Environ. Sci. Technol. 1990, 2 4 , 676-683
Pierson, W. R.; Brachaczek, W. W. Enuiron. Sci. Technol. 1983, 17, 757-760. Gorse, R. A., Jr. Enoiron. Sci. Technol. 1984, 18, 500-507. Schlaug, R. N.; Carlin, T. J. Aerodynamicsand Air Quality Management of Highway Tunnels. NTIS No. PB80-143803; Report No. FHWA-RD-78-185;Federal Highway . Administiation, Washington, DC, 1979. Schlaug, R. N. Users Guide for the TUNVEN and DUCT Programs. NTIS No. PB80-141575;Report No. FHWARD-78-187;Federal Highway Administration,Washington, DC, 1980. Chang, T. Y.; Modzelewski,S. W.; Norbeck, J. M.; Pierson, W. R. Atmos. Enuiron. 1981, 15, 1011-1016. Regulatory Support Document Proposed Organic Emission Standards and Test Procedures for 1988 and Later Methanol Vehicle and Engine. U S . EPA Office of Mobile Sources, Ann Arbor, MI, 1986. Nichols, R. J.; Clinton, E. L.; King, E. T.; Smith, C. S.; Wineland, R. J. A View of Flexible Fuel Vehicle Aldehyde Emissions. SAE Paper 881200; Society of Automotive Engineers, Inc., 400 Commonwealth Dr., Warrendale, PA, 1988.
(9) Gold, M. D.; Moulis, C. E. Effects of Emission Standards on Methanol Vehicle-Related Ozone, Formaldehyde and Methanol Exposure. APCA Paper No. 88-41.4; Pittsburgh, PA, 1988. (10) Ingalls, M. N. Estimating Mobile Source Pollutants in
Microscale Exposure Situations. EPA-460/3-81-021;U.S. EPA, Ann Arbor, MI, 1981. Ingalls, M. N.; Garbe, R. J. Ambient Pollutant Concentrations from Mobile Sources in Microscale Situations. SAE Paper 820787; Society of Automotive Engineers, Inc., 400 Commonwealth Dr., Warrendale, PA, 1982. (11) Gorse, R. A., Jr.; Norbeck, J. M. J.Air Pollut. Control Assoc. 1981, 31, 1094-1096. (12) Traynor, G. W.; Anthon, D. W.; Hollowell, C. D. Atmos.
Environ. 1982,16, 2979-2987.
(13) Nazaroff, W. W.; Cass, G. R. Enuiron. Sci. Technol. 1986, 20, 924-934.
(14) U.S. EPA, Fed. Regist. 40 CFR Part 86, April 11, 1989. (15) US.EPA, Fed. Regist. 42 FR 32954, June 28, 1977. Received for review August 10, 1989. Accepted December 11, 1989.
Effect of Soil Moisture on the Sorption of Trichloroethene Vapor to Vadose-Zone Soil at Picatinny Arsenal, New Jerseyt James A. Smith* US. Geological Survey, 810 Bear Tavern Rd., Suite 206, West Trenton, New Jersey 08628
Cary T. Chiou, James A. Kammer, and Daniel E. Kite U S . Geological Survey, Box 25046, MS 407, Denver Federal Center, Denver, Colorado 80225
This report presents data on the sorption of trichloroethene (TCE) vapor to vadose-zone soil above a contaminated water-table aquifer at Picatinny Arsenal in Morris County, NJ. To assess the impact of moisture on TCE sorption, batch experiments on the sorption of TCE vapor by the field soil were carried out as a function of relative humidity. The TCE sorption decreases as soil moisture content increases from zero to saturation soil moisture content (the soil moisture content in equilibrium with 100% relative humidity). The moisture content of soil samples collected from the vadose zone was found to be greater than the saturation soil-moisture content, suggesting that adsorption of TCE by the mineral fraction of the vadose-zone soil should be minimal relative to the partition uptake by soil organic matter. Analyses of soil and soil-gas samples collected from the field indicate that the ratio of the concentration of TCE on the vadose-zone soil to its concentration in the soil gas is 1-3 orders of magnitude greater than the ratio predicted by using an assumption of equilibrium conditions. This apparent disequilibrium presumably results from the slow desorption of TCE from the organic matter of the vadose-zone soil relative to the dissipation of TCE vapor from the soil gas.
Introduction From 1960 to 1981, wastewater from metal-plating and degreasing operations was discharged into two unlined 'The use of trade names in this report is for identification purposes only and does not constitute endorsement by the U S . Geological Survey. 676
Environ. Sci. Technol., Vol. 24, No. 5, 1990
wastewater lagoons and an unlined overflow dry well adjacent to building 24 (Figure 1) at Picatinny Arsenal in Morris County, NJ. As a result, the unconfined sand and gravel aquifer that underlies the site has been contaminated by several chlorinated organic compounds. The plume of groundwater contamination extends from building 24 to Green Pond Brook. The major component of the organic contamination in the groundwater is trichloroethene (TCE); the approximate areal distribution of the TCE is shown by the isoconcentration lines in Figure 1, which are based on an extensive groundwater sampling program conducted in October-November 1987. Details of the groundwater contamination, as well as additional information on site hydrology, lithology, and source contamination history are reported elsewhere (1-6). This study investigates the potential effect of soil-moisture content on the sorption of TCE vapor by vadose-zone soil above the groundwater solute plume a t Picatinny Arsenal in order to gain a better understanding of the system parameters mediating the dynamics of vapor movement in the vadose zone. This study also describes the relation between the concentrations of TCE in the soil gas and underlying shallow groundwater along the main axis of the plume.
