Macro- and Micro-Purge Soil-Gas Sampling Methods for the Collection of Contaminant Vapors

Purging influence on soil‐gas concentrations for volatile organic compounds (VOCs), as affected by sampling tube inner diameter and sampling depth (i.e., system volume) for temporary probes in fine‐grained soils, was evaluated at three different field sites. A macro‐purge sampling system consisted of a standard, hollow, 3.2‐cm outer diameter (OD) drive probe with a retractable sampling point attached to an appropriate length of 0.48‐cm inner diameter (ID) Teflon^® tubing. The macro‐purge sampling system had a purge system volume of 24.5 mL at a 1‐m depth. In contrast, the micro‐purge sampling systems were slightly different between the field sites and consisted of a 1.27‐cm OD drive rod with a 0.10‐cm ID stainless steel tube or a 3.2‐cm OD drive rod with a 0.0254‐cm inner diameter stainless steel tubing resulting in purge system volumes of 1.2 and 7.05 mL at 1‐m depths, respectively. At each site and location within the site, with a few exceptions, the same contaminants were identified in the same relative order of abundances indicating the sampling of the same general soil atmosphere. However, marked differences in VOC concentrations were identified between the sampling systems, with micro‐purge samples having up to 27 times greater concentrations than their corresponding macro‐purge samples. The higher concentrations are the result of a minimal disturbance of the ambient soil atmosphere during purging. The minimal soil‐gas atmospheric disturbance of the micro‐purge sampling system allowed for the collection of a sample that is more representative of the soil atmosphere surrounding the sampling point. That is, a sample that does not contain an atmosphere that has migrated from distance through the geologic material or from the surface in response to the vacuum induced during purging soil‐gas concentrations. It is thus recommended that when soil‐gas sampling is conducted using temporary probes in fine‐grained soils, the sampling system use the smallest practical ID soil‐gas tubing and minimize purge volume to obtain the soil‐gas sample with minimal risk of leakage so that proper decisions, based on more representative soil‐gas concentrations, about the site can be made.     

A New System for Ground Water Monitoring

This paper describes a new system for ground water monitoring, "the BAT System," which includes the following functions: (a) sampling of ground water in most types of soils, (b) measurement of pore water pressure, and (c) in situ measurement of hydraulic conductivity. The system can also be used for tracer tests. The system utilizes a permanently installed filter tip attached to a steel or PVC pipe. Installation is normally performed by pushing the tip down to the desired depth. The filter tip can also be buried beneath a landfill. The primary feature of the new system is that the filter tip contains a self-sealing quick coupling unit, which makes it possible to temporarily connect the filter tip to adapters for various functions, e.g. water sampling and for measurement of pore pressure and hydraulic conductivity. The new technique makes sampling of both pressurized water and gas possible. Samples are obtained directly in hermetically sealed, pre-sterilized sample cylinders. Sampling of ground water and measurement of pore pressure can be repeated over a long period of time with undiminished accuracy. This technique is also well-adapted for taking water samples from different strata in a soil profile, in both the saturated and unsaturated zones. Actual installations range from 0.5 to 60m depth.     

A Triggered Depth‐Dependent Sampling System to Overcome the Carry‐Over Effects of the Membrane Interface Probe

The membrane interface probe (MIP) is widely used for the in situ characterization of volatile organic compounds (VOCs) in the subsurface. A main problem using the MIP system is the carry‐over effect of VOCs during the transport from the point of measurement to the detector using a conventional transfer line. This effect results in compound specific retention times, which is shown in disproportionately high measuring signals after the actual penetration of contaminated zones. In consequence, the lower extent of contamination is not clearly identifiable and may be overestimated. The presented field study presents an evaluation of different methods to overcome the carry‐over effect, especially with regard to the required measurement times that are needed to wait for a complete disappearance of the detector signals before forwarding the probe. This was accomplished by comparing data collected with a MIP system with (1) unheated transfer line and (2) a system including a heated transfer line to data collected with a system using (3) a depth‐dependent triggered sampling behind the membrane including two transfer lines. A comparison with analytical results from soil samples gave a good correlation for all three methods. Furthermore, it could be shown that the use of a heated transfer line has a time improvement of 30% compared to an unheated transfer line while the depth dependent triggered sampling using two separate transfer lines yielded a time improvement of over 90%. These results confirm the benefit of the latter method, particularly for the use in highly contaminated sediments.     

