Assessing site contamination
last updated 08/00
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Sources: Tri-service SCAP website, Science and Engineering Associates website
(http://aec-www.apgea.army.mil/prod/usaec/et/scaps/intro.htm and http://www.seabase.com/tech/spark.shtml)
Keywords: contaminated soil, contamination, site assessment, petroleum contamination, heavy metals, VOCs, explosives.
Cost-effective remediation of contaminated soils requires detailed knowledge about the level and extent of contamination at each site. Over the past five years the U.S. military has developed a Site Characterization and Analysis Penetrometer System (SCAPS), which allows rapid on-site assessment of levels of several key pollutants.
Urban regeneration in the U.S. and Europe is becoming increasingly dependent on the redevelopment of 'brownfield sites' - land previously used (but no longer needed) by manufacturing and industry. Such land often lies close to the center of cities, and is potentially a very valuable commodity. However, former industrial activity on the sites has often left a legacy of soils contaminated with unacceptably high levels of petroleum products, heavy metals, and/or VOCs.
While a number of techniques exist for the remediation of contaminated soils, one of the largest problems is often the initial site assessment. It can be a difficult, expensive and time-consuming process to determine the exact extent of site contamination; often involving the digging of test pits and wells, and the analysis of samples by commercial laboratories. The inherent inaccuracies in these processes also generate further costs; to be sure of meeting regulatory requirements contractors often remove (or treat) more soil than is strictly necessary. As a result there is a great potential market for a system that can help contractors assess levels of contamination without the need for large-scale test bores and pits.
The U.S. Military has a large number of contaminated sites and over the past five years has developed a system for the rapid assessment of contaminated sites. Site Characterization and Analysis Penetrometer System (SCAPS) is a truck-based mobile laboratory that has saved the forces millions of dollars through providing accurate assessment of sites across the United states.
The core of the SCAPS system is a truck-mounted 20 tonne cone penetrometer (see figure 1). Cone Penetrometer Technology (CPT) involves using hydraulic equipment and push rods to force a cone-shaped 5 cm diameter probe into the soil to take various measurements. Although the technology was developed in the Netherlands in 1934, it was first widely used (for geotechnical measurements) in the late 1980s. It is only in the last few years that sensors have been developed which allow CPT to be used for the assessment of soil and groundwater contamination.
Figure 1: SCAPS system
Using a penetrometer has a number of advantages over drilling boreholes and wells. It is many times quicker, disturbance to the site is much less, and the only waste generated is from the steam cleaning of the push rods after use. Additionally, because the holes created by SCAPS are so small, they can simply be filled with grout, which prevents contamination spreading from sampling point to sampling point. (The grout, which is a mixture of cement, bentonite, and water, is pumped through an internal tube in the cone penetrometer, the cone tip is ejected and the hole is filled with grout as the push rods are retracted.)
The SCAPS system uses a number of sensors to allow it to evaluate contamination by Petroleum, Oil and Lubricants (POL), heavy metals, VOCs, and explosives.
Petroleum, Oil and Lubricants (POL)
Petroleum, oil and lubricant contamination is detected by SCAPS using a Laser-Induced Fluorescence (LIF) Sensor. Light from an ultraviolet (UV) laser is channelled down a fiber optic cable into the cone penetrometer. It then passes through a sapphire window in the side of the probe, out into the surrounding soil. The UV radiation causes the POL contamination to fluoresce (emit light). This light is transmitted up a second fiber-optic cable and analysed in real-time in the SCAPS on-board laboratory. This allows continuous collection of data as the probe passes down through the soil.
The SCAPS LIF has obtained numerous evaluations and certifications through state and federal regulatory agencies in the United States. A cost/benefit analysis conducted by DOE (DOE report no. LAUR-91-4016) indicates that at least 25 to 35 percent cost reductions can be achieved with the SCAPS LIF technology. The patented Tri-Service SCAPS LIF Sensor is licensed, commercially available, and in use worldwide.
The SCAPS system can use two techniques for the detection and measurement of heavy metals: the Laser Induced Breakdown Spectroscopy (LIBS) sensor (for unsaturated soils) and the X-Ray Fluoresence (XRF) sensor (usable in both saturated and unsaturated soils).
X-Ray Fluorescence Sensor
The XRF Sensor uses tried and tested technology to allow the SCAPS system to detect heavy metals in both saturated and unsaturated soils. The XRF can detect heavy metals at levels below 100 ppm, up to the full depth allowable by the penetrometer.
The SCAPS XRF Metals Sensor operates by detecting the characteristic wavelengths of x-rays emitted by metal atoms in the soil when excited by an x-ray source. The sensor's probe tip contains an x-ray source, which is used to bombard soil particles. These incident x-rays cause metal atoms to fluoresce at specific wavelengths. These emitted x-rays are detected at the probe tip and allow identification (and quantification) of metal concentrations in real time.
Figure 2: XRF detector schematic
XRF offers several advantages over optical spectroscopies. XRF excites atomic states which are mostly independent of the chemical state of atoms and requires no sample preparation. The relatively high energy of the x-rays make them penetrate any type of matter for distances of several microns to several millimeters, regardless of optical transparency. Any type of atom with sufficiently energetic core levels can be detected in any matrix.
LIBS sensors are a new technology. Two alternate prototypes are being trialled - one developed by the Naval Research and Development (NRaD), the other by U.S. Army Engineer Waterways Experiment Station (USAEWES). After evaluation and selection of the final design, the LIBS sensor will be submitted for regulatory acceptance.
