Comparison of Two Integrated Methods for the Collection and Analysis of Volatile Organic Compounds in Ground Water

The principal difficulties with determinations of volatile organic compounds (VOCs) in ground water are the reliability of sampling procedures and analytical methods. Two integrated methods have been developed for routine sampling, processing, and analysis of VOCs in ground water. These methods involve in situ collection of ground water using a modified syringe sampler from PVC piezometers or using dedicated glass syringes from stainless steel multilevel bores. The samples are processed in the syringe using purge and trap or microsolvent extraction and analyzed by GC/MSD.
The modified purge-and-trap method is time-consuming and limited to volatile organic compounds. However, it is extremely sensitive and flexible: the volume of sample used can be varied by the use of different-size glass syringes (sample volumes from 1 to 100 mL).
In cases where extremely low sensitivity (<10 mg 1⁻¹) is not critical, the microextraction technique is a more cost-effective method, allowing twice as many samples to be analyzed in the same time as the purge-and-trap method. It enables less volatile compounds such as polynuclear aromatic hydrocarbons, phenol, and cresols to be analyzed in the same GC run. Also, the microextraction method can be used in the field to avoid delays associated with transportation of ground water samples to the laboratory.     

Sampling of VOCs with the BAT Ground Water Sampling System

In the BAT ground water sampling system, a stainless steel probe with a porous filter element is pushed vertically to the desired sampling depth. An evacuated glass sampling tube is then lowered down the penetration rods where it makes contact with the filter via a hypodermic needle and draws a pore fluid sample.
An investigation of the system was carried out at a number of sites contaminated by leaking underground gasoline storage tanks. Ground water samples obtained using the BAT system and adjacent monitoring wells were analyzed for volatile organic compounds (VOCs).
Because the BAT system is an in situ penetration device with a small filter length, it is possible to determine variations in contaminant concentration with depth. BAT samples in general exhibited higher recovery of VOCs than did bailer samples from adjacent monitoring wells screened over large intervals.
Much higher levels of VOCs were recovered when the probe was used with its 316 stainless steel filter than when using the high-density polyethylene (HDPE) filter. Significant sorption apparently occurred on the latter filter.
Because the BAT sample tubes are sealed and remain a closed system, the in situ water pressure is maintained. No significant loss of VOCs was found in sampling tubes containing headspace. Samples from the upper tube in the cascaded setup with headspace recovered levels of VOCs as high, or in a few cases higher, than the lower, no-headspace tubes.     

Modeling Fate and Transport of Volatile Organic Compounds (VOCs) Inside Sewer Systems

Hazardous waste site investigations have shown that volatile organic compounds (VOCs) can be transported via sewer pipes and migrate into indoor spaces. Despite field data confirming the presence of this exposure pathway, there is lack of context-based numerical models that provide guidance to characterize and predict VOCs concentration in sewer gas at vapor intrusion sites. Particularly, this poses a challenge when assessing and mitigating risks associated with these exposure pathways. Therefore, a numerical model has been developed to simulate the concentration of VOCs in sewer gas in different stages throughout the sewer lines. The developed model considers various input parameters, including temperature, sewer liquid depth, groundwater depth, and sewer construction characteristics to incorporate local and operational conditions. The model's output is verified using field data from a sewer system constructed near a Superfund site. Moreover, a sensitivity analysis was conducted to evaluate the model's response to variation of the external input parameters. To the best of our knowledge, this study is the first attempt to model VOCs concentration in sewer gas, particularly to address vapor intrusion. The developed model can be used as a numerical tool to support the development of sewer assessment guidelines, risk assessment studies, and mitigation strategies.     

An Evaluation of Some Systems for Sampling Gas-Charged Ground Water for Volatile Organic Analysis

Loss of volatile organics during sampling is a well-recognized source of bias in ground water monitoring; sampling protocols attempt to minimize such loss. Such bias could be enhanced for ground water highly charged with dissolved gases such as methane. Such ground water was the object of this study. A positive-displacement bladder pump, a momentum-lift pump and a suction-lift, peristaltic pump were employed in sampling both methane-charged ground water for volatile aromatic hydrocarbons and a CO₂-charged reservoir water for volatile chlorinated hydrocarbons. In both cases, the suction-lift pump produced samples with a significant negative bias (9 to 33 percent) relative to the other methods. Little difference between samples produced by the other pump Systems was noted at the field site, but in sampling the reservoir, the bladder pump produced samples that were 13 to 19 percent lower in halocarbon concentration than were samples from the momentum-lift pump.
These negative biases are tentatively interpreted as losses due to volatilization during sampling. Slightly greater negative biases occur for compounds of higher volatility as estimated from their Henry's law constants. Additional studies appear to be warranted in order to adequately establish the scientific basis for recommending protocols for sampling ground water in which degassing could enhance the loss of volatile organics during sampling.     

