Total Organic Carbon Determinations in Natural and Contaminated Aquifer Materials, Relevance and Measurement

摘要

Quantifying the total organic carbon (TOC) content of soils and aquifer materials is essential for understanding subsurface chemistry during environmental site characterization. Contaminant fate and transport, microbial ecology, and effective treatment methodology are all influenced by the quantity of both naturally occurring and anthropogenic organic carbon present. Unfortunately, the determination both in natural and contaminated settings is complicated by many factors. These include the system chemistry and heterogeneity, sampling methodology, and analytical limitations. Accurate determination becomes increasingly difficult as the total carbon (TC):TOC ratio increases, due to carbonate components of the TC and the difficulty of removing them without affecting the TOC. Interlaboratory data shows variations of 39% between laboratories analyzing the same sample, even with appropriate pretreatment. Further errors occur, enhanced by the limited sample sizes used in most instruments, when widely dispersed or heterogeneously distributed carbon particles are present. Additional complexity is introduced when the sample is from the saturated zone and contaminated with volatile organic compounds (VOCs). This is illustrated by data from the aviation gasoline and JP-4 jet fuel spills at the Traverse City Coast Guard Air Station bioremediation projects. Carbonate removal is impossible in this case and requires a separate determination, doubling the TOC measurement inaccuracy. This, combined with volatiles losses during handling and potential transport of unreacted volatiles through the instrument, results in observed coefficients of variation of 52%, with values 73% lower than a gas chromatographic (GC) method. Standardizing TOC determinations of subsurface materials, from both the saturated and unsaturated zones, must be accomplished for consistent interlaboratory results. Standard reference aquifer materials are necessary. Attempts to increase sample integrity, from field collection to analysis, particularly with volatile contaminants present, are needed. An instrument design prototype that should be useful for water-saturated, volatiles contaminated aquifer materials is presented. Cooperation between field investigators, analysts, and instrument manufacturers will be required to improve data quality and usefulness.