Environmental Issues—Persistent Problems and Emerging Challenges

Abstract

A field study was performed to evaluate indoor air concentrations and vapor intrusion (VI) of tetrachloroethylene (PCE) and other chlorinated solvents at a commercial retail site in Dallas, TX. The building is approximately 40 yr old and once housed a dry cleaning operation. Results from an initial site characterization were used to select sampling locations for the VI study. The general approach for evaluating VI was to collect time-integrated canister samples for off-site U.S. Environmental Protection Agency Method TO-15 analyses. PCE and other chlorinated solvents were measured in shallow soil gas, subslab soil-gas, indoor air, and ambient air. The subslab soil gas exhibited relatively high values: PCE ≤2,600,000 parts per billion by volume (ppbv) and trichloroethylene ≤170 ppbv. The attenuation factor, the ratio of indoor air and subslab soil-gas concentrations, was unusually low: approximately 5 x 10^-6 based on the maximum subslab soil-gas concentration of PCE and 1.4 x 10^-5 based on average values.     

Emission rates of chlorinated volatile organics from new and used consumer products found during vapor intrusion investigations: Impact on indoor air concentrations

Consumer products can emit chlorinated volatile organic compounds (CVOCs) that complicate vapor intrusion (VI) assessments. Assessment protocols acknowledge the need to remove these products during VI investigations, but they can be problematic to identify and locate. Predicting if the products cause detectable air concentrations is also difficult since emission rate information is limited and can vary with product use and age. In this study, the emission rates of 1,2-dichloroethane, trichloroethene, tetrachloroethene, and carbon tetrachloride from four consumer products identified as indoor sources during VI field investigations were measured under laboratory conditions using a flow through system. Emissions of PCE from an adhesive container tube ranged from 1.33 ± 1.13 μg/min (unopened) to 23.9 ± 2.93 μg/min (previously opened). The laboratory-measured emission rates were used to estimate indoor air concentrations, which were then compared to concentrations measured after the products placed were into an actual residence. The estimated and measured indoor air concentrations were generally comparable, showing that emission rate information can be used to determine the relative impact of internal CVOC sources.     

U.S. EPA SITE demonstration of AWD Technologies' AquaDetox/SVE system.

Under the Superfund Innovative Technology Evaluation (SITE) Program, a technology developed by AWD Technologies, Inc. was demonstrated in September 1990. This paper presents the major results of the SITE demonstration of AWD Technologies' AquaDetox/SVE treatment system designed for simultaneous on-site treatment of contaminated groundwater and soil-gas. The groundwater and soil at the demonstration site were contaminated with trichloroethylene (TCE) and tetrachloroethylene (PCE). The AWD technology was evaluated on the basis of the removal efficiencies of TCE and PCE from the contaminated groundwater and soil-gas. The conclusions drawn from these evaluations are: (1) the system achieved removal efficiencies as high as 99.99 percent for groundwater and 99.9 percent for soil-gas; (2) the effluent groundwater was in compliance with the regulatory discharge requirements of 5 micrograms/L each for TCE and PCE for all test runs; (3) the demonstrated 1,000 gpm system has an estimated capital cost of $4.3 million and annual operating and maintenance cost of approximately $820,000.     

U.S. EPA SITE Demonstration of AWD Technologies’ AquaDetox/SVE System

Under the Superfund Innovative Technology Evaluation (SITE) Program, a technology developed by AWD Technologies, Inc. was demonstrated in September 1990. This paper presents the major results of the SITE demonstration of AWD Technologies’ AquaDetox®ISVE treatment system designed for simultaneous on-site treatment of contaminated groundwater and soil-gas. The groundwater and soil at the demonstration site were contaminated with trichloroethylene (TCE) and tetrachloroethylene (PCE). The AWD technology was evaluated on the basis of the removal efficiencies of TCE and PCE from the contaminated groundwater and soil-gas. The conclusions drawn from these evaluations are: (1) the system achieved removal efficiencies as high as 99.99percent for groundwater and 99.9 percent for soil-gas; (2) the effluent groundwater was in compliance with the regulatory discharge requirements of 5 fig/L each for TCE and PCE for all test runs; (3) the demonstrated 1,000 gpm system has an estimated capital cost of $4.3 million and annual operating and maintenance cost of approximately $820,000.     