Screening of Ground Water Samples for Volatile Organic Compounds Using a Portable Gas Chromatograph

A portable gas chromatograph was used to screen 32 ground water samples for volatile organic compounds. Seven screened samples were positive; four of the seven samples had volatile organic substances identified by second-column confirmation. Four of the seven positive, screened samples also tested positive in laboratory analyses of duplicate samples. No volatile organic compounds were detected in laboratory analyses of samples that headspace screening indicated to be negative. Samples that contained volatile organic compounds, as identified by laboratory analysis, and that contained a volatile organic compound present in a standard of selected compounds were correctly identified by using the portable gas chromatograph. Comparisons of screened-sample data with laboratory data indicate the ability to detect selected volatile organic compounds at concentrations of about 1 microgram per liter in the headspace of water samples by use of a portable gas chromatograph.     

Groundwater Sampling at ISCO Sites: Binary Mixtures of Volatile Organic Compounds and Persulfate

In situ chemical oxidation involves the introduction of a chemical oxidant into the subsurface for the purpose of transforming groundwater contaminants into harmless by‐products. Owing to oxidant persistence, groundwater samples collected at hazardous waste sites may contain both the contaminant(s) and the oxidant in a “binary mixture.” Binary mixtures composed of sodium persulfate (2.5 g/L; 10.5 mM) and volatile organic compounds (VOCs) (benzene, toluene, m‐xylene, perchloroethylene, trichloroethylene) were analyzed to assess the impact on the quality of the sample. A significant decline (49 to 100%) in VOC concentrations was measured in binary mixtures using gas chromatography (GC) purge and trap, and GC mass spectroscopy headspace methods. Preservation of the binary mixture samples was achieved through the addition of ascorbic acid (99 to 100% VOC average recovery). High concentrations of ascorbic acid (42 to 420 mM) did not interfere in the measurement of the VOCs and did not negatively impact the analytical instruments. High concentrations of ascorbic acid favored the reaction between persulfate and ascorbic acid while limiting the reaction between persulfate and VOCs. If an oxidant is detected and the binary sample is not appropriately preserved, the quality of the sample is likely to be compromised.     

Decontaminating Materials Used in Ground Water Sampling Devices: Organic Contaminants

In these studies, the efficiency of various decontamination protocols was tested on small pieces of materials commonly used in ground water sampling devices. Three materials, which ranged in ability to sorb organic solutes, were tested: stainless steel (SS), rigid polyvinyl chloride (PVC), and polytetrafluoroethylene (PTFE). The test pieces were exposed to two aqueous test solutions: One contained three volatile organic compounds (VOCs) and one nitroaromatic compound, and the other contained four pesticides. Also, three types of polymeric tubing were exposed to pesticide solutions. Generally, the contact times were 10 minutes and 24 hours for sorption and desorption.
The contaminants were removed from the nonpermeable SS and the less-sorptive rigid PVC test pieces simply by washing with a hot detergent solution and rinsing with hot water. Additional treatment was required for the PTFE test pieces exposed to the VOCs and for the low-density polyethylene (LDPE) tubing exposed to the pesticide test solution. Solvent rinsing did not improve removal of the three VOCs from the PTFE and only marginally improved removal of the residual pesticides from the LDPE. However, a hot water and detergent wash and rinse followed by oven drying at approximately 105°C was effective for removing the VOCs from the PTFE and substantially reduced pesticide contamination from the LDPE.     

Case History of a Large-Scale Air Sparging/Soil Vapor Extraction System for Remediation of Chlorinated Volatile Organic Compounds in Ground Water

A large-scale air sparging/soil vapor extraction (AS/SVE) project constructed within coastal plain sediments in New Jersey has demonstrated substantial progress toward remediating ground water through removal of volatile organic compounds (VOCs). Potential concerns identified prior to project implementation regarding hydraulic mounding, reduction in hydraulic conductivity, development of air channels, and the absence of hydraulic containment were assessed and addressed through testing and operational features incorporated into the project. At the project site, AS/SVE has successfully reduced the presence of many VOCs to undetectable levels, while reducing the concentrations of the remaining VOCs by factors of two to 500. The physical agitation caused by air sparging, and incomplete transformation from sorbed and nonaqueous phases to the vapor phase, appears to temporarily increase VOC concentrations and/or mobility of dense nonaqueous phase liquids (DN APLs) within source areas at the project site, but this is addressed in terms of subsequent removal of VOCs by properly placed downgradient treatment lines and VOCs by properly placed downgradient treatment lines and DNAPL recovery wells. This case study identifies and evaluates project-specific features and provides empirical data for potential comparison to other candidates AS/SVE sites.     