Background Recent scientific evidence indicates that natural soil functions as a dual sorbent for the uptake and release of nonionic organic compounds (7-9). The mineral surfaces of the soil function as a conventional solid adsorbent and the soil organic matter functions as a partition medium. Mineral adsorption is characterized by vapor or solute condensation onto the mineral surface by physical and/or
Not subject to US. Copyright. Published 1990 by the American Chemical Society
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2b Figure 1. Map of study area showing soil, groundwater, and soil-gas sampling sites, along with depth-averaged groundwater isoconcentration lines for trichloroethene.
chemical bonding forces, whereas the partition uptake by soil organic matter is effected by molecular forces common to solution, similar to the extraction of an organic solute by an organic solvent such as 1-octanol. Soil-moisture content is a critical factor in determining the relative contributions of mineral and organic soil fractions to soil uptake of nonionic organic compounds (7, 9). In aqueous systems, sorption of nonionic organic compounds to soil is controlled by partition into soil organic matter, as supported by linear isotherms, dependence of soil uptake on organic matter content, noncompetitive sorption between solutes, and low heats of sorption (9-11). The predominance of the partition uptake by organic matter in water-saturated soil presumably is a result of the suppression of mineral adsorption of organic com-
pounds by a much stronger competitive adsorption of water on the soil-mineral surfaces. By contrast, in the absence of a polar solvent such as water, the sorption of a nonionic compound by soil is caused largely by adsorption onto minerals. For example, the uptake of parathion by dry soil from hexane (a relatively nonpolar solvent) exhibits competitive sorption, nonlinear isotherms, and relatively high heats of sorption (12, 13). Similarly, the sorption of nonionic organic compounds to dry soil from the vapor phase has been shown to decrease with increasing soil-moisture content (7, 14-16). Given these observations, it appears that contaminant sorption to river sediment or to soil below a groundwater table can be described by means of a partition model, because sufficient water is present to prevent significant Environ. Sci. Technol., Vol. 24, No. 5, 1990 877
solute adsorption on mineral surfaces. However, it is unclear whether this condition can be met in the prediction of contaminant sorption to soil from the vadose zone because the moisture content of subsurface soil varies from land surface to the water table. For example, the mineral surfaces of a surficial soil in close contact with the atmospheric humidity are not likely to be water-saturated. As a result, contaminant uptake by adsorption may be important relative to uptake by partition for this soil, whereas the adsorptive effect of minerals will be relatively insignificant for the water-saturated subsurface soils in close proximity to the water table. Because the transport of contaminants in the vapor phase can cause the migration of both soil-gas and groundwater contamination, accurate description of the vapor's sorption to the soil in the vadose zone is intrinsic to predicting the ultimate fate of the contaminant. This problem was explored by quantifying the effect of soil moisture on TCE vapor sorption to soil. Then, the degree of saturation of the mineral surfaces of vadose-zone soil from Picatinny Arsenal was determined and a predictive expression was developed to define equilibrium conditions for TCE vapor sorption. Finally, the TCE vapor-soil distribution in the field was quantified and compared to the predicted equilibrium distribution. In addition, data that describe the vertical concentration profile of TCE vapor in the vadose-zone soil gas and their relation to shallow groundwater contamination are presented.
Materials and Methods Sample Collection. Soil samples used in this study were collected from the vadose zone at locations 1-4 and 7 in Figure 1. The samples were collected with either a motor-driven hollow-stem auger or a split-spoon sampler manually driven to the desired depth. Soil-gas samples were removed from the vadose zone by using probes constructed of 3-mm-i.d. stainless steel tubes cut to the desired probe depth. The bottom 6 cm of each probe was slotted and covered with a 60-mesh (0.25-mm) stainless-steelscreen. Nests of three probes were installed in the vadose zone at Picatinny Arsenal by augering a 5-cm-diameter hole to the desired sampling depth. The deepest vapor probe was inserted into the hole, and general-purpose sand was added to surround the screen. Both an intermediate- and a shallow-depth probe subsequently were added to the borehole. A 30-cm layer of bentonite was then placed between the screened intervals of nested probes to minimize vertical transport of soil gas during sampling. At land surface, the vapor probes were fitted with Swagelok connectors and caps to maintain a gas-tight seal. Nests of vapor probes were installed at locations 1-6 in Figure 1at depths ranging from 0.46 to 3.1 m below land surface. Approximately 2 months elapsed between the installation of the probes and the first soil-gas sampling and analysis. Gas samples were collected in 125-mL glass sampling bulbs with Teflon stopcocks at both ends. Initially, laboratory studies were conducted to determine if the Teflon stopcocks caused a significant reduction in the concentration of TCE in the sampling bulbs relative to bulbs with glass stopcocks. No significant difference ( p = 0.05) was observed, and the bulbs with Teflon stopcocks were chosen because of their ease of disassembly for cleaning and their durability in the field. Gas samples from the field were collected by drawing soil gas from the probes simultaneously through two of the sampling bulbs. The bulbs were connected in parallel to the vapor probe by a glass Y-joint. A peristaltic pump 678
Environ. Sci. Technol., Vol. 24, No. 5, 1990
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50 100 150 200 250 300 350 400 450 500 550
DISTANCE FROM BUILDING 24 (B24),METERS
Figure 3. Cross section showing site topography, soil-gas and water sampling locations, trichloroethene concentrations in soil 9 s and water, and water-table position. The water-table position and trichloroethene concentrations in parentheses were measured in March 1988. All other concentrations were measured in February 1988.
