A new method for quantifying contaminant flux at hazardous waste sites

Transcription

1 Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings ofthe Groundwater Quality 2001 Conference held at Sheffield. UK. June 2001). IAHS Publ. no A new method for quantifying contaminant flux at hazardous waste sites KIRK HATFIELD Department of Civil Engineering, kliatfsce.ufl.edit University of Florida, Gainesville, Florida 32611, USA MICHAEL D. ANNABLE Department of Environmental Engineering Sciences, University of Florida, Gainesville, Florida 32611, USA SUZANNAH KUHN Malcolm Pirnie Inc., Tampa, Florida 33605, USA P. SURESH C. RAO School of Civil Engineering, Purdue University, West Lafayette, Indiana , USA TIM CAMPBELL Department of Civil Engineering, University of Florida, Gainesville, Florida 32611, USA Abstract A new method has been developed and laboratory tested for measuring both contaminant and groundwater fluxes at hazardous waste sites. The method uses a sorptive permeable media that is placed either in a borehole or monitoring well to intercept contaminated groundwater and release resident tracers. The material is placed in a groundwater flow field for a specified period of time and removed for extraction and analysis. By quantifying the fraction of resident tracer lost and the mass of contaminant sorbed, groundwater flow and contaminant flux can be calculated. This approach requires knowledge of the tracer and contaminant partitioning characteristics with the sorptive media, and an estimate of the media and aquifer permeability contrast. The new approach has been preliminarily tested in the laboratory using a packed aquifer model. Experimental results indicate good estimates of contaminant and groundwater fluxes. This method has potential applications for: (a) measuring contaminant flux from sites in order to meet flux based regulatory end points, (b) quantifying flux across planes within a plume to assess natural attenuation and loads to surface water, and (c) quantifying contaminant flux from a source zone before and after remedial efforts. Key words contaminant flux; groundwater flow; tracers INTRODUCTION AND BACKGROUND INFORMATION The need to measure contaminant flux in groundwater has increased as remedial activities have been attempted at a growing number of sites (Einarson & Mackay, 2001). Risk assessment at contaminated sites is dependent on flux from source zones including DNAPLs (Feenstra et al, 1996). A new method that has been developed for direct in situ measurement of both cumulative water and contaminant fluxes in groundwater is presented here. Such measurements have many applications in long-term monitoring, aquifer restoration, natural attenuation, and contaminant source remediation. In

2 26 Kirk Hatfield et al. situ measurements of contaminant flux generate critical data needed to optimize the design and assess the performance of proposed contaminant source and groundwater remediation systems. These fluxes, when integrated over a source area, produce estimates of contaminant source strength and contaminant mass loads to groundwater and surface water. Furthermore, from contaminant fluxes measured downgradient from on-going remediation activities, it is feasible to verify the performance of existing technologies, assess cumulative benefits, and estimate prevailing environmental risks. The new method involves a device, hereafter referred to as a "flux meter", that is a self-contained permeable unit that is inserted into a well or boring such that it intercepts groundwater flow but does not retain it. The interior composition of the flux meter is a matrix of hydrophobic and hydrophilic permeable sorbents that retain dissolved organic and/or inorganic contaminants present in fluid intercepted by the unit. The sorbent matrix is also impregnated with known amounts of one or more fluid soluble "resident tracers". These tracers are leached from the sorbent at rates proportional to the fluid flux. After a specified period of exposure to groundwater flow, the flux meter is removed from the well or boring. Next, the sorbent is carefully extracted to quantify the mass of all contaminants intercepted by the flux meter and the residual masses of all resident tracers. The contaminants' masses are used to calculate time-averaged contaminant mass fluxes, while residual resident tracer masses are used to calculate cumulative fluid flux. Depth variations of both water and contaminant fluxes can be measured in an aquifer from a single flux meter by vertically segmenting the exposed sorbent packing, and analysing for resident tracers and contaminants. Thus, at any specific well depth, an extraction from the locally exposed sorbent yields the mass of resident tracer remaining and the mass of contaminant intercepted. This data is used to estimate local cumulative water and contaminant fluxes. As indicated above, resident tracers are used to estimate total fluid flux. As water flows through the meter, soluble tracers are leached from the sorbing matrix and lost from the meter. Figure 1 displays two hypothetical cross-sections of a meter configured as a circular column (such as one installed in a monitoring well). Crosssection A reveals a single resident tracer uniformly distributed over the cross-section before any fluid has flowed through the meter. Cross-section B reflects the subsequent spatial distribution of tracer after exposure to a fluid flow field. Here the tracer has been displaced to the right and leached from the section in a manner consistent with the assumption that fluid streamlines are parallel within the circular domain of the device. A B Fig. 1 Flux meter cross-sections. A: initial tracer distribution, B: displaced tracer distribution after exposure to a fluid flow field.

