The prediction of migratory patterns of both light non-aqueous phase and dense non-aqueous liquid phases (LNAPL and DNAPL) in the environment1 is considered to be one of the more complex and teasing problems associated with hydrology and the effect of liquid contaminants on water quality.
To a certain extent, the LNAPL is effortlessly made when contaminants are encountered at the vadose air-water interface even though more complexities are easily detected which relate to solubility of contaminants not only with water but also between DNAPL and LNAPL components.
Table 1. Typical volatile chlorinated hydrocarbon contaminants
||carbon tetrachloride (CT)
||1,1,2,2-tetrachloroethane (1,1,2,2 TCA)
One of the most commonly encountered environmental contaminant groups refers to the volatile chlorinated hydrocarbon (v-CHC) group of compounds (see Table 1), many of which are suspect carcinogens. Most members of this group are, to varying degrees, miscible with water and in this way they can enter biological systems. These compounds have anthropogenic origins and are often associated with some form of industrial activity where they were often used as metal degreasing agents during plating, machining, or fabrication.
To begin with, it is essential to ascertain the extent of contamination and the speed of infiltration or migration of the contaminant plume and usually during the lifetime of environmental remediation projects. This process mostly depends on a series of strategically placed wells, which when sampled, are capable of providing information on the contaminant flux over a period of time2. In this way, it is possible to ascertain the dynamics and concentration data in three dimensions that is then summed in the form of an isopleth contour plot which can be most usefully manipulated by computer.
It is important to envisage the collection of copious quantities of analytical chemistry data before detailed maps can be drawn. Although the collection of some information, based on the type of contaminants present, can be automated in a partial manner at the well-head by various sensing devices, the v-CHC’s are particularly complex to analyze because of their relative insensitivity as perceived by some of the more commonly available devices. Therefore, the analytical process as well as the generation of analyte specific concentration data can be very expensive and time consuming.
There is a clear requirement for site methods which will rapidly and economically emulate traditional analytical chemistry laboratory data with adequate accuracy to both assess and map concentration gradients. The availability of ultra-rapid gas chromatographs3-6 fitted with nonanalyte particular detectors provides a clear advantage to mapping plume migration and also to site survey in terms of environmental chemistry.
A field-portable ultra-rapid GC utilizing a surface acoustic wave detector (SAW) has been regularly used in labs besides being taken into the field in a wide range of proving expeditions. Figure 1 shows the basic layout of the instrument.
Figure 1. Ultra-rapid SAW/GC component schematic
To detect VOCs, sampled air is pumped through a Tenax packed trap for a pre-selected time. The trap is then fired electro-capacitativly and the desorbed vapors directed through a heat controlled rotary valve to a short GC column.
It is possible to thermally ramp the GC column and then direct the effluent chromatographed vapors onto the surface of the SAW. The SAW is generally set to resonate close to 500 Mhz and is highly sensitive to any impinging vapors. The corresponding diminution frequency caused by surface loading of the SAW oscillator is recorded and displayed in the form of an integram in a Windows based proprietary software adapted to run on an associated lap-top PC.
An evolving chromatogram produced from the differential of the integram is simultaneously displayed by the computer. The form of a traditional chromatogram is mimicked by the differential, which will usually display a typical negative inflection following each chromatographic peak which is the normal result of differentiating a signal through a change in sign and describes the physical effect of adsorption followed by desorption of each analyte from the SAW’s surface. A typical chromatogram registered by the SAW detector is shown in Figure 2.
Figure 2. Lab standard of TCE and PCE used for retention time calibration and quantitation
Routine monitoring of a mature DNAPL plume at Lawrence Berkeley Laboratories has helped in demonstrating the useful application7 this instrument in the field. The plume has been migrating over a period of many years following the underlying statigraphy in a steep gradient hillside location. The original contamination was obtained from metal cleaning operations after world war II at the synchrotron facility.
Routine monitoring of plume migration is done from well sites strategically located on the hillside where soil gas samples are readily available. The ease and rapidity with which well-head analytical chemistry data can be accumulated was augmented because a 19 mm rubber septum port has been used to secure each well.
The sensing head of the SAW/GC was fitted with a water trap in order to moderate the potentially overwhelming effect of water vapor on the SAW. This sensing head was fitted with a male leur fitting which allowed a short 27 swg needle to be used by puncturing the well-head septum directly. In this way, sample procurement was reduced based on the time taken to approach the well head and puncture the septa.
