Project Number: 2519-C9V01
Project Chief: Don Vroblesky
Cooperator: U.S. Environmental Protection Agency
Period of Project: 2006 to present
THIS PROJECT HAS BEEN COMPLETED AND IS BEING ARCHIVED IN ITS FINAL CONFIGURATION
The U.S. Geological Survey (USGS) invented an approach whereby a simple head-space analysis of tree cores in vial can be used to determine the distribution of volatile organic compounds (VOCs) in groundwater. The method is now being used worldwide.
Although much remains to be discovered regarding the uptake of contaminants by trees, a variety of influences on contaminant concentrations in tree cores have been examined in recent studies. Vroblesky and others (2004, Groundwater Monitoring and Remediation, v. 24, p. 124-138) examined the influence of rainfall infiltration, differences in concentrations from differing sides of the tree, changes in tree-core concentrations over time, depth to the water table, and potential influences from bacterial infestation of the tree. Recent work also has shown that vapor-phase VOC concentrations in the soil zone also are detectable in tree cores, sometimes with more impact than groundwater VOC concentrations. Thus, tree coring has potential for mapping vadose-zone as well as phreatic-zone applications. This leads to the possibility that tree-core VOC concentrations could have application to vapor-intrusion studies.
Because of the increase in the use of tree coring as a reconnaissance tool for examining subsurface VOC contamination and for monitoring phytoremediation progress, there is a need for a guidance document on this method. Users of the method should be made aware of the strengths and limitations of the method as well as the preferred approach for collecting the data. A user's guide for the method would provide the necessary information in one document.
The objective of the proposed work is to produce a document that would function as a user's guide to tree coring as a method for obtaining information on VOCs in the subsurface.
A major task involved in this effort is compiling and summarizing existing publications on the application of tree coring to examining subsurface VOC concentrations. Much information regarding this task already has been examined, but is disseminated in a variety of publications.
In addition, however, some questions remain regarding the use of this method. The questions include the maximum length of time allowed between core collection and insertion into a sealed vial. Another question concerns the type of sampling vial to be used. Standard volatile organic analysis (VOA) vials sometimes are used in tree-coring investigations, but these vials are technically only to be used for water samples, not gas samples. Are these vials adequate, or should septated crimp-top vials be recommended? Does heating the vials containing tree cores produce different concentrations than allowing the vials to equilibrate at room temperature? In addition, there may be other minor technique questions that will become apparent as development of the user's guide progresses.
Therefore, the scope includes a variety of tasks. The first task is a literature review. The second task is a compilation of the literature to produce a document addressing the state-of-the-art understanding of tree coring as applied to groundwater contamination investigations. The final task is to perform field tests to examine the questions cited above and selected additional questions that may arise during the course of the investigation.
The approach used in this investigation will be to conduct a literature review of information related to tree coring as an aid to investigating subsurface VOC contamination. The literature will be compiled and condensed to produce a document detailing the state-of-the-art understanding of the method in terms of approach, strengths, weaknesses, range of application (including types of applicable compounds), and case studies.
A second part of the approach will be to conduct field investigations to answer selected questions regarding the mechanics of the sampling approach, as outlined in the scope above. Cliff Casey (Southern Division Naval Facilities Engineering Command, Charleston, South Carolina) has given verbal permission to use trees located at Solid Waste Management Unit (SWMU) 17, Naval Weapons Station Charleston, South Carolina, for this aspect of the investigation. The target trees at SWMU 17 have been previously sampled by the USGS and are known to contain substantial concentrations of chlorinated ethenes.
Examination of the question regarding the maximum length of time between sample collection and isolation of the sample in a vial will be examined by collected several tree cores in close proximity from the same tree. The cores will be allowed to stand in open air for various lengths of time prior to sealing them in vials. After an appropriate equilibration time, they will be analyzed to examine potential VOC losses. To examine the influence, if any, of the type of collection vial, duplicate core samples will be collected in differing vials and the resulting concentrations will be compared. This will be done on several pairs of cores to allow statistically meaningful results. The expected product from this investigation is a publication outlining a preferred approach to tree-core collection for VOC investigations and examining strengths and weaknesses of the approach. The format of the publication and the outlet series is subject to negotiation.
Production of a user's guide to the use of tree-coring as a method for obtaining data on subsurface volatile organic compounds will allow potential users of the method to have a central location to find relevant information on the method. It also will streamline their efforts by allowing them to optimize their chances of having a successful outcome from such an investigation. For example, it may not be widely known that VOC concentrations in tree cores can be rapidly lowered by dilution following rainfall infiltration (Vroblesky and others, 2004). Thus, it would be of value to have this information available so that potential users could plan tree-coring surveys to not immediately follow a rainfall event. Knowledge that tree-core VOC concentrations sometimes may more closely approximate soil-vapor concentrations than groundwater concentrations may aid the user in data interpretation. These and other considerations could be of great benefit when compiled in a single publication.