Environ. Sci. Technol. 2003, 37, 2302-2309
Performance Evaluation of a Permeable Reactive Barrier Using Reaction Products as Tracers STAN MORRISON* Environmental Sciences Laboratory, 2597 B 3/4 Road, Grand Junction, Colorado 81503
A method incorporating laboratory analysis of constituents that formed as reaction products was developed and used to determine the flux of groundwater through a zerovalent iron-based permeable reactive barrier (PRB) installed to treat U-contaminated groundwater. Concentrations of three nonvolatile constituents (Ca, U, and V) that formed as reaction products in the PRB were analyzed in 279 samples. Areal distributions of the reaction products indicate that groundwater flowed through all portions of the PRB and that nearly the entire volume of reactive material is treating the groundwater. Almost 9 t of calcium carbonate precipitated in the PRB during the first 2.7 yr of operation, but only 24 kg of combined U- and V-bearing minerals precipitated during the same period. Concentration gradients of Ca, U, and V dissolved in the groundwater indicate that a hydraulically upgradient portion of the PRB lost some reactivity during the first 2.7 yr of operation. Calculations that partially couple porosity changes to ZVI reactivity suggest that loss of reactivity may be more limiting than porosity reduction for long-term performance of the PRB. Calculations using groundwater concentration gradients and solid-phase concentrations indicate that the mean groundwater flux ranged from 11 to 24 L/min, considerably less than the design flux of 185 L/min. Flux values calculated with all three constituents were in good agreement. This method provides a more accurate determination of groundwater flux than is possible with flow sensor measurements, dissolved tracers, or Darcy’s law computations.
Introduction Withdrawals of groundwater in the United States increased from 26 to more than 75 billion gal/d from 1950 to 1995 (1). More than 20,000 sites nationwide have groundwater contamination (1). Few sites with contaminated groundwater, particularly those with inorganic contamination, have been remediated to regulated concentration standards largely because of the high costs using existing technologies (2-5). Because of the high cost of groundwater cleanup, the U.S. Environmental Protection Agency advocated the development of cost-effective innovative technologies (6). To help reduce costs, a passive remediation technology termed permeable reactive barrier (PRB) was introduced in 1991 at a field demonstration site at the Canadian Forces Base Borden in Ontario (7). Since 1991, more than 70 PRBs have been installed at sites with contaminated groundwater (8). * Phone: (970)248-6373;
[email protected]. 2302
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fax:
(970)248-7628;
e-mail:
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A PRB is an engineered zone of reactive material placed in the subsurface that causes destruction or containment of contaminants dissolved in the groundwater flowing through it. Most PRBs use zerovalent iron (ZVI) as the reactive material. ZVI degrades chlorinated solvents (9) and stabilizes metals and radionuclides (10, 11). The corrosion of ZVI causes changes in the hydrogen ion and electron potentials in the treated groundwater. These changes trigger reactions with dissolved contaminants, causing their destruction or containment by immobile mineral phases. Analytical results of samples from PRBs that have been in place for several years generally indicate that concentrations of contaminants leaving PRBs remain low. However, the flow regime through PRBs is difficult to quantify, and estimates of the groundwater flux are usually imprecise. Groundwater flux measurements are typically based on Darcy’s law using water table gradient and hydraulic conductivity estimates, results from downhole flow velocity meters, or dissolved tracer tests. Often, these methods provide conflicting results. In comparisons at the same measurement station, Wilson et al. (12) observed that three different types of downhole flow velocity meters rarely provided the same velocities and flow directions. Measurements with flow meters indicate that the flow velocity within boreholes is heterogeneous. Sharp gradients and uncertainties in the hydraulic conductivity field and water table gradients make application of Darcy’s law imprecise. Dissolved tracers have been used to define local flow velocities. Because they can be deployed at only a few widely spaced locations to avoid overlap of the tracer plumes, large uncertainties exist in using these data to determine the mean flux of groundwater through a PRB. A PRB at Monticello, UT, has operated for 2.7 yr to remove U, V, and other contaminants associated with an abandoned uranium ore processing mill (13). An evaluation of column studies and field data from the Monticello site indicates that as ZVI irreversibly corrodes, pE values decrease, pH values increase, carbonate minerals precipitate, iron redistributes, and U and V are removed from solution (14). Because some elements remain in the PRB as the groundwater passes through, they can be used as tracers to indicate former flow paths. If concentration gradients in the groundwater are also known, a mean groundwater flux can be determined. The purpose of this study was to develop a method to determine the mean flux of groundwater through the Monticello PRB and to establish if the groundwater flow was evenly distributed or preferred certain paths. Data used to accomplish this goal include results of laboratory analyses of 279 PRB core samples for Ca, U, and V concentrations and 10 groundwater sampling events during the initial 2.7 yr of operation. Field Site Description. The Monticello PRB was installed in June 1999 to remediate groundwater contaminated by seepage from uranium mill tailings (13). Sheet piling construction was used to install the PRB; two impermeable wing walls were constructed of soil-bentonite slurry. The slurry walls are 29.6 and 73.2 m long, and the PRB is 31.4 m in length (Figure 1). The three parallel zones of the PRB are composed of, from upgradient to downgradient, 0.6 m of 1.3-cm gravel mixed with 13 vol % 0.85-4.75-mm ZVI (gravel/ ZVI zone), 1.2 m of 0.85-2.36-mm ZVI (ZVI zone), and 0.6 m of 1.3-cm gravel (Figure 1). Both the PRB and the slurry walls are keyed into impermeable shale bedrock at depths of 3.4-4 m. The system nearly spans the width of an alluvial valley, but small gaps that were left at each end of the slurry walls because of landowner concerns permit some of the 10.1021/es0209565 CCC: $25.00
2003 American Chemical Society Published on Web 04/11/2003
FIGURE 1. Schematic of Monticello PRB and slurry walls. Reproduced with permission from Academic Press (13). groundwater to bypass the PRB. Morrison et al. (13) provide additional details of the construction.