Both LIBS protoypes work by focussing high-power pulsed lasers, which generate a plasma in the soil outside the probe. For a brief period in the process the plasma emits light, and it is the wavelengths of this light that indicates the contaminants present. Furthermore, the brightness of the light at a metals wavelength indicates how much of that metal is present. This information is analyzed onboard the SCAPS truck (using a spectrophotometer) to obtain qualitative and quantitative data.
Figure 3: LIBS detector results
The sensor developed by NRaD is configured with the laser in the SCAPS truck utilizing a fiber optics system, while the USAEWES configuration houses the laser and fiber optics components within the probe. Commercial probes are now also available from Science & Engineering Associates Inc (www.seabase.com).
Volatile Organic Compounds (VOCs) remain relatively difficult to measure in-situ. However, the SCAPS system provides two rapid and cost-effective methods for their analysis. The first, designed for analysis of VOCs in groundwater, uses commercially available Hydropurge® samplers. The second, used for measurement of VOCs in soil vapor, uses thermal desorption. Both methods ultimately use Ion Trap Mass Spectrometry for quantification of the VOCs.
VOCs in groundwater with helium sparging
In this method, the cone penetrometer is used to push commercially-available Hydropunch ® and Powerpunch probes into the soil. The probes are pushed to the desired depth and the push pipes are retracted, exposing a screen in the probe to the groundwater. Once sufficient groundwater has entered the probe and come to equilibrium with water in the surrounding soil (usually about 15-20 minutes) a sparge module is lowered into the probe.
The sparge module purges the VOCs from the groundwater using Helium gas. The helium containing the VOCs is piped up to an Ion Trap Mass Spectormeter (ITMS) where the contaminants are analyzed in real-time. The ITMS, using the conditional EPA method 8265, is capable of detecting most VOCs qualitatively and quantitatively in the low part per billion (ppb) range.
Figure 4: VOC sampling schematic
The reliability of in situ, direct sparging of VOC analytes from groundwater in concert with the ITMS has been successfully demonstrated at numerous sites in conjunction with the Environmental Security Technology Certification Program (ESTCP). The technology is currently being evaluated by the California Environmental Protection.
Thermal Desorption Sampler
The Thermal Desorption VOC Sampler principle of operation is based on capturing a known volume of soil in situ and heating the soil plug while purging the released VOCs. The Sampler is pushed to the ground depth and an interior rod retracts the penetrometer tip. The probe is then pushed further into the soil, collecting a 5 gram soil plug in the sample chamber. The soil plug is heated, releasing the VOCs. These VOCs are carried to the surface by an inert gas, where they are trapped on an adsorbent media. The trap is then thermally desorbed into an onboard, field portable ITMS where the contaminants are analyzed in near-real time.
Figure5: VOC detection using thermal desorption
After each sample, the soil plug is then expelled from the sample chamber. The sample chamber is heated and purged to remove any residual contamination. This process can be repeated at multiple depths during a single push. Upon completion of the push, grouting is required to seal the penetrometer hole in order to ensure there is no seepage or cross-layer contamination.
The Thermal Desorption Sampler can also be used as a vapor sampler in the vadose zone by applying a vacuum to the transfer line to draw soil vapors to the surface where they are trapped, desorbed, and analyzed by the ITMS in near-real time.
The reliability of in situ, thermal desorption of VOCs from soil in concert with the ITMS has been successfully demonstrated at various sites in conjunction with the ESTCP.
Camp Pendleton, California
(SCAPS system saved $600,000)
In mid 1995, the US Navy was planning to spend $620,000 to dewater or move 19,000 cubic yards of soil from a site containing fuel residue. A SCAPS investigation of the site produced a more accurate "picture" of the contamination boundaries, showing the plume was smaller than indicated by a prior investigation. The SCAPS results showed dewatering wasn't necessary and only half the original amount of soil needed excavation and treatment. Total savings: more than a year of work and $600,000.
For more information about this project, please contact: Mr. Rod Soule - (619) 556-9506 900RSOUL@pwcsd.nosc.mil
FISC Fuel Farm, Point Loma, California
(SCAPS system saved $1 Million)
An initial analysis of this site suggested the presence of more than 9,000 tons of diesel-contaminated soile. The Navy's proposal to excavate and remediate the soil with thermal desorption would have cost approximately $1 million. However a SCAPS investigation indicated the contamination was mostly near the surface and did not extend to the water table. The data allowed the Navy to close the site without spending any money on remediation levels of contamination were deemed acceptable by San Diego County regulators.
For more information about this project, please contact: Mr. Rod Soule - (619) 556-9506 900RSOUL@pwcsd.nosc.mil
Longhorn Army Ammunition Plant, Texas
(SCAPS system saved $186,000)
SCAPS and gas chromatography gave the Army Corps of Engineers Tulsa District a fast, cost-effective method to investigate storage tank sites in the plant's production area. Analyzing volatile organics using traditional methods would have cost $12,000; SCAPS did the job for $2,040. The installation of 71 traditional wells earlier in the project, took two drills rigs and crews about 60 days to complete; a SCAPS crew needed only 15 days to install, sample and analyze 112 temporary wells saving $186,000. Disposal of the 400 drums of waste soil and water generated during the traditional sampling cost about $500,000. The SCAPS equipment produced only 3 drums of waste water and cost only $1500 for disposal.
For more information about this project, please contact: Ms. Liz Herman - (918) 669-7150
The SCAPS penetrometer and mobile laboratory system combines a number of exisitng and new technologies to create a unique tool for the rapid, accurate and cost-effective assessment of contaminated soils. In the United States, the SCAPS system is now available for lease from the US military, and it is estimated that its use can save more than 33% of the cost of soil remediation, by providing accurate assessment of the extent of contamination. Such a system has much potential to save companies around the world millions of dollars in the remediation of brownfield sites.