3 A new method for quantifying contaminant flux at hazardous waste sites 27 The assumption of parallel streamlines within the device is consistent with what Strack & Haitjema (1981) demonstrated for a homogeneous permeable element of circular geometry situated in a locally homogeneous aquifer of contrasting permeability. The permeability contrast between the flux meter and the aquifer can produce aquifer flows that converge or diverge near the meter. However, the specific discharge within the circular bounds of the meter remains spatially uniform. Strack & Haitjema (1981) presented the following relationship between the specific discharge in the device, q and the undisturbed aquifer flow, q 0 : ^ = (1) q 0 k + k 0 where k and ko are the respective permeabilities of the flux meter and the aquifer. The mass of resident tracer remaining within section B of Fig. 1 can be used to estimate the cumulative fluid volume intercepted by the meter. Assuming reversible, linear and instantaneous resident tracer partitioning between the sorbent and water, the dimensionless cumulative volume of water, \, intercepted by the flux meter at a specified well depth is obtained iteratively using the following equation: 1- ( sin 2 %M R (2) where MR is the relative mass of tracer retained in the flux meter sorbent at the particular well depth. The specific discharge through the device, q, can then be calculated using: * = (3) where: r is the radius of the flux meter cylinder, 9 is the water content in the device, R c t is the retardation of the resident tracer on the sorbent, and t is the sampling duration. The actual specific discharge ofthe aquifer, q 0 is then found from q and equation (1). Because in most field applications the magnitude of flow is unknown, multiple resident tracers should be used to represent a broad range of tracer retardation factors. Likewise, multiple tracers provide for flux meters designed for both long- and shortterm sampling duration. The contaminant mass retained on the sorbing porous matrix can be used to estimate solute flux intercepted by the meter. The measured flux is valid over the dimensions of porous medium contributing flow into the device. For example, a meter designed to sample the entire vertical depth of an aquifer could be used to characterize horizontal groundwater and contaminant fluxes continuously over the vertical extent of an aquifer. Assuming reversible, linear and instantaneous contaminant partitioning between the sorbent and water, the contaminant mass flux (J c ) can be determined using: J c =, qm < (4) nr 2 L(l-M RC )QR dc where M C is the mass of contaminant sorbed and L is the length of the sorbent matrix or the vertical thickness of aquifer interval interrogated, R t i c is the retardation of contaminant on the sorbent, MRC is the relative mass of a hypothetical resident tracer retained after time period t where that tracer has the same retardation is R c i c.