induced gas flow through each bulb at a rate of approximately 225 mL/min. After lo00 mL of soil gas had passed through the bulbs, the effluent stopcocks on the sampling bulbs were closed. Three to four seconds later, the influent stopcocks were also closed. This procedure was adapted to prevent the development of pressures less than atmospheric inside the sampling bulb. Pressure inside the sampling bulb was periodically measured through the bulb's septum port with a water manometer, and the measured pressures were consistently within 5% of atmospheric pressure. Immediately following the sample collection, a gaseous field surrogate (bromochloromethane) was added by syringe to each bulb through a septum port on each bulb's side. The gas samples subsequently were stored in the dark at approximately 20 "C during transportation to the laboratory for same-day analysis. A schematic diagram of the sampling configuration is shown in Figure 2. Shallow groundwater samples were collected at sites 1-6 (Figure 1)from permanently installed stainless-steelprobes (identical with the probes used for vapor sampling) and a peristaltic pump fitted with Teflon tubing. The depth of the water table below land surface at the site ranged
from 1.5 to 3.7 m; the groundwater samples were collected through probes screened 0.5-2 m below the water table. Figure 3 shows the vertical location of soil-gas and water probes along with the site topography and water-table position. Laboratory Sorption Isotherms. Three types of sorption isotherms were generated in the laboratory with soils collected from Picatinny Arsenal: a single-vapor isotherm, a dual-vapor isotherm, and a water isotherm. Single-vapor isotherms were used to describe the sorption of TCE vapor or water vapor onto the dry vadose-zone soil. These isotherms were generated by using a static sorption chamber described by Chiou et al. (8). Briefly, either water vapor or TCE vapor (previously purified by vacuum distillation) was introduced into the sorption chamber containing 100-200 mg of vacuum-dried soil on an electrical microbalance. The soil samples were passed through a 2-mm sieve and mixed thoroughly prior to analysis to ensure a homogeneous, representative sample. The mass of vapor sorbed to the soil was determined by the increase in the weight of the soil sample a t equilibrium. The corresponding vapor pressure of TCE or water in the system was measured with a Baratron pressure gauge. A dual-vapor isotherm was used to quantify the sorption of TCE vapor to vadose-zone soil at 40% relative humidity (RH). A stream of nitrogen gas containing TCE and water vapor was passed over 1-2 g of soil until equilibrium was reached. The soil sample then was extracted with a 1:l (by volume) mixture of hexane and acetone. Following dilution in hexane, the extract was analyzed by direct-injection gas chromatography using a flame-ionization detector. The gaseous concentration of TCE was measured by passing the gas stream through two microbubble impingers containing hexane, followed by gas chromatographic analysis of the hexane solution. The relative humidity was measured by recording the weight change of a glass tube containing 2-3 g of magnesium perchlorate as a function of total gas-flow rate and trapping time. Details of this experimental procedure have been described by Chiou and Shoup (7). Sorption of TCE from water to soil (a water isotherm) was quantified by using the conventional batch-equilibration method. Known masses of soil, water, and TCE were combined in 25-mL centrifuge tubes and were shaken at constant temperature (25 "C) for 24 h. The tubes were centrifuged at 3000g (g = 9.81 m/s2) for 45 min, and the resulting supernatant was extracted with hexane. The extract then was analyzed by direct-injection gas chromatography using a flame-ionization detector. The concentration of TCE on the soil in each centrifuge tube was determined by subtracting the mass of TCE in solution from the total mass of TCE. Water, Soil, and Vapor Analyses. Soil-gas samples were analyzed in the laboratory for the presence of TCE by purge-and-trap gas chromatography. The entrance and exit ports of the sampling bulb were attached to a purge-and-trap concentrator by a Teflon line and a heated nickel line, respectively. After the stopcocks on the sampling bulb were opened, any TCE in the soil-gas sample was displaced from the bulb onto the adsorbent trap by the flow of purge gas (helium). Approximately 560 mL of helium was passed through the sampling bulb at a rate of 40 mL/min. This purge volume consistently was found to displace greater than 99% of the TCE from the sampling bulb onto the adsorbent trap (17). The TCE was subsequently desorbed from the trap onto a 60-m by 0.75-mm-i.d. capillary column enclosed in a gas chromatograph. The concentration of TCE vapor was quantified
