Because several technologies including chemical, geophysical , radionuclide, and sampling and sampler emplacement exist to evaluate contamination in soil, determining the level of contamination is no easy task. While site characterization studies are done to measure the pollution present at the sites, how the measurement is done relates to soil type, geographical area, suspected pollutants, and more. In fact, to help choose the best analytical method of measurement, the Environmental Protection Agency (EPA) offers an Environmental Monitoring Methods Index to assist engineers in comparing and evaluating the available methods. Right now, the index contains information on more than 3,400 abstracts of different methods. This article focuses primarily on advances in chemical technologies (see table) used to measure contaminants.
During a typical site characterization, soil samples are collected and analyzed for chemical and radioactive content. Traditionally, the level of pollution has been measured by collecting large soil samples and sending them to a lab for analysis. Yet the cost of this process is often prohibitive since so many samples are needed - and tests cost between $200 and $2,000 for results that take anywhere from two to eight weeks, according to Marc Wise, Ph.D., and member of Oak Ridge National Laboratory (ORNL) organic chemistry instrumentation group in Oak Ridge, TN. To help reduce costs, many researchers have developed field-based testing methods.
|Colorimetric Test Strip||single-measurement, portable technology using wet-chemistry nonimmunoassay tests||Nitrates, TNT, RDX, and HMX||easy-to-use, potentially cost effective, real-time data||potential interference caused by nitrite, creation of soil slurry needed|
|Cone Penetrometer||Uses a fiber-optic based, laser-induced fluorescence sensor system||Petroleum hydrocarbons||Real-time, cost-effective, accurate measurements, 3D mapping, fingerprint capable, no soil cuttings, quick decontamination||Expensive for some locations; naturally occurring fluorescent material can cause false positives; tough to maneuver in small spaces; limited by rough terrain|
|Fiber Optic Chemical Sensor||Coating-based sensors on fiber optics that monitor change in refractive index in soil gas||TPH, benzene, toluene, ethylbenzene, BTEX, halogenated VOCs||Easy-to-use, portable, potentially cost effective, rapid turnaround||Possible interference from chlorinated VOCs; concentration of contaminants affects results time|
|Gas Chromatography||Separates and analyzes target analytes in complex mixtures||Halogenated and nonhalogenated VOCs, SVOCs, PCBs, TPH, pesticides and dioxins||Low detection limit, quick turnaround, portable, high quality data, good correlation with EPA data; simultaneous analysis for BTEX and other hydrocarbon compounds||Suitable for thermally stable compounds only; high learning curve; extraction time modifications required; poor extraction of diesel fuels with highly organic soil|
|Immunoassay||Uses antibodies to bind to specific substances||Halogenated VOCs, PAHs, TPH, BTEX, PCBs, organic pesticides, mercury||Near real-time, reproducible results; correlation with lab results; low rate of false positives; can define boundaries of pollutant||High rate of false positives in results from PCB and pesticide kits; can't identify individual PAHs; not appropriate for peat or bog|
|X-ray Fluorescence||Operates on principle of energy dispersive XRF spectometry||Heavy metals||No investigative derived waste; correlates with lab results; real-time, quick turnaround; determine multiple analytes simultaneously; consistent data quality; little sample preparation||Limit on penetration depth; heavy portable units; hard to obtain low detection limits|
PAH = polycyclic aromatic hydrocarbons
BTX = benzene, toluene, and p-xylene
BTEX = benzene, toluene, ethylbenzene, and xylenes
TPH = total petroleum hydrocarbons
PCB = polychlorinated biphenyls
RDX = royal demolition explosive
HMX = her majesty's explosive
VOC = volatile organic compounds
Immunoassay field analysis generates results in real time with cost-effectiveness. Three PCB immunoassay tests have been approved by the EPA Environmental Technology Verification pilot program: D TECH, EnviroGard, and RaPID Assay System from Strategic Diagnostics, Inc.
The kits produce results in just 30 to 60 minutes and measure contamination directly within the soil, reducing the number of samples that need to be analyzed off-site. Each usesa specific antibody to detect the analyte of interest. Currently, they can be used for PCBs, triazines, organophosphates , cylodienes, and sulfonyleureas. However, immunoassay kits can't screen more than one analyte nor can they identitfy analytes; they just measure. In addition, reagent stability is often a problem.