Methods Section Field Study. Seventy vertical 4.1-cm-diameter cores were obtained in February 2002 by pushing a core barrel into the subsurface with percussion using a model 8MR1 Geoprobe. Core recovery averaged 57% ( 3%; uncertainty nomenclature used throughout this paper indicates (2 standard errors (SE). The range defined by 2 SE provides approximately a 95% confidence of containing the true mean. Core recovery was approximately the same in the gravel/ZVI zone as in the ZVI zone. The recovered portions of the cores were intact and showed little disruption from the drilling vibrations. The reason for less than 100% recovery was likely because of the buildup of friction and the inability of the equipment to push the core up the full length (1.2 m) of the core barrel. Each core collected from the saturated zone was sectioned into 15.2-cm lengths with a hand saw and placed in plastic zippered bags for transport to the laboratory; 614 samples were collected. Groundwater samples were collected 10 times at nearly equal time intervals since installation of the PRB in June 1999. The wells are 2.5 cm diameter with 1.5-m-long well screens made of poly(vinyl chloride) positioned at the bottom of the alluvium or reactive media. Samples were collected after purging 1 L (about 1 bore volume) with a peristaltic pump and pH values had stabilized. Measurements of pH values were made in a flow-through cell. Samples for analysis of Ca, U, and V were filtered (0.45-µm filter) into plastic laboratory bottles and preserved with nitric acid at a pH value of less than 2. Sampling Strategy. Random sampling within segments (15) was used to select transect locations for coring. This sampling plan combines aspects of simple random sampling but ensures that sample locations are spread along the entire length of the PRB. A transect line trending perpendicular to the front edge of the PRB was located randomly within each of 10 equally proportioned segments of the PRB. Cores were collected at six random locations along each transect line, four in the ZVI zone and two in the gravel/ZVI zone. Samples were not collected at 5 of these 60 locations because of drilling problems. Fifteen additional cores were collected at intermediate locations throughout the area of the PRB for a total of 70 core holes (Figure 2). Four samples from each core hole
FIGURE 2. Core and well locations. Width scale is expanded by a factor of 10. were selected randomly from the population of 15.2-cmlong samples and were digested and analyzed for Ca, U, and V concentrations. Core Sample Preparation and Analysis. The contents of the plastic bags were placed in aluminum pans and dried in a convection oven at 105 °C. Weights were determined to within 0.1 g before and after drying to measure moisture contents. Average moisture contents of the gravel/ZVI and ZVI zones were 9.5 and 15.6%, respectively; these values do not represent in situ moisture contents because water was lost during field sampling. Analytical results of seven samples from the upper portion of three cores in the ZVI zone were omitted from the data because their low moisture contents, ranging from 0.57 to 4.45%, indicated that they were from the unsaturated zone. Dry weight density was determined using a constant volume of 201 cm3 for the 15.2-cm-long core samples. Three samples of parent (material prior to use in the PRB) gravel/ZVI and three samples of parent ZVI were processed in the same manner as the core samples. Half-gram splits of 279 dried core samples were digested with 10 mL of concentrated nitric acid in a microwave oven at a power level of 1000 W with Method 3051 (16). One of every 20 samples was digested in duplicate. Method 3051 digested more than 90 wt % of the ZVI and removed all the Ca, U, and V. Several samples were tested with a total digestion method that involved five sequential steps with hot concentrated acids to confirm the total removal of Ca, U, and V concentrations. Nitric acid microwave digestions were used instead of total digestions because of the higher cost of the total digestion method and the equivalent removal of Ca, U, and V. Semiquantitative values of carbonate concentrations in dried samples were determined by measuring carbon dioxide gas emission after treating with hydrochloric acid. One gram of sample was placed in a 60-mL plastic syringe, and 10 mL VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Frequency distributions of dry weight density. of 5% hydrochloric acid was introduced via a second syringe. Carbon dioxide pushes the syringe out against atmospheric pressure. The distance the syringe is displaced is a measure of the carbonate concentration in the sample. The system was calibrated with a variety of calcite masses, and the calibration curve was used to convert values of syringe displacement to carbonate content in the PRB samples. The method has a detection limit of about 0.5% calcite. Uranium concentrations in the digestates were analyzed by inductively coupled plasma mass spectrometry with Method 6020 (16); Ca and V were analyzed by inductively coupled plasma atomic emission spectrometry with Method 6010B (16). Preserved groundwater samples were analyzed with these same methods.