Nickel and Chromium in Ground Water Samples as Influenced by Well Construction and Sampling Methods

An investigation of elevated concentrations of nickel and chromium in certain ground water samples collected at Williams Air Force Base (AFB) indicated that type 304 stainless steel well materials are the source. Chloride in the ground water has apparently caused crevice corrosion of the stainless steel well screens installed during site characterization. An evaluation of site geochemistry suggested that chromium released from the well screen would precipitate, while nickel would remain dissolved. Thus, low-flow purging and sampling significantly reduces the chromium found in the ground water samples because such sampling minimizes the collection of artificially entrained particulates. In contrast to chromium, nickel concentrations did not decrease during low-flow purging and sampling, indicating that it is dissolved. Nickel and chromium concentrations are both low following high-volume purging when turbidity levels are stabilized below 10 nephelometric turbidity units prior to sampling. In the latter case, chromium concentration is low because particulate collection is minimized, and nickel concentration is low because of increased dilution. Based on these results, it is recommended that elevated levels of nickel and chromium in ground water samples collected from stainless steel monitoring wells be carefully evaluated, because well materials may be the source. In addition, although low-volume purging is increasingly becoming the sampling method of choice, high-volume purging may be a useful means of determining whether the well materials influence nickel and chromium concentrations.     

Design, Installation, and Uses of Combination Ground Water and Gas Sampling Wells

Waste disposal sites with volatile organic compounds (VOCs) frequently contain contaminants that are present in both the ground water and vadose zone. Vertical sampling is useful where transport of VOCs in the vadose zone may effect ground water and where steep vertical gradients in chemical concentrations are anticipated. Designs for combination ground water and gas sampling wells place the tubing inside the casing with the sample port penetrating the casing for sampling. This physically interferes with pump or sampler placement. This paper describes a well design that combines a ground water well with gas sampling ports by attaching the gas sampling tubing and ports to the exterior of the casing. Placement of the tubing on the exterior of the casing allows exact definition of gas port depth, reduces physical interference between the various monitoring equipment, and allows simultaneous remediation and monitoring in a single well. The usefulness and versatility of this design was demonstrated at the Idaho National Engineering and Environmental Laboratory (INEEL) with the installation of seven wells with 53 gas ports, in a geologic formation consisting of deep basalt with sedimentary interbeds at depths from 7.2 to 178 m below land surface. The INEEL combination well design is easy to construct, install, and operate.     

Progressive Packer/Zone Sampling Method for VOC Plume Delineation in Consolidated Aquifers

The progressive packer/zone sampling method was used to identify the bottom of a plume of volatile organic compounds (VOCs) in the parts-per-million (ppm) range using one well in each of three separate locations. The method involves progressively drilling a 20-foot length of borehole through casing, setting an inflatable packer at the top of the drilled zone, purging the zone of three volumes of water using the airlift method, sampling the zone in situ through the packer string using a bailer, then repeating the procedure.
A plume consisting of chlorinated VOCs, alcohols, and vinyl chloride occurs in a low-yielding fractured bedrock aquifer located in the Passaic Formation at a site in central New Jersey. The thickness of the plume in total VOC concentrations exceeding 1 ppm was determined using the progressive packer/zone sampling method to a depth of 200 feet. The first borehole was completed as a monitoring well in the "hottest" zone encountered during testing. Additional wells were then clustered with this exploratory well to delineate the plume in the parts-per-billion (ppb) range. Cross contamination from previously sampled zones was not a problem as long as total VOCs in the ppm range were targeted and the sample interval was properly purged.
Instead of using a multiple well cluster consisting of an indefinite number of wells to determine the bulk thickness of a plume at a specific location, information from one borehole may suffice during the exploratory phase. Costs to the client and cross contamination potential to the aquifer can be minimized by limiting the number of boreholes needed for vertical delineation.     

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.     

