Issue Date: September 1, 2008
Sleuthing Out Contamination
PERCHLORATE. BENZENE. Dissolved organic material. Uranium. All of these chemical species can have adverse effects on human and environmental health and can be found in groundwater in various parts of the U.S. After such contaminants are detected in groundwater, several common questions arise: What is the original source? Is the contaminant natural or synthetic? How would natural degradation or a remediation project affect the chemistry of the contaminant?
At a symposium in the Division of Agrochemicals at the American Chemical Society meeting in Philadelphia last month, four researchers explained how they use sophisticated analytical instrumentation and field-ready, nearly real-time sensors to answer such questions.
Groundwater contamination can start out as a big mystery, said Suresh Mislankar, a product responsible scientist with Bayer CropScience and one of the symposium organizers. Researchers such as those at the session use analytical techniques to do the detective work needed to determine the origin of contaminants and how best to treat them, he said.
Robert L. Cook, an environmental analytical chemist at Louisiana State University and a symposium organizer, said he considers this type of detective work, which is sometimes referred to as environmental forensics, as "an emerging and dynamic area of environmental research." He notes that some groundwater contamination mysteries have legal implications, such as assigning liability for cleaning up a chemical spill. Other contaminants are natural, and their origin can give scientists a better understanding of how nature operates.
Perchlorate contamination, for example, can stem from natural or synthetic sources. Each source has a unique isotopic signature. Stable isotopic analysis with isotope-ratio mass spectrometry (IRMS) can help distinguish between natural and synthetic origins as well as provide information about mechanisms of natural perchlorate formation, said Paul B. Hatzinger, a senior research scientist with Shaw Environmental in Lawrenceville, N.J.
Perchlorate can affect the thyroid's ability to take up iodide and even small amounts of perchlorate can lead to improper functioning. Widespread perchlorate contamination of groundwater, exceeding microgram-per-liter levels in many parts of the U.S., prompted analytical work, he explained.
To do the analysis, Hatzinger and his colleagues first had to create a database of isotopic signatures from perchlorate samples of known origin. They collected samples of synthetic perchlorate from various materials, including fireworks, road flares, gunpowder, and even household bleach, to assess the extent to which these sources contribute to groundwater contamination.
THEY OBTAINED SAMPLES from natural sources too. For example, perchlorate is known to be present in the Atacama Desert of Chile, where millions of tons of nitrate fertilizer were mined and used on some U.S. crops until the mid-20th century.
Then, the researchers collected groundwater samples at a number of locations with known perchlorate contamination, including sites in West Texas, Southern California, Long Island, and New Mexico.
Because perchlorate concentrations in the environment are often low, field collection can be labor intensive, Hatzinger said. To obtain enough perchlorate for analysis from soil, he and his colleagues recently extracted the compound by passing about 2,000 gal of water through about 5 tons of soil. That water then went through an ion-exchange resin device about the size of a soda can, so it took several days to collect 5 mg of perchlorate anion for IRMS, he explained. Sampling a well doesn't involve soil, but often requires passing a similar amount of water through a resin column.
Back in the lab, Hatzinger and coworkers extract perchlorate from the resin, purify it, and analyze it for stable isotopes of both oxygen and chlorine to get a signature. Finally, they compare signatures of perchlorate from synthetic and natural sources to those of the samples derived from groundwater and wells.
Hatzinger said the isotopic data from some locations, including Long Island and Southern California, clearly indicate multiple sources, with signatures resembling those of synthetic perchlorate in some wells and Chilean natural perchlorate in others. Because the areas had a history of agriculture nearby, a legacy of Chilean perchlorate permeates the groundwater in those regions, as it does in many locations, Hatzinger said.
But the signatures from West Texas and New Mexico groundwater samples were not similar to either natural or synthetic signatures. Hatzinger and coworkers are carrying out further studies to determine the origin of perchlorate in those samples.
R. Paul Philp, an environmental geochemist at the University of Oklahoma, Norman, also uses IRMS to study groundwater contaminants, but rather than tracing their origins, he is more interested in the contaminants' fate.
He and his colleagues use signatures of carbon and hydrogen isotopes to track the natural degradation of groundwater contaminants such as benzene compounds, which are carcinogenic and are used in numerous industrial applications.
Even if a site is contaminated with only a single compound, analyzing samples can get complicated. The stable isotope approach in combination with gas chromatography and GC/MS is particularly powerful for discriminating multiple sources of a single compound in one area, Philp said. Manufacturers sometimes use different starting materials for the same product, and thus each product has a different fingerprint.
Right now, many companies responsible for contaminated sites prefer to remediate by letting native microorganisms digest contaminants into more harmless compounds, Philp said. But they need a way to determine whether the contaminant is actually degrading.
Concentration alone is not a defensible measure. "Using changes in concentration can be misleading because concentrations can change as a result of dilution and dispersion, not necessarily microbial degradation," Philp said.
Isotope analysis can help distinguish the cause of concentration changes. If a compound that contains carbon is degrading, the lighter isotope, 12C, is removed more rapidly than the heavier isotope, 13C. In general, enrichment of the heavier isotope as the concentration of the compound decreases is "strong evidence that natural attenuation is taking place," Philp said.
