Please replace article in Technical Highlights of the ORNL REVIEW with the following for the Web version:

ORNL Develops Technique To Rapidly
Measure Mercury in Water, Sediment, and Soil


An economical, user-friendly technique for quickly measuring concentrations of mercury in water, soil, and sediment on site has been devised by scientists at ORNL. The technique, which avoids the costs of laboratory analyses, is expected to save thousands of dollars if used to characterize contamination and verify cleanup of mercury-contaminated sites.  

Already the costs of developing the mercury analysis technique have been recovered through costs avoided. In this case, ORNLÕs mercury analyzer was used to produce detailed maps of mercury-contaminated soils along the floodplain of Lower East Fork Poplar Creek in Oak Ridge. Now, the ORNL technique will be used to verify that contaminated soil has been removed and replaced with clean soil.

The federal government has decided to excavate soil from areas in the floodplain having mercury concentrations above 400 parts per millionÑthe selected remedial goal option. According to this plan, about 27,000 cubic yards of soil will be removed from two sites along the creek and taken to a permitted landfill at the Oak Ridge Y-12 Plant for disposal. 

Mercury concentrations in floodplain soil must be measured to ensure that all soil with mercury concentrations above 400 parts per million has been identified and to verify that remediation has been effective in meeting the remedial goal.

To refine the extent of mercury contamination on the floodplain, workers collected samples and measured the concentration of mercury in each sample. By determining where surface soil contains 400 or more parts per million of mercury, they then estimated the amount and location of soil that must be excavated. Preparations for disposing of the contaminated soil in the landfill were then completed. 

ÒWe first prepared and processed up to 150 samples in about three days,Ó says Ralph Turner, developer of the mercury analysis technique and a researcher in ORNLÕs Environmental Sciences Division. ÒIn this case, sample preparation involved drying, crushing, and digesting the soil samples, but samples of naturally moist soil can be digested without first drying and crushing.

ÒTechnicians dried, pulverized, and digested the soil samples. It then takes about 3 minutes to do an analysis on a soil sample and 2 minutes to analyze a water sample using our technique. Along with workers from Jacobs Engineering, we processed nearly 1100 soil samples from the East Fork Poplar Creek floodplain and refined the area that requires excavation.Ó

Labor and material for one nationally recognized laboratory analysis technique cost about $90 a sample, but for ORNLÕs mercury analysis technique, they cost only $35 a sample. Based on these costs, Turner says, if the mercury analyzer were used to direct and confirm cleanup, the government would save several million dollars in floodplain remediation costs. The savings come from reduction in the cost of analysis and in the time that soil excavators and handlers are idle. 

DOE and the Environmental Protection AgencyÕs (EPAÕs) Region IV have approved use of this technique for analyzing mercury levels in the East Fork Poplar Creek floodplain before remediation begins. ÒOur technique,Ó Turner says, Òis now being used to confirm that cleanup of the East Fork Poplar Creek floodplain has achieved allowable levels. EPA has approved use of the technique for this activity provided that a few samples also be analyzed by a conventional method.Ó

In the ORNL technique, the mercury-containing soil samples are chemically treated before analysis to transform the mercury into an easily detectable form. To liberate the mercury from the soil particles, the soil sample is digested using aqua regiaÑa mixture of hydrochloric and nitric acids often used for dissolving platinum and gold. 

For both water and soil samples, stannous chloride is added. The tin in stannous chloride, which has been used for many years in conventional mercury analyses, supplies the electrons to reduce oxidized mercury to the metallic element, which tends to escape from water to the air as a vapor. 

The 1-liter plastic bottle containing the sample is then shaken by hand, causing about one-third of the elemental mercury in the soil solution to leave it as a vapor and to mix with air in the headspaceÑthe space between the top of the solution and the bottle cap. ÒThis partitioning of the volatile elemental mercury between a liquid and a gas according to HenryÕs Law is important,Ó Turner says, Òbecause the analyzer detects and measures only mercury vapor in air or some other gas, not mercury in water.Ó 

The amount of mercury vapor in the bottleÕs headspace is measured using a commercially available portable analyzer. The analyzer takes advantage of mercuryÕs affinity for gold; the electrical conductivity of a gold foil in the analyzer is affected by the amount of mercury attracted to it, so the measured change in conductivity indicates the mercury concentration. 

The battery-powered mercury analyzer, which can be rented, is about the size of a big loaf of bread. Other materials used with it can easily be packed in a shoebox, except for the bottle in which the headspace measurements are made.

The development of the mercury analysis technique was supported by the Environmental Restoration Division of DOEÕs Oak Ridge Operations.