Field Sampling of Residual Aviation Gasoline in Sandy Soil

Two complementary field sampling methods for the determination of residual aviation gasoline content in the contaminated capillary fringe of a fine, uniform, sandy soil were investigated. The first method featured field extrusion of core barrels into pint-size Mason jars, while the second consisted of laboratory partitioning of intact stainless steel core sleeves. The barrel extrusion procedure involved jar headspace sampling in a nitrogen-filled glove box, which delineated the 0.7m thick residually contaminated interval for subsequent core sleeve withdrawal from adjacent boreholes. Soil samples removed from the Mason jars (in the field) and sleeve segments (in the laboratory) were subjected to methylene chloride extraction and gas chromatographic analysis to compare their aviation gasoline content. The barrel extrusion sampling method yielded a vertical profile with 0.10m resolution over an essentially continuous 5.0m interval from the ground surface to the water table. The sleeve segment alternative yielded a more resolved 0.03m vertical profile over a shorter 0.8m interval through the capillary fringe. The two methods delivered precise estimates of the vertically integrated mass of aviation gasoline at a given horizontal location, and a consistent view of the vertical profile as well. In the latter regard, a 0.2m thick lens of maximum contamination was found in the center of the capillary fringe, where moisture filled all voids smaller than the mean pore size. The maximum peak was resolved by the core sleeve data, but was partially obscured by the barrel extrusion observations, so that replicate barrels or a half-pint Mason jar size should be considered for data supporting vertical transport analyses in the absence of sleeve partitions.     

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.     

Cone Penetrometer Tests and HydroPunch® Sampling: A Screening Technique for Plume Definition

Cone penetrometer tests and HydroPunch® sampling were used to define the extent of volatile organic compounds in ground water. The investigation indicated that the combination of these techniques is effective for obtaining ground water samples for preliminary plume definition. HydroPunch samples can be collected in unconsolidated sediments and the analytical results obtained from these samples are comparable to those obtained from adjacent monitoring wells. This sampling method is a rapid and cost-effective screening technique for characterizing the extent of contaminant plumes in soft sediment environments. Use of this screening technique allowed monitoring wells to be located at the plume boundary, thereby reducing the number of wells installed and the overall cost of the plume definition program.     

Low-Flow Purging and Sampling of Ground Water Monitoring Wells with Dedicated Systems

A field study was conducted to assess purging requirements for dedicated sampling systems in conventional monitoring wells and for pumps encased in short screens and buried within a shallow sandy aquifer. Low-flow purging methods were used, and wells were purged until water quality indicator parameters (dissolved oxygen, specific conductance, turbidity) and contaminant concentrations (chromate, trichloroethylene, dichloroethylene) reached equilibrium. Eight wells, varying in depth from 4.6 to 15.2 m below ground surface, were studied. The data show that purge volumes were independent of well depth or casing volumes. Contaminant concentrations equilibrated with less than 7.5 I. of purge volume in all wells. Initial contaminant concentration values were generally within 20 percent of final values. Water quality parameters equilibrated in less than 10 L in all wells and were conservative measures for indicating the presence of adjacent formation water. Water quality parameters equilibrated faster in dedicated sampling systems than in portable systems and initial turbidity levels were lower.     

The Hydropunch™: An In Situ Sampling Tool for Collecting Ground Water from Unconsolidated Sediments

The Hydropunch™ is a stainless steel and Teflon® sampling tool that is capable of collecting a representative ground water sample without requiring the installation of a ground water monitoring well. To collect a sample, the Hydropunch (Patent #4669554) is connected to a small-diameter drive pipe and either driven or pushed hydraulically to the desired sampling depth. As the tool is advanced, it remains in the closed position, which prevents soil or water from entering the Hydropunch. Once the desired sampling depth is obtained, the tool is opened to the aquifer by pulling up the drive pipe approximately 1.5 feet (0.46m). In the open position, ground water can flow freely into the sample chamber of the tool. When the sample chamber is full, the Hydropunch is pulled to the surface. As the tool is retracted, check valves close and trap the ground water in the sample chamber. At the surface the sample is transferred from the Hydropunch to an appropriate sample container. The tool is a fast, inexpensive alternative for collecting ground water samples from a discrete interval. It is excellent for vertical profiling or defining the areal extent of a contaminant plume.     