by using a calibrated Hall electrolytic conductivity detector (HECD). The large sample size (125 mL) discharged to the adsorbent trap, in combination with the sensitivity of the HECD for halogenated organic compounds, yielded a quantitation limit of 40 ng/L. Percent recovery of the field surrogate, bromochloromethane, typically was between 90 and 110%. Analyses of soil-gas samples with surrogate recoveries less than 80% were discarded. Information on the preparation of gaseous calibration standards, analytical method precision, sample holding time, additional chromatographic conditions, and purge volumes is given by Kammer and Smith (17). The presence and concentration of TCE in groundwater samples were positively determined by purge-and-trap concentration with capillary gas chromatography/mass spectrometry followed by comparison of mass spectra with authentic standards (18). Soil samples collected for moisture content analysis were transported to the laboratory (in an ice-filled cooler) in either sealed glass jars or as undisturbed cores capped at each end to prevent moisture loss. Because some of these samples were also used to generate water vapor sorption isotherms, it was important to obtain a homogeneous sample. Therefore, prior to moisture content analysis, the samples were passed through a 2-mm sieve, and 20-30 g of each sieved sample was placed in a preweighed evaporation dish. The dishes containing the samples were heated to a constant weight a t a temperature of 105 f 5 "C. The percent moisture content was calculated on the basis of the dry weight of the soil sample. Analysis of soil samples for TCE was performed in accordance with the procedure described by Sawhney et al. (19). Soil samples were transported to the laboratory as undisturbed cores in an ice-fiiled cooler. At the laboratory, each core was transferred to a glass jar and shaken vigorously to homogenize the soil core. Approximately 35 g of soil was removed from the jar and combined with 25 mL of methanol in a 40-mL centrifuge tube with a Teflon-lined septum cap. The tube was shaken vigorously on a vortex mixer for 60 s, inverted, and then incubated a t 75 f 5 "C for 36 h. Halfway through the incubation period, the tubes were cooled to room temperature, shaken again for 60 s, and returned to the incubator. A t the end of the incubation period, the samples were centrifuged a t 2000g for 60 min, and 10 mL of the supernatant was combined with 100 mL of water and 30 mL of hexane in a 200-mL flask. The mixture was shaken vigorously for 60 s and the phases were allowed to separate. The hexane phase was analyzed by direct-injection gas chromatography using an electron-capture detector. The moisture content and organic carbon content of each soil sample also were determined.
Results and Discussion Figure 4 presents two TCE vapor-soil isotherms (0 and 40% RH) and one TCE water-soil isotherm (100% RH) for surficial soil collected from site 2 (Figure 1). The soil sample has an organic carbon content of 4.02% and is 60% sand, 26.5% silt, and 13.5% clay. The predominant clay mineral is dioctahedral chlorite. The dependent variable in Figure 4 is soil uptake, in milligrams of TCE per gram of soil. The independent variable for the 0 and 40% R H isotherms is the vapor pressure of TCE, P, normalized to the saturation vapor pressure, Po. The independent variable for the 100% RH isotherm is the aqueous concentration of TCE, C, normalized to its aqueous solubility, S. The data normalization facilitates comparison of vapor-soil isotherms and water-soil isotherms. Saturation vapor pressures were determined as a function of temperature by plotting existing data for log PO (the Environ. Sci. Technol., Vol. 24, No. 5, 1990
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Table I. Comparison between Field- and Saturation Soil-Moisture Contents (SSMC) of Six Soil Samples Collected from t h e Vadose Zone
0-percent relative humidity 40-percent relative humidity 100-percent 100-percent relative relative humidity humidity (from water)
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depth, m
03/4/87 03/4/87 07/30/87 08/3/87 08/6/87 12/3/87
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Flgure 4. Soil uptake of trichloroethene vapor as a function of the vapor's partial pressure (P)normalized to saturation vapor pressure (Po) at 0 and 40% RH, and soil uptake of trichloroethene from water as a function of its aqueous concentration (C) normalized to its aqueous solubility (S). The soil sample was collected at site 2 from a depth of approximately 0.0-0.01 m.
