Organic Carbon Sampling and Methodology Project: Yakima Railroad Area Report prepared for the Department of Ecology March 1997 Christene L. Albanese and *Richelle M. Allen-King Department of Geology, Washington State University, Pullman, WA 99164-2812 Rick Roeder, Department of Ecology, Central Regional Office, Yakima, WA 98902-3401 *Corresponding Author: R.M.Allen-King; email: [email protected]; phone 509-335-1180 Table of Contents Introduction ..........................................................................................................................1 Descriptions of Established Analytic TOC Methods ...........................................................2 Materials and Methods .........................................................................................................5 Sorbents ..................................................................................................................5 Sample Preparation ..................................................................................................6 Analytical Procedures ..............................................................................................7 Difference Method .......................................................................................7 Pre-acidification: WSU ................................................................................7 Pre-acidificaion: Manchester Environmental Lab .......................................9 Error Analysis ..........................................................................................................9 Results and Discussion ......................................................................................................10 Method Comparison...............................................................................................10 Difference and pre-acidification methods ..................................................10 Method Choice: YRRA..............................................................................11 Sample Preparation and Field Sampling Effects ...................................................11 Interlaboratory Comparison .......................................................................11 Grains Size and Roots ................................................................................12 Error Analysis ........................................................................................................14 Summary ............................................................................................................................17 References ..........................................................................................................................18 Appendix A - Yakima Railroad Area Data Yakima Railroad Area TOC Data ...................................................................... A-1 Analytical Replicates of YRRA Samples ........................................................... A-2 TOC vs Depth, YRRA Samples .......................................................................... A-3 Appendix B - Error Propagation Method ........................................................................B-1 Appendix C - Quality Assurance Project Plan (QAPP) ...................................................... C Organic Carbon Sampling and Methodology Project: Yakima Railroad Area March 16, 1997 INTRODUCTION The magnitude of sorption, distribution of a contaminant between the subsurface solids and groundwater, is a critical parameter in determining site specific clean-up levels which are protective of groundwater. Sorption of many of the most frequently detected groundwater contaminants, such as chlorinated solvents and other hydrophobic organic chemicals (HOC), is directly proportional to the fraction organic carbon (foc) content, also expressed as total organic carbon (TOC), of the sediments (e.g. Schwarzenbach and Westall, 1981). Even in subsurface sediments with very low foc a direct correlation between foc and the magnitude of HOC sorption, Kd, has been determined. Accurate measurements of foc, in conjunction with standard physicochemical property information for the contaminant of interest (such as solubility, octantol-water partitioning coefficient, or organic carbon partitioning coefficient), can be used to estimate site specific sorption. Thus, accurate measurements of organic carbon are of particular importance to environmental scientists and remediation planners. Subsurface geologic samples, such as those comprising most aquifers, often have low TOC, and present particular analytical challenges because most analytical techniques for measuring TOC were developed for use on organic rich, carbonate poor, surface soils. Although a number of studies have been conducted to compare various aspects of organic carbon analytical techniques (e.g. Powell et al., 1989; Lee and Macalady, 1989) there are still questions as to which method should be used for different types of sediments. The methods described in this report measure TOC, defined as all organic carbon that is nonvolatile, including dissolved organic carbon. The four primary goals of this work were: 1) to summarize the strengths and limitations of existing analytical approaches for TOC in geologic materials; 2) to compare the two approaches to TOC analysis which are most appropriate for subsurface materials using a test sample set with different chemical and geological attributes (method comparison); 3) to apply an appropriate technique to a set of Washington samples; and 4) to address TOC sampling and preparation effects. The method comparison included an evaluation of two of the approaches using a test set of six geologic samples with varying proportions of TOC and carbonate (inorganic carbon, IC) contents, and a comparison of acid treatment methods. The samples analyzed in this study consist of four samples supplied by Washington State University and 36 samples from the Yakima Railroad Area (YRRA) supplied by the Washington State Department of Ecology. This report will be useful to Ecology staff: in evaluating TOC analyses, in selecting appropriate sampling and analytical methods for particular samples, and in providing data for a set of Washington samples. 1 Organic Carbon Sampling and Methodology Project: Yakima Railroad Area March 16, 1997 DESCRIPTIONS OF ESTABLISHED ANALYTIC TOC METHODS There are four basic approaches to measuring TOC in soils and sediments, outlined in Table 1. The first approach assumes that the amount of carbonate in the sample is negligible relative to the amount of TOC. TOC can then be measured by one analysis using total carbon methods which do not require extensive sample preparation. The second approach is to measure total carbon and carbonate (inorganic carbon, IC) by two separate analyses to determine the percent of each. The computed difference is the TOC. The third approach requires pre-acidification of the sample to remove carbonate prior to total carbon analysis of the remaining carbon, TOC. The fourth approach measures TOC directly by a titration method, also known as wet oxidation. A known amount of dichromate is added to the sample which should oxidize all organic compounds, assuming all organic carbon has an average valence of zero. TOC is determined by titrating all remaining unreduced dichromate. Table 1 - Four Basic Approaches to Total Organic Carbon Measurement Approach Advantages Disadvantages Limitations 1 Total Carbon One Analysis Large errors if IC is (Assume IC is present negligible) 2 Difference (TC-IC) Minimize Sample Two separate High IC samples Handling analyses show high errors 3 Pre-Acidification Minimal Errors Use of Acid Acid must be (to remove IC) compatible with instrumentation 4 Direct Measure of TOC Simple titration Only good for Does not completely procedure organic rich oxidize all organic samples carbon IC - inorganic carbon, TC- total carbon, TOC - total organic carbon On page 4, Table 2 supplies additional information, outlining specific methods for each approach, supplying descriptions of appropriate sample types, and listing references for each method. Although four approaches have been identified from a review of the literature, only two, the difference and pre-acidification approaches, measure TOC consistently and accurately for subsurface geologic materials. Approach 1 is not appropriate unless the IC is negligible relative to the TOC. Approach 4 was developed to quantify larger TOC values in surface soils and sludge material rich in organics. This approach is not appropriate for the analytical accuracy required to determine site specific sorption (Plumb, 1981). Wet oxidation methods do not completely oxidize more refractory organic matter, thus can underestimate the “true” TOC (Powell et al., 1989). Lee and Macalady (1989) found that the standard deviation for wet oxidation replicates were 6 to 20 times greater than for high temperature combustion. The accuracy of the difference method, Approach 2, depends on the accuracy of the two separate analyses of total carbon and carbonate (Table 2). Although total carbon methods which use high 2 Organic Carbon Sampling and Methodology Project: Yakima Railroad Area March 16, 1997 temperature combustion are reproducible, the inorganic methodology is not as consistent from lab to lab, especially if dolomite is present in the sample (Nelson and Sommers, 1996, Caughey and Barcelona, 1994). This approach yields reasonable results when the absolute error of the carbonate measurement is low compared to the TOC. In