Sampling Fauna in Stream Sediments as well as Groundwater Using One Net Sampler

Groundwater bores act as traps. Net samplers are regularly used for sampling this type of trap for fauna. To enable direct comparisons of faunal communities in groundwater bores and stream sediments, stream sediment tubes were built similar to groundwater bores and were sampled with net samplers for fauna. These stream sediment tubes consisted of a tube anchored in the stream sediment, also called interstitial space. To test the efficacy of this trap method in stream sediments, it was compared to another type of trap, Hahn's trap. Faunal communities sampled by a net in the stream sediment tubes did not differ hugely from fauna in Hahn's trap samples. Physical and chemical factors of sampled water in both the stream sediment tubes, the surrounding interstitial sediments and the second type of traps, Hahn's traps, showed that water in both the tubes and Hahn's traps was closely related to interstitial water. The net sampler is inexpensive and easy to handle. It is suggested that sampling stream tubes with nets may be an appropriate method for long‐term monitoring studies.     

Comparison of Methods for Sampling Dissolved Nitrogen in a Fractured Carbonate-Rock Aquifer

As part of an agricultural non-point-source study in the Conestoga River head waters area in Pennsylvania, different methods for collecting ground water samples from a fractured carbonate-rock aquifer were compared. Samples were collected from seven wells that had been cased to bedrock and drilled as open holes to the first significant water-bearing zone. All samples were analyzed for specific conductance, dissolved oxygen, and dissolved-nitrogen species. Water samples collected by a point sampler without pumping the well were compared to samples collected by a submersible pump and by a point sampler after pumping the well. Samples collected by using a point sampler, adjacent to major water-bearing zones in an open borehole without pumping the well, were not statistically different from samples collected from the pump discharge or from point samples collected adjacent to major water-bearing zones after pumping the well. Samples collected by using a point sampler without pumping the well at depths other than those adjacent to the water-bearing zones did not give the same results as the other methods, especially when the water samples were collected from within the well casings. It was concluded that, for the wells at this site, sampling adjacent to major water-bearing zones by using a point sampler without pumping the well provides samples that are as representative of aquifer conditions as samples collected from the pump discharge after reaching constant temperature and specific conductance, and by using a point sampler after pumping the well.     

Comparison of Temporal Trends in VOCs as Measured with PDB Samplers and Low-Flow Sampling Methods

The effectiveness of passive diffusion bag (pdb) samplers in the measurement of selected volatile organic compounds (VOCs) is dependent on a number of factors. At some sites and wells, pdb sampling methods provide an attractive alternative to other sampling methods. In this discussion, I provide two examples of comparisons of temporal trends in tetrachloroethylene (PCE) concentrations from passive and low-flow sampling methods. At the example field site, large changes in PCE concentrations occurred over the deployment period(s) of the pdb samplers, yet the concentrations from the pdb samples are similar to the low-flow samples and the overall trends are the same.     

Sampling VOCs with Porous Suction Samplers in the Presence of Ethanol: How Much Are We Losing?

Porous suction samplers have been widely used to obtain ground water samples from the vadose zone. However, previous studies identified different mechanisms that may compromise the sample's representativeness, such as volatilization and sorption. This issue is particularly important when dealing with volatile organic compounds (VOCs) as in gasoline spills. Ethanol is common in modern fuels and so may be present in ground water contamination from fuel releases. The objective of this work was to evaluate the losses of VOCs in the presence of ethanol when using porous suction samplers. Laboratory experiments were performed using a ceramic porous suction sampler to sample test solution containing benzene, toluene, xylenes, trimethylbenzenes, naphthalene, and different volumetric fractions of ethanol. Significant losses were found up to 30% for ethylbenzene. Ethanol was found to affect the accuracy of the readings by two main mechanisms: first, negatively, by increasing the headspace in the sampling tube, and second, positively, increasing partition to the aqueous phase due to the cosolvent effect and therefore decreasing the mass loss by volatilization. As a consequence, the highest losses of VOCs were found at intermediate ethanol volume fractions: 10% and 20% (v/v). The losses can be anticipated by measuring the ratio of gas to water in the sampling line and then by applying simple partition models considering cosolvency by ethanol. The importance of adequate purging when using porous suction samplers was also shown.     