Spectral data identifies material based on how they absorb and reflect light. The H.E.L.P. Mate 2000 from Hanby Environmental Laboratory Procedures in Houston , TX., is a colorimetric procedure that "utilizes very vigorous Friedel-Crafts reactions to precipitate brilliantly colored products from fuels, solvents, and oils," says Hanby, president. The signals are produced by distinctive but transient (15 - 20 minutes) chromophores created by the reaction. Generated signals are reflected to a Charge Coupled Device (CCD) in the spectrometer. The large signal-to-noise ration from the readings allows use of reflective light spectroscopy, for the first time allowing UV spectral regions to be utilized for molecular analysis.
According to Hanby, "the robustness of the signal allows very sensitive (sub-ppm) detection, and the unique 'fingerprint' produced in the UV and visible spectral regions scanned provide repeatable qualitative analysis for the petroleum substances this method is used for."
The advantages of both systems according to Hanby are "rapidity, sensitivity, accuracy, ease-of-use, and economy - and there are no other portable spectrometers available to perform this UV/Vis reflectance analysis."
While cone penetrometer technology (CPT) isn't new, advances in the sensor field have given it new life. CPT provides continuous, subsurface, screening-quality, real-time data (physical, electrical, and chemical measurements) and minimizes disturbance to the subsurface because hole diameters measure less than 2" and no drilling fluids are used. It can also be adapted for new sensors as they come along.
The penetrometer rod has a steel cone at its end that is hydraulically pushed into the ground while measurements are collected and transported to the surface. Penetration rates are usually 40 to 50 feet per hour although they can go as high as 180 feet. As the rod progresses into the ground, a computer reads data from sensors located in both the tip and the side of the probe.
The new dual mass dynamic cone penetrometer from Kessler Soils Engineering, Inc in Springfield, VA, has a dual mass hammer that gives accurate readings in both soft and high strength soils. According to Ken Kessler, president, the company is working with the US Army Corps of Engineers on several projects which include a way to provide data on the moisture content of soil and an automated data collection device that would be used to collect and store data in the field. They're also "conducting investigations into the correlation between data (mm/blow) from the DCP and k, R, modulus, SPT, n and CBR." In addition, they produce disposable cones that mount on an adapter screwed onto the penetration rod to alleviate difficulty in retrieving the test cone from the soil.
Applied Research Associates in Albuquerque, NM, is working on a sonic-enhanced cone penetrometer to enhance the penetration capabilities of a CPT, thereby improving its suitability for dense non-aqueous phase liquid characterization and monitoring. The overall goal is to develop a technique to cut through soils otherwise impenetrable by either static or sonic CPTs.
While typical industrial sites being monitored for pollutants don't often have explosives to contend with, 50 of the Superfund sites do. Fiber optic sensors which are used to remotely monitor contaminants in the vadose zone and in groundwater are based on the ability of fused quartz optical fibers to transmit probe signals of visible or near infrared light making them ideal of evaluating sites for explosives. The signals are immune to interference and provide real-time multi-point monitoring to detect low part-per-million to part-per-billion gas-phase levels of contaminants.
Dr. Paul Charles and colleagues at the Naval Research Center in Washington DC have developed a field-portable continuous flow fluorescence based immunosensor to detect soil contaminants such as PCBs, TNT, and RDX. The sensor uses antibodies as recognition elements for specific antigens and the "antibodies are covalently immobilized on a solid support matrix" that is subsequently saturated with a fluorescence analog of the targeted contaminant such as cyanine dye. According to Charles, "the derivatized matrix is prepacked into a micro column with a continuous flow stream of buffer that removes nonspecifically bound fluorescent analog. After a stable baseline is obtained, sample injections of the desired pollutant into the flow stream displaces the fluorescence analog from the immobilized antibody on the solid support. A signal response over background from the displaced fluorescence analog is measured and integrated by an in-line fluorometer." They've been able to detect low level concentrations: TNT and RDX at 20 parts per billion and PCBs at 1.0 part per million.
Both a fiber-optic biosensor and the continuous flow immunosensor, will be used at the Umatilla Army Depot Activity and Submarine Base in Bangor. The sensors will measure the proportional level of fluorescent activity caused by the introduction of the sample to the system. The fiber-optic biosensor shows contaminant molecules competing with fluorescent antibodies for a binding site on the fiber optic core.
X-radiographs of sediment samples and cores are a standard part of many geologic investigations and traditionally were accomplished with film-based x-rays that sometimes would then be digitized. Just this month (March) Varian Imaging Products in Palo Alto, CA, released the VIP-9 SH/1, a real-time, digital x-ray fluoroscopic imager designed for energy uses up to 1,000 kV in industrial and scientific applications. Using amorphous silicon fabricated into a sensor panel, the sensing technology allows receptors large enough to use in general x-ray imaging, replacing traditional x-ray film.