Philp is collaborating with an environmental company to investigate the natural attenuation of extensive contaminant plumes in groundwater at military bases in the Midwest. The researchers are doing long-term degradation monitoring of chlorinated solvents, such as tetrachloroethylene, a toxic compound used in base facilities for cleaning military equipment, from jet engine parts to vehicles.
Philp and his colleagues are also using IRMS analysis to track sources of some contaminants at the military bases by matching sample signatures to international standards or to signatures of solvents they have collected from different manufacturers.
MEANWHILE, other environmental detectives use techniques not based on isotopic analysis. For example, William T. Cooper, an analytical and environmental chemist at Florida State University, and graduate student Daniel Osborne have used an MS technique to determine whether landfill leachate can be treated sufficiently to permit it to go to wastewater treatment plants for further cleanup.
When rain mixes with garbage in landfills, the resulting dark-colored liquid gathers in the facility's drainage system. If spilled in the environment, this landfill leachate can wreak havoc on groundwater because the leachate's high concentrations of easily oxidizable organic matter steals oxygen from native aerobic organisms in soil. In addition, its dark color can block sunlight needed for photosynthesis.
"This water is pretty nasty stuff," and a regular wastewater treatment plant can't process it, Cooper said.
But a new process Cooper's group developed shows promise for pretreating the nasty stuff. They used the process to treat samples of leachate collected from the Tallahassee Solid Waste Facility in Florida. Oxidizing the dark-colored organic matter by ozonolyis and ultraviolet radiation for two hours cleaned up the landfill leachate enough that it could be sent to a wastewater facility for further processing, he said.
Cooper and his colleagues pioneered a Fourier transform ion cyclotron resonance MS (FT-ICRMS) method that can detect the photochemical degradation of complex mixtures of high-molecular-weight dissolved organic matter one molecule at a time. The technique helped them identify the molecular changes in the complex mixtures that give rise to changes in their photochemical properties during treatment.
Several hundred highly unsaturated organic molecules, many containing sulfur, appear to be responsible for the dark color of landfill leachates, Cooper said. Once these compounds are removed photochemically, the leachate's color—indicating the bulk of its organic contaminants—disappears. Then the liquid is ready to go to a standard wastewater treatment plant for further purification, Cooper explained.
As the work of Hatzinger, Philp, and Cooper demonstrates, analytical sleuthing often depends on lab-based instruments. But because field sites are usually far from labs, lab instruments often cannot do real-time analysis. Delays between collecting samples in the field and analyzing them in the lab can be anywhere from several hours to many weeks and can affect progress of the research.
To bridge collection and analysis, some researchers use field sensors. For example, Diane A. Blake, a biochemist at Tulane University School of Medicine in New Orleans, is developing sensors with low parts-per-billion sensitivity and the ability to do analysis immediately after sampling. "We can give you an answer in an hour or less out in the field," she said. "That's as close to real-time as you can get."
Blake's field sensors use immunoassays based on antibodies targeted at groundwater contaminants such as uranium. Her uranium sensor eliminates the time needed to truck potentially uranium-containing samples back to a lab for testing with a kinetic phosphorescence analyzer, an instrument that detects the presence of lanthanides in solution.
Her uranium sensor is based on a fluorescently labeled antibody that binds to a uranium-chelate complex. The sensor detects a change in fluorescence when the antibody binds to chelated uranium in an environmental sample.
Blake is developing sensor prototypes in conjunction with Sapidyne, in Boise, Idaho. This summer, she and her students tested the uranium sensors at a Department of Energy site in Rifle, Colo., that has groundwater contaminated with uranium from mine tailings. At the site, scientists from DOE and several universities have stimulated naturally occurring microbes to clean up groundwater by reducing uranium(VI) to uranium(IV), which is insoluble and precipitates out. Blake and her students have monitored the microbes' progress in the field with their automated, battery-operated sensors.
THE SENSORS are much more versatile in terms of analytes than many lab instruments, Blake added. She can use different antibodies and chelating ligands in the sensor to detect, for example, caffeine from environmental surface waters or cadmium in blood serum. The sensor format is up to three orders of magnitude more sensitive than microwell-plate-based immunoassays, depending on the antibody she uses.
Blake said the almost real-time nature of the sensors could be valuable in locations that experience periodic episodes of abrupt contamination. An example is a Louisiana bayou known to be heavily contaminated with lead, but the lead is usually sequestered in muddy bottom sediments. Every so often, that lead dissociates from the sediments, travels into the nearby soil, and contaminates groundwater supplies.
The contamination stops almost as quickly as it starts, and no one has been able to confirm what causes the sudden change because researchers need to wait at least a week for analytical results to come back from the lab. Sensors can be used to investigate such phenomena much more quickly.
Analytical techniques may not solve all of the problems that arise when contaminants are found in the environment. But they can certainly help bring researchers closer to solving the mysteries of contaminant sources and determining optimal treatment approaches.
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