XRF Marks the Spot

The benefits of X-ray fluorescence are helping change the way people look at brownfield remediation

• By James Martin
• Mar 01, 2006

This case study describes the use of portable X-ray fluorescence (XRF) instrumentation in site characterization and corrective remediation for heavy metals and other contaminants, a process that allowed the environmental engineers to accelerate remediation efforts as the client prepared to invest in the redevelopment opportunities of a commercial 26-acre site in Massachusetts.

Brownfield Background
Many cities across the United States are facing tight budgets due to state spending cuts, rising costs, and a weak economy, but the U.S. Conference of Mayors believes developing contaminated vacant lots and industrial sites (brownfields) could help their financial health. A survey found that redeveloping brownfield sites could generate more than 575,000 new jobs and $1.9 billion per year in new tax revenue for cities.

However, when compared to the trouble-free buying and selling of properties in normal (or undeveloped) areas, developers of brownfield sites can face significant costs, delays, and risks because they must assess site contaminants and develop remediation plans. A typical assessment can cost up to $40,000 and can take weeks or even months to complete.

Government tax incentives and assistance programs are helping to reduce the burden and make brownfield development more attractive. New technologies are also helping. One breakthrough has come in the form of handheld XRF instruments. The X-ray fluorescence analyzer can be used to identify contaminant metals in a soil sample in the field, right on site in seconds or minutes.

Soil Testing and Portable XRF
Field screening XRF tests can provide a number of benefits to cleanup efforts, including:

• Fast site surveys, with quality assurance on results to complement lab analysis. Contaminants and other analytes of interest can be measured simultaneously and evaluated. The same soil samples can be sent to the lab for confirmation.
• Reduction of overall testing time and costs. One can identify specific areas and establish boundaries in a site for clean up, reducing the overall effort and costs.
• Assurances of a properly completed site clean up. This benefit ensures that remediation contractors do not leave prematurely.
• Minimization of the amount of soils with hazardous waste that has to be removed or processed. On-site in-situ testing ensures only soils having hazardous wastes above limits are removed for costly disposal or processing.

A portable XRF analyzer consists of three basic components: an X-ray source; an X-ray detector; and electronics for signal processing, calculations, and results. The XRF directs a beam of X-rays at a soil sample. The atoms of the various elements in the soil absorb the X-rays and re-emit X-rays unique to their atomic structure (i.e. characteristic X-rays). Modern portable XRF systems use X-ray tubes rather than radioactive isotopes. X-ray tubes have numerous advantages, including no NRC regulations, ease of travel, faster testing, and lower detection limits.

Portable XRF analyzers use an X-ray detector, conditioning electronics, and on-board PDA (personal digital assistant) software to quantify various levels of elements in a sample. A detector measures the energy of each X-ray that strikes it and counts the total number of X-rays in discrete energy bands. The electronics and software process this information to yield concentration data for various elements in soils or other samples.

In-situ XRF Testing and Results
EnviroScience Consultants Inc. provided on-site screening for lead and arsenic (Pb and As) in the soil at a commercial property in response to a Remediation Action Measure Plan that was required by the Massachusetts Department of Environmental Protection.

The screening team used portable XRF analysis to perform on-the-spot analysis. More than 150 in-situ tests of the soil were performed at the site to screen for lead and arsenic (Pb, As), and over 40 tests to screen for barium (Ba). Barium testing was suspended per the request of the licensed site professional after the first day. Short test times allowed a large number of samples to be analyzed, and as a result, a more complete, accurate, and detailed site description was produced. Field screening provided a fast and cost-effective method to thoroughly delineate contamination at the site.

Table 1 contains the minimum and maximum concentrations of arsenic, barium, and lead found at the site using the Innov-X hand held XRF. All results are recorded in parts per million (ppm). Typical testing times were in the range of 30 to 45 seconds.

The handheld XRF enabled the engineers to determine site contaminant boundaries, monitor remediation, pre-screen soil samples, and minimize costly off-site lab testing. A common thread throughout these uses is the rapid identification and quantification of the elements present to allow fast and accurate analysis in the field. Post-remediation sampling and field screening with XRF demonstrated the levels of Pb in the soil to be below the regulatory threshold.

Challenges of Site Cleanup
At larger sites, there is often a trade-off between the numbers of tests needed to properly delineate the contamination patterns, project schedule, and cost. In general, operators can use portable XRF either in-situ (i.e. tests directly on the ground) or for collected, bagged samples. In either case, test results provide concentrations for more than 20 metals within seconds. Because a large volume of tests can be performed quickly, sites may be rapidly "scanned" for contaminants and patterns, allowing informed decisions for sample collection, preparation, and additional analysis. Accurate field testing helps limit the amount of contaminated soil removed to that where contaminants exceed the acceptable level.

Field Screening Precautions
It is crucial for XRF operators to understand potential sources of error for field-screening work, particularly in-situ analysis. Method detection limits depend on several factors, including X-ray excitation source and strength, analytes of interest, type of detector used, times to test samples, physical and chemical matrix effects, sample preparation, and interference from the spectrum of other elements present. Accuracy can be greatly affected by sample preparation. This is largely due to the way in which accuracy is established -- by sending comparative samples to the lab. Without thorough sample preparation that produces a uniform, homogeneous sample type, an XRF analysis followed by a lab analysis risks comparing "apples to oranges."

In general, in-situ testing is used for field screening, and prepared samples are generated for quantitative XRF analysis and laboratory confirmation. An XRF is a screening instrument only -- it cannot be used for any other purpose within the construct of the Massachusetts Contingency Plan. XRF analysis can be used to indicate impacted areas, but only laboratory analyses can determine if an area is not impacted.

Conclusions and Further Considerations
After an initial in-situ XRF survey, the results are used to identify areas that warrant a more thorough investigation. Typically these are areas of low contaminant concentrations or levels near the clean up requirements as identified by the in-situ testing. In these cases, operators often collect samples and prepare them to confirm the in-situ findings with laboratory-accurate results. Portable XRF almost always yields better accuracy when samples are prepared as they would be for XRF lab analysis. Collected, prepared samples are ideal for laboratory analysis because they provide a direct comparison with portable XRF results. XRF testing is non-destructive, allowing the same prepared sample (or split) analyzed via portable XRF to also be analyzed by a laboratory system. For prepared-sample analysis, each sample is first prepared by sifting, grinding, and homogenizing the selected soil. The prepared samples are placed in standard test cups and analyzed with the portable XRF unit. Site-specific calibrations are not necessary as the analyzer's PDA software automatically corrects for variations in sample chemistry and density.

This article originally appeared in the 03/01/2006 issue of Environmental Protection.