Tropospheric concentrations of the chlorinated solvents,tetrachloroethene and trichloroethene,measured in the remote northern hemisphere

A fully automated twin ECD gas chromatograph system with sample enriching adsorption–desorption primary stage was deployed on two field campaigns – Ny-Ålesund, Svalbard, Arctic Norway (July–September 1997), and the RRS Discovery CHAOS cruise of the northeast Atlantic (April–May 1998). Concentrations of an extensive set of halocarbons were detected at hourly intervals at pptv levels. We present here the results obtained for the chlorinated solvents, tetrachloroethene (PCE) and trichloroethene (TCE). Average baseline PCE and TCE concentrations of 1.77 and 0.12 pptv, respectively, were recorded in Ny-Ålesund. During pollution incidences, concentrations rose to 5.61 (PCE) and 3.18 pptv (TCE). The cruise data showed average concentrations ranging from 4.26 (PCE) and 1.66 pptv (TCE) for air masses originating over the North Atlantic and Arctic open oceans, to maxima of 15.59 (PCE) and 17.51 pptv (TCE) for polluted air masses from Northern Europe. The data sets emphasise the difficulties in defining remote sites for background tropospheric halocarbon measurements, as Ny-Ålesund research station proved to be a source of tetrachloroethene. The data also suggest possible oceanic emissions of trichloroethene in the sub-tropical ocean.     

PCB-blood levels in teachers, working in PCB-contaminated schools

In order to get more information about potential health hazards due to indoor air PCBs the present study investigated the PCB indoor concentration in schools as well as the blood levels of 6 PCB-indicator congeners in teachers from these schools. 151 teachers (78 male and 73 female; mean age 48 years) from 3 contaminated and 2 control schools participated in the study. Maximal indoor air values for total PCBs (6 PCB-indicator congeners times 5) in schools ranged from 1587 to 10655 ng/m3. Blood analyses indicated an increase in mean PCB 28 level from 0.036 (control group) to 0.098 microg/l in teachers from a school with heavy contamination of low chlorinated PCB. But there was no significant increase of PCB 138, 153 and 180 in blood above the normal background concentrations in any of the contaminated schools (mean values of all groups: PCB 138 = 0.66, 153 = 0.95, 180 = 0.70 microg/l blood). The results of blood analyses and additional toxicokinetic calculations suggested that inhalative PCB-uptake in the most contaminated schools caused a minor increase above mean background-PCB concentrations in blood. In conclusion, despite high PCB indoor air levels in schools, there was only a moderate increase in blood concentrations of teachers, mainly due to congeners with low chlorination (PCB 28 to PCB 101).     

Characterizing the occurrence, sources, and variability of radon in Pacific Northwest homes

A compilation of data from earlier studies of 172 homes in the Pacific Northwest indicated that approximately 65 percent of the 46 homes tested in the Spokane River Valley/Rathdrum Prairie region of eastern Washington/northern Idaho had heating season indoor radon (222Rn) concentrations above the U. S. EPA guideline of 148 Bq m-3 (4 pCi L-1). A subset of 35 homes was selected for additional study. The primary source of indoor radon in the Spokane River Valley/Rathdrum Prairie was pressure-driven flow of soil gas containing moderate radon concentrations (geometric mean concentration of 16,000 Bq m-3) from the highly permeable soils (geometric mean permeability of 5 x 10(-11) m2) surrounding the house substructures. Estimated soil gas entry rates ranged from 0.4 to 39 m3h-1 and 1 percent to 21 percent of total building air infiltration. Radon from other sources, including domestic water supplies and building materials was negligible. In high radon homes, winter indoor levels averaged 13 times higher than summer concentrations, while in low radon homes winter levels averaged only 2.5 times higher. Short-term variations in indoor radon were observed to be dependent upon indoor-outdoor temperature differences, wind speed, and operation of forced-air furnace fans. Forced-air furnace operation, along with leaky return ducts and plenums, and openings between the substructure and upper floors enhanced mixing of radon-laden substructure air throughout the rest of the building.     