Results Parameter Estimation. Mean density was estimated from frequency distributions after excluding outliers that are more than 2 standard deviations (SD) from the mean (Figure 3). Outliers accounted for 5 and 6% of data collected in the gravel/ZVI zone and the ZVI zone, respectively, and resulted from imprecise cutting of the core in the field. Mean densities are 1.851 ( 0.022 g/cm3 for cores from the gravel/ZVI zone and 2.238 ( 0.018 g/cm3 for cores from the ZVI zone. The density values measured in the PRB are similar to laboratorydetermined values on parent material: 1.74 g/cm3 for gravel/ ZVI and 2.21 g/cm3 for ZVI. Saturated thickness is estimated at 2.01 ( 0.07 m from frequency distributions of water levels in 40 monitor wells averaged over five evenly spaced sampling events that represent the variation in water table elevations because of seasonal effects. On the basis of as-built geometry, the total volumes (Vt) of the saturated gravel/ZVI and ZVI are 37.9 and 75.7 m3, respectively. On the basis of the mean densities, the masses (dry weight basis) of the gravel/ZVI and ZVI zones are 70.2 and 169.4 t, respectively. Mean concentrations of Ca, U, and V measured in three parent gravel/ZVI samples are 2371, 0.22, and 34.7 mg/kg, respectively. In the parent ZVI, the mean concentrations of Ca, U, and V are 21.0, 0.06, and 60.7 mg/kg, respectively. Laboratory-determined effective porosity values are 42% for gravel/ZVI and 70% for ZVI. Hydraulic conductivity of the parent ZVI is 0.062 cm/s as determined from falling head permeameter testing. Hydraulic conductivity of the alluvial aquifer is about 0.013 cm/s based on pump testing. Spatial Distributions. Spatial distributions were examined on contour maps using means of the concentrations in samples from four random depths in each core hole. Mean concentrations of samples from individual cores from the gravel/ZVI zone averaged from 15.1 to 46.8 g/kg of Ca, from 70 to 596.9 mg/kg of U, and from 30.2 to 1168.3 mg/kg of V. These concentrations are relatively evenly distributed along 2304
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FIGURE 4. Contour map of means of solid-phase Ca concentrations in samples from four random depths (g/kg). the full length of the PRB (Figures 4-6). Bivariant plots (not presented) of Ca, U, or V concentrations in the gravel/ZVI zone indicate no relationship between concentrations and distance from the hydraulically upgradient front of the gravel/ ZVI zone. However, concentrations of Ca, U, or V in samples from a single core vary by as much as a factor of 10. Bivariant plots (not presented) of Ca, U, and V concentrations indicate a lack of correlation with depth. Mean concentrations of Ca in core samples from the ZVI zone range from 0.8 to 33.5 g/kg (Figure 4). Uranium concentrations are near those of the parent material (0.06 mg/kg) in samples from throughout the ZVI zone, except in two cores collected less than 0.25 m from the contact with the gravel/ZVI zone (Figure 5). The highest concentration of U in samples from the ZVI zone is 10.5 mg/kg, which is significantly less than concentrations in most samples from the gravel/ZVI zone. Vanadium concentrations are near the concentration in the parent sample (60.7 mg/kg) in samples from throughout the ZVI zone (Figure 6). Concentrations of Ca, U, and V in the PRB. The concentrations of Ca, U, and V are log-normally distributed in samples collected in the PRB. The frequency distributions of the logarithmic concentrations are exemplified by U (Figure 7). The geometric means (Cs) of concentrations in samples from the gravel/ZVI zone are 22.7 g/kg of Ca, 162 mg/kg of U, and 107 mg/kg of V (Table 1). The coefficient of variation (η) indicates the relative spread of the data distribution. As indicated by a smaller η, Ca has a tighter distribution than U or V in samples from the gravel/ZVI zone. Mean values for U and V concentrations in samples from the ZVI zone were not calculated because they are close to the concentrations in samples from the parent ZVI. The geometric mean of Ca concentration in samples from the ZVI zone is 11.0 g/kg (Table 1). Concentrations of Ca, U, and V in Groundwater. The groundwater concentrations of Ca, U, and V (Cw) in samples
FIGURE 7. Frequency distributions of log U concentrations.