Sampling for Toxic Contaminants in Ground Water

In this paper, we relate recent developments in ground water sampling techniques to the practical application of sampling for toxic contaminants in ground water. We address the choices that must be made in choosing equipment for a particular project by going through a step-by-step procedure for collecting a ground water sample from a typical monitoring well. Ground water sampling topics that are discussed include: choice of equipment for purging and sampling a well, monitoring for purged ground water indicators and quality assurance/quality control.     

The Effect of Air Bubbles and Headspace on the Aqueous Concentrations of Volatile Organic Compounds in Sampling Vials

The presence of headspace and air bubbles in volatile organic analysis sampling vials lowers the actual aqueous concentration of these compounds due to the partitioning of solutes into the gaseous phase. This could make the sample invalid for analysis.
In this work, the effects of air bubbles and headspace on the aqueous concentration of 60 volatile organic compounds listed in U.S.Environmental Protection Agency (U.S. EPA) Method 8260 were evaluated experimentally and theoretically. The results showed that for air to water ratios of 1 to 20 and less, there was no significant effect on the aqueous concentrations of target organic solutes in the sampling vials. When the air to water ratio was increased to 1 to 10, the recovery rates of four organic compounds were lower than the control. Laboratory experiments on sampling vials showed that the presence of air bubbles or headspace with the volumetric air to water ratios of 1 to 20 and less do not produce any significant effect on the original concentrations for most targeted volatile organic compounds.
The experimental results also indicated that in 40 mL sampling vials with headspace range of 2 to 8 mL, the recovery rates of most volatile organic compounds with high values of Henry's law constant (> 0.01 Atm m³/mol. at 25°C) were larger than the calculated rates.     

A Comparison of Sampling Mechanisms Available for Small-Diameter Ground Water Monitoring Wells

The objective of most ground water quality monitoring programs is to obtain samples that are "representative" or that retain the physical and chemical properties of the ground water in an aquifer. Many factors can influence whether or not a particular sample is representative, but perhaps the most critical factor is the method or type of sampling device used to retrieve the sample.
The sampling equipment available today ranges from simple to highly sophisticated, and includes bailers, syringe devices, suction-lift pumps, gas-drive devices, bladder (Middelburg-type) pumps, gear-drive and helical rotor electric submersible pumps and gas-driven piston pumps. New devices are continually being developed for use in small-diameter wells in order to meet the needs of professionals engaged in implementing elaborate ground water monitoring programs.
In selecting a sampling device for a monitoring program, the professional must consider a number of details. Among the considerations are: the outside diameter of the device, the overall impact of the device on ground water sample integrity (including the materials from which the sampling device and associated equipment are made and the method by which the device delivers the sample), the capability of the device to purge the well of stagnant water, the rate and the ability to control the rate at which the sample is delivered, the depth limitations of the device, the ease of operating, cleaning and maintaining the device, the portability of the device and required accessory equipment, the reliability and durability of the device, and the initial and operational cost of the device and accessory equipment. Based on these considerations, each of the devices available for sampling ground water from small-diameter wells has its own unique set of advantages and disadvantages that make it suitable for sampling under specific sets of conditions. No one sampling device is applicable to all sampling situations.     

Field Comparison of Micropurging vs. Traditional Ground Water Sampling

Micropurge sampling of ground water wells has been suggested as a possible replacement to traditional purge and sample methods. To compare methods, duplicate ground water samples were collected at two field sites using iraditional and micropurge methods. Samples were analyzed for selected organic and inorganic constituents, and the results were compared statistically. Analysis of the data using the nonparametric sign test indicates that within a 95 percent confidence interval, there was no significant difference between the two methods for the site contaminants and the majority of analytes. These analytical results were supported by visual observations with the colloidal borescope, which demonstrated impacts on the flow system in the well when using traditional sampling methods. Under selected circumstances, the results suggest replacing traditional sampling with micropurging based on reliability, cost, and waste minimization.