Contaminated site remedial investigation and feasibility removal of chlorinated volatile organic compounds from groundwater by activated carbon fiber adsorption

Groundwater contaminated by dense, non-aqueous phase liquids (DNAPLs) such as chlorinated solvents has become a serious problem in some regions of Taiwan. The sources of these contaminants are due to industrial discharges. These chlorinated volatile organic compounds (VOCs) have been proven to be carcinogenic to humans. The groundwater is used for domestic drinking water supply in some cities of Taiwan and the severely contaminated groundwater has to be treated in order to meet the requirement of drinking water standards. This study covers two areas of work. In the first part, polluted groundwater samples were collected from the contaminated site and analytical results indicated measurable concentrations of 12 representative chlorinated VOCs in water samples. The primary VOCs detected included trichloroethene (TCE), tetrachloroethene (PCE), 1,1,2-trichloroethane (1,1,2-TCA), and 1,1-dichloroethene (1,1-DCE). Second, to remove VOCs groundwater was treated using adsorption on activated carbon fiber (ACF). This involved pumping groundwater through vessels containing ACF. Most VOCs, including TCE, PCE, 1,1,2-TCA, and DCE, were readily adsorbed onto ACF and are removed from the water stream. Our study showed that the technology was able to significantly reduce chlorinated VOCs concentrations in groundwater.     

Numerical modeling of oxygen exclusion experiments of anaerobic bioventing

A numerical and experimental study of transport phenomena underlying anaerobic bioventing (ABV) is presented. Understanding oxygen exclusion patterns in vadose zone environments is important in designing an ABV process for bioremediation of soil contaminated with chlorinated solvents. In particular, the establishment of an anaerobic zone of influence by nitrogen injection in the vadose zone is investigated. Oxygen exclusion experiments are performed in a pilot scale flow cell (2 x 1.1 x 0.1 m) using different venting flows and two different outflow boundary conditions (open and partially covered). Injection gas velocities are varied from 0.25 x 10(-3) to 1.0 x 10(-3) cm/s and are correlated with the ABV radius of influence. Numerical simulations are used to predict the collected experimental data. In general, reasonable agreement is found between observed and predicted oxygen concentrations. Use of impervious covers can significantly reduce the volume of forcing gas used, where an increase in oxygen exclusion efficiency is consistent with a decrease in the outflow area above the injection well.     

Review of concentrations and chemistry of trichloroacetate in the environment

This paper reviews the concentrations of trichloroacetate (TCA) in the atmosphere-plant-soil system. Data originate mainly from Europe. The median TCA concentration in rainwater and canopy drip decreased until 1995. From then the median TCA concentration in rainwater remains rather constant while for canopy drip later data are not available. The same seems to hold for concentrations in air although a very limited data set is available. The median concentrations in coniferous needles and groundwater are constant for the period observed. The median TCA concentrations in soil decreased until 1992 and then remained constant.The TCA formation from chlorinated solvents in the atmosphere may explain a substantial percentage of the TCA amount in the atmosphere. The TCA concentrations in rainwater and canopy drip indicate that there will be other sources contributing to 10-50%. Waste incineration, biomass burning and natural formation in the marine boundary layer are potential candidate sources of TCA, but nothing can be said as yet on their TCA emission rates. Anthropogenic emissions of chlorine could also be a source.TCA can be formed from chlorinated solvents by biota. However, for coniferous trees the uptake of TCA from soil may be the predominant route. Biotic and abiotic reactions can cause to formation of TCA in soil, but also formation of TCA from chlorinated solvents by biota that excrete TCA, may contribute. Mass balance calculations of the bioactive soil top layer show that the production rate of TCA in certain soil types could be substantial. The mass balance calculations could not distinguish between natural and anthropogenic sources in soil.     

