Some toxic chemicals buried in soil may not be as hazardous as once thought because their toxicity decreases over time, according to a Cornell University scientist.

During the long time many toxic organic chemicals remain in soil, some seem to slowly become hidden within the soil structure. In this hidden form, they are less available to living organisms, according to Martin Alexander, the Liberty Hyde Bailey Professor of Soil Science at Cornell.

The result: some soil may be less hazardous than originally believed.

"Assessments of the hazards from those sites have not taken into account the slow processes that may take place and possibly reduce the impact of toxic compounds deposited in the soil, particularly those compounds which do not leach out to contaminate underlying aquifers," Alexander said. "In recent years, evidence has accumulated that the availability of certain organic compounds changes as the compounds reside in soil for some time -- a process that has been termed 'aging.' "

His research, "How Toxic Are Toxic Chemicals in Soils," was published in the November issue of the American Chemical Society's journal, Environmental Science & Technology. Soil scientists have long examined what happens when chemicals -- such as chlorinated hydrocarbons once widely used as insecticides -- get into the soil. Alexander and his colleagues began looking into the behavior of organic pollutants in soil about four years ago, examining why and how some of these toxic compounds became hidden and unavailable in the soil.

The problem is monitoring such chemicals. Scientists were puzzled by why bioremediation of some chemicals slowed and possibly stopped when microbes responsible for the bioremediation were still present in the soil. What process could account for this strange behavior?

"From day zero, many compounds are broken down by microbes," Alexander said. "Then in time the chemical is hidden and becomes unavailable within the soil structure where it may no longer present a problem. That's why we believe the soil containing some compounds may not be as hazardous as once thought."

Researchers studied which chemicals dissipated at what rates. Compounds such as DDT become less and less available to microbes over time. Alexander believes that microbial activity then contributes to a compound's effective disappearance.

"Even with single compounds, the times and percentages of chemicals that become unavailable may vary greatly," Alexander said. "A relationship to soil type or climate is not evident from the available data, but a systematic study was never conducted to establish such a relationship.

"These investigations show that some abiotic processes make organic compounds less readily available to microorganisms and probably to animals and plants as well," he said.