Pollutant plume delineation from tree core sampling using standardized ranks Agung Wahyudi a, * , Patrick Bogaert a , Stefan Trapp b , Ji  rina Machá  cková c a Earth and Life Institute, Environmental Sciences, Université Catholique de Louvain, Croix du Sud, 2 bte L7.05.05, B-1348, Louvain-la-Neuve, Belgium b Department of Environmental Engineering, Technical University of Denmark, Lyngby, Denmark c AECOM CZ s.r.o., Liberec, Czech Republic article info Article history: Received 20 June 2011 Received in revised form 7 November 2011 Accepted 9 November 2011 Keywords: Tree core sampling Standardized rank Contaminated site Spatial mapping abstract There are currently contradicting results in the literature about the way chloroethene (CE) concentrations from tree core sampling correlate with those from groundwater measurements. This paper addresses this issue by focusing on groundwater and tree core datasets in CE contaminated site, Czech Republic. Preliminary analyses revealed strongly and positively skewed distributions for the tree core dataset, with an intra-tree variability accounting for more than 80% of the total variability, while the spatial analyses based on variograms indicated no obvious spatial pattern for CE concentration. Using rank trans- formation, it is shown how the results were improved by revealing the initially hidden spatial structure for both variables when they are handled separately. However, bivariate analyses based on cross- covariance functions still failed to indicate a clear spatial correlation between groundwater and tree core measurements. Nonetheless, tree core sampling and analysis proved to be a quick and inexpensive semi-quantitative method and a useful tool. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Chloroethenes (CE) are among the most encountered contami- nants at industrial sites, as they were widely used as cleaning and degreasing agents (Doherty, 2000). CE are of particular interest and concern here because they are known to be carcinogenic (Shapiro et al., 2004), neurotoxic, and associated with kidney failure in human beings (James et al., 2009). In the sub-surface, CE are characterized by high densities, low solubilities, and resistance to bio-degradation and chemical degradation (Ma and Burken, 2002). CE-related pollutions can last for a long period of time in the sub- surface. From a monitoring viewpoint, early examination of sites supposedly contaminated with CE is done by taking samples from soil and groundwater sub-surface. The collection of samples generally relies on invasive methods which are costly, time- consuming and may require heavy equipments e.g. for sampling boreholes (Schumacher et al., 2004). Moreover, these methods are sometimes difficult to deploy on the factory sites where boreholes installation may be hindered by buildings or buried structures. There is thus a growing demand for faster, inexpensive and less invasive indirect methods. Tree core sampling can be considered as one of them. Generally, the use of trees growing on contaminated sites (either naturally or artificially) can be classified into three main applica- tions which are (i) phytoremediation processes (James et al., 2009; Cherian and Oliveira, 2005; Gao and Zhu, 2003), (ii) delineation of a pollutant plume through tree core sampling (Vroblesky et al., 1999; Ma and Burken, 2002; Gopalakrishnan et al., 2007), and (iii) more recently, the monitoring of CE fate in the shallow groundwater (i.e. at less than 10 m depth) (Schumacher et al., 2004; Larsen et al., 2008). Particularly, for delineating the CE plume distribution, tree core sampling is of direct interest here as it is considered as an inexpensive, quick and environmental friendly method compared to direct and more invasive methods. However, this method also raised some important issues, as trees are not acting as ideal samplers of the sub-surface contaminants. More precisely, recent studies led to contradicting results about how CE concentrations measured from tree core samples correlate with those obtained from groundwater. Ma and Burken (2002), Vroblesky et al. (1999), and Gopalakrishnan et al. (2007) suggest there is a clear and strong correlation between the two, while Schumacher et al. (2004) and Sorek et al. (2008) indicate it is not the case, or at least that it is far from obvious. Whereas, Larsen et al. (2008) report a monotone, but not linear relation. For CE pollutions characterized by the presence of a contami- nation plume, it is expected that consistently high concentrations are to be found inside the plume area, whereas by comparison off- plume concentrations will be typically very low, thus creating * Corresponding author. E-mail address: agung.wahyudi@uclouvain.be (A. Wahyudi). Contents lists available at SciVerse ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2011.11.010 Environmental Pollution 162 (2012) 120e128