TABLE 1. Geometric Means (Cs) ( 2 Standard Errors (SE) and Coefficients of Variation (η) for Ca, U, and V Concentrations in Core Samplesa constituent
-2 SE
Ca (g/kg) U (mg/kg) V (mg/kg)
Gravel/ZVI Zone 20.3 22.7 124 162 83 107
Ca (g/kg)
9.25
Cs
ZVI Zoneb 11.0
+2 SE
η
25.2 212 138
0.054 0.288 0.291
13.0
0.117
a
FIGURE 5. Contour map of means of solid-phase U concentrations in samples from four random depths (mg/kg).
Concentrations in the parent materials were subtracted, so the values represent the concentrations that resulted from precipitation from groundwater. b U and V are not presented because of the large number of nondetectable values.
presents well locations. The concentrations used for each time (Cit) are the means of the concentrations in samples from the five upgradient wells (i):
( ) 5
∑C
t
i
i)1
10
Cw )
∑ t)1
5
(1)
10
Similarly, mean concentrations in samples collected hydraulically downgradient of the gravel/ZVI zone were calculated from 10 sampling events in 5 downgradient wells. The upgradient concentrations used for the ZVI zone are equivalent to the downgradient concentrations used for the gravel/ZVI zone. The downgradient concentrations for the ZVI zone are means from the 10 sampling events using the means of samples from the 10 downgradient wells shown in Figure 2. Concentration gradients (∆Cw) are the differences between the mean upgradient concentrations and the mean downgradient concentrations in each of the two zones; the standard errors were calculated from the means for each time period (Table 2).
Discussion
FIGURE 6. Contour map of means of solid-phase V concentrations in samples from four random depths (mg/kg). collected hydraulically upgradient of the gravel/ZVI zone for the 2.7-yr period was estimated by averaging concentrations from the 10 nearly evenly spaced time intervals; Figure 2
Data collected during this investigation were used to (i) evaluate mass-balance evidence for determining reaction mechanisms, (ii) evaluate the tendency for groundwater flow to bypass some areas of the PRB, (iii) determine the mass and volume of mineral precipitation that has accumulated in the PRB in 2.7 yr, (iv) determine the loss of porosity caused by mineral precipitation and potential for loss of permeability, and VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Geometric Means ( 2 SE and Coefficients of Variation (η) for Ca, U, and V Groundwater Concentration Gradients (∆Cw) across Gravel/ZVI and ZVI Zones constituent
-2 SE
mean ∆Cw
+2 SE
η
Ca (mg/L) U (µg/L) V (µg/L)
Gravel/ZVI Zone 16.6 47.0 241 334 300 330
77.4 427 360
1.020 0.442 0.141
152.0
0.445
Ca (mg/L)
85.0
ZVI Zonea 118.4
a
U and V are not presented because of the large number of nondetectable values.
