by Ioana Petrisor, Ph.D.
Trees provide beauty and health to our planet. They support life and create scenic landscapes. Beyond all these benefits, trees provide sustainable environmental tools and solutions to many contamination issues. Phytoremediation, a proven remediation technique, involves the use of trees to extract toxic chemicals from the subsurface and prevent the spread of contaminants. This ability of trees to extract contaminants expands their use from site clean-up and rehabilitation to other important environmental fields including site assessment and environmental forensics. The emerging use of trees for site assessments and forensic studies is still a relatively new technique.
Tree-ring fingerprinting to age-date contamination
One of the most innovative and perhaps also one of the least known applications of trees relates to their use in environmental forensics for accurate age-dating, source identification, and reconstruction of subsurface transport of contaminants over time. Such applications provide reliable evidence in litigation, helping to identify responsible parties and to cover remediation costs. In addition, trees provide useful characterization tools at non-litigated sites, enabling efficient remediation design.
The forensic use of trees is based on some basic principles governing the well-established science of dendrochronology which uses information from tree growth rings to reconstruct past climates. The general principle is based on the fact that in most temperate climates trees produce yearly growth rings which are visibly demarcated, can be measured, and are directly related to the tree health during the time of their formation. Therefore, ring width patterns may provide information related to climatic factors influencing tree health over time. Similarly, environmental forensics uses the information stored in tree growth rings to reconstruct past contamination events. This use of trees involves the measurement of both ring width and chemical composition via a technique usually referred to as dendroecology or tree-ring fingerprinting.
Tree-ring fingerprinting provides the width and the chemical elemental composition of growth rings from trees exposed to contamination. When contaminants get into the root zone, they are absorbed and transported up the trunk along with water and nutrients and are circulated only through the outermost ring formed in that particular year. Once in the tree, contaminants migrate through the tree trunk toward branches and leaves. During their migration, contaminants leave traces in the ring cells of the outermost ring. Traces consist of so called “elemental markers”. These are elements (other than carbon, hydrogen and oxygen – the building blocks of trees) present in targeted contaminants such as: chlorine for chlorinated solvents, lead for leaded gasoline, sulphur for Diesel and fuel oils, and nickel and vanadium for crude oils and heavy distillates.
By measuring elemental markers ring by ring, we can observe when chemical peaks occur and link those to the year(s) when the contamination entered he tree.This allows us to age-date contamination events. As rings form on a yearly basis, contamination may be age-dated to the year which is more precise than any other age-dating technique available today. Moreover, the accurate age-dating capability may extend as much in the past as the tree age allows (i.e., hundreds or even thousands of years) and the method can be applied in the absence of contamination at the time of sampling (e.g., at mitigated sites). Note that this age-dating technique is more precise when the selected trees are located close to the suspected source. When more trees are available in the study area (at different distances from the source), the reconstruction of plume subsurface movement over time becomes possible through tracking chemical elemental peaks in consecutive years in various trees.
Sampling the trees for fingerprinting studies is easy and fast (10-15 minutes per tree) using hand-held increment borers (Figure 1) which extract tree core samples. The extracted core (Figure 2) should be dried at room temperature, sanded in order to produce a polished surface (Figure 3) and then analyzed using microdensitrometry images for ring width measurements and energy-dispersive X-Ray fluorescence (EDXRF) for chemical elemental analysis. More information on the forensic use of trees, as well as on sampling and analysis techniques can be found in my recent text book, Environmental Forensics Fundamentals: A Practical Guide (Petrisor, 2014).
Trees also provide sustainable tools in site assessment. A method referred to as “phytoscreening” uses tree core samples to detect and map subsurface contamination without the need for disruptive soil borings or monitoring wells. The method is fast and accurate as shown by USGS. This process is simpler than tree-ring fingerprinting as it involves the testing of contaminants from the whole core sample with no ring-by-ring analysis needed. Phytoscreening may also be performed using tree branches instead of tree cores, making the method even more simple and time-effective (Gapalakrishnan et al., 2007).
Both aesthetically pleasing and nearly ubiquitous, trees provide some of the most accurate, sustainable, and innovative environmental tools and the means to decontaminate our planet. There is no doubt that we are going to see an increase use of these natural tools in site investigations and remediation in the years to come.
Publications regarding phytoscreening include those by Sorek et al. (2008); Larsen et al. (2008); & Burken et al. (2011)
Burken, J.G., Vrobleski, D.A., Balouet, J.C. 2011. Phytoforensics, Dendrochemistry, and Phytoscreening: New Green Tools for Delineating Contaminants from Past and Present. Environmental Science & Technology 45(15): 6218-6226.
Gapalakrishnan, G., Brgri,, M.C., Minsker, B.S., Werth, C.J. 2007. Monitoring Subsurface Contamination Using Tree Branches. Groundwater Monitoring & Remediation 27(1): 65-74.
Larsen, M., Burken, J.,Machackova, J., Karlson, U.G., Trapp, S. 2008. Using Tree Core Samples to Monitor Natural Attenuation and Plume Distribution after a PCE Spill. Environ. Sci. Technol. 42: 1711-1717.
Petrisor, I.G. 2014. Environmental Forensics Fundamentals: A Practical Guide. CRC Press, Taylor & Francis Group, 395 pages.
Sorek, A., Atzmon, N., Dahan, O., Gerstl, Z., Kushisin, L., Laor, Y., Mingelgrin, U., Nasser, A., Ronen, D., Tsechansky, L., Weisbrod, N., Graber, E.R. 2008. “Phytoscreening”: The Use of Trees for Discovering Subsurface Contamination by VOCs. Environ. Sci. Technol. 42: 536-542.
About the Author:
Dr. Petrisor is Great Ecology’s Senior Biochemist and a leading environmental forensic scientist with over 20 years of experience. She specializes in determining the source and age of environmental contaminants using innovative forensic fingerprinting techniques. She has served as an expert witness, is the Editor-in-Chief of the Environmental Forensics Journal, and recently published a text book, Environmental Forensics Fundamentals: A Practical Guide, on forensic fingerprinting techniques.