QUESTION FROM DECEMBER’S CHEMICAL PROCESSING
|We recently had a spill at our chemical plant during the night. The night-shift supervisor didn’t write it up, so we weren’t sure of the exact location. Since our plant is in the Southeast, the soil is sandy and we were worried about the spill contaminating any water wells in the area.
Is there an easy way to locate, track and identify a chemical spill? Is some sort of equipment available that can do this?
Conduct a geophysical survey
The answer to this challenge is more straightforward than it might appear at first. There are some simple things that you or an environmental consultant can do to locate the spill quickly and relatively inexpensively.
The first is a geophysical survey. The technique requires a moderately expensive transmitter/receiver combination to measure electrical conductivity in the ground. The equipment is portable and relatively easy to use, but you might want to hire a trained operator to interpret the results. Two or three types of transmitters/receivers are commercially available for rental.
The first type is a ground penetrating radar (GPR) survey. The device is wheeled or dragged over the ground with the antennas pointing downward. The readings show up as radar traces. This is excellent for finding buried drums and metallic objects, but it is often blind if there is a high clay concentration in the soil, or if there is the presence of salt water. The second type is an electromagnetic (EM) survey device. The equipment is portable and can measure the conductivity of soils to a depth of about 6 meters. The third type is a resistivity survey, which is slightly more difficult to use and requires a geologist to conduct and interpret.
I have used an EM device to locate buried oil. The conductivity data was cross-referenced with physical survey data, and the result was placed on a grid structure, which was then plotted using 3-D topographic mapping software. The location of the oil (crude in this case) corresponded to the low points in the topographic survey profiles. One caution is in order, and that is that the EM survey will detect changes in conductivity from any of a number of sources. If there is disturbance in the soil from excavation or other activities, including the presence of pipelines or buried metals, the EM survey will show those as well, although those tend to be shown as high points on the survey plot.
Another type of survey you can use to locate volatile compounds is a soil gas or vapor survey. In this type of survey, a shallow probe is driven into the ground and the air in the soil is sampled and analyzed by a gas chromatograph. Depending upon the type of soil and the volatility of the compound, it might be possible to conduct this type of survey using a photoionization detector or other handheld device, if it has sufficient sensitivity.
The cost of a geophysical survey will often be between $3,000 and $5,000, but it can save many thousands of dollars compared to drilling wells to detect underground contamination.
The cost of a vapor survey will be about the same, but it depends upon the porosity of the soil and the volatility of the compound you are seeking. The results are quite accurate, but in some cases the detection ability for chemicals dissolved in water is very low because of the low vapor pressure due to the low solubility of the compound in water (Henry’s Law). In desert areas, underground plumes have been tracked through 100 ft. or more of soil. In the Southeast, even in clay soils, it is often possible to locate a plume at depths greater than 30 ft.
By comparison, well construction and exploration will cost between $30 and $50 per foot, plus a one-day mobilization charge of $500 to $1,000. A well survey can easily cost upward of $10,000, and many can go up to $30,000 or higher. The well installation directly locates the product, tells you how thick it is and how much is dissolved in the groundwater, but when you drill a monitoring well, it can often cost more than $500 per well, plus the cost of the analyses.
You might ultimately have to drill monitoring wells to satisfy the regulatory authorities and protect the public health, but that’s a concern you will have anyway, and will have to be addressed as part of the cleanup plan.
David Russell, P.E., president
Global Environmental Operations Inc., Lilburn, Ga.
Know the properties of the chemical
If you know which chemical was spilled, the MSDS might be a good source for information about the physical and chemical properties, which might be used to locate the general area of the spill. There are several easy ways to help locate a spill: a portable Flame Ionization Detector (FID) for many hydrocarbon spills; chemical-specific detector tubes (although not very accurate); and your lab might be able to perform wet-chemistry tests on surface-soil samples. Visual tests, such as soil discoloration, are also a good way to find the general location of the spill.
In addition to the MSDS, the manufacturer of the chemical can offer specific information about spill detection.
Unfortunately, tracking the spill as it migrates through the soil is difficult because of scant data available on the subject. Some chemicals behave according to the octanol-water partition coefficient, which is an index relating the extent to which the chemical will stay in the soil or will dissolve in groundwater. Qualitatively, the migration depends on the physical and chemical properties of the liquid, the texture of the soil and the presence of other chemicals at the spill site.