(v) provide a preliminary assessment of PRB longevity. Calcium, U, and V were selected for this study because they are nonvolatile and their concentrations decreased significantly in groundwater as it flowed through the PRB. Concentrations of U and V are useful for this method in groundwater systems in which they are contaminants. Calcium (and CO2) is useful for evaluations of most ZVIbased PRBs, including those used to treat organic contaminants, because Ca is present in all groundwater systems. The methodology used in the study is easy to implement, and the results are more accurate than commonly used methods (e.g., tracers and flow sensors) for evaluating groundwater flow direction and flux through PRBs. Because the constituents are nonvolatile and do not degrade, solidphase samples can be collected at one time without the addition of preservatives and analyzed later. Reaction Mechanisms. Two mechanisms have been suggested for U uptake by ZVI: reductive precipitation (14, 17, 18) and adsorption on oxidative reaction products (19). The reddish-orange color of some of the cores collected from the gravel/ZVI zone indicates that some ZVI has oxidized. A calculation was made to determine if sufficient amorphous ferric oxyhydroxide (AFO) adsorption sites may have been available to account for the observed concentration of solidphase U. The mean concentration of U in the gravel/ZVI zone (162 mg/kg) when normalized to the concentration of Fe in the gravel/ZVI zone (0.00022 mol of U/mol of Fe) is about half the maximum possible adsorption density (0.0004 mol of U/mol of Fe) for U on AFO; adsorption data from Morrison et al. (20) were used to calculate the maximum U adsorption density at the dissolved carbonate concentrations and pH values of the Monticello groundwater samples. The maximum adsorption density is possible only if all the ZVI in the gravel/ZVI zone has been oxidized to AFO; visual inspections and results of qualitative magnetic separations of core samples indicate that much of the original quantity of ZVI is still present in samples from the gravel/ZVI zone. A better quantification of the amount of AFO is needed to confirm that adsorption is a viable mechanism. Relative Reaction Rates and Preferential Flow. All the V and 99.2% of the U concentrations in the PRB were deposited in the gravel/ZVI zone, but substantial quantities of Ca were deposited in both gravel/ZVI and ZVI zones. This distribution suggests that the uptake reactions for U and V are rapid relative to those for Ca, a finding that is consistent with results of column experiments (14). Tracer tests conducted at the Monticello PRB indicate heterogeneity that caused preferential flow on a local (decimeter to meter) scale (21). However, the spatial distributions of Ca, U, and V concentrations in the solid phases suggest that groundwater has moved through the PRB relatively uniformly along its length. Flow has not bypassed any substantial portion of the PRB. Thus, nearly all the ZVI is being utilized to remove contaminants. 2306
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TABLE 3. Mass of Ca, U, and V Deposited in PRB (Ms) during the First 2.7 yr of Operation Based on Equation 2 mass (Ms) constituent
gravel/ZVI zone
ZVI zone
total
Ca (t) U (kg) V (kg)
1.6 11.4 7.5
1.9 ∼0 ∼0
3.5 11.4 7.5
TABLE 4. Mass and Volume of Minerals Deposited in the PRB during First 2.7 yr of Operation mineral
gravel/ZVI zone
ZVI zone
total
calcite (t) calcite (m3) uraninite (kg) uraninite (L) V trioxide (kg) V trioxide (L)
4.0 1.5 12.9 1.6 11.0 2.3
4.8 1.7 ∼0 ∼0 ∼0 ∼0
8.8 3.2 12.9 1.6 11.0 2.3
Despite the large mass of calcium carbonate deposited in the PRB, the cores appear to have substantial permeability. Water flowed freely from the cores during extraction from the sample tubes. No hardpan was encountered in the PRB, indicating that calcium carbonate had not completely cemented any portions of the PRB. Total Masses and Volumes of Minerals Deposited. Total masses of Ca, U, or V deposited in the PRB solids (Ms) during the first 2.7 yr of operation are calculated as the product of the mean solid-phase concentration in the gravel/ZVI or ZVI zone (Cs) and the mean dry weight mass of the ZVI in the zone (Mz) (Table 3):
Ms ) CsMz
(2)
Calcium occurs in carbonate minerals (CaCO3) as indicated by approximately stoichiometric amounts of Ca and CO2 and by mineral composition determined with an electron microprobe on four samples. Electron microprobe analysis identified calcium carbonate in ZVI corrosion rims in all four samples. Calcium carbonate minerals were identified by stoichiometric concentrations of calcium and, in some grains, by a distinct radiating crystal fabric characteristic of aragonite. The mineralogy of U and V was not determined. To estimate the mass and volume of mineral matter that was deposited, Ca, U, and V were assumed to deposit as calcite (CaCO3), uraninite (UO2), and vanadium trioxide (V2O3) with mineral densities of 2.71, 8.0, and 4.87 kg/L, respectively (densities from refs 22 and 23). Calcite deposited in the PRB during the 2.7-yr period has a mass of 8.8 t and occupies 3.2 m3 (Table 4). The volume of calcite deposited in the gravel/ ZVI zone (1.5 m3) represents 9.3% of the pore space available at the time of installation (15.9 m3). The volume occupied by U and V minerals (1.6 and 2.3 L, respectively) is minor as compared to that occupied by calcite. The volume of calcite deposited in the ZVI zone (1.7 m3) represents 3.2% of the initial pore space (53.0 m3). Groundwater Flux. Unfiltered samples were collected with the filtered (0.45 µm) samples from five downgradient wells in one sampling event (July 2002). Concentrations of Ca and alkalinity in the unfiltered samples were similar to concentrations in the filtered samples, indicating that no particulates containing these constituents were exiting the PRB (Table 5). Similarly, loss of U and V by particulate transport was insignificant. Calcium, U, and V do not volatilize and, therefore, are conserved within the chemical system. Thus, any mass removed from the groundwater must reside
TABLE 5. Comparison of Filtered and Unfiltered Results for Calcium and Alkalinity for Downgradient Samples Collected in July 2002 calcium (mg/L)
alkalinity (mg/L CaCO3)
filtered
unfiltered
filtered
unfiltered
1 2 3 4 5
71 150 96.8 59 104
66.5 201 98.3 58.2 101
125 232 141 98 173
125 naa 153 92 170
na, not analyzed.
