Arsenic (As) mobility and transport in the environment are strongly influenced by arsenic's associations with solid phases in soil and sediment. We have tested a sequential extraction procedure intended to differentiate the following pools of solid phase arsenic: loosely and strongly adsorbed As; As coprecipitated with metal oxides or amorphous monosulfides; As coprecipitated with crystalline iron (oxyhydr)oxides; As oxides; As coprecipitated with pyrite; and As sulfides. Additions of As-bearing phases to wetland and riverbed sediment subsamples were quantitatively recovered by the following extractants of the sequential extraction procedure: As adsorbed on goethite, 1 M NaH2PO4; arsenic trioxide (As2O3), 10 M HF; arsenopyrite (FeAsS), 16 N HNO3; amorphous As sulfide, 1 N HCI, 50 mM Ti-citrate-EDTA, and 16 N HNO3; and orpiment (As2S3), hot concentrated HNO3/H2O2. Wet sediment subsamples from both highly contaminated wetland peat and less As-rich sandy riverbed sediment were used to test the extraction procedure for intra-method reproducibility. The proportional distribution of As among extractant pools was consistent for subsamples of the wetland and for subsamples of the riverbed sediments. In addition, intermethod variability between the sequential extraction procedure and a single-step hot concentrated HNO3/H2O2 acid digestion was investigated. The sum of the As recovered in the different extractant pools was not significantly different than results for the acid digestion.

Reconstructing the history of ambient levels of metals by using tree-ring chemistry is controversial. This controversy can be resolved in part through the use of selective microanalysis of individual wood cells. Using a combination of instrumental neutron activation analysis and secondary ion mass spectrometry, we have observed systematic inhomogeneity in the abundance of toxic metals (Cr, As, Cd, and Pb) within annual growth rings of Quercus rubra (red oak) and have characterized individual xylem members responsible for introducing micrometer-scale gradients in toxic metal abundances. These gradients are useful for placing constraints on both the magnitude and the mechanism of heavy metal translocation within growing wood. It should now be possible to test, on a metal-by-metal basis, the suitability of using tree-ring chemistries for deciphering long-term records of many environmental metals. M aterials such as continental ice and lake, ocean, and wetland sediments are currently used to develop records of past chemical conditions. However, the geographic distribution of suitable deposits is limited; furthermore, dating of these materials can be uncertain. Because of their relative ease of dating, large geographic range, and record length extending back to the preindustrial era, tree rings have long intrigued scientists from many disciplines as monitors of environmental conditions. Nearly three decades of work, however, have produced a literature that is highly polarized between those who have demonstrated that various tree species faithfully record and preserve records of environmental metal contamination and those who have documented that measured dendrochemical patterns of metals do not correlate with known changes in past environmental conditions (1).We contend that the reasons for this polarization stem from our general inability to address several fundamental questions. What are the contaminant pathways of metals within the treesoil-groundwater-atmosphere system? Is the uptake of nonessential metals proportional to ambient levels in the immediate environment of the tree? Once taken up into the stem wood, do the metals stay where they first interact with the xylem tissues or are they mobile? And finally, how long are these complex biogeochemical signals preserved within the stem wood? To determine the efficacy of using dendrochemical approaches to monitor metal loading histories, these processes that govern both the uptake and storage of metals within each potential biomonitoring species must be evaluated. In this paper, we propose methodologies that aid in quantifying these difficult-to-evaluate interdependent processes.We believe the dichotomy of observations concerning the use of tree-ring chemistry to monitor metals is a consequence of two closely related underlying problems: an inadequate understanding of the physiological controls on the rates of metal translocation within trees, and a reliance on analytical approaches that require the complete digestion and hence homogenization of bulk wood tissues. ...