Indoor and outdoor carbon monoxide concentration relationships at different microenvironments in the Athens area

Simultaneous measurements of indoor and outdoor carbon monoxide (CO) concentrations conducted at two different microenvironments in Athens, Greece, using a non-dispersive infrared analyzer, are described in this paper. The two selected microenvironments, an office and a public school, were located in the vicinity of two streets with heavy traffic, near the center of Athens. A statistical correlation analysis of indoor concentration levels with outdoor concentrations monitored at the school and at the office, as well as with meteorological parameters and outdoor concentrations monitored at a fixed monitoring site, was conducted. Hourly indoor concentrations at the office and at the school showed a significant positive correlation with outdoor concentrations measured at both measurement locations (with correlation coefficient values R=0.74 and R=0.83 respectively) and at the fixed site (with R=0.70 and R=0.67 respectively). The correlation between indoor and outdoor concentrations was even better when hourly concentrations averaged over a 4 h period were considered (correlation coefficient values between indoor and outdoor concentrations measured at the office and at the school were R=0.85 and R=0.92 respectively and the correlation coefficient value between indoor and outdoor fixed site concentrations was R=0.75 for both sites). Mean hourly outdoor concentrations at the fixed monitoring site explained approximately 56% (R=0.75) of the variation of outdoor concentrations at the office and approximately 68 % (R=0.83) of the variation of outdoor concentrations at the school. The mean daily indoor to outdoor (I/O) ratio ranged between 0.74 and 1.00 at the office and between 0.53 and 0.89 at the school.     

Continuous monitoring of ultrafine,fine, and coarse particles in a residence for 18 months in 1999-2000

Continuous monitors were employed for 18 months in an occupied townhouse to measure ultrafine, fine, and coarse particles; air change rates; wind speed and direction; temperature; and relative humidity (RH). A main objective was to document short-term and long-term variation in indoor air concentrations of size-resolved particles (0.01-20 microm) caused by (1) diumal and seasonal variation of outdoor air concentrations and meteorological variables, (2) indoor sources such as cooking and using candles, and (3) activities affecting air change rates such as opening windows and using fans. A second objective was to test and compare available instruments for their suitability in providing real-time estimates of particle levels and ancillary variables. Despite different measuring principles, the instruments employed in this study agreed reasonably well for particles less than 10 microm in diameter. The three instruments measuring fine and coarse particles (aerodynamic diameter between 0.3 and 20 microm) agreed to within 30% in their overall estimates of total volume. Two of these instruments employed optical scattering, and the third used an aerodynamic acceleration principle. However, several lines of evidence indicated that the instrument employing aerodynamic acceleration overestimated concentrations for particle diameters greater than 10 microm. A fourth instrument measuring ultrafine and accumulation-mode particles (0.01-1 microm) was operated with two different inlets providing somewhat different particle size ranges. The two inlets agreed in the ultrafine region (< 0.1 microm) but diverged increasingly for larger particles (up to 0.445 microm). Indoor sources affecting ultrafine particle concentrations were observed 22% of the time, and sources affecting fine and coarse particle concentrations were observed 12 and 15% of the time, respectively. When an indoor source was operating, particle concentrations for different sizes ranged from 2 to 20 times the average concentrations when no indoor source was apparent. Indoor sources, such as cooking with natural gas, and simple physical activities, such as walking, accounted for a majority (50-90%) of the ultrafine and coarse particle concentrations, whereas outdoor sources were more important for accumulation-mode particles between 0.1 and 1 microm in diameter. Averaged for the entire year and including no periods when indoor sources were apparent, the number distribution was bimodal, with a peak at approximately 10 nm (possibly smaller), a shallow minimum at approximately 14 nm, and a second broad peak at approximately 68 nm. The volume distribution was also bimodal, with a broad peak at approximately 200 nm, a minimum at approximately 1.2 microm, and then an upward slope again through the remaining size fractions. A database was created on a 5-min averaging time basis. It contains more than 90,000 measurements by two of the instruments and approximately 30,000 by the two optical scattering instruments. About 4500 hour-long average air change rates were also calculated throughout the year using a dedicated gas chromatograph with electron capture detection (GC/ECD). At high air change rates [> 0.8 air changes per hour (hr(-1))], particle concentrations were either elevated (when no source was present) or depressed (when an indoor source was operating) by factors of up to 2 compared with low air change rates.     

