INTERLABORATORY STUDY OF A METHOD FOR DETERMINING NONVOLATILE ORGANIC-CARBON IN AQUIFER MATERIALS

The organic carbon fraction in aquifer materials exerts a major influe nce on the subsurface mobilities of organic and organic-associated con taminants. The spatial distribution of total organic carbon (TOC) in a quifer materials must be determined before the transport of hydrophobi c organic pollutants in aquifers can be modeled accurately. Previous i nterlaboratory studies showed that it is difficult to measure TOC conc entrations <0.1% in aquifer materials, when total inorganic carbon (TI G) concentrations are >1%. We have tested a new analytical method desi gned to improve the accuracy and precision of nonvolatile TOC quantita tion in geologic materials that also contain carbonate minerals. Four authentic aquifer materials and one NIST standard reference material w ere selected as test materials for a blind collaborative study. Nonvol atile TOC in these materials ranged from 0.05 to 1.4%, while TIC range d from 0.46 to 12.6%. Sample replicates were digested with sulfurous a cid, dried at 40 degrees C, and then combusted at 950 degrees C using LECO or UIC instruments. For the three test materials that contained > 2% TIC, incomplete acidification resulted in a systematic positive bia s of TOC values reported by five of the six laboratories that used the test method. Participants did not have enough time to become proficie nt with the new method before they analyzed the test materials. A seve nth laboratory successfully used an alternative method that analyzed s eparate liquid and solid fractions of the acidified sample residues.