saturation vapor pressure, in mmHg) against 1 / T (the inverse of absolute temperature) (20). The resulting Clausius-Clapeyron relation gives log Po = -1817.6(1/T) 7.957 (1)
+
which allows PO to be determined accurately at each temperature. The aqueous solubility of TCE at 25 "C is 1100 mg/L (21). The isotherms in Figure 4 illustrate that as relative humidity increases, the soil uptake of TCE decreases and the isotherm shapes become more linear. At 0% RH, soil uptake of TCE appears to be caused mainly by adsorption onto mineral surfaces, as evidenced by the similarity of the isotherm's shape to that of a conventional Brunauer type-I1 adsorption isotherm (22). At higher relative humidity, the strong adsorptive competition of water for the soil-mineral surface reduces TCE adsorption by the minerals. At 100% RH, TCE uptake by soil is in all likelihood predominated by partition into the soil organic matter. The data shown in Figure 4 agree with the data of Spencer et al. (14) and Spencer and Cliath (16),who showed that the vapor densities of dieldrin and lindane decreased when the soil moisture was decreased below the saturation level (e.g., below the moisture content corresponding to 100% RH). Chiou and Shoup (7) demonstrated similar moisture effects for the vapor sorption of benzene, m-dichlorobenzene,and 172,4-trichlorobenzenein their discussion of the respective roles of minerals and organic matter in soil sorption. The isotherms in Figure 4 help explain the sorptive behavior of volatile organic compounds that are transported upward into the vadose zone from a contaminated water-table aquifer. Near the water table, the mineral surfaces of the soil are likely to be saturated with water, and vapor sorption should be limited to uptake by partition into the soil organic matter; the equilibrium distribution would therefore follow the 100% RH isotherm (9). By contrast, soil within a few millimeters of land surface may be expected to have below-saturation moisture content and therefore exhibit greater sorption of a nonionic contaminant because of the adsorptive effect of minerals (provided that the soil composition remains relatively unchanged) (9). The 40% RH isotherm is an example of equilibrium vapor uptake by a water-unsaturated soil as a function of the ambient humidity. Although TCE sorption to soil in equilibrium with 0% RH may not be relevent in actual field situations. these data demonstrate 680
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Flgure 5. Water vapor uptake by two soils collected from site 2 as a function of the vapor's partial pressure (P)normalized to saturation vapor pressure (Po).
the sensitivity of the sorption process to moisture. The results in Table I and Figure 5 provide additional information about the sorptive behavior of TCE throughout the vadose zone and above the contaminant plume at Picatinny Arsenal. Table I compares the fieldmoisture content with the saturation soil-moisture content (SSMC) of six soil samples collected from the vadose zone over a 9-month period. The field-moisture content is the mass of water per unit mass of soil in the vadose zone and was determined by the difference between the soil sample's wet and dry weight. The SSMC for each soil sample was determined by extrapolating a laboratory-generated water-vapor isotherm for the soil to the soil-moisture content corresponding to PIP" = 1.0. Because the vapor phase is saturated with water vapor and is in equilibrium with the soil at P / P = 1.0, the soil also is saturated with water vapor. Examples of water-vapor isotherms for two of the soils in Table I are shown in Figure 5. At PIP" = 1.0, the SSMC of the shallow soil is approximately 60 mg/g, or 6 % ; the SSMC of the deeper soil is approximately 10 mg/g, or 1%. The difference between the SSMC of these two soil samples probably is caused by differences in mineral type and content of the soils. The soil used for the upper isotherm in Figure 5, for example, has a higher clay and silt content than the soil used for the lower isotherm in the figure. The upper isotherms in Figures 4 and 5 compare the uptake of TCE and water vapor by the same soil sample. The isotherms indicate that water vapor sorbs strongly to the soil (presumably by adsorption on minerals) relative to TCE. This observation lends further support to the hypothesis that increasing soil moisture decreases adsorption of TCE onto soil-mineral surfaces. The data in Table I show that the field-moisture content of each soil sample is greater than its corresponding SSMC. Application of the nonparametric sign test to the data in
550-
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TRICHLOROETHENE CONCENTRATION, IN MICROGRAMS PER LITER
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Table I confirms that the moisture content of vadose-zone soil above the contaminant plume is greater than SSMC at a 95% confidence level. This result, in combination with the observed suppression of TCE sorption by moisture (Figure 4), suggests that sufficient moisture is present in the vadose zone to limit TCE uptake to partition into the soil organic matter. Therefore, equilibrium TCE sorption can be approximated by using the aqueous-phase isotherm (Figure 4) along with the Henry's law constant of the organic contaminant. I t is important to point out that the amount of moisture required to saturate the mineral surfaces of the soil (and therefore to displace organic vapors from minerals) is far less than required to occupy the void space of soil particles. Therefore, @thoughthe porous media may be considered "unsaturated" (on the basis that air occupies some fraction of the pore space), the mineral surfaces of the soil are actually saturated with water molecules. In this study, the handling of soil samples prior to moisture content analyses may have resulted in a small loss of moisture by evaporation; however, this would only cause an underestimation of the soil-moisture content and therefore would not change the conclusion that the fieldmoisture content is greater than SSMC. The large fieldmoisture content for the surface soil sample (0.0-0.01-m depth) in Table I is worth noting. This sample was collected while there was melting snow on the ground. During dry conditions, the moisture content of surficial soil may be expected to be less than saturation. Prior to the measurement of the vaporsoil distribution of TCE in the field, a reconnaissance of the vertical and areal concentrations of TCE in the soil gas in the vadose zone and underlying shallow groundwater was conducted. Some of the results of this reconnaissance are shown in Figures 3 and 6. Figure 3 presents the concentrations of TCE in soil-gas samples collected during February 1988 and the concentrations of TCE in soil-gas and underlying groundwater samples collected during March 1988. The water-table position corresponds to the data from March. The deepest sampling point at each of the six sites was used to collect water samples. In general, changes in the concentration of TCE in the shallow groundwater closely parallel changes in the concentration of TCE in the soil gas. These data support the use of sail-gas analyses to delineate shallow groundwater contamination. Figure 6 presents the variation of the vertical concentration of TCE in soil gas sampled at site 3 during February, July, October, and December 1988. Based on the sample analysis, the TCE vapor concentration was found to decrease with distance above the water table and this
c
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-
Predicted equilibrium TCE distribution 10
0
0.0
Figure 6. Depth profile of trichloroethene (TCE) in soil-gas samples collected from site 3. Values in parentheses identify the distance between the water table and land surface at the time of the sampling.
