loam, whih would result in smaller differenes between the P -S urve determined with a pressure ell and the true urve. Although both media exhibited onsiderable hysteresis, it was most pronouned for the medium sand ontaining TCE and water. The average air

5 00 FS. Sand Loation TCE/Air entry value for the TCE/air systems was.5 m for the medium sand and. m for the sandy lay loam. For the TCE/water systems the average TCE entry value was.0 m for the medium sand and m for the sandy lay loam. The average measured TCE residual saturation for the sand was 0.05 when TCE was displaed by air and when it was displaed by water. P (m Water) 0 Drainage Wetting Figure TCE drainage and imbibition urve at loation for a Flintshot. Ottawa sand ontaining TCE and air. FS. Sand Loation TCE Displaed by Water File twsan.dat 5 P (m Water) Figure Water imbibition urve at loation for a Flintshot. Ottawa sand ontaining TCE and water. 5

6 FS. Sand Loation Water Displaed by TCE File wtsan.dat P (m Water) Figure Water drainage urve at loation for a Flintshot. Ottawa sand ontaining TCE and water. FS. Sand Loation TCE/Water 0 Drainage P (m Water) Wetting Figure Water imbibition and drainage urve at loation for a Flintshot. Ottawa sand ontaining water and TCE.

7 00 9 Loation TCE Displaed by Air File tasl.dat 7 5 P (m Water) Figure TCE drainage urve at loation for a sandy lay loam ontaining TCE and air. 00 Loation Air Displaed by TCE File atsl.dat P (m Water) Figure 9. TCE imbibition urve at loation for a sandy lay loam ontaining TCE and air. 7

8 00 Drainage P (m Water) 0 Wetting Loation TCE/Air Figure 0. TCE drainage and imbibition urve at loation for a sandy lay loam ontaining TCE and air. Loation TCE Displaed by Water File twsl.dat 0 P (m Water) Figure. Water imbibition urve at loation for a sandy lay loam ontaining TCE and water.

9 0 P (m Water) Loation Water Displaed by TCE File wtsl.dat Figure. Water drainage urve at loation for a sandy lay loam ontaining TCE and water. Drainage 0 P (m Water) Wetting Water-TCE Figure. Water imbibition urve (ombined data of loations 5,, and 7) and drainage urve (ombined data of loations and ) for a sandy lay loam ontaining TCE and water. 9

10 J. H. Dane and B. C. Missildine are with Auburn University, Auburn University, AL 9. M. Oostrom is with the Battelle Paifi Northwest Laboratory, Rihland, WA 995. James W. Weaver is the EPA Projet Offier (see below). The omplete report, entitled "Determination of Capillary Pressure - Saturation Curves Involving TCE, Water and Air for a Sand and a," (Order No. PB9-0 75/AS; Cost: $7.00, subjet to hange) will be available only from National Tehnial Information Servie 55 Port Royal Road Springfield, VA Telephone: The EPA Projet Offier an be ontated at: Robert S. Kerr Environmental Researh Laboratory U.S. Environmental Protetion Ageny Ada, OK 70 United States Environmental Protetion Ageny Center for Environmental Researh Information Cininnati, OH 5 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-5 Offiial Business Penalty for Private Use $00 EPA/00/SR-9/005 0