TABLE 6. Groundwater Flux through the PRB during First 2.7 yr of Operation (L/min) constituent
zone
Ca Ca U V
gravel/ZVI ZVI gravel/ZVI gravel/ZVI
12.9 7.2 14.5 11.4
mean
higha
24.0 11.3 24.0 16.0
75.0 18.3 43.6 22.8
a Low and high values incorporate the combined uncertainty of 2 SE for solid-phase concentrations and aqueous-phase concentration gradients.
in the solid phases of the PRB. Mean groundwater flux (Qw) for the investigation period (t ) 2.7 yr) was calculated from the total mass (Table 3) and the mean groundwater concentration gradients (Table 2) for Ca, U, and V:
Qw ) Ms/t∆Cw
dr
samples required (n)
samples collected
uranium/gravel/ZVI
0.288 0.291
calcium/gravel/ZVI
0.054
calcium/ZVI
0.117
32 128 33 131 2 5 6 21
119
vanadium/gravel/ZVI
0.10 0.05 0.10 0.05 0.10 0.05 0.10 0.05
119 119 163
estimate the number of samples (n) required for a desired value of relative error (dr) (15):
n ) (Z1-R/2η/dr)2
groundwater flux (Qw) lowa
η
constituent/zone
well
a
TABLE 7. Number of Samples Required for 95% Confidence Level
(3)
Mean groundwater fluxes calculated with eq 3 range from 11.3 to 24.0 L/min (Table 6). The calculated means from these four independent measurements are reasonably consistent, providing confidence that the true groundwater flux is close to this range. The 2 SE values for the means of both the solid-phase concentrations and the groundwater concentration gradients expand the calculated range of groundwater flux to 7.2 to 75.0 L/min. Data for Ca concentrations in the ZVI zone and V concentrations in the gravel/ZVI zone are best suited for calculating groundwater flux because their distributions are the most constrained. The groundwater flux calculated with these data ranges from 7.2 to 22.8 L/min with a combined mean of 13.5 L/min. Calculated values of groundwater flux using the Ca distribution in the gravel/ZVI zone have a relatively wide range, even though the concentration distribution in the solid phase has a low coefficient of variation (Table 1). This wide range is attributable to an increase in the Ca concentrations in groundwater exiting the gravel/ZVI over time (the gradient decreased), resulting in a high coefficient of variation for the groundwater concentration gradient (Table 2). The decreasing Ca concentration gradient indicates that the gravel/ZVI zone was losing reactivity during the 2.7-yr period. Loss of reactivity is also indicated by an increase in the mean U concentration exiting the gravel/ZVI zone from 0.2 to 185 µg/L during the 2.7-yr period. Number of Samples Required. Coring and the number of samples collected and analyzed share the major costs of using solid-phase concentrations to assess performance of a PRB using the methods described in this study. Distributions of solid-phase concentrations developed during this study were used to determine the number of samples required to estimate the true means within a 95% confidence limit. The coefficient of variation (η), presented in Table 1, is used to
(4)
where Z1-R/2 for the 95% confidence interval is equal to 1.96 (Table 7). For U, 32 samples are required to have 95% confidence that the sample mean is within 10% of the true mean. To have the same confidence of being within 5% of the true mean, 128 samples are needed. Because of the tight distribution of Ca concentrations in the gravel/ZVI zone, only two samples are needed to have 95% confidence of being within 10% of the true mean. Thus, if the distributions are known or can be reliably assumed, the number of samples required may be considerably fewer than were collected for this study. Additional samples would be required to adequately define preferential flow paths through a PRB. Longevity. This study identified two mechanisms that can limit the longevity of PRBs: reduction of reactivity and reduction of porosity. Deposition of carbonate minerals on ZVI surfaces could affect both processes. Reactivity decreases as a PRB ages, causing a decrease in the rate of porosity loss. A simple model of PRB performance was formulated that incorporates a partial coupling between reactivity loss and porosity loss and is supported by the data from this study. The rate of removal of U from groundwater by ZVI decreases with decreased surface area. If reactivity decreases because of mineral precipitation on ZVI surfaces, then the rate of reactivity would likely be rapid initially because of the higher density of available surface sites and would decrease exponentially as reaction sites are coated. A first-order rate expression is used to portray this loss of ZVI reactivity, expressed as the loss of ZVI surface area (Sz) over time (t):
-dSz/dt ) λSz
(5)
where λ is a rate constant. Equation 5 is used to model the loss of ZVI surface area in the gravel/ZVI zone as indicated by the decrease in the gradient of dissolved U over time (Figure 8). The rate constant was used as a fitting parameter to provide reasonable consistency between the model and the field data. By using an initial surface area of 1 m2/g for ZVI that was estimated from data provided by Johnson et al. (24), the initial surface area of ZVI (Sz0) is 291 and 2240 m2/L in the gravel/ ZVI and ZVI zones, respectively. Effluent from the gravel/ ZVI zone exceeded the Monticello standard of 30 µg/L after about 0.5 yr of PRB operation. At that time, the gravel/ZVI zone had about 220 m2/L of reactive surface area based on eq 5; the surface area of the ZVI zone would decrease to 220 m2/L in about 6 yr from the time of installation (Figure 9). Porosity (Φ) decreases as mineralization coats ZVI surfaces. Assuming that the change in surface area is linearly related to the change in porosity, the porosity is given by
Φ ) Φ0 - (Tc)(Sz0 - Sz) VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. Change in U concentration gradient across the gravel/ ZVI zone and modeled (λ ) 0.001/d) surface area loss using eq 5. Both curves were normalized to the value at time 0. Uranium gradients are the differences between incoming and outgoing groundwater concentrations (both are averages from five locations).