A controlled field experiment on groundwater contamination by a multicomponent DNAPL: dissolved-plume retardation

A natural gradient emplaced-source (ES) controlled field experiment was conducted at the Borden aquifer research site, Ontario, to study the transport of dissolved plumes emanating from residual dense nonaqueous-phase liquid (DNAPL) source zones. The specific objective of the work presented here is to determine the effects of solute and co-solute concentrations on sorption and retardation of dissolved chlorinated solvent-contaminant plumes. The ES field experiment comprised a controlled emplacement of a residual multicomponent DNAPL below the groundwater table and intensive monitoring of dissolved-phase plumes of trichloromethane (TCM), trichloroethylene (TCE), and perchloroethylene (PCE) plumes continuously generated in the aquifer down gradient from gradual source dissolution. Estimates of plume retardation (and dispersion) were obtained from 3-D numerical simulations that incorporated transient source input and flow regimes monitored during the test. PCE, the most retarded solute, surprisingly exhibited a retardation factor approximately 3 times lower than observed in a previous Borden tracer test by Mackay et al. [Water Resour. Res. 22 (1986) 2017] conducted approximately 150 m away. Also, an absence of temporal trend in PCE retardation contrasted with the previous Borden test. Supporting laboratory studies on ES site core indicated that sorption was nonlinear and competitive, i.e. reduced sorption of PCE was observed in the presence of TCE. Consideration of the effects of relatively high co-solute (TCE) concentration (competitive sorption) in addition to PCE concentration effects (nonlinear sorption) was necessary to yield laboratory-based PCE retardation estimates consistent with the field plume values. Concentration- and co-solute-based sorption and retardation analysis was also applied to the previous low-concentration pulse injection test of Mackay et al. [Water Resour. Res. 22 (1986) 2017] and was able to successfully predict the temporal field retardation trends observed in that test. While it is acknowledged that other "nonideal transport" effects may contribute, our analysis predicts differences in the PCE retardation magnitude and trend between the two experiments that are consistent with field observations based on the marked solute concentration differences that resulted from contrasting source conditions. Solute and co-solute concentration effects have heretofore received little attention, but may have wide significance in aquifers contaminated by point-source pollutants because many plumes contain mixed solutes over wide concentration ranges in strata that are likely subject to nonlinear sorption.     

Assessing in situ mineralization of recalcitrant organic compounds in vadose zone sediments using delta13C and 14C measurements

Few techniques exist to measure the biodegradation of recalcitrant organic compounds such as chlorinated hydrocarbons (CHC) in situ, yet predictions of biodegradation rates are needed for assessing monitored natural attenuation. Traditional techniques measuring O2, CO2, or chemical concentrations (in situ respiration, metabolite and soil air monitoring) may not be sufficiently sensitive to estimate biodegradation rates for these compounds. This study combined isotopic measurements (14C and delta13C of CO2 and delta13C of CHCs) in conjunction with traditional methods to assess in situ biodegradation of perchloroethylene (PCE) and its metabolites in PCE-contaminated vadose zone sediments. CHC, ethene, ethane, methane, O2, and CO2 concentrations were measured over 56 days using gas chromatography (GC). delta13C of PCE, trichloroethylene (TCE) and cis-1,2-dichloroethylene (DCE), delta13C and 14C of vadose zone CO2 and sediment organic matter, and delta13C, 14C, and deltaD of methane were measured using a GC-isotope ratio mass spectrometer or accelerator mass spectrometer. PCE metabolites accounted for 0.2% to 18% of CHC concentration suggesting limited reductive dechlorination. Metabolites TCE and DCE were significantly enriched in (13)C with respect to PCE indicating metabolite biodegradation. Average delta13C-CO2 in source area wells (-23.5 per thousand) was significantly lower compared to background wells (-18.4 per thousand) indicating CHC mineralization. Calculated CHC mineralization rates were 0.003 to 0.01 mg DCE/kg soil/day based on lower 14C values of CO2 in the contaminated wells (63% to 107% modern carbon (pMC)) relative to the control well (117 pMC). Approximately 74% of the methane was calculated to be derived from in situ CHC biodegradation based on the 14C measurement of methane (29 pMC). 14C-CO2 analyses was a sensitive measurement for quantifying in situ recalcitrant organic compound mineralization in vadose zone sediments for which limited methodological tools exist.     