Field TCE distribution
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reduction appears to be linear and in accord with a diffusion-dominated transport phenomenon in a relatively homogeneous soil (23,24). It is interesting to note that extrapolation of the lines in Figure 6 to the vertical axis indicates that the TCE vapor concentration approaches zero at 0.2-0.6 m below land surface. Indeed, TCE was not detected in any soil-gas sample collected within 0.75 m of land surface during the February sampling. The temporal variation of the vertical concentration profile of TCE at site 3 is at least partly influenced by the temperature variation of the shallow groundwater. Increased groundwater bmperatures cause an increase in the TCE vapor pressure and, therefore, an increase in the steady-state concentration profile of TCE in the vadosezone soil gas. For the February, July, October, and December sample collection periods, the respective shallow groundwater temperatures were 8.9, 16.0, 13.2, and 9.3 "C. Therefore, increased water temperatures in July and October generally resulted in increased concentrations of TCE in the soil gas. Finally, Figure 7 compares the distribution of TCE between soil gas and soil organic carbon observed in the field to the distribution predicted from a conventional aqueous-phase isotherm along with the water solubility and saturation vapor pressure data for TCE. The field TCE distribution in Figure 7 (upper plot) was quantified by first sampling the soil gas at sites 1-3 in duplicate. Next, soil samples were collected from the same depths and at distances of approximately 0.5 m from the points of collection of the gas samples. The concentration of TCE in the gas and soil samples then was quantified. The soil-gas concentrations were converted to partial pressures (in mmHg) by using the ideal gas law and then were normalized to saturation vapor concentrations. Saturation vapor concentrations were calculated as a function of temperature in the vadose zone at the point of soil-gas collection by use of eq 1. In addition, the moisture and organic carbon content of the soil samples were measured. The concentration of TCE on the soil is expressed on a dry-weight basis and is normalized to the soil organic carbon content. The mass of TCE dissolved in the aqueous phase of the soil sample as calculated from the Henry's law constant of TCE and the concentration in the gas phase was less than 3% of the total mass of TCE sorbed to the sample. Therefore, the concentration of TCE in the soil samples was not adjusted to account for the mass of TCE in the aqueous phase. The predicted equilibrium TCE distribution in soil in Figure 7 (lower plot) is taken from the 100% RH isotherm Environ. Sci. Technol., Vol. 24, No. 5, 1990
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depicted in Figure 4 with normalization to the soil's organic carbon content (4.02%). This predicted distribution corresponds to a soil-organic carbon-water partition coefficient (K,) of 63, which is comparable to previously reported values (25, 26). In considering the vertical TCE distribution in the vadose zone, it is important to recognize that, in such a nonclosed system, the TCE vapor may be far from being in sorption equilibrium with the vadose-zone soil. The extent of this disequilibrium is presumably a function of how rapidly the TCE vapor is sorbed and desorbed by the soil and the rate the vapor moves out of the vadose zone into the atmosphere. It is evident from the discontinuous vertical scale in Figure 7 that the field TCE distribution between the soil and soil gas is from 1 to 3 orders of magnitude greater than the distribution predicted from an equilibrium aqueous-phase isotherm. This disequilibrium suggests that TCE vapor is subjected to a kinetically slow desorption relative to the rate at which TCE vapor is dissipated by vapor loss to the atmosphere and degraded by other processes. Therefore, a possible explanation for the disequilibrium may be the kinetically slow desorption of TCE from soil that has a history of gross contamination. In 1981, the discharge of TCE-containing wastewater to the unlined lagoons and overflow pit adjacent to building 24 was discontinued and the lagoons and pit were excavated. However, post-1982 groundwater monitoring at the site shows little evidence that TCE concentrations have decreased at the site, despite the continued influx of relatively pristine groundwater (27). This observation, and the results shown in Figure 7, suggest that slow TCE desorption from soil contaminated by more than 20 years of wastewater discharge continues to contaminate groundwater and soil gas at the site. The rate of TCE desorption from the soil is not great enough to allow the attainment of an equilibrium condition with the incoming groundwater and, consequently, with the overlying soil gas. This slow desorption phenomena has been documented elsewhere. For example, Steinberg et al. (28)observed that 1,2-dibromoethane (EDB) in agricultural soil (subjected to years of exposure to the soil fumigant) is highly resistant to desorption into water and to volatilization. Similarly, Pignatello et al. (29) studied the fate of EDB in vadosezone soil from a former tobacco field in Connecticut. Although EDB was last applied to the field in 1967, they quantified EDB concentrations as high as 32 pg/kg 20 years later. They noted that the EDB could not be extracted from the soil samples with water over a 20-day period and that the EDB was not available for biodegradation. However, more than 80% of added [14C]EDBwas degraded over a 22-day period. Sawhney et al. (19)studied the effectiveness of different extraction methodologies on field soil that had previously been fumigated with EDB. They observed that only the most extreme extraction methodologies (e.g., 24-h methanol extraction at 75 OC) produced high EDB recoveries. However, all the tested extraction methodologies were satisfactory when applied to uncontaminated soil that was freshly spiked with EDB in the laboratory. They also noted that air-drying of the field soil did not cause a measurable loss of sorbed contaminant. These results indicate that the sorbed contaminant may be highly resistant to desorption and that the rate of contaminant desorption decreases with increasing time of exposure to the contaminant. Hysteretic desorption has also been reported for a variety of halocarbons, including TCE (30,31). Soil-water mixtures that were spiked with TCE and allowed to equilibrate for a 57-day period were then sequentially 882
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extracted 16 times with deionized water. The final sorption coefficient was approximately 200 times greater than predicted by using an assumption of equilibrium conditions (30). Also, the slowly desorbing contaminant fraction of soil samples with different contaminant histories was found to increase nonlinearly with sorption time and applied concentration (31). These results suggest that long-term contamination produces a fraction of the sorbed contaminant that is relatively resistant to desorption and may offer a possible explanation for the disequilibrium observed in the vadose zone at Picatinny Arsenal.