FIGURE 10. Modeled change in hydraulic conductivity with time for gravel/ZVI (G/Z) and ZVI zones. and spanning the length of the PRB. Valves could be used to initially direct contaminated groundwater to a perforated distribution pipe near the hydraulically downgradient edge of the PRB. If the distribution pipe is close to the PRB exit, the residence time in the ZVI zone is less than it would be for groundwater passing through the entire width of the PRB. The shorter residence time would result in a smaller increase in pH values and less precipitation of calcium carbonate. As hydraulically downgradient zones become less reactive, hydraulically upgradient zones in the PRB could be activated by directing flow through a different distribution pipe. This design takes advantage of the finding that some contaminants, such as U and V, are removed from solution faster than calcium carbonate.
Acknowledgments FIGURE 9. Modeled change in ZVI surface area (Sz) with time for gravel/ZVI and ZVI zones. where Φ0 and Sz0 are initial values and Tc is a proportionality constant equivalent to the thickness of an evenly distributed coating of mineral; Tc is determined by fitting eq 6 to 9.3% porosity loss in the gravel/ZVI zone and 3.2% loss in the ZVI zone in the first 2.7 yr. Failure of the PRB is expected if hydraulic conductivity (K) of the PRB approaches that of the alluvial aquifer. Hydraulic conductivity is assumed to be related to Φ3/(1 Φ)2 as indicated by the theoretical analysis of pore flow developed by Carmen (25). According to this relationship, K is expected to decrease rapidly initially but approach an asymptote at about 0.034 cm/s for the gravel/ZVI zone and 0.052 cm/s for the ZVI zone (Figure 10). Thus, the K of the PRB is not expected to decrease to the value of the alluvial aquifer (0.013 cm/s). Unfortunately, the chemical reactions occurring at mineral surfaces leading to loss of reactivity are poorly understood, as is the effect of mineral precipitation on K. A better understanding of these processes is needed to have confidence in predictions of PRB longevity. Design Implications. The precipitation of noncontaminant-bearing minerals is an important factor in the longevity of PRBs. Calcite mineralization is evident throughout the Monticello PRB, but contaminants are confined to the gravel/ ZVI zone, indicating that contaminants precipitate with less residence time. Therefore, design strategies that decrease residence time could be effective in prolonging the life span of the PRB. A design that could improve the performance of some PRBs includes a series of buried perforated pipes parallel to 2308
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Installation and 2 yr of monitoring of the PRB at Monticello was funded by the DOE Office of Science and Technology through the Accelerated Technology Deployment Program. Funding for this study was provided through the DOE Grand Junction Office Monticello Program Operable Unit III. This project was possible through the coordinated efforts of Kristen McClellen (MFG, Inc.), Joel Berwick (DOE Grand Junction Office), Paul Mushovic (U.S. Environmental Protection Agency), and David Bird (State of Utah). Insights provided by these individuals and those of Clay Carpenter and Jody Waugh (S. M. Stoller Corporation) and Tim Bartlett’s (MFG, Inc.) many years of careful observation and analysis of the groundwater system were essential to this study. Three anonymous reviewers graciously provided thoughtful critiques that improved the manuscript. The Environmental Sciences Laborary is operated by the S. M. Stoller Corp. for the U.S. Department of Energy Grand Junction Office under DOE Contract DE-AC13-02GJ79491.