Variability of tropospheric hydroperoxides at a coastal surface site in Antarctica

The annual cycles of hydrogen peroxide (H₂O₂) and methylhydroperoxide (MHP) have been investigated at a remote site in Antarctica in order to study seasonal variations as well as chemical processes in the troposphere. The measurements have been performed from March 1997 to January 1998 and in February 1999 at the German Antarctic research station Neumayer which is located at 70°39′S, 8°15′W. The obtained time series for hydrogen peroxide and methylhydroperoxide in near-surface air represents the first all-year measurements in Antarctica and indicates clearly the occurrence of seasonal variations. During polar night mean values of 0.054±0.046 ppbv (range<0.03–0.11 ppbv) for hydrogen peroxide and 0.089±0.052 ppbv (range<0.05–0.14 ppbv) for methylhydroperoxide were detected. At the sunlit period higher Mixing ratios were found, 0.20±0.13 ppbv (range<0.03–0.91 ppbv) for hydrogen peroxide and 0.19±0.10 ppbv (range<0.05–0.89 ppbv) for methylhydroperoxide. Occasional long-range transport of air masses from mid-latitudes caused enhanced peroxide concentrations at polar night. During the period of stratospheric ozone depletion we observed peroxide mixing ratios comparable to typical winter levels.     

Flow patterns influencing the seasonal behavior of surface ozone and carbon monoxide at a coastal site near Hong Kong

Surface O₃ and CO were measured at Cape D’Aguilar, Hong Kong during the period of January 1994 to December1996 in order to understand the temporal variations of surface O₃ and CO in East Asia–West Pacific region. The isentropic backward trajectories were used to isolate different air masses reaching the site and to analyze the long-range transport and photochemical buildup of O₃ on a regional scale. The results show that the diurnal variation of surface O₃ was significant in all seasons with daily O₃ production being about 20 ppbv in fall and 10 ppbv in winter, indicating more active photochemical processes in the subtropical region. The distinct seasonal cycles of O₃ and CO were found with a summer minimum (16 ppbv)–fall maximum (41 ppbv) for O₃ and a summer minimum (116 ppbv)–winter maximum (489 ppbv) for CO. The isentropic backward trajectory cluster analyses suggest that the air masses (associated with regional characteristics) to the site can be categorized into five groups, which are governed by the movement of synoptic weather systems under the influence of the Asian monsoon. For marine-originated air masses (M-SW, M-SE and M-E, standing for marine-southwest, marine-southeast and marine-east, respectively) which always appear in summer and spring, the surface O₃ and CO have relatively lower mixing ratios (18, 16 and 30 ppbv for O₃, 127, 134 and 213 ppbv for CO), while the continental air masses (C-E and C-N, standing for continent-east and continent-north, respectively) usually arrive at the site in winter and fall seasons with higher O₃ (43 and 48 ppbv) and CO (286 and 329 ppbv). The 43 ppbv O₃ and 286 ppbv CO are representative of the regionally polluted continental outflow air mass due to the anthropogenic activity in East Asia, while 17 ppbv O₃ and 131 ppbv CO can be considered as the signature of the approximately clean marine background of South China Sea. The very high CO values (461–508 ppbv) during winter indicate that the long-range transport of air pollutants from China continent is important at the monitoring site. The fall maximum (35–46 ppbv) of surface O₃ was believed to be caused by the effects of the weak slowly moving high-pressure systems which underlie favorable photochemical production conditions and the long-range transport of aged air masses with higher O₃ and its precursors.     

Characterizing the Occurrence,Sources, and Variability of Radon in Pacific Northwest Homes

A compilation of data from earlier studies of 172 homes in the Pacific Northwest indicated that approximately 65 percent of the 46 homes tested in the Spokane River Valley/Rathdrum Prairie region of eastern Washington/northern Idaho had heating season indoor radon (²²²Rn) concentrations above the U. S. EPA guideline of 148 Bq m^?3 (4 pCi L^?1). A subset of 35 homes was selected for additional study. The primary source of indoor radon in the Spokane River Valley/Rathdrum Prairie was pressure-driven flow of soil gas containing moderate radon concentrations (geometric mean concentration of 16,000 Bq m^?3) from the highly permeable soils (geometric mean permeability of 5 × 10^?11 m²) surrounding the house substructures. Estimated soil gas entry rates ranged from 0.4 to 39 m³h^?1 and 1 percent to 21 percent of total building air infiltration. Radon from other sources, including domestic water supplies and building materials was negligible. In high radon homes, winter indoor levels averaged 13 times higher than summer concentrations, while in low radon homes winter levels averaged only 2.5 times higher. Short-term variations in indoor radon were observed to be dependent upon indoor-outdoor temperature differences, wind speed, and operation of forced-air furnace fans. Forced-air furnace operation, along with leaky return ducts and plenums, and openings between the substructure and upper floors enhanced mixing of radon-laden substructure air throughout the rest of the building.     