A Method to Evaluate the Vertical Distribution of VOCs in Ground Water in a Single Borehole

Volatile organic compounds (VOCs) are present in multiple water-bearing zones beneath and downgradient of Lawrence Livermore National Laboratory. This area is composed of interfingering unconsolidated alluvial sediments with hydraulic conductivities ranging over four orders of magnitude. The more permeable sediments exhibit moderate hydraulic interconnection horizontally and less interconnection vertically, and appear to consist largely of interconnected stream channel deposits. To optimize selection of monitoring well screened intervals in this complex environment, a technique that enables collection of saturated formation samples from each water-bearing zone without contamination from other VOC-containing zones was developed, tested, and implemented. The technique utilizes a wireline punch-coring system that allows the drill bit to be replaced with a core barrel without removing the drill rod from the borehole. To help ensure that a sample from one water-bearing zone is not contaminated by VOCs from another zone, the drilling fluid is replaced with new fluid before each sampling run. Overnight chemical analysis by gas chromatography enables field personnel to know the vertical distribution of VOCs as drilling proceeds. Since its first use in 1985, the technique has successfully characterized the presence or absence of VOCs in ground water in 123 of 140 wells, many with concentrations in ground water in the low parts-per-billion range. Our sampling technique is a cost-effective and rapid method of evaluating the vertical distribution of VOCs in ground water in a complex hydrogeologic environment.     

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.     

Effect of Different Sampling Methodologies on Measured Methane Concentrations in Groundwater Samples

Analysis of dissolved light hydrocarbon gas concentrations (primarily methane and ethane) in water supply wells is commonly used to establish conditions before and after drilling in areas of shale gas and oil extraction. Several methods are currently used to collect samples for dissolved gas analysis from water supply wells; however, the reliability of results obtained from these methods has not been quantified. This study compares dissolved methane and ethane concentrations measured in groundwater samples collected using three sampling methods employed in pre‐ and post‐drill sampling programs in the Appalachian Basin. These include an open‐system collection method where 40 mL volatile organic analysis (VOA) vials are filled directly while in contact with the atmosphere (Direct‐Fill VOA) and two alternative methods: (1) a semi‐closed system method whereby 40 mL VOA vials are filled while inverted under a head of water (Inverted VOA) and (2) a relatively new (2013) closed system method in which the sample is collected without direct contact with purge water or the atmosphere (IsoFlask^®). This study reveals that, in the absence of effervescence, the difference in methane concentrations between the three sampling methods was relatively small. However, when methane concentrations equaled or exceeded 20 mg/L (the approximate concentration at which effervescence occurs in the study area), IsoFlask^® (closed system) samples yielded significantly higher methane concentrations than Direct‐Fill VOA (open system) samples, and Inverted VOA (semi‐closed system) samples yielded lower concentrations. These results suggest that open and semi‐closed system sample collection methods are adequate for non‐effervescing samples. However, the use of a closed system collection method provides the most accurate means for the measurement of dissolved hydrocarbon gases under all conditions.     

The Sensitivity of Four Monitoring Well Sampling Systems to Low Concentrations of Three Volatile Organics

Four state-of-the-art ground water sampling systems were analyzed to determine their reliability in providing representative samples of the volatile chlorinated hydrocarbons trichloroethylene (TCE), perchloroethylene (PCE), and 1,1,1-trichloroethane (TCA) from a simulated monitoring well. The sampling systems studied represent four commonly used devices, including a stainless steel and Teflon® piston pump, a Teflon bailer, a Teflon bladder pump, and a PVC air-lift pump.
Controlled laboratory sampling experiments were conducted in a tank and well test chamber designed to approximate field conditions. A well purging and sampling procedure was used in the test apparatus to determine the accuracy and precision of each device for detecting low concentrations of the compounds in ground water. The compounds selected are some of the most ubiquitous hazardous contaminants found in shallow aquifers near hazardous waste sites throughout the United States.
No significant statistical difference was found among the four sampling systems in detecting the compounds.