Literature Cited (1) U.S. Environmental Protection Agency. National Water Quality Inventory 1998 Report to Congress; EPA816-R-00-013; U.S. Government Printing Office: Washington, DC, 2000. (2) Mackay, D. M.; Cherry, J. A. Environ. Sci. Technol. 1989, 23, 630-636. (3) Travis, C. C.; Doty, C. B. Environ. Sci. Technol. 1990, 24, 14641466. (4) Bredehoeft, J. Ground Water 1992, 30, 834-835. (5) MacDonald, J. A.; Kavanaugh, M. C. Environ. Sci. Technol. 1994, 28, 362A-368A. (6) Clay, D. R. Furthering the Use of Innovative Treatment Technologies in OSWER Programs; PB91-921336; U.S. Environmental Protection Agency: Washington, DC, 1991. (7) Gillham, R. W.; Blowes, D. W.; Ptacek, C. J.; O’Hannesin, S. F. In In Situ Remediation: Scientific Basis for Current and Future Technologies; Gee, G. W., Wing, N. R., Eds.; Battelle Press; Columbus, OH; 1994; Part 1, pp 913-930.
(8) Vogan, J. EnviroMetal Inc., personal communication. (9) Gillham, R. W.; O’Hannesin, S. F. Ground Water 1994, 32, 958967. (10) Blowes, D. W.; Ptacek, C. J.; Jambor, J. L. Environ. Sci. Technol. 1997, 31, 3348-3357. (11) Naftz, D. L., Morrison, S. J., Fuller, C. C., Davis, J. A., Eds. Handbook of Groundwater Remediation Using Permeable Reactive Barriers Applications to Radionuclides, Trace Metals, and Nutrients; Academic Press: New York, 2002. (12) Wilson, J. T.; Mandell, W. A.; Paillet, F. L.; Bayless, E. R.; Hanson, R. T.; Kearl, P. M.; Kerfoot, W. B.; Newhouse, M. W.; Pedler, W. H. An Evaluation of Borehole Flowmeters Used to Measure Horizontal Ground-Water Flow in Limestones of Indiana, Kentucky, and Tennessee, 1999; Water-Resource Investigations Report 01-4139; U.S. Geological Survey: Indianapolis, 2001. (13) Morrison, S. J.; Carpenter, C. E.; Metzler, D. R.; Bartlett, T. R.; Morris, S. A. In Handbook of Groundwater Remediation Using Permeable Reactive Barriers Applications to Radionuclides, Trace Metals, and Nutrients; Naftz, D. L., Morrison, S. J., Fuller, C. C., Davis, J. A., Eds.; Academic Press: New York, 2002; pp 371-399. (14) Morrison, S. J.; Metzler, D. R.; Carpenter, C. E. Environ. Sci. Technol. 2001, 35, 385-390. (15) Gilbert, R. O. Statistical Methods for Environmental Pollution Monitoring; Van Nostrand Reinhold: New York, 1987. (16) U.S. Environmental Protection Agency. Test Methods for Evaluating Solid Waste, 3rd ed.; U.S. Government Printing Office: Washington, DC, 1994; SW-846, Vol. 1A.
(17) Cantrell, K. J.; Kaplan, D. I.; Wietsma, T. W. J. Hazard. Mater. 1995, 42, 201-212. (18) Gu, B.; Liang, L.; Dickey, M. J.; Yin, X.; Dai, S. Environ Sci. Technol. 1998, 32, 3366-3373. (19) Fiedor, J. N.; Bostick, W. D.; Jarabek, R. J.; Farrell, J. Environ. Sci. Technol. 1998, 32, 1466-1473. (20) Morrison, S. J.; Spangler, R. R.; Tripathi, V. S. J. Contam. Hydrol. 1995, 17, 333-346. (21) Liang, L.; Korte, N. E.; Moline, G. R.; West, O. R. Long-Term Monitoring of Permeable Reactive Barriers Progress Report; Oak Ridge National Laboratory Report ORNL/TM-2001/1; ORNL: Oak Ridge, TN, 2001. (22) Hurlbut, C. S., Jr.; Klein, C. Manual of Mineralogy, 19th ed.; John Wiley & Sons: New York, 1977. (23) Sax, N. I.; Lewis, R. J., Sr. Hawley’s Condensed Chemical Dictionary, 11th ed.; Van Nostrand Reinhold: New York, 1987. (24) Johnson, T. L.; Scherer, M. M.; Tratnyek, P. G. Environ. Sci. Technol. 1996, 30, 2634-2640. (25) Carmen, P. C. Trans. Inst. Chem. Eng. 1937, 15, 150-166.
Received for review September 27, 2002. Revised manuscript received February 14, 2003. Accepted March 5, 2003. ES0209565
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