4 28 Kirk Hatfield et al. LABORATORY TESTING OF THE FLUX METER Laboratory aquifer model experiments were conducted to assess potential application of the flux meter. The experiments included resident tracer desorption to estimate groundwater flow, and sorption of a single contaminant surrogate to estimate contaminant flux. The sorbent for the flux meter was sand mixed with liquid hexadecane (HD), a long chain hydrophobic hydrocarbon which contained predetermined amounts of resident tracers. 2,6-dimethyl-2-heptanol represented the aquifer contaminant. The hexadecane/water partition coefficient for each resident tracer and the contaminant surrogate are listed in Table 1. The sorbent and tracers used here are based on previous research using partitioning tracers to quantify non-aqueous phase liquid (NAPL) saturation (Jin et al, 1995; Annable et al, 1998; Rao et al, 2000). Table 1 Tracers used. Contaminant surrogate: 2,6-dimethyl-2-heptanol 70 Resident tracers: 2,4-dimethyl-3-pentanol 12 6-methyl-2-heptanol 32 K\iw 2-ethyl-l-hexanol 42 A plastic container with dimensions of 52 x 30 cm, and 37 cm deep was used to create the aquifer model. The box was packed under water with sand to a height of 16 cm. The water contained 200 mg 1 2,6-dimethyl-2-heptanol. The two ends of the container were used for flow injection and extraction and were packed with coarse gravel. This was done to provide a constant head across the width of the box, and a uniform gradient across the length of the box. The sand used in the main section of the box was commercial grade medium sand having a hydraulic conductivity of 2.0 cm min" 1. The water table was set to a height of 16 cm. The flux meters were constructed from 0.3 mm mesh screen wrapped to form a 3.5 cm diameter tube. Each flux meter had exposed screen between 1 and 13 cm from the bottom of the model aquifer. The applied flow rate was 2.5 ml min" 1 giving a Darcy flux of cm min" 1. This flow contained the contaminant surrogate at 200 mg 1"' to produce an actual solute flux of mg cm" 2 h"'. Figure 2 shows the aquifer model with flux meters in place. The sorbent used inside the flux meter was mesh Ottawa sand with trapped HD containing the resident tracers. Sufficient HD tracer mixture was combined with Fig. 2 Installed flux meters.

5 A new methodfor quantifying contaminant flux at hazardous waste sites 29 Ottawa sand to produce a residual NAPL saturation of 0.21 and a resultant NAPL/sand sorbent permeability of 4.5 cm min"'. Given that clean Ottawa sand has a porosity of 0.29, and the HD saturation was 0.21, the resultant sorbent water content, 0 was The bulk density of the Ottawa sand with the HD was 1.85 kg 1"', with a mass fraction of organic carbon of Resident tracer concentrations in the HD were 7500 mgl" 1. The sorbent was packed into four flux meters to a total height of 16 cm. A sample of the sorbent was extracted separately to determine the initial resident tracer concentration. The devices were then refrigerated to freeze the HD to minimize mobilization during the insertion process in the 3D box. Flux meters were inserted into the 3D box using a larger diameter tube that was pushed in and the interior sand removed (similar to a well installation). The devices were then slowly emplaced and the larger pipes were removed allowing the surrounding sand to collapse. Four flux meters were inserted in a fence-row alignment 32 cm from the injection end of the box (Fig. 2). These devices were removed at four different times (6, 14, 26, and 40 h). Immediately after removing each meter, the hole was filled with the sand. The sorbent from each meter was extracted using acetone and analysed in triplicate using a gas chromatograph flame ionization detector for HD and tracers. This data was used to determine the amount of contaminant surrogate intercepted by the flux meter, Mc, and the amount of residual resident tracer remaining, Mr- Values of Mr were normalized to initial control extractions to obtain for each resident tracer the relative mass retained, MR. The mass of 2,6-dimethyl-2-heptanol intercepted by the flux meter was used to obtain M c. The dimensionless contaminant mass intercepted, Ma was then calculated by normalizing M c to the theoretical maximum based on a calculation of equilibrium partitioning. Experimental values of MR were then compared with theoretical values derived from equations (l)-(3). Theoretical values of Mci were generated using equations (l)-(4) and the relationship MRQ = 1 - Met- RESULTS Measured resident tracers masses from each flux meter were compared to the extraction of the control media to obtain a relative mass, MR, at the time, t, when each device was pulled. Values of MR greater than 0.33 were used with the following equation to estimate the specific discharge of flow through the meter: M R =h 0.6/ R D Qr valid for M R = >0.33 (5) Figure 3 is a plot of MR values from two resident tracers vs 0.6t/(rR ir Q). Fitting equation (5) to this data produced a best-fit value of q equal to cm min"'. From equation (1), box-aquifer values of specific discharge and contaminant flux are obtained by dividing meter-derived estimates by Thus, the estimated box aquifer specific discharge was cm min" 1, which was within 2.5% of the true flux. Using the metered water flux of cm min" 1, Fig. 4 was constructed to illustrate the relationship between theoretical and experimental MR and the dimension-

