DETERMINATION OF NANOGRAM PER LITER CONCENTRATIONS OF VOLATILE ORGANIC-COMPOUNDS IN WATER BY CAPILLARY GAS-CHROMATOGRAPHY AND SELECTED-ION MONITORING MASS-SPECTROMETRY AND ITS USE TO DEFINE GROUNDWATER-FLOW DIRECTIONS IN EDWARDS-AQUIFER, TEXAS

method has been developed to measure nanogram per liter amounts of sel ected volatile organic compounds (VOCs) including dichlorodifluorometh ane, trichlorofluoromethane, cis-1,2-dichloroethene, trichloroethene, tetrachloroethene, and the isomers of dichlorobenzene in water. The me thod uses purge-and-trap techniques on a 100 mL sample, gas chromatogr aphy with a megabore capillary column, and electron impact, selected i on monitoring mass spectrometry. Minimum detection levels for these co mpounds ranged from 1 to 4 ng/L in water. Recoveries from organic-free distilled water and natural groundwater ranged from 70.5% for dichlor odifluoromethane to 107.8% for 1,4-dichlorobenzene. Precision was gene rally best for cis-1,2-dichloroethene, tetrachloroethene, and the dich lorobenzene isomers and worst for dichlorodifluoromethane and trichlor ofluoromethane. Blank data indicated persistent, trace-level introduct ion of dichlorodifluoromethane, 1,4-dichlorobenzene and tetrachloroeth ene to samples during storage and shipment at concentrations less than the method reporting limits. The largest concentrations of the select ed VOCs in 27 water samples from the Edwards aquifer near San Antonio, TX, were from contined-zone wells near an abandoned landfill. The res ults defined a zone of water with no detectable VOCs in nearly all of the aquifer west of San Antonio and from part of the confined zone ben eath San Antonio.