Summary Soil-moisture content suppresses sorption of TCE vapor to soil up to SSMC (saturation soil-moisture content), presumably by preferential adsorption of water to the soil-mineral surfaces. Above SSMC, soil uptake of TCE vapor is attributed largely to partition into the soil organic matter. At Picatinny Arsenal, sufficient moisture is present in the vadose zone above the contaminant plume to minimize TCE adsorption to the mineral surfaces of the soil. In general, the concentration of TCE in vadose-zone soil gas at Picatinny Arsenal parallels the concentration of TCE in shallow groundwater. The concentration of TCE in the soil gas also decreases linearly with depth above the water table, suggesting that the transport of TCE in the vadose zone is caused primarily by molecular diffusion. The distribution of TCE between the soil gas and soil was measured in duplicate at six locations. The results show a strong disequilibrium in which the TCE concentration on the soil phase is greater than the concentration predicted by use of water isotherm data that accounts for soil organic carbon and assumes an equilibrium condition. The discrepancy between predicted and observed results may be caused by slow TCE desorption from a soil previously exposed to high levels of contamination relative to the rate of TCE dissipation from the soil gas by diffusion to the atmosphere or other degradation processes. Registry No. Trichloroethene, 79-01-6. Literature Cited Fusillo, T. V.; Ehlke, T. A.; Martin, M. Open-File Rep.U.S. Geol. Surv. 1987, No. 87-395. Imbrigiotta,T. E.; Martin, M. Water-Resour. Inuest. Rep. (US.Geol. Surv.) 1988, No. 88-4220, 343-350. Imbrigiotta,T. E.; Martin, M.; Sargent, B. P.; Voronin, L. M. Water-Resour. Invest. Rep. (U.S. Geol. Surv.) 1988, No. 88-4220, 351-360.
Martin, M. Water-Resour. Invest. Rep. ( U S . Geol. Suru.) 1988, NO.88-4220, 377-384. Wilson, S. A.; Taggart, J. E. Water-Resour. Invest. Rep. (U.S. Geol. Suru.) 1988, No. 88-4220, 385-388. Wilson, B. H. Water-Resour.Invest. Rep. ( U S . Ceol. Surv.) 1988, N O .88-4220, 389-396. Chiou, C. T.; Shoup, T. D. Enuiron. Sci. Technol. 1985,19, 1196-1 200.
Chiou, C. T.; Kile, D. E.; Malcolm, R. L. Enuiron. Sci. Technol. 1988,22,298-303. Smith, J. A.; Witkoweki, P. J.; Chiou, C. T. Rev. Environ. Contamin. Toxicol. 1988, 103, 127-151. Chiou, C. T.; Peters, L. J.; Freed, V. H. Science 1979,206, 831-832.
Smith,J. A.; Witkowski, P. J.; Fusillo, T. V. U S . Geol. Suru. Circ. ( U S . ) 1988, No. 1007. Yaron, B.; Saltzman, S. Soil Sci. SOC.Am. Proc. 1972,36, 583-586.
Chiou, C. T.; Shoup, T. D.; Porter, P. E. Org. Geochem. 1985, 8, 9-14.
Spencer, W. F.; Cliath,M. M.; Farmer, W. J. Soil Sci. SOC. Am. Proc. 1969, 33, 509-511.
Environ. Sci. Technol. 1990,2 4 , 683-688
Spencer, W. F.; Farmer, W. J.; Cliath, M. M. Res. Rev. 1973,
National Water Well Association: Dublin, OH, 1986; pp
49, 1-40.
420-441.
Spencer, W. F.; Cliath, M. M. Soil. Sci. SOC.Am. R o c . 1970,
Marrin, D. L.; Kerfoot, H. B. Enuiron. Sci. Technol. 1988,
34, 574-578.
22, 740-745.