"This new model permits use of the imager up to 1,000 kV, which is necessary for inspection of large castings, thick laminations, and other high density objects," says David Gilblom, general manager of Varian Imaging Products. "This high voltage can be used as long as the primary beam is confined to the imager active area. The electronics are arranged so that they are never in the path of the primary X-ray beam."
Cindy Bruckner-Lea, PhD and senior research scientist within the Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory in Washington and her colleagues are working on an automated nucleic acid processing module that allows concentration of DNA on the order of milliliters "that bridges the gap between real world samples and minature detectors."
Nucleic acid-based techniques can be used for bacterial identification and characterization in soil because of such molecular biology techniques as polymerase chain reaction (PCR). However, as Bruckner-Lea points out, "the routine use of nucleic acid detection methods in many environmental applications is limited by the time and labor required for manual sample handling for nucleic acid purification, and removal of inhibitory compounds that interfere with subsequent manipulations such as PCR and fluorescence detection."
As Bruckner-Lea says, "this concentration and purification step will be critical for applications such as the environmental analysis of bacterial pathogens and the monitoring of food processing lines which require low detection limits. The system is compatible with any downstream detector and the procedures and methods can be easily modified for specific applications. A tiny renewable micro-column is used to capture, purify, and then release DNA. Since the micro-column is automatically disposed of after each use, this system is suitable for field use with little or no operator intervention."
Once the measurement method has been determined, treatment follows. And here, the field of bioremediation is booming. Bioremediation treats contaminated soils by utilizing living organisms to degrade organic compounds and reduce or eliminate hazards resulting from accumulations of toxic chemicals and other hazardous wastes.
Environmental Directions, Inc, (EDI) of Roanoke, VA, carries out treatability tests with to engineer site-specific enzymes to assist in reconciling laboratory results with on-site testing. Barry Osborne, vice president, says, "in the laboratory, we isolate exactly the enzyme responsible for the breakdown of a site-specific contaminant. Factors such as the acidity or alkalinity of soil can introduce a significant variant in the development of an enzyme. We isloate an indigenous enzyme, which we then grow and enhance in order to accelerate contaminant breakdown."
This proprietary enzyme, Microzymeğ, is a site-specific procedure for selecting microbial populations, growing and stabilizing these microbes and designing nutrient additions specific to meet the needs of each site which involves a three-step procedure.
Initially, EDI receives a sample of the material to be degraded and background samples, as well as a complete analysis of the geographic and climatic conditions of the area surrounding the site. While maintaining similar environmental conditions, naturally occurring organisms are isolated and selectively adapted to degrade the contaminant of concern.
To date, EDI has been successful in treating sites up to 10,000 tons. EDI's experience has indicated the validity of laboratory testing up to such site sizes, although Osborne notes that the treatment of larger sites, which may exhibit a variety of geographical and geological
characteristics, has not yet been tackled.
Once contamination has been determined and measured, a popular cleanup is method is bioremediation, which treats contaminated groundwater and soils by utilizing living organisms to degrade organic compounds and reduce or eliminate hazards resulting from accumulations of toxic chemicals and other hazardous wastes.
AGI Technologies, of Bellevue, WA, is in the process of commercializing two intrinsic bioremediation test kits. The first is designed to measure extremely low concentrations of dissolved hydrogen in contaminated soil samples. Whilst the technology for measuring such concentrations has been available since 1983, it has involved the use of a high-priced reduction gas analyzer, and has been largely employed in the semiconductor industry.
AGI Technologies set about the task of designing an accessible testing kit capable of detecting dissolved hydrogen in concentrations of "parts per quadrillion". The basis of the kit is currently subject to a patent application, and Patrick J Evans, PhD, Associate Chemical Engineer at AGI, was understandably coy about revealing the methodology underlying the kit, but noted that its development "had not been easy."
AGI is confident that its development will make low concentration dissolved hydrogen testing accessible to a wider range of consultants and environmental agencies than is currently the
case, thus augmenting both the quality and quantity of data.
AGI's second major development has been in the development of a ferric iron assay. "This is an area of testing which has simply been unavailable until now," said Evans. "Testing has hitherto been confined to ferrous iron, which is an unreliable indicator of current and future contaminant degradation.
"By developing a test for the detection of ferric iron, we are focusing on the active agent, rather than measuring the existence of ferrous iron, which is the end result of a process of degradation." Detection of ferric iron in contaminated sub-soil is a key element in the quantification of the degradation of carcinogenic vinyl chlorides. AGI's new testing kit therefore opens up the possibility of accurately detecting this key element in active bioremediation.