The Colloid Threat: Small, Nontoxic Particulates May Enhance Water Pollution
Raloff, Janet, Science News
The Colloid Threat
Eight years ago, scientists from Argonne National Laboratory near Chicago began studying the contamination of a shallow aquifer underlying part of another federal lab. Since 1963, workers had routinely dumped liquids from the central waste-treatment plant onto the ground of Mortandad Canyon at Los Alamos (N.M.) National Laboratory (LANL). Though these wastes contained residual low-level concentrations of plutonium and americium, LANL analyses suggested they offered no cause for concern. Laboratory calculations predicted each radioactive contaminant would tightly bind to the soil, allowing the pollutants to travel a few meters at most.
At radioactive-waste sites in Hanford, Wash., and Sheffield, Ill., field studies by the Argonne team found that soil indeed quickly ties up trace quantities of escaping radionuclides, notes William R. Penrose, a member of the 1982 Argonne team and now an environmental scientists with Transducer Research Inc. in Naperville, Ill. But at Los Alamos the radioactive wastes have not behaved as expected. They've migrated into the underground aquifer, more than 2 miles from where workers first dumped them.
Though LANL's tainted aquifer has never served as a source of drinking water, the radioactive wastes' lengthy migration raised a troubling question: Why had pollution-transport models failed to predict it?
The answer now appears to be colloids -- particulates too small to settle out of water, ranging from a nanometer to a micron in size. In the February ENVIRONMENTAL SCIENCE AND TECHNOLOGY, Penrose and his colleagues at Argonne and LANL report data indicating that LANL's radioactive wastes have been trapped by colloids and are therefore prevented from binding to much larger soil particles. Their work is one of two reports in that issue presenting evidence that conventional methods for predicting the movement of pollutants through the water in soils, streams and underground aquifers can greatly underestimate the hazards posed by a range of pollutants -- from organic chemicals leaking out of landfills or waste dumps to radioactive wastes and pesticides applied to soil.
Penrose says LANL's pollutants-migration model -- typical of most designed to characterize the transport of insoluble chemicals in soil or water -- erred by assuming plutonium and americium would behave as if they were "free" to chemically bind to large particles, like those making up soil. Instead, the team found that colloids of some yet unidentified material had trapped (and probably encapsulated) the toxic radionuclides, thereby shielding these pollutants from chemical processes that might otherwise allow them to bind to the soil -- or settle out into bottom sediment should they ever reach groundwater.
Filtering the aquifer's water showed that the colloids trapping americium are different--and much smaller--than those transporting plutonium. Not only might this allow the americium to move through water faster than the plutonium, but it also apparently explains why americium is a more persistent polluter. Plutonium levels at the most distant groundwater-sampling well (3,390 meters) are just one-thousandth those at the well nearest the waste's discharge. Americium levels, by contrast, remain constant between wells, indicating that once it enters water, no further removal occurs, Penrose says.
Both radioactive contaminants "are well within safety limits" at the most distant water-sampling well, Penrose says, and remain within the lab's boundaries. He therefore believes the real lesson here is in forecasting what might happen if high levels of other water-insoluble contaminants -- radioactive or otherwise -- are released into similarly colloid-laden waters.
A second paper in the same journal indicates that for very insoluble organic chemicals--such as DDT, PCBs and dioxin -- microscopic emulsions (stable mixtures) of surfactants and oil may play a water-pollution-enhancing role conceptually similar to that of the colloids. …