Peterson, M. S.; Lion, L. W.; Shoemaker, C. A. Environ. Sci. Technol. 1988,22, 571-578. Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983, 17, 227-231. Sargent, B. P.; Green, J. W.; Harte, P. T.; Vowinkel, E. F. Open-File Rep.-U.S. Geol. Surv. 1986, No. 86-58. Steinberg, S. M.; Pignatello,J. J.; Sawhney,B. L. Enuiron. Sci. Technol. 1987,21, 1201-1208. Pignatello, J. J.; Frink, C. R.; Marin, P. A.; Droste, E. X. J . Contam. Hydrol., in press. Pignatello, J. J. Environ. Toxicol. Chem., in press. Pignatello, J. J. Environ. Toxicol. Chem., in press.
Kammer, J. A,; Smith, J. A. Water-Resour. Invest. Rep. (US.Geol. Surv.) 88-4220 1988, No. 88-4220, 617-624. Wershaw, R. L.; Fishman, M. J.; Grabbe, R. R.; Lowe, L. E. US.Geological Survey Techniques of Water-Resources Investigations; U.S. Geological Survey: Washington, DC, 1982; Book 5, Chapter A3. Sawhney,B. L.; Pignatello,J. J.; Steinberg,S. M. J. Environ. Qual. 1988,17, 149-152.
Weast, R. C., Ed. Handbook of Chemistry and Physics; CRC Press, Inc.: Boca Raton, FL, 1988-1989. Dilling, W. L. Environ. Sci. Technol. 1977, 2 1 , 405-409. Adamson, A. W. Physical Chemistry of Surfaces; John Wilev and Sons: New York, 1976. Brueil, C. J.; Hoag, G. E. Proceedings of Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection and Restoration; Houston, TX;
Received for review December 5, 1988. Revised manuscript received November 27, 1989. Accepted December 20, 1989.
Study of Copper( I I ) Association with Dissolved Organic Matter in Surface Waters of Three Mexican Coastal Lagoons Anne M. Hansen,*st James 0. Leckie,**$Enrique F. Mandelli,* and R. Scott Altmannt
Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, California 94305 Copper binding by dissolved organic matter has been evaluated by potentiometric titrations on surface waters from three Mexican coastal lagoons. A copper-selective electrode technique was utilized to measure cupric ion activity. A model that accounts for the variation in binding intensity as a function of the degree of surface loading was employed to calculate the binding constants of the complex formation between cupric ion and the organic ligands in solution. Small amounts of strongly complexing ligands were found to be present in the dissolved organic fraction. These naturally occurring ligands may be responsible for the availability of some micronutrients and for the inactivation of toxic heavy metals in the studied lagoons.
Introduction Increased understanding of the composition and the concentration of dissolved organic ligands in natural waters and ligand interaction with metal ions has been the focus of numerous studies (1-5). Organic metal complexing substances have two principal sources: humic substances derived from the chemical and microbial decomposition of organic material (6) and extracellular metabolic compounds excreted from algae (7,8). The major functional groups of humic substances include carboxylic acids, phenolic and alcoholic hydroxyl groups, and keto functional groups. The structure of aquatic humic substances is unknown (9). Fulvic acids contain considerably more groups of acidic nature, particularly carboxyl and phenolic OHs, than do humic acids (10-12). Since carboxyl and hydroxyl groups of humic 'Institub de Ciencias Nucleares, Universidad Nacional Auunoma de MBxico, 04510 Mexico City, Mexico. Environmental Engineering and Science, Department of Civil Engineering,Stanford University, Stanford, CA 94305. 8 Instituto de Ciencias del Mar y Limnologia, Universidad Nacional Autdnoma de MBxico, 04510 Mexico City, Mexico.
*
0013-936X190/0924-0683$02.5010
substances are most reactive with cations, fulvic acids have a higher affinity for interactions with metal ions (13,14). It is generally agreed that fulvic acids have two general types of functional groups: salicyclic and dicarboxylic. These general types of sites may be presented in a great variety of combinations, and it is likely that no two carboxyl groups are chemically identical. Therefore, metal complexation by humic and fulvic materials must be associated by not one but a continuum of binding energies (2, 4, 15, 16). The evaluation of interactions between metals and organic ligands present in natural waters has lately received considerable attention. Evidence for strong organic complexation has been demonstrated under controlled experimental conditions with samples from natural environments such as lakes and rivers, and with fulvic and humic acids extracted from soils (17-22). Observations of metal-binding characteristics of humic-type compounds (2),hydrous oxides (23,241,and natural sediments (25)have revealed similarities in behavior. The major similarities are the following: (1)that a variation in binding intensity is exhibited as a function of degree of site occupancy; (2) that the binding intensity for the same solid depends on the cation. This means that strong binding sites for one metal are not necessarily preferred binding sites for other metal ions; and (3) different binding energy sequences are expected for the same cations with respect to different ligand systems. Bresnahan and coworkers (20) observed a different complexing behavior of copper(I1) with soil fulvic acid than with water fulvic acid. Mantoura and colleagues (22) studied dissolved ligands from different sources and found a wide range of values for the complexing energy of any particular metal. In any case, comparison of published results should be done with much care, since the energies of binding vary as a function of the degree of loading of the ligand system. The number of protons released when a metal ion adsorbs varies with the degree of occupation of the ligand
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