Diffusion scrubber technique used for measurements of atmospheric ammonia

A diffusion scrubber (DS) was developed to measure trace levels of gaseous ammonia in ambient air. The sampling resolution time for this method is 10 min and the detection limit is estimated to be 0.01 ppbv. The response to the NI-I₃ concentrations is found to be dependent on the relative humidity in the ambient air and the temperature. The method is calibrated by using a diluted NH₃ cylinder gas, and the concentrations of the calibration gas were in the range 0.02–2 ppbv during the test. Sampling performed with the DS-method is compared to sampling performed by a filter pack and a continuous flow denuder (AMANDA). The DS-method shows good agreement with the continuous flow denuder and the filter pack.     

Measurement of Volatile Organics at Part per Billion Concentrations Using a Cold Trap Inlet and High Speed Gas Chromatography

Direct inlet gas chromatography (GC) is becoming one of the most frequently used techniques for measurement of volatile organics in air. Although GC is an effective tool, its usefulness as a field method can be limited by retention times of several minutes or more, and by the limits of detection. In order to address these issues, a high speed GC system featuring a capillary cold trap inlet and variable speed electrometeramplifier was developed and tested.

The gas cooled inlet was used to collect and focus organic vapors from injection volumes of up to 1 ml. Resistance heating of the metal cold trap produced a narrow injection band that allowed simple separations to be completed in 5 to 20 seconds. Use of a variable speed electrometer-amplifier allowed the response time and noise suppression characteristics to be adjusted to the needs of the specific analysis.

Simple mixtures of organics including aromatics, alkanes and chlorinated hydrocarbons were separated in 20 seconds or less. The limits of detection for 13 test compounds ranged from less than 1 part per billion by volume (ppbv) to about 50 ppbv. Data presented here indicate that high speed GC using a cold trap inlet may be a feasible approach for near real-time measurement of volatile organics in ambient air.     

A controlled field experiment on groundwater contamination by a multicomponent DNAPL: creation of the emplaced-source and overview of dissolved plume development

A unique field experiment has been undertaken at the CFB Borden research site to investigate the development of dissolved chlorinated solvent plumes from a residual dense non-aqueous phase liquid (DNAPL) source. The "emplaced-source" tracer test methodology involved a controlled emplacement of a block-shaped source of sand containing chlorinated solvents below the water table. The gradual dissolution of this residual DNAPL solvent source under natural aquifer conditions caused dissolved solvent plumes of trichloromethane (TCM), trichloroethene (TCE) and perchloroethene (PCE) to continuously develop down gradient. Source dissolution and 3-D plume development were successfully monitored via 173 multilevel samplers over a 475-day tracer test period prior to site remediation research being initiated. Detailed groundwater level and hydraulic conductivity data were collected. Development of plumes with concentrations spanning 1-700,000 micrograms/1 is described and key processes controlling their migration identified. Plumes were observed to be narrow due to the weakness of transverse dispersion processes and long due to advection and significant longitudinal dispersion, very limited sorptive retardation and negligible, if any, attenuation due to biodegradation or abiotic reaction. TCM was shown to be essentially conservative, TCE very nearly conservative and PCE, consistent with its greater hydrophobicity, more retarded yet having a greater mobility than observed in previous Borden field tests. The absence of biodegradation was ascribed to the prevailing aerobic conditions and lack of any additional biodegradable carbon substrates. The transient groundwater flow regime caused significant transverse lateral plume movement, plume asymmetry and was likely responsible for most of the, albeit limited, transverse horizontal plume spreading. In agreement with the widespread incidence of extensive TCE and PCE plumes throughout the industrialized world, the experiment indicates such solvent plumes are likely to be highly mobile and persistent, at least in aquifers that are aerobic and have low sorption potential (low foc content).