Characterization of Organic Matter in Soil and Aquifer Solids

The focus of this work was the evaluation of analytical methods to determine and characterize fractions of subsurface organic matter. Major fractions of total organic carbon (TOC) include: particulate organic carbon (POC) in aquifer material, dissolved organic carbon (DOC) and both volatile (VOC) and non-volatile (NVOC) organic carbon sub-fractions. POC makes up the bulk of TOC in contaminated and uncontaminated subsurface soils and aquifer materials. The volatile subfraction of POC can be determined quantitatively when minimally disturbed sub-cores are preserved immediately in the field. Methanol and acid addition (i.e. , HCl, NaHSO 4 ) to pH 2 are adequate preservatives for specific volatile organic compound determinations. An interlaboratory round-robin test to improve acidification and removal methods for carbonates in total carbon using sulfurous acid (H 2 SO 3 ) showed sensitivity to several factors. These factors include: operator care, acid strength and carbon content, and particularly, the incomplete removal of inorganic carbon at high total carbon to organic carbon ratios. Stable isotopic characteristics of NVOC from fuel contaminated and organic-enriched environments were found to be quite sensitive to the stable isotopic signatures of natural organic matter. The extractability of POC by a range of high to medium polarity solvents resulted in the observations that relatively little POC was extractable and water extracted comparable amounts to 1:1 mixtures of 0.01M KOH in methanol:toluene. Introduction Organic matter in subsurface systems is a complex mixture of natural organic substances, fossil fuels and a variety of synthetic compounds. The transport and fate of organic contaminants is quite dependent on the nature and distribution of organic carbon in general. Dispersion, sorption and degradation are processes which affect organic compound transport and fate. The estimation of the influence of these processes depends heavily on the quantitative determination of fractions of organic carbon in soils and aquifer materials (Powell et al., 1989). Conventional contaminant analytical methods have focused on constituents in fuels and synthetic mixtures (e.g., solvents, plasticizers and other chemicals) (Keith, 1991). Methods for determining volatile and non-volatile organic carbon (i.e., VOC and NVOC) in dissolved (DOC) and particulate (POC) fractions have seen relatively little attention in the literature or practice of subsurface environmental chemistry (Thurman, 1985). Methods for the determination of major carbon subfractions, as well as the specific organic compounds of which they are composed, must be based on quantitative preservation, separation, and analytical methods which lend themselves to routine practice. In this way, the roles, identity, and fates of specific organic contaminants may be incorporated into processlevel hydrogeological investigations. Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109-2099. Office of Environmental Chemistry, Illinois State Water Survey, Champaign, IL 61820. Departments of Geology and Chemistry, Western Michigan University, Kalamazoo, MI 49008. EPA/600/S-97/001 April 1997