The site at Lawrence Berkeley Laboratories (Figure 3) is a small site with buildings that are quite densely packed and because of this the wells were relatively few in number.
Figure 3. At the steeply sloping well-head gathering soil gas chromatographic data on migrating DNAPL contaminants
The air temperature differed considerably from one group of well sites to another which needed continual recalibration of the instrument with tedlar bags comprising of two of the known contaminants, TCE and PCE essential. Failure to constantly update the calibration in terms of retention times would have resulted in false peak identification owing to a temperature dependent shift in retention times. The lower panel of Figure 4 shows an indication of this phenomenon with drift time of 0.12 seconds and 0.24 seconds for the two analytes.
Figure 4. Site generated chromatograms with a TCE/PCE standard which is used to indicate the presence of these analytes
Figure 4 shows a typical identification profile, where TCE and PCE have been detected by matching their retention times with known standard vapors provided in situ using a tedlar bag comprising of the two standards in vapor form at 1 ppm concentration. Since the sample acquisition time is 10 seconds and the average chromatogram is just 15 seconds, data collection proceeded at 25 seconds per run which provides sufficient time to run standards between each data set.
Generally, the chromatograms from each well head are quite complex and in these data sets the ubiquitous presence of a late eluting peak is observed at around 9 seconds. This peak is mostly associated with the reduction products of PCE and TCE to which the SAW is even more sensitive.
This article has efficiently demonstrated that chromatograms that are accurate and reproducible can be produced in the field that closely mimic those derived during analytical chemical laboratory analysis. Although the resolution provided by the SAW/GC does not compare with laboratory-based equipment for the most part this feature is not needed.
The positive benefit during ground well monitoring or field screening is that data is obtained in an extremely rapid manner which permits the selection of only those samples containing analytes of interest for further in depth analysis at an off-site analytical laboratory.
Additionally, the SAW is very sensitive providing detection levels in the low ppb range and its sensitivity is not linked to any chemical feature of the analyte. This last attribute guarantees that analytes such as the volatile chlorinated hydrocarbons can be effortlessly detected as the SAW is essentially a mass-sensitive detector.
In order to ensure portability, the SAW/GC itself is a compact device using a solid state SAW detector: it is lightweight, compact, and does not require additional supply of fuel gas. When manufactured in quantity, the unit cost is likely to decrease as seen in the case with most lithographic microelectronic fabrications.
When this detector is coupled to a short, fast capillary GC column through a trap and valve, the viability of an ultra-rapid general purpose field gas chromatograph begin to become apparent.
- McAllister P.M and Chiang C.Y. A Practical Approach to Evaluating Natural Attenuation of Contaminants in Ground Water GWMR Spring 1994 161-173.
- Parker, L.V. The Effects of Ground Water Sampling Devices on Water Quality: A Literature Review GWMR Spring 1994 130-141.
- Wohltjen and R. Dessy, “Surface Acoustic Wave Probe for Chemical Analysis: I. An introduction and instrument description; II. Gas Chromatography detector; III. Thermomechanical Polymer Analyzer” Anal. Chem. Vol 52, pp. 1458-1475, August 1979
- Ballantine and H. Wohltjen, “Use of SAW Devices to Monitor ViscoElastic Properties of Materials”, Proceedings 1988 Ultrasonics Symposium, pp. 559-562.
- Lec, J. Vetelino, R. Falconer and Z. Xu, “Macroscopic Theory of Surface Wave Gas Microsensors”, Proceedings 1988 Ultrasonics Symposium, pp. 585-588.
- Frye, S.J. Martin, R.W. Cernosek, K.B. Pfeifer, and J.S. Anderson, “Portable Acoustic Wave Sensor Systems”, Proceedings of the 1991 Ultrasonics Symposium, pp.311-316., IEEE No. 1051-0117/91/0000-0311.
- EPA Subsurface Characterization and Monitoring Techniques A Desk Reference Guide: Volume 1 Solids & Ground Water Appendices A and B. Office of Research and Development, Washington DC 20460. May 1993.
This information has been sourced, reviewed and adapted from materials provided by Electronic Sensor Technology.
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