6 30 Kirk Hatfield et al. Slope = q =.007 cm min" 1 Slope = q =.007cm/min Theory: Equation (5) Mo ethyl-1-hexanol A 6-methyl-2-heptanol Fig. 3 Specific discharge determination using M R values from two resident tracers vs 0.6t/(rRfi). - Theory: Equation (2) 2,4 dimethyl-3-pentanol A 6-methyl-2-heptanol 2-ethyl-1-hexanol Dimensionless cumulative flow, i; 1.5 Fig. 4 Resident tracer mass remaining, M R vs dimensionless cumulative flow. Theory 2,6-dimethyl-2-heptanol Dimensionless cumulative flow, cj Fig. 5 Dimensionless contaminant mass intercepted M a vs dimensionless cumulative flow. less cumulative flow t, = tql(2rr c $). This figure clearly demonstrates a loss of each resident tracer consistent with theory. Figure 5 portrays the dimensionless cumulative interception of 2,6-dimethyl-2- heptanol or M a vs dimensionless cumulative flow t, = tql(2rr c fi). The theoretical

7 A new method for quantifying contaminant flux at hazardous waste sites 31 relationship between Mci and is derived using the relationship MRC = 1 - M C t to obtain MRC, and then substituting the value of M RC for MR in equation (2) to find The theoretical curve shown in Fig. 5 was derived using the previous metered water flux of cm min"' and the known the influent concentration of 2,6-dimethyl-2-heptanol. Figure 5 clearly demonstrates an excellent comparison of M cj values predicated on theoretical and measured masses of intercepted 2,6-dimethyl-2-heptanol. Based on the 2,6-dimethyl-2-heptanol mass intercepted at hours 6, 14, and 26, the averaged metered contaminant flux was mg cm" 2 h"'. When this flux is reduced by the factor 1.38 (see equation (1)), the estimated box contaminant flux was mg cm" 2 h"', which was within 6.8% of the true contaminant flux. CONCLUSIONS The basic concept behind a new method for measuring groundwater flow and contaminant transport was presented. The approach consists of placing a sorbent with resident tracers into the ground to intercept a flow field. The resident tracer removal will be a function of the groundwater flow and the mass capture of contaminants can be used to estimate flux. The viability of the method was assessed in a laboratory aquifer model. The resident tracers were removed and contaminant was sorbed in accordance with the theory presented. This method shows great promise as a new tool to assess groundwater contamination at hazardous waste sites. The approach may have utility for assessing remedial performance, evaluating natural and enhanced bioremediation, and for quantifying risk to humans and ecosystems. The method is currently being tested at a field site. Acknowledgements This research was partially funded by the US DoD under the Environmental Security Technology Certification Program (ESTCP project CU-0114). REFERENCES Aimable, M. D., Rao, P. S. C, Graham, W. D Hatfield, K. & Wood, A. L. (1998) Use of partitioning tracers for measuring residual NAPL: results from a field-scale test. J. Environ. Engng 124(6), Einarson, M. D. & Mackay, D. M. (2001) Predicting impacts of groundwater contamination. Environ. Sci. Technol. 35, 66A-73A. Feenstra, S., Cherry, J. A. & Parker, B. L. (1996) Conceptual models for the behavior of nonaqueous phase liquids (DNAPLs) in the subsurface. In: Dense Chlorinated Solvents and Other DNAPLs in Groundwater (ed. by J. F. Pankow & J. A. Cherry), Waterloo Press, Portland, Oregon, USA. Jin, M., Delshad, M., Dwarakanath, V., McKinney, D. C, Pope, G. A., Sepehrnoori, K., Tilburg, C. & Jackson, R. E. (1995) Partitioning tracer test for detection, estimation, and remediation performance assessment of subsurface nonaqueous phase liquids. Wat. Resour. Res. 31(5), Rao, P. S. C, Aimable, M. D. & Kim, H. (2000) NAPL tracer techniques for site characterization and remediation technology performance assessment: recent developments and applications../. Contam. Hydrol. 45, Strack, O. D. L. & Haitjema, H. M. (1981) Modeling double aquifer flow using a comprehensive potential and distribution singularities. 2. Solution for inhomogeneous permeabilities